PROF. DANIEL ARIZTEGUI (Orcid ID : 0000-0001-7775-5127) Accepted Article Article type : Original Research Article Banded Iron Travertines at the Ilia Hot Spring (Greece): An interplay of biotic and abiotic factors leading to a modern Banded Iron Formation analog? Christos Kanellopoulos1,2, Camille Thomas2, Nikolaos Xirokostas3 and Daniel Ariztegui2,* 1 National and Kapodistrian University of Athens, Department of Geology and Geoenvironment, Panepistimioupolis, Ano Ilissia, 15784 Athens, Greece, e-mail: ckanellopoulos@gmail.com 2 Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Genève, GE, Switzerland, e-mail: Camille.Thomas@unige.ch , daniel.ariztegui@unige.ch 3 Institute of Geology and Mineral Exploration (I.G.M.E.), 1st Spirou Louis St., Olympic Village, 13677, Acharnae, Greece, e-mail: nikosxirokostas@yahoo.com * Correspondent author: daniel.ariztegui@unige.ch Abstract A hot spring at Ilia in the Greek Island of Euboea precipitates iron-rich travertine at an oregrade concentration (up to 35.3 wt% Fe). This hydrothermal chemical sediment system deposits bands of iron oxihydroxides (ferrihydrite), millimetres to centimetres thick, alternating with calcium carbonate dominated layers, creating “Banded Iron Travertine” (BIT). The ferrihydrite laminae display a dendritic texture formed of spherical nodules often covering filaments identified as bacterial stalks of Zetaproteobacteria. These microaerophilic iron-oxidizing bacteria were identified by their 16S rRNA gene sequences in ferrihydrite- This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/dep2.55 This article is protected by copyright. All rights reserved. enriched samples from areas under high water flow. They were missing in the Accepted Article aragonite/calcite-dominated samples exhibiting features of aerial exposure and cyanobacteria instead. These characteristics, and the relative depletion in Fe-rich layers of redox sensitive elements like Mn and Ce, as well as the presence of halite in Ca-rich layers, suggest that the bands form by successive changes in hydrothermal flow. This allowed microaerophilic iron oxidation to form Fe-rich layers, while Ca-rich bands precipitated when the hydrothermal water had time to equilibrate with the atmosphere. This seawater-dominated hydrothermal system is enriched in reduced iron and rapidly precipitating carbonates and ferrihydrite in the form of bands, having similarities to “Banded Iron Formation” (BIF). BIF represent archives of Earth’s primitive biogeochemistry although the combined abiotic and biotic processes that have likely led to their formation are not fully resolved. Diagenesis and metamorphism have a strong imprint on BIF. Thus, continuous efforts are pursued to identify modern analogues that could help unravel their formation. Although carbonate is not a common feature of BIFs, Ilia system provides an interesting analog for their depositional processes and potential microbial-mineral associations they may have hosted. It also presents pre-diagenesis facies association and mineralogy that could bring new clues for unraveling BIF modes of formation and the salient biogeochemical conditions characteristic of their original depositional environment. Keywords Calcium carbonates, ferrihydrite, hydrothermal springs iron-rich thermogenic travertines, Zetaproteobacteria. This article is protected by copyright. All rights reserved. INTRODUCTION Accepted Article Among the most important iron ore deposits on Earth are the spectacular and enigmatic Banded Iron Formations (BIFs). Despite extensive studies, their modes of depositions have not been fully resolved; although one of the prevalent models is that BIFs are thought to be the result of the reaction between hydrothermal reduced iron, and photosynthetically produced oxygen. Microbes are generally recognized as important contributors to the specific sedimentological conditions of these deposits, both regarding the processes of primary iron oxidation and secondary iron reduction (Posth et al., 2013). Since BIFs likely hold information on some of the earliest forms of life on Earth, they are extensively investigated and analogs for their mode of formation are thoroughly searched for. Active hydrothermal iron-rich deposits are among the targets of this search. They can be found in terrestrial environments i.e. iron-rich thermogenic travertines (Takashima & Kano, 2005; Takashima et al., 2011; Kanellopoulos et al., 2017) and in submarine hydrothermal environments (Pichler & Veizer, 1999; Kilias et al., 2013; Bortoluzzi et al., 2017). In these systems, the presence of microbial activity is most often identified at redox interfaces and has been suggested to play an active role in the formation of iron minerals such as iron oxides (Widdel et al., 1993; Konhauser, 1998; Kappler & Newman, 2004). Hot springs are ideal environments for iron oxidation, since the hydrothermal fluid is often enriched in Fe(II) and free of oxygen. In such conditions, a steep redox gradient develops in the water immediately adjacent to the spring vent through uptake of atmospheric O2 (Takashima et al., 2008). In near-neutral pH hydrothermal springs, the Fe-rich deposits consist primarily of ferric hydroxides (commonly as ferrihydrite). These ferric hydroxides precipitate readily abiotically in oxygenated environments or their precipitation can be microbially mediated and influenced (Konhauser, 1998; Chan et al., 2011). When microbes are involved, rates of iron mineralization can increase four-fold (Kasama & Murakami, 2001). These processes This article is protected by copyright. All rights reserved. generally occur at redox interfaces, where iron-oxidizing bacteria are commonly found and Accepted Article where microaerophilic conditions dominate (Konhauser, 1998). Such conditions have been suggested in the formation of BIFs, and iron-oxidizing bacteria as associated with the formation of iron bands in Precambrian BIFs (Holm, 1987, Chaudhuri et al. 2001; Konhauser et al. 2002). However, the study of iron mineralization in hydrothermal systems is often hampered by the poor accessibility of active sites, and the instable characteristics of ferrihydrite therein. This study characterizes the iron-rich thermogenic travertines of Ilia, on the Greek island of Euboea. The depositional system hosts almost pure ferrihydrite lamina which, together with fast precipitation rates, high iron enrichment and specific hydrothermal chemistry (Kanellopoulos et al., 2017) provides a unique setting for the study of laminated iron chemical sediments. The purpose of the work presented in this paper is to understand the depositional processes leading to the formation of laminated iron-rich deposits in Ilia and to further link them to BIF depositional models. This study documents the geochemistry of the Fe-rich hydrothermal fluids from the Ilia area, the macro-facies and micro-facies, the petrographical characteristics, mineralogical and geochemical composition of the newly formed iron-rich thermogenic travertine, and the microbial communities associated with the different facies of this deposit. The Fe-rich travertine is compared with active hydrothermal systems elsewhere and all of the available data synthesized to propose a model for Banded Iron Travertine (BIT) deposition, with possible analogy to the formation of BIFs. The goal is to provide a depositional model associated to an easily accessible and dynamic system precipitating banded iron-rich chemical sediments. This article is protected by copyright. All rights reserved. GEOLOGICAL SETTING Accepted Article Ilia is located in the north-western part of Euboea Island, Greece (Fig. 1). The geological setting of the region is unique since it is located at the western extremity of the North Anatolian Fault and is in a back-arc position with respect to the South Aegean Active Volcanic Arc (Pe-Piper & Piper, 2002; Vött, 2007; Ring et al., 2010). It is one of the most neo-tectonically active areas in Greece dominated by extensional tectonics similarly to the rest of the Aegean Sea. Ilia’s hot spring i.e. artesian borehole (38o 51΄ 07΄΄N, 23o 07΄ 44΄΄E) is one surface manifestation of a large geothermal system with other surface manifestations of hot springs in northern Euboea island and the neighboring area of Sperchios (mainland; Kanellopoulos, 2011; Kanellopoulos et al., 2017). This hydrothermal system is controlled by active tectonics and supplied with heat by a deep magma chamber (Kanellopoulos et al., 2017), identified at depths of 7-8 km under the Northern Euboean Gulf (Karastathis et al., 2011), which has the Plio-Pleistocene trachyandesitic volcanic center of Lichades as a surface manifestation. A salient feature of Euboea`s hot springs is the dominance of thermogenic travertine deposition (Kanellopoulos, 2011, 2012, 2013) including the Fe±As -rich ore-bearing travertines of Ilia (Kanellopoulos, 2011, 2012; Kanellopoulos et al., 2017). The hydrothermal fluids of northern Euboea’s hot springs have near neutral pH and a sodium-chloride composition. They are similar in their chemical characteristics and are classified as deep geothermal fluids, with volcanic origin affinities (Kanellopoulos et al., 2017). Based on geochemical and isotopic studies (Shimizu et al., 2005; Kanellopoulos et al., 2011; Dotsika, 2015; Kanellopoulos et al., 2017) their chemistry is controlled mainly by contribution of three components: i) high seawater participation, ii) deeper magmatic fluids, and iii) the local bedrock. The chemical composition of the thermogenic travertine, together with drilling data (Gkioni-Stavropoulou, 1998; Hatzis et al., 2008; Kanellopoulos et al., This article is protected by copyright. All rights reserved. 2017), strongly suggest that the source of calcium for the extensive travertine deposits of Accepted Article Euboea is the dissolution by geothermal fluids of marble/limestone located below the metamorphic sequence of Aedipsos and Ilia. MATERIAL AND METHODS Hot water samples Hydrothermal fluid samples from the hot spring/artesian borehole were collected from the mouth of the pipe (Fig. 2A through D). Physicochemical parameters such as temperature, pH, electrical conductivity (E.C.) and total dissolved solids (T.D.S.) were measured in-situ, using portable probes. The pH-meter was calibrated with standard buffer solutions of pH 4.0 and 7.0 before any measurements were taken. The pH measurement error, including accuracy and reproducibility, is better than ±0.05 pH units. Temperature was measured with the probe connected to the pH-meter and the error is estimated to be less than ±0.3 °C. Moreover, a thermal imaging camera was used in order to present through thermal images the temperature changes at the sampling site. Thermal imaging camera is a type of thermographic camera, which is reading infrared radiation as visible light and allow us to see areas of heat through water or solid rock. Water was collected in polyethylene bottles for laboratory analyses. Aliquots of 250 mL intended for the determination of trace elements were acidified to pH < 2 with analytical grade HNO3 (Suprapur 65%). All samples were stored in a portable cooler containing ice packs, transported to the laboratory and refrigerated at 4 °C until analysis. One-liter aliquots (non-acidified) were filtered in the laboratory before major ion determinations. The major ion concentrations were measured using spectrophotometry, titration and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES; Perkin Elmer This article is protected by copyright. All rights reserved. Optima 5300 DV). The trace element concentrations were measured using Inductively Accepted Article Coupled Plasma-Mass Spectrometry (ICP-MS; Perkin Elmer Sciex Elan 6100). Travertine samples The hydrothermal spring of Ilia vents from an artesian borehole accessible from the road that run along the beach. The water then flows along a 3-5 m long pipe and out onto the beach (Fig. 2). Samples of thermogenic travertine were collected from the top part of the borehole, from the venting point of the pipe and from the deposits at the beach (Fig. 2). The mineralogical composition of the main mineral phases in these travertines was identified with X-Ray Diffraction (XRD) using a Siemens Model 5005 X-ray diffractometer, Cu Ka radiation at 40 kV, 40 nA, 0.020o step size, and 1.0 sec step time. The XRD patterns were evaluated using the EVA program of the Siemens DIFFRACplus and the D5005 software package. The minor mineral phases were identified by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The SEM-EDS analyses were carried out using a Jeol JSM 5600 SEM instrument, equipped with an Oxford ISIS 300 micro analytical device. The travertine samples were analysed for whole rock geochemistry after drying and pulverizing in an agate mortar and mill to less than 0.075 mm. The samples were digested with a mixture of HCl-HNO3-HF acids and analysed with Inductively Coupled Plasma– Optical Emission Spectroscopy method (ICP-OES; Perkin Elmer Optima 5300 DV) for Ca, Na, P, S, Si, and by Inductively Coupled Plasma-Mass Spectrometry method (ICP-MS; Perkin Elmer Sciex Elan 6100) for a series of trace elements, including rare earth elements (REE). Analytical data quality was test with internal standards, certified and in-house reference materials in random positions within the analytical batch and by blank analysis and duplicate analysis of a proportion of the samples (Ramsey et al., 1987). This article is protected by copyright. All rights reserved. Molecular Biology Accepted Article Samples for microbiological studies were collected at the vent of the artesian borehole (Ilia Drill for ID-labelled samples, Fig. 3A and B) and at the pipe outlet (Ilia Pipe for IPlabelled samples, Fig. 3C and D). They experienced different environmental conditions at time of sampling, which are detailed in Table 1. Hard rock samples were taken using a chisel and a hammer cleaned and sterilized with ethanol. Soft material was sampled using a sterile scalpel. All samples were stored in 50 mL falcon tubes or zip-lock bags and kept at -20°C until further processing. Hard rock samples were then crushed under clean conditions in a sterilized agate mortar. Two grams of processed sample were used for DNA extraction using the MOBIO power soil kit. Triplicate DNA amplification was realized with a total of ~10 ng of DNA per triplicate using universal primer 515F (5′-GTGYCAGCMGCCGCGGTA-3′) and 909R (5′- CCCCGYCAATTCMTTTRAGT-3′) for the V4-V5 hypervariable region of the 16S rRNA gene (Wang & Qian, 2009), with indexes integrated following the dual-indexing procedure described by Kozich et al. (2013). Product triplicates were then pooled, quantified using Picogreen assay (Life Technologies, Carlsbad, USA) and pooled equimolarly. The final pool was concentrated using a SpeedVac Plus SC110A Savant and purified with CleanNA beads (Moka science). Sequencing was performed by Fasteris (Geneva, Switzerland) on an Illumina Miseq with 2x250 cycles, with settings of 7.5 Gb yield (including PhiX), an error rate of 2.5% and Q30 at 75%. The analysis yielded 5.3 Gb of sequences with an error rate within quality specifications. Adapters were removed using trimmomatic (Bolger et al., 2014), paired-ends reads were joined using ea-utils (Aronesty, 2011) quality checks were performed using FastQC, and samples were demultiplexed using the Fasteris in-house script. Fasteris also did all the downstream work described above. This article is protected by copyright. All rights reserved. DNA-derived 16S rRNA gene sequences were processed using Mothur (Schloss et al., Accepted Article 2009). Samples were dereplicated, aligned, and filtered by length. Chimeras were removed using uchime (Edgar et al., 2011), and taxonomic affiliation was then realized using the method of Wang et al. (2007) at a cutoff of 80% against the Silva SSU database 123 (Quast et al., 2013). Operational Taxonomic Units (OTU) were then defined at a 97% similarity and used for similarity analysis. Random subsampling was realized based on the smallest number of obtained sequences in one sample after singleton removal (see Tables S1 and S2 for more details). A matrix was built at the phylum level (excluding phyla that accounted for less than 1 % of the total community) and Principal Component Analysis (PCA) was performed using Past (Hammer et al., 2001). Sequences were submitted to NCBI and can be found under the accession number MH385389 - MH388018. RESULTS Water Chemistry Based on repeated measurements conducted over the sampling period and the previous years (Kanellopoulos et al., 2017), the physicochemical parameters of the hydrothermal fluid have only limited variation, with temperature varying from 60.9 to 63.7 °C and acidic pH values varying from 5.88 to 6.45 (Table 2, Fig. 2). Table 2 shows the physicochemical and chemical parameters analyzed in situ and in the laboratory. Chloride is the dominant anion of the Ilia fluid (1.28% Cl). Based on the hydrochemical analyses (Table 2), the fluid is characterized by Na-Cl hydrochemical type. Specific enrichment exists in major elements such as Fe (13.3 mg.L-1) and Ca (1570 mg.L-1) and in trace elements such as As (up to 250 μg.L-1), Mn (500 μg.L-1), Ni (up to 80 μg.L-1) and ammonium cation (up to 1.9 mg.L-1 ΝΗ4+, Table 2). It also shows concentrations of Sr of 24 mg.L-1, B of 9.7 mg.L-1, F of 3.8 mg.L-1, and Li of 3.4 mg.L-1. This article is protected by copyright. All rights reserved. Morphology of the travertine deposit Accepted Article Ilia’s hot water artesian borehole deposits thermogenic travertine at the discharge point, along the pipe, at the mouth of the pipe and at its outflow on the beach, building up the sides of a keel that slowly rises in height with time (Figs 2 and 3). Precipitation also occurs below the pipe as the water flows on the wall and onto the beach (Figs 2E, F and 3D) although deposition practically stops only a few metres from the pipe. Travertine deposition is also taking place inside the borehole (Fig. 3A). Periodic intervention of the local authorities to remove the deposits prevents the borehole and pipes from becoming completely clogged. The thermogenic travertine (Fig. 4) at the discharge point consists of separate Fe-rich (brownish-metallic colour) and Ca-rich (orange-yellowish colour) laminae/bands up to a few centimetres thick (Fig. 4B through D). The travertine macro-facies consists of laminated specular, knob-like and botryoid shapes (Fig. 4) which evolves into a dense layered crystalline travertine crust at the pipe mouth (Figs 2A, B, E, F and 4A, B) forming bands (Fig. 4C and DF) or thin layers (Fig. 4E and F) usually botryoidal in shape. A vertical cross section through several samples revealed a well-developed laminated texture in all of them (Fig. 4C through F), with band thickness ranging from submillimetre (Fig. 4F) to centimetre scale (Fig. 4C and D). Based on field observations, Ilia deposition is mainly orange in colour, suggesting Ca-rich component dominates. This colour expands along the hot water flow, until it reaches the sea. Dark-brown to black coloured surfaces (Fe-rich component) are observed where the hot water with a high flow-rate reaches the ground; in the top part of the borehole where water-level changes due to degassing (Fig. 3A); and inside the pipe, especially at water level, which changes depending on the water flow (Fig. 2C through F). This article is protected by copyright. All rights reserved. Mineralogical Study and microfacies of the travertine deposit Accepted Article Samples from Fe-rich and Ca-rich components were separated and analyzed by XRD (Fig. 5A). Based on these analyses, the Fe-rich component is composed mainly of two-line ferrihydrite and to a lesser extent of aragonite and calcite (Fig. 5B), as previously described by Kanellopoulos (2012) and Kanellopoulos et al. (2017). The Ca-rich component is composed mainly of aragonite and calcite with a minor contribution from ferrihydrite and halite (Fig. 5C). Under the optical microscope, the Fe-rich and Ca-rich bands are markedly distinct (Fig. 6A). The Ca-rich bands are mainly laminated (Fig. 6B), consisting of needle-like crystals of aragonite, growing normal to the substrate (Fig. 6C). The crystals are elongated parallel to the c-axis, exhibiting straight extinction. The substrate is usually a thin lamina of silt-sized micritic aggregates (Fig. 6C) or another Fe-rich lamina. In several cases, parallelogramshaped areas of aragonite, with rounded edges, are observed developing within Fe-rich masses (Fig. 6D), suggesting the complex 3-D spatial spreading of the two main components (i.e. Fe-rich and Ca-rich components). Under the optical microscope the Fe-rich components mainly consist of dark red to brown ferrihydrite. Combined optical microscope observations and SEM images show two ferrihydrite facies. The first type consists of laterally aligned shrub-like structures extending and branching upwards and forming laminae (Figs 6E through G and 7A, B). Typically, they are elongated, dense masses consisting of ferrihydrite micritic clumps, usually with spherical nodules creating “branches” and dendritic fabric (Figs 6E, F and 7F through H). Based on the classification scheme of Chafetz and Guidry (1999), they could be characterized as woody shrub or bush (i.e. bacterial shrubs). These shrubs are usually developed above aragonitic laminae (Fig. 6E). In some cases, dark-tinted axes (filament-like), potentially consisting of organic matter, can be observed encased within the ferrihydrite shrubs (Fig. 6G). The This article is protected by copyright. All rights reserved. filaments extend and branch upwards, over a distance corresponding to the thickness of the Accepted Article lamina. Revealed under SEM, the iron-rich components of the studied samples are commonly composed of fine-scale laminae of the iron-rich mineral phase (ferrihydrite) alternating with calcium-rich (aragonite) laminae over a few tens of micrometres (Fig 7A). Ferrihydrite has been observed in several cases to fill empty space between ferrihydrite shrubs, or between aragonite crystals (Fig. 7E) creating very dense laminae. However, in several cases empty spaces between the dendrite aggregate was observed inside the laminae (Fig. 7B). Additionally, ferrihydrite was identified in a second type of facies consisting of dense laminae, with very limited porosity (Figs 6D, H and 7C through E). In this case, no crystalline habit or microfacies of the ferrihydrite could be identified. Although, the ferrihydrite laminae followed the geometry dictated by the substrate (i.e. Ca-rich laminae or bedrock). Minor amounts of sulphides such as pyrite, As-rich pyrite, arsenopyrite, stibnite, galena, Fe-rich sphalerite, chalcopyrite and oxides, and non-metallic mineral phases such as barite, fluorite, and some REE-bearing phases, can be found in the studied samples. All of these minerals occur as very small crystals (few μm in size), in accordance to Kanellopoulos et al. (2017). Travertine geochemistry Samples from Fe-rich and Ca-rich components were separated to analyze their bulk geochemical composition (Table 3). The Fe-rich component of the Ilia travertine is enriched compared to the Ca-component (and typical travertines) in Fe (35.3%), As (2.6%), and Na (4%) for major elements and in Y (323 mg/Kg), Nd (96 mg/Kg), Dy (63 mg/Kg) for trace and rare earth elements (REE). In contrast, the Ca-rich components of the Ilia travertine is less enriched in all of these elements (11% Fe, 931 mg/Kg As, 0.08% Na, inc. all the REE), but This article is protected by copyright. All rights reserved. has higher concentrations of certain elements like Mn (330 mg/Kg), S (0.27 %) and other Accepted Article metals such as Co (0.24%), Cr (2.68%) and Ni (1.58%). Both components are presenting comparable concentrations to elements such as Cu (Fe-rich component: 6 mg/Kg and Ca-rich component: 8.4 mg/Kg), Mg (Fe-rich component: 1915 mg/Kg and Ca-rich component: 1650 mg/Kg), Pb (Fe-rich component: 4.7 mg/Kg and Ca-rich component: 2.9 mg/Kg), U (Fe-rich component: 240 μg/Kg and Ca-rich component: 290 μg/Kg), Zn (Fe-rich component: 63 mg/Kg and Ca-rich component: 48 mg/Kg). Normalization of the whole rock chemical data to the average composition of Upper Continental Crust (UCC, Rudnick & Gao, 2003) and Post-Archaean Australian Shale (PAAS, Taylor & McLennan, 1985; Fig. 8) indicates that both laminae are enriched in elements such as As, Fe and S. The Fe-rich component in particular is enriched 5440 times in As, compared to UCC abundances and almost 10 times in Fe, compared to UCC and PAAS abundances. Additionally, the Fe-rich component shows a clear enrichment in REE compared to UCC and PAAS, by more than 10 times (Fig. 8). The two studied components show similarities with respect to light REE depletion, but the Ca-rich component has small positive anomalies in Ce and Y, which are not present in the Fe-rich component. Analysis of microbial communities based on 16S rRNA gene sequences. Microbial communities differ from each other based on sample lithology and exposure to water/air (Fig. 9A). Bacteria-associated reads are dominant in all samples, but samples ID1 and ID4 have 19% and 26% of archaeal reads respectively, and IP2 and IP3 reach 44% and 28% (Fig. 9A). Principal component analysis shows the relationship between samples and the main phyla that characterize their relative composition (Fig. 9B). Samples IP2 and ID2 are marked by a high number of Aquificae of the Hydrogenotherma genus (40% and 65% respectively). They This article is protected by copyright. All rights reserved. also share Zetaproteobacteria of the Mariprofundales genus with IP3 and ID3 (11% for IP2, Accepted Article 5% for ID2, 3% for ID3 and 7% for IP3). IP3 and ID3, together with IP1 and ID5, are clearly characterized by Alphaproteobacteria and Cyanobacteria. Alphaproteobacteria are generally represented by unclassified members as well as Rhodobacterales, Rhodospiralles, Erythrobarcteraceae and Rhizobiales. ID1 and ID4 are both characterized by a large number of Thaumarchaeota (15% and 10%, respectively), Planctomycetes (34% and 17%, respectively), Chloroflexi (28% and 26%), Deltaproteobacteria (9% and 10%) and finally Deferribacteres (5% and 8%). Nitrospira are found in ID4, ID3 (13%) and form a significant portion of the Thermodesulfovibrio associated sequences. Sample ID2, however, is dominated by unclassified Nitrospira sequences (11% of the total relative abundance). Clades involved in iron reduction and oxidation are abundantly represented in the samples of the Ilia travertine. Members of the iron-oxidizing Zetaproteobacteria represent 3% of the community in ID2 and ID3. This clade is even more abundant in the Ilia pipe samples, where they correspond to 10% and 7% of IP2 and IP3 microbial communities, respectively. The presence of unclassified Rhodobacteracea may also be related to phototrophic iron oxidation, but this is not certain. DISCUSSION Hydrothermal fluid The hot spring/artesian borehole of Ilia degases mainly CO2 (D`Alessandro et al., 2014) and deposits Fe±As -rich thermogenic travertine (Kanellopoulos, 2011; 2012; Kanellopoulos et al., 2017). It is part of a large single hydrothermal system, which occurs at the northwestern part of Euboea island, and at the neighboring area of mainland i.e. Sperchios area (Kanellopoulos et al., 2017, Fig. 1). The hydrothermal fluids of NW Euboea`s hot springs are near neutral pH, sodium-chloride, with very close chemical relation between them This article is protected by copyright. All rights reserved. (Kanellopoulos et al., 2017). Ilia’s hydrochemical fluid type is Na-Cl (Table 2), with chloride Accepted Article concentrations (i.e. 1.28% Cl) comparable to Aegean Sea seawater (2.2% Cl, Athanasoulis et al., 2016). However, Ilia’s hydrothermal fluid has higher concentration of Sr (24 mg.L-1) and similar concertation of Li (3.4 mg.L-1) comparable to Aegean Sea seawater (5.8 mg.L-1 for Sr; and 3.4 mg.L-1 for Li; Athanasoulis et al., 2016). The Ilia hot spring/artesian borehole is located only few metres from the seashore. The dominant anion and cation in Ilia’s hydrothermal fluid are chloride (up to 1.28%) and sodium (up to 0.7%). Additionally, the fluid shows high concentrations of a series of ions and trace elements (conservative constituents), typically associated with seawater or/ and are likely to be of deep geothermal origin such as F, SiO2, and Li. In accordance with the findings of previous studies (D`Alessandro et al., 2014; Dotsika, 2015; Vakalopoulos et al., 2016; Kanellopoulos et al., 2017) these enrichments suggest high seawater participation and a magmatic contribution. Moreover, hydrothermal fluid from the Ilia area shows the highest concentrations of Fe (up to 13300 μg.L-1) and As (250 μg.L-1) compared to any of the other hot springs on NW Euboea island (Kanellopoulos et al., 2017, Table 1). Kanellopoulos et al. (2017, 2018) based on mineralogical and geochemical observations such as i) the presence of elements in their native form, such as Fe, Pb, Cu, ii) the presence of native alloys such as Au±Cu-Ag and the enrichment of metalloids such as As, Sb and iii) the abundance of REE, suggested that the system has a magmatic contribution. As a result, metals and metalloids were mainly derived from magmatic fluids, which after mixing with heated sea waters deposited native elements, sulphide mineralization at depth, and As-Fe-rich travertines in the surface (Kanellopoulos et al., 2017; 2018). The repeated measurements conducted during the last years (including wet and dry seasons, Kanellopoulos et al., 2017) have shown that the physicochemical parameters of the hydrothermal fluid are relatively stable with temperatures varying only from 60.9 to This article is protected by copyright. All rights reserved. 63.7 °C and pH values from 5.9 to 6.45 (Table 1, Fig. 2). The results reported here are in line Accepted Article with previously published values. Terrestrial thermogenic Fe-rich travertines with ferrihydrite have been reported in Shionaha (Takashima & Kano, 2005) and Okuoku-Hachikurou, Japan (Takashima et al., 2008, 2011). Additionally, the presence of ferrihydrite in hydrothermal environments has also commonly been reported in submarine vents (chimneys and hydrothermal sediments) such as the Basiluzzo Islet, Italy (Bortoluzzi et al., 2017), Kolumbo, Greece (Kilias et al., 2013) and Tutum Bay in Papua New Guinea (Pichler & Veizer, 1999). By comparing Ilia’s hydrothermal fluid with these occurrences (Table 2), the studied fluids show similar pH values (slightly acidic: 5-6.6). The temperatures of the other known terrestrial ferrihydrite depositing hot springs in Japan are 20-35oC lower, potentially allowing the development of non-thermophilic organisms. Regarding iron, Ilia’s hydrothermal fluid shows higher concentrations than the terrestrial hot springs of Japan (Takashima & Kano, 2005; Takashima et al., 2008) and the hydrothermal vent from Tutum Bay (Papua New Guinea; Pichler & Veizer, 2004). However, iron concentrations are lower when compared to the Basiluzzo Islet (Italy; Bortoluzzi et al., 2017) and Kolumbo (Greece; Kilias et al., 2013) submarine vents. Hence, Ilia hot spring shares some similar physicochemical characteristics with the fluids of the above-mentioned areas. Additionally, based on these characteristics Ilia can be ranked among the most iron enriched terrestrial hot springs worldwide, with near neutral pH. Iron-rich travertine composition Based on their strong differences such as their color and also their facies and chemical composition, it is clear that the Fe-rich and Ca-rich laminae of the travertine are formed under very different conditions. In particular, spatial discrimination of Fe-deposits from Ca-deposits is observed, with a dark colour (representative of iron oxides of the Fe-rich laminae) only This article is protected by copyright. All rights reserved. present when there is hot water flowing at a relatively high speed. When the flow rate Accepted Article decreases (away from the vent) or where the is no water flow (on the sides of the pipe for example), the orange colour, representative of the Ca-rich component, dominates. These observations strongly suggest that the differential deposits of the Fe-rich and Ca-rich component are controlled by variations in the flow rate of hot water venting from the spring. As a result, it is expected that the laminae are controlled by these changes. The elements As, Sc, and Y are clearly enriched in the Fe-rich component compared to the Ca-rich component. The strong binding potential of metal oxides towards arsenic could explain this difference in the ferrihydrite-rich component (Smedley & Kinniburgh, 2002). In contrast, the Ca-rich component of the Ilia travertine clearly has higher concentrations of Mn (330 mg.Kg-1) compared to the Fe-rich component. This may be related to the fast oxidation of Mn2+ in changing redox conditions, which may have varied between the two different phases. Metals such as Co and Ni follow the same trend. Similarly, the positive Ce anomaly observed in the Ca-rich component could be linked to the sensitivity of Ce to oxidative conditions (Bau & Koschinky, 2009). Hence, enrichment is observed in elements that become insoluble when oxidized in the Ca-rich component, potentially pointing towards changes in redox conditions during the precipitation of this phase. Microbial communities of the travertine deposits of Ilia Communities of the thermogenic travertines of Ilia vary with changes in water flow, distance to the vent, aerial exposure and natural redox conditions (Table 1, Fig. 9A). The two samples that experienced the highest water flow (ID2 and IP2) are marked by an abundance of thermophilic organism of the Aquificae phylum. Sequences of this phylum are uniquely attributed to the family Hydrogenothermaceae, which consists of chemolithoautotrophic microaerophilic thermophiles (Takai & Nakagawa, 2014). The other phyla consist mainly of This article is protected by copyright. All rights reserved. unclassified sequences, except for Zetaproteobacteria, which are attributed to the Fe- Accepted Article oxidizers of Mariprofundaceae family (Emerson et al., 2010). Interestingly, these Feoxidizers are characteristic of marine environments like the Loihi seamount (Emerson & Moyer, 2012), Santorini marine hydrothermal springs (Handley et al., 2010) or of Basiluzzo Islet in the Aeolian islands (Bortoluzzi et al., 2017). Communities obtained from IP3 and ID3 share similarities allowing a continuum to be drawn between the environments of ID2 and IP2. Aquificae and Zetaproteobacteria sequences are still present (3.3% and 7.2%). Additionally, species richness is higher, with photosynthetic Cyanobacteria in IP3 and a large number of Bacteroidetes and Alphaproteobacteria. These are widespread clades for which metabolic styles are hard to distinguish at the phylum to family level. However, the coexistence of Cytophagia, Erythrobacteraceae, Rhodospirillaceae, and Rhodobacteraceae clades suggests a dynamic environment in photic zone (Table 1), with different sources of carbon vis autotrophy and heterotrophy under microaerophilic to aerobic conditions. Sequences obtained from samples ID5 and IP1 show a clear dominance of aerobic metabolisms. No iron-related clade was obtained and visual examination clearly showed photosynthetic activity (green-pigmented travertine interpreted as containing Cyanobacteria) for these two samples (Figs 3A, C), as well as in other areas of the Ilia system where water flow was slow or nonexistent (because of daily to monthly flow variations). It is suggested that the high-water temperature prevents the development of Cyanobacteria in other light exposed systems. This would differentiate this system from the Okuoku-Hachikurou hot spring (Takashima et al., 2011). But as soon as the water flow rate drops or diverges, Cyanobacteria take over and dominate the travertine assemblage. Samples ID1 and ID4 show completely different communities (Fig. 9B) marked by anaerobic clades. Sequences are attributed to members of the Planctomycetes, Chloroflexi unclassified or attributed to the Anaerolinae class, which consists only of strict anaerobes commonly found in hot spring This article is protected by copyright. All rights reserved. microbial mats (Hanada & Pierson, 2006), Defferibacter and Deltaproteobacteria. From the Accepted Article latter class, sequences that could be assigned to an order were either related to Desulfobacterales, Desulfoarculales or Syntrophobacterales, which all point towards organisms involved in sulphate reduction. Defferibacter-associated sequences could only be assigned to the thermophilic anaerobic Caldithrix genus that consists of fermenters potentially using nitrate as an electron acceptor (Alauzet & Jumas-Bilak, 2014). Overall, this environment is likely anaerobic and its microbial assemblage relies mainly on the anaerobic degradation of organic carbon, potentially involving sulphate and nitrate reduction. The 16S rRNA gene analysis revealed the presence of diverse microbial communities, which could be associated with the deposition of ferrihydrite. The compositional variability of the microbial community is indicative of the heterogeneity and dynamic nature of the studied system. Microaerophilic conditions dominate at the vent, where water starts to equilibrate with the ambient oxygen. When the water flow decreases, oxygen content increases and microaerophilic organisms are replaced by aerobic autotrophs and heterotrophs. In buried settings fermenters and other sulphate reducers degrade the organic matter previously deposited by runoff or past microbial production. Biologically induced ferrihydrite precipitation Ferrihydrite precipitates from oxidation of Fe2+ to Fe3+ and from rapid hydrolysis of Fe3+ in abiotic or biologically aided processes (Jambor et al., 1998; Konhauser, 1998). In the case of the Ilia travertine, several lines of evidence suggest that ferrihydrite is mainly precipitated by bacterial activity in microaerophilic conditions. This conclusion is supported by the presence of fabrics showing a close association of ferrihydrite with filamentous textures resembling sheaths formed by Zetaproteobacteria (Figs 6G and 7G, H). They are covered with ferrihydrite particles, and are strikingly similar to the structures described in other This article is protected by copyright. All rights reserved. settings exhibiting the same organisms (Takashima et al., 2008; Handley et al., 2010; Accepted Article Bortoluzzi et al., 2017). In submarine or continental hydrothermal systems, ferrihydrite is interpreted as precipitated by sheathed or stalked iron oxidizers. These sheathed or stalked iron oxidizers are generally encountered in loose sediments without any textural characteristics. Until now, only Takashima et al. (2008) have identified well-preserved erected filaments encased in dendritic structures, in their case in the Shionoha iron-rich travertine deposits although these were preserved in calcite. Encased-filaments from Ilia are visually similar to the Shionoha structures and could possibly be formed by analogous microbe-mineral interactions. In Shionoha spring (Takashima et al., 2011) the central, darker part of the dendritic fabric of the ferrihydrite spherical aggregates has been interpreted as microbially encrusted filaments. Zetaproteobacteria Mariprofundaceae, identified in the Ilia travertine 16S rRNA gene library, is known to form these ferrihydrite-covered stalks (Emerson et al., 2007; Handley et al., 2010). In Ilia deposits, they were found in environments of high water flow, which experienced moderate oxygenation that could have allowed their development under microaerophilic conditions (Konhauser, 1998; Takashima et al., 2008). All of the Ilia system data thus suggests microbially aided deposition of ferrihydrite by Zetaproteobacteria under microaerophilic conditions. The second type of ferrihydrite facies, described as dense laminae with limited porosity and no crystalline habit, has been attributed to abiotic precipitation by Kanellopoulos et al. (2017). The findings presented here cannot rule out this hypothesis, and it is possible that ferrihydrite precipitation continues to occur once it has been initiated by the iron-oxidizing bacteria community, potentially evolving from the shrub ferrihydrite facies to porosity-filling by small ferrihydrite spherulites. A geomicrobiological investigation at the lamina scale will be necessary to untangle these processes. This article is protected by copyright. All rights reserved. Proposed model for the precipitation of Banded Iron Travertine (BIT) Accepted Article The two types of bands (i.e. Fe-rich and Ca-rich) observed in the BIT display different mineralogical, morphological, chemical and microbiological features, all related to the distinct processes of deposition. Fe-rich bands show relative depletion in elements that are the most sensitive to redox conditions (Mn and Ce), implying more reduced conditions during their precipitation than for the Ca-rich bands. Microbiological analyses of organisms living under these conditions support this kind of redox process, all displaying a significant fraction of iron-oxidizing bacteria from the Zetaproteobacteria class. Hence, it is suggested that these microorganisms only develop where microaerophilic conditions prevail, and that they are responsible for most of the primary ferrihydrite precipitation within the iron bands. Due to the significant variations in water flux at the spring, these microaerophilic environments may develop at different levels of the borehole and pipe system. During periods of higher flow, the upcoming reduced spring water will be oxidized further away from the vent, and the site where microaerophilic Fe-oxidizers can develop and precipitate ferrihydrite will move away as well. In conditions of lower flow, fully oxidized conditions may develop in areas that used to be microaerophilic. In such conditions, biologically induced ferrihydrite precipitation would not occur and the CaCO3 precipitation takes over as exposure to the atmosphere allows degassing and travertine formation (Fig. 10). Potentially, the development of Cyanobacteria in such settings would enhance this CaCO3 formation. Areas of standing water will most likely develop such conditions and evaporative processes accompanying the travertine deposition will allow co-precipitation of halite, as observed in the XRD data (Fig. 5). In the case of the Okuoku-Hachikurou hot spring, a symbiotic relationship between Cyanobacteria and Fe-OB was suggested to explain the laminated texture of iron-rich deposits. These laminations were thought to reflect changes in photosynthetic intensity (Takashima et al., 2011). In the Ilia travertines, co-occurrence of Zetaproteobacteria and This article is protected by copyright. All rights reserved. Cyanobacteria was not observed, likely because of the more dynamic with higher Accepted Article temperature. The interlayering change of Fe-rich and Ca-rich bands is explained by changes in water flow coming from the vent. It allowed the biologically driven precipitation of the iron bands to alternate with mainly abiotically driven precipitation of the CaCO3 bands in the same location. These changes may be link to tide cycles as seawater fuels the hydrothermal spring system, or by magmatic pulses. Pulses have been observed on a daily basis in the Euboea hydrothermal system (unpublished results). While cyanobacteria may contribute to the carbonate precipitation in this system, the high precipitation rates suggest that it is unlikely that they play the major role in carbonate precipitation. Iron-rich travertines as analogs to BIF The Ilia travertines are mainly the result of seawater circulation within a hydrothermal system that subsequently leads to the formation of iron-rich deposits, when interacting with oxidative environments, at a geologically fast rate. It leads to deposition of almost pure ferrihydrite laminae alternating with another chemically precipitated facies (here calcium carbonate) that resemble BIF characteristics. In particular, the Ilia travertines have iron enrichment of up to 35 wt%, only comparable with systems like Tutum Bay in Papua New Guinea (Pichler & Veizer, 1999) and the Endeavour Segment Main Field (Hannington et al., 2005). Even the Ca-rich bands exhibit higher iron concentrations than many active submarine hydrothermal deposits (Table 3), which emphasizes the enormous iron enrichment exhibited by the Ilia travertine and corroborates its resemblance to BIF. As an example, Algoma-type BIF ore grade varies from 15 to 45 wt% (Bekker et al., 2010, 2014; Posth et al., 2010). Other similarities with the general geological setting of Algoma-type BIFs (Cannon, 1986; Bekker et al., 2010, 2014) are apparent in the Ilia case. The Ilia travertine forms in a back-arc region, after circulation within deeper bedrock including ophiolitic-ultramaphic rocks. Algoma-type This article is protected by copyright. All rights reserved. BIFs were likely deposited in island arc/back arc basins (Veizer, 1983; Bekker et al., 2010, Accepted Article 2014) associated with similar (ultra)mafic volcanics (Gourcerol et al., 2016). The entangling of biogenic and abiotic processes is also a salient feature of BIF deposition. Several hypotheses have been suggested concerning the biological influence on BIF deposition (Bekker et al., 2010, 2014): the anoxygenic phototrophic iron-oxidizing bacteria, which can couple Fe(II) oxidation to the reduction of CO2 (Widdel et al., 1993; Ehrenreich & Widdel, 1994), the cyanobacterial model for BIF formation (Cloud, 1973) or the chemolithoautotrophic iron-oxidizing bacteria living in a neutral pH environment (Holm, 1989). In the case of the BIT of Ilia, the latter model likely explains the process of iron precipitation in the Fe-rich bands. This process however relies on the existence of suboxic conditions (incomplete oxygenation of the spring water at the Ilia vent). Similar suboxic conditions could have been encountered during the time of BIF deposition, when oxygen production fueled by ancestral cyanobacterial activity was only being initiated. Although, there are some BIF with carbonate bands such as the Cauê Iron Formation in Brazil (Spier et al., 2007; Teixeira et al., 2017), BIFs do not typically contain carbonate bands. The mineralogy of BIF generally consists of silica as chert, magnetite, hematite, Ferich silicate minerals such as stilpnomelane, minnesotaite, greenalite, and riebeckite, carbonate minerals such as siderite, ankerite, calcite, and dolomite, and in some cases sulfides such as pyrite and pyrrhotite (Barrett et al., 2010, 2014; Koeksoy et al., 2016). The silica considered to have been originated either from the sediment–water interface in colloidal form, by absorption on iron oxyhydroxides from seawater and scavenged with organic matter (Krapez et al., 2003; Grenne & Slack, 2005 Fischer & Knoll, 2009; Bekker et al., 2010; 2014), or from replacement of the original sediment, formed beneath the sediment–water interface (Krapez et al., 2003; Bekker et al., 2014). However, all BIFs have undertaken significant modifications either during diagenesis, or during metamorphism. So, their This article is protected by copyright. All rights reserved. mineralogy is the results of a series of factors, including the bulk composition of the original Accepted Article sediment, the diagenetic conditions, the metamorphic conditions, and the post-depositional fluid flow. Especially, the effects of temperature and pressure have led to mineralogy replacement, recrystallization, and from change to deletion of primary textures (Klein, 2005; Barrett et al., 2014). The BIT of Ilia interestingly present poorly diagenetically transformed structures and chemical components that could help untangle the primary features of BIFs. For example, the presence of both, ferric and ferrous minerals in BIFs suggests an average oxidation state of Fe2.4+ (Klein & Beukes, 1992), which means that 60% of the Fe in BIFs is made up of Fe(II). The BIF mineralogical composition reflects significant post-depositional alteration under diagenetic and metamorphic conditions. The Fe-rich bands, which consist of magnetite and hematite, are most likely to have formed from an initial Fe(III) oxyhydroxide phase such as ferrihydrite (Klein, 2005; Posth et al., 2008; Bekker et al., 2010, 2014; Gauger et al., 2015). The BIT of Ilia have an almost pure ferrihydrite composition in their Fe-rich bands, which could represent the primary composition of Fe-rich bands in BIFs and therefore may provide a model of BIF depositional characteristics. Moreover, the BIT of Ilia also procures a model for potential diagenetic processes in precursor bands. The existence of fermenters and sulphate reducers for example, in these Ferich reducing environments (Fig. 9) attests of organic matter recycling already taking place within sites of actively forming iron bands. The Ilia spring is therefore an interesting and easily accessible laboratory to investigate the initiation of iron mineral precipitation, micro to macro-banding formation and iron and carbon early syn-sedimentary recycling. Overall, the Ilia BIT shares many textural, mineralogical, chemical, and depositional similarities with Algoma-type BIFs. Given its intense precipitation rates, strong iron enrichment and easy access, it is suggested that a thorough investigation of the biotic and abiotic processes This article is protected by copyright. All rights reserved. involved in this dynamic small-scale system will help in understanding the suite of complex Accepted Article processes that have led to BIFs as we know them today. Acknowledgments CK would like to thank the Institute of Geology and Mineral Exploration (I.G.M.E.) - Division of geothermal energy and thermo-metallic waters for the support and collaboration. CT and DA acknowledge the technical and funding support of the University of Geneva as well as the Swiss National Science Foundation (project N° 200021_166308/1). Conflict of Interest All authors declare no conflict of interest. This article is protected by copyright. All rights reserved. REFERENCES Accepted Article Alauzet, C. and Jumas-Bilak, E. (2014) The Phylum Deferribacteres and the Genus Caldithrix. In The Prokaryotes: Other Major Lineages of Bacteria and The Archaea, (eds. E. Rosenberg, E. F. DeLong, S. Lory, E. Stackebrandt, and F. Thompson). Springer Berlin Heidelberg, Berlin, Heidelberg. pp., 595–611. Aronesty, E. (2011) ea-utils: Command-line tools for processing biological sequencing data. Expr Anal Durham, NC. Athanasoulis, K., Vougioukalakis, G., Xenakis, M., Kavouri, K., Kanellopoulos, C., Christopoulou, M., Statha, F., Rigopoulos, P., Spagakos, N., Tsigkas, T. and Papadatou, M. (2016) Diachronic monitoring of hot springs and geothermal fields of Greece. I.G.M.E., Athens (in Greek). Bau, M. and Koschinsky, A. (2009) Oxidative scavenging of cerium on hydrous Fe oxide: Evidence from the distribution of rare earth elements and yttrium between Fe oxides and Mn oxides in hydrogenetic ferromanganese crusts. Geochemical Journal, 43, 37–47. Bekker, A., Planavsky, N., Krapez, B., Rasmussen, B., Hofmann, A., Slack, J., Rouxel, O. and Konhauser, K.O. (2014) Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry. In Treatise on Geochemistry (eds Holland, H. and Turekian, K.). Elsevier, Netherlands, vol. 9, pp. 561-628. Bekker, A., Slack, J.F., Planavsky, N., Krapež, B., Hofmann, A., Konhauser, K.O. and Rouxel, O.J. (2010) Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes. Economic Geology, 105, 467–508. Bolger, A.M., Lohse, M. and Usadel, B. (2014) Trimmomatic: a flexible trimmer for Illumina sequence data. Bioinformatics, 30(15), 2114-2120. This article is protected by copyright. All rights reserved. Bortoluzzi, G., Romeo, T., La Cono, V., La Spada, G., Smedile, F., Esposito, V., Accepted Article Sabatino, G., Di Bella, M., Canese, S., Scotti, G., Bo, M., Giuliano, L., Jones, D., Golyshin, P.N., Yakimov, M.M. and Andaloro, F. (2017) Ferrous iron- and ammoniumrich diffuse vents support habitat-specific communities in a shallow hydrothermal field off the Basiluzzo Islet (Aeolian Volcanic Archipelago). Geobiology, 15, 664–677. https://doi.org/10.1111/gbi.12237 Cannon, W. F. (1986) Descriptive model of Algoma Fe, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, 198 pp. Chafetz, H. S. and Guidry, S. A. (1999) Bacterial shrubs, crystal shrubs, and ray-crystal shrubs: bacterial vs. abiotic precipitation. Sedimen. Geol., 126, 57–74 Chan, C.S., Fakra, S.C., Emerson, D., Fleming, E.J. and Edwards, K.J. (2011) Lithotrophic iron-oxidizing bacteria produce organic stalks to control mineral growth: Implications for biosignature formation. ISME J., 5, 717–727. Chaudhuri, S., Lack, J.G. and Coates, J.D. (2001) Biogenic magnetite formation through anaerobic biooxidation of Fe(II). Appl Environ Microbiol 67, 2844–2848. Cloud, P. (1973) Paleoecological significance of the banded iron-formation. Econ Geol 68, 1135–1143. D`Alessandro, W., Brusca, L., Kyriakopoulos, K., Bellomo, S. and Calabrese, S. (2014) A geochemical traverse along the “Sperchios Basin e Evoikos Gulf” graben (Central Greece): Origin and evolution of the emitted fluids. Marine and Petroleum Geology, 55, 295-308. De Ronde, C.E.J., Stoffers, P., Garbe-Schonberg, D., Christensin, B.W., Jones, B., Manconi, R., Browne, P.R.L., Hissmann, K., Botz, R., Dany, B.W., Schmitt, M. and This article is protected by copyright. All rights reserved. Battershill, C.N. (2002) Discovery of active hydrothermal venting in Lake Taupo, New Accepted Article Zealand. Journal of Volcanology and Geothermal Research, 115, 257-275. Dotsika, E. (2015) H-O-C-S isotope and geochemical assessment of the geothermal area of Central Greece. Journal of Geochemical Exploration, 150, 1-15. https://doi.org/10.1016/j.gexplo.2014.11.008 Edgar, R.C., Haas, B.J., Clemente, J.C., Quince, C. and Knight, R. (2011) UCHIME improves sensitivity and speed of chimera detection. Bioinforma., 27, 2194–2200. Ehrenreich, A. and Widdel, F. (1994) Anaerobic oxidation of ferrous iron by purple bacteria, a new type of phototrophic metabolism. Appl. Environ. Microbiol., 60, 4517– 4526. Emerson, D, Fleming, E.J. and McBeth, J.M. (2010) Iron-oxidizing bacteria: an environmental and genomic perspective. Annu. Rev. Microbiol., 64, 561–583. Emerson, D. and Moyer, C.L. (2012) Neutrophilic Fe-Oxidizing Bacteria are abundant at the Loihi Seamount Hydrothermal Vents and Play a Major Role in Fe Oxide Deposition. Appl. Environ. Microbiol., 68, 3085–3093. Emerson, D., Rentz, J.A., Lilburn, T.G., Davis, R.E., Aldrich, H., Chan, C. and Moyer, C.L. (2007) A novel lineage of proteobacteria involved in formation of marine Feoxidizing microbial mat communities. PLoS One, 2. Fischer, W.W. and Knoll, A.H. (2009). An iron shuttle for deepwater silica in Late Archean and Early Paleoproterozoic iron formation. Geological Society of America Bulletin, 121, 222–235. Gauger, T., Konhauser, K. and Kappler, A. (2015) Protection of phototrophic iron(II)oxidizing bacteria from UV irradiation by biogenic iron(III) minerals: Implications for This article is protected by copyright. All rights reserved. early Archean banded iron formation: Geology, v. 43, p. 1067–1070. Accepted Article https://doi.org/10.1130/G37095.1 Gkioni-Stavropoulou, G. (1998) Hydrogeological study of hot and mineral springs of Euboean – Maliac gulf. IGME, Athens (in Greek). Glasby, G. P., Iizasa, K., Hannington, M., Kubota, H. and Notsu, K. (2008) Mineralogy and composition of Kuroko deposits from northeastern Honshu and their possible modern analogues from the Izu-Ogasawara (Bonin) Arc south of Japan: Implications for mode of formation. Ore Geology Reviews, 34, 247-560. Gourcerol, B., Kontak, D.J., Thurston, P.C. and Petrus, J.A. (2018) Results of LA-ICPMS sulfide mapping from Algoma-type BIF gold systems with implications for the nature of mineralizing fluids, metal sources, and deposit models. Mineralium Deposita. https://doi.org/10.1007/s00126-017-0788-7 Gourcerol, B., Thurston, P.C., Kontak D.J., Côté-Mantha O. and Biczok, J. (2016) Depositional setting of Algoma-type banded iron formation. Precambrian Research, 281, 47–79. Grenne, T. and Slack, J.F. (2005). Geochemistry of jasper beds from the Ordovician Løkken ophiolite, Norway: Origin of proximal and distal siliceous exhalites. Economic Geology, 100, 1511–1527. Hammer, Ø., Harper, D. and Ryan, P. (2001) PAST: Paleontological statistics software package for education and data analysis. Palaeontol. Electron., 4, 9. Hanada, S. and Pierson, B.K. (2006) The Family Chloroflexaceae. In The Prokaryotes: Volume 7: Proteobacteria: Delta, Epsilon Subclass, (eds. M. Dworkin, S. Falkow, E. Rosenberg, K.-H. Schleifer, and E. Stackebrandt). Springer New York, New York, NY. pp., 815–842. This article is protected by copyright. All rights reserved. Handley, K.M., Boothman, C., Mills, R.A., Pancost, R.D. and Lloyd, J.R. (2010) Accepted Article Functional diversity of bacteria in a ferruginous hydrothermal sediment. ISME J., 4, 1193–1205. Hannington, M.D., De Ronde, C.E.J. and Petersen, S. (2005) Sea-floor Tectonics and Submarine Hydrothermal Systems (eds Hedenquist, J.W., Thompson, J.F H., Goldfarb, R.J., Richards, J.D.) Econ. Geol. 100th Anniv. Vol., 111–141. Harnmeijer, J. (2010) Squeezing Blood from A Stone: Inferences into the life and depositional environments of the early Archaean. Ph.D. thesis, University of Washington. Hatzis, M., Kavouridis, T., Bakalopoulos, P. and Xenakis, M. (2008) Investigation and determination of Northern Euboea geothermal fields. IGME, Athens (in Greek). Holm, N.G. (1987) Possible biological origin of banded iron-formations from hydrothermal solutions. Orig. Life, 17, 229–250. Holm, N.G. (1989) The 13 C/12C ratios of siderite and organic matter of a modern metalliferous hydrothermal sediment and their implications for banded iron formations. Chem. Geol., 77, 41–45. Jambor, J.L. and Dutrizac, J.E. (1998) Occurrence and constitution of natural and synthetic ferrihydrite, a widespread iron oxyhydroxide. Chem. Rev., 98, 2549–2585. Kanellopoulos, C. (2011) Geochemical research on the distribution of metallic and other elements in the cold and thermal groundwater, soils and plants in Fthiotida Prefecture and N. Euboea. Environmental impact. Ph.D. Thesis, National and Kapodistrian University of Athens, Greece (in Greek with English abstract). Kanellopoulos, C. (2012) Distribution, lithotypes and mineralogical study of newly formed thermogenic travertines in Northern Euboea and Eastern Central Greece. Open This article is protected by copyright. All rights reserved. Geosciences (former Central European Journal of Geosciences), 4(4), 545-560. Accepted Article https://doi.org/10.2478/s13533-012-0105-z Kanellopoulos, C. (2013) Various morphological types of thermogenic travertines in northern Euboea and Eastern Central Greece. Bulletin of the Geological Society of Greece, XLVII/4, 1929-1938. http://dx.doi.org/10.12681/bgsg.10958 Kanellopoulos, C., Mitropoulos, P., Valsami-Jones, E. and Voudouris, P. (2017) A new terrestrial active mineralizing hydrothermal system associated with ore-bearing travertines in Greece (northern Euboea Island and Sperchios area). Journal of Geochemical Exploration 179, 9-24, 10.1016/j.gexplo.2017.05.003. Kanellopoulos, C., Valsami-Jones, E., Voudouris, P., Stouraiti, C., Moritz, R., Mavrogonatos, C. and Mitropoulos, P. (2018) A New Occurrence of Terrestrial Native Iron in the Earth’s Surface: The Ilia Thermogenic Travertine Case, Northwestern Euboea, Greece. Geosciences, 8, 297, doi:10.3390/geosciences8080287 Kappler, A. and Newman, D.K. (2004) Formation of Fe(III)-minerals by Fe(II)- oxidizing photoautotrophic bacteria. Geochim. Cosmochim. Acta, 68, 1217–1226. Karastathis, V.K., Papoulia, J., Di Fiore, B., Makris, J., Tsambas, A., Stampolidis, A. and Papadopoulos, G.A. (2011) Deep structure investigations of the geothermal field of the North Euboean Gulf, Greece, using 3-D local earthquake tomography and Curie Point Depth analysis. Journal of Volcanology and Geothermal Research, 206, 106–120. Kasama, T. and Murakami, T. (2001) The effect of microorganisms on Fe precipitation rate at neutral pH. Chem. Geol., 180, 117–128. Kilias, S. P., Nomikou, P., Papanikolaou, D., Polymenakou, P. N., Godelitsas, A., Argyraki, A., Carey, S., Gamaletsos, P., Mertzimekis, T. J., Stathopoulou, E., Goettlicher, J., Steininger, R., Betzelou, K., Livanos, I., Christakis, C., Bell, K.C. and This article is protected by copyright. All rights reserved. Scoullos, M. (2013) New insights into hydrothermal vent processes in the unique Accepted Article shallow-submarine arc-volcano, Kolumbo (Santorini), Greece. Nat. Sci. Rep., 3, 2421. Klein, C. (2005) Some Precambrian banded iron-formations (BIFs) from around the world: their age, geologic setting, mineralogy, metamorphism, geochemistry, and origin. Am. Mineral., 90, 1473–1499. Klein, C. and Beukes, N.J. (1992) Time distribution, stratigraphy, and sedimentologic setting, and geochemistry of Precambrian iron-formations. In Schopf, J. W., and Klein, C. (eds.), The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge: University of Cambridge, pp. 139–146 Koeksoy, E., Halama, M., Konhauser, K.O. and Kappler, A. (2016) Using modern ferruginous habitats to interpret Precambrian banded iron formation deposition. International Journal of Astrobiology, 15(3), 205-217. https://doi.org/10.1017/S1473550415000373 Konhauser, K.O. (1998) Diversity of bacterial iron mineralization. Earth-Science Rev., 91– 121. Konhauser, K.O., Hamade, T., Raiswell, R., Morris, R.C., Ferris, F.G., Southam, G. and Canfield, D.E. (2002) Could bacteria have formed the Precambrian banded iron formation? Geology, 30, 1079–1082. Kozich, J.J., Westcott, S.L., Baxter, N.T., Highlander, S.K. and Schloss, P.D. (2013) Development of a dual-index sequencing strategy and curation pipeline for analyzing amplicon sequence data on the miseq illumina sequencing platform. Appl. Environ. Microbiol., 79(17), 5112–5120. This article is protected by copyright. All rights reserved. Krapez, B., Barley, M.E. and Pickard, A.L. (2003). Hydrothermal and resedimented origins Accepted Article of the precursor sediments to banded iron formations: Sedimentological evidence from the early Palaeoproterozoic Brockman Supersequence of Western Australia. Sedimentology, 50, 979–1011. Lavrushin, V., Kuleshov, V. and Kikvadze, O. (2006) Travertines of the Northern Caucasus. Lithol. Miner. Resour. 41 (2), 137–164. Pe-Piper, G. and Piper, D. (2002) The igneous rocks of Greece, the anatomy of an orogeny. Ge-bruder Borntraeger, Berlin. Pichler, T. and Veizer, J. (1999) The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater. Mar. Chem., 64, 229–252. Pichler, T. and Veizer, J. (2004) The precipitation of aragonite from shallow-water hydrothermal fluids in a coral reef, Tutum Bay, Ambitle Island, Papua New Guinea. Chemical Geology, 207, 31– 45 Posth, N.R., Hegler, F., Konhauser, K.O. and Kappler, A. (2008) Alternating Si and Fe deposition caused by temperature fluctuations in Precambrian oceans. Nat. Geosci., 1(10), 703–708. Posth, N.R., Konhauser, K.O. and Kappler, A. (2010) Banded iron formations. In: Thiel, V., Reitner, J. (eds) Encyclopedia of geobiology. Springer, Hiedelberg. Posth, N.R., Konhauser, K.O. and Kappler, A. (2013) Microbiological processes in banded iron formation deposition. Sedimentology, 60, 1733–1754. This article is protected by copyright. All rights reserved. Price, R.E. and Pichler, T. (2005) Distribution, speciation and bioavailability of arsenic in a Accepted Article shallow-water submarine hydrothermal system, Tutum Bay, Ambitle Island. Chem. Geol. 224, 122–135. Quast, C., Pruesse, E., Yilmaz, P., Gerken, J., Schweer, T., Yarza, P., Peplies, J. and Glöckner, F.O. (2013) The SILVA ribosomal RNA gene database project: improved data processing and web-based tools. Nucleic Acids Res., 41, D590-6. Ramsey, M.H., Thompson, M. and Banerjee, E.K. (1987) A realistic assessment of analytical data quality from inductively coupled plasma atomic emission spectrometry. Analytical Proceedings, 24, 260–265. Ring, U., Glodny, J., Will, T. and Thomson, S. (2010) The Hellenic Subduction System: High-Pressure Metamorphism, Exhumation, Normal Faulting, and Large-Scale Extension. Annual Review of Earth and Planetary Sciences, 38, 45-76. Rudnick, R. L. and Gao, S. (2003) Composition of the continental crust. In: Rudnick, R. L. (ed.), The Crust, Elsevier, 3, 1-64. Schloss, P.D., Westcott, S.L., Ryabin, T., Hall, J.R., Hartmann, M., Hollister, E.B., Lesniewski, R., Oakley, B.B., Parks, D.H., Robinson. C.J., Sahl, J.W., Stres, B., Thallinger, G.G., Van Horn, D.J. and Weber, C.F. (2009) Introducing mothur: opensource, platform-independent, community-supported software for describing and comparing microbial communities. Appl. Environ. Microbiol., 75, 7537–7541. Shimizu, A., Sumino, H., Nagao, K., Notsu, K. and Mitropoulos, P. (2005) Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece. Journal of Volcanology and Geothermal Research 140 (4), 321–339. Smedley, P.L. and Kinniburgh, D.G. (2002) A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochemistry, 17, 517–568. This article is protected by copyright. All rights reserved. Smith, D. (2008) From slab to sinter: the magmatic-hydrothermal system of Savo volcano, Accepted Article Solomon Islands. PhD Thesis University of Leicester, U.K. Spier, C.A., de Oliveira, S.M.B., Sial, A.N. and Rios F.J. (2007) Geochemistry and genesis of the banded iron formations of the Caue Formation, Quadrilatero Ferrifero, Minas Gerais, Brazil. Precambrian Research, 152, 170–206. Takai, K. and Nakagawa, S. (2014) The Prokaryotes. Springer Berlin Heidelberg. Takashima, C. and Kano, A. (2005) Depositional processes of travertine developed at Shionoha hot spring, Nara Prefecture, Japan. J. Geol. Soc. Japan, 111, 751–764. Takashima, C., Kano, A., Naganuma, T. and Tazaki, K. (2008) Laminated Iron Texture by Iron-Oxidizing Bacteria in a Calcite Travertine, Geomicrobiology Journal, 25(3-4), 193202. https://doi.org/10.1080/01490450802081887 Takashima, C., Okumura, T., Nishida, S., Koike, H. and Kano, A. (2011) Bacterial symbiosis forming laminated iron-rich deposits in Okuoku-hachikurou hot spring, Akita Prefecture, Japan. Isl. Arc, 20, 294–304. Taylor, S. R. and McLennan, S. H. (1985) The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp. Teixeira, N.L., Caxito, F.A., Rosière, C.A., Pecoits, E., Vieira, L., Frei, R., Sial, A.N. and Poitrasson, F. (2017). Trace elements and isotope geochemistry (C, O, Fe, Cr) of the Cauê iron formation, Quadrilátero Ferrífero, Brazil: Evidence for widespread microbial dissimilatory iron reduction at the Archean/Paleoproterozoic transition. Precambrian Research, 298, 39-55. Vakalopoulos, P., Xenakis, M., Vougioukalakis, G., Kanellopoulos, C., Christopoulou, M. and Statha, F. (2016). Medium – high enthalpy geothermal exploration in Edipsos area. IGME, Athens (in Greek). This article is protected by copyright. All rights reserved. Veizer, J. (1983) Chemical diagenesis of carbonates: theory and application of trace element Accepted Article technique. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable Isotopes in Sedimentary Geology, Vol. 10. Society of Economic Paleontologists and Mineralogists Short Course Notes, pp. III-1–III-100. Vött, A. (2007) Relative sea level changes and regional tectonic evolution of seven coastal areas in NW Greece since the mid-Holocene. Quaternary Science Reviews, 26(7-8), 894919. Wang, Q., Garrity, G.M., Tiedje, J.M. and Cole, J.R. (2007) Naïve Bayesian classifier for rapid assignment of rRNA sequences into the new bacterial taxonomy. Appl. Environ. Microbiol., 73, 5261–5267. Wang, Y. and Qian, P.Y. (2009) Conservative fragments in bacterial 16S rRNA genes and primer design for 16S ribosomal DNA amplicons in metagenomic studies. PLoS One 4(10). https://doi.org/10.1371/journal.pone.0007401 Widdel, F., Schnell, S., Heising, S., Ehrenreich, A., Assmus, B. and Schink, B. (1993) Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature, 362, 834–836. This article is protected by copyright. All rights reserved. Accepted Article Table 1: Microbiological sample description and characteristics Site Water flow light exposure air exposure composition commentaire ID1 Borehole no no burried black mat black unconsolidated material, EPS like ID2 Borehole high no no ferrihydrite crust black crust ID3 Borehole moderate moderate no ferrihydrite crust black crust on leaf ID4 Borehole no no burried black crust underneath black crust ID5 Borehole low yes yes mixed mat black crust cyano mat in contact with black IP1 Pipe mouth low yes yes CaCO3 crust cyano mat in pipe IP2 Pipe mouth high moderate no mixed crust below pipe IP3 Pipe mouth moderate moderate no mixed crust below pipe off light little flow This article is protected by copyright. All rights reserved. Table 2. Physiochemical parameters, major ions and trace elements concentrations from the Accepted Article studied hot spring and others terrestrial hot springs and hydrothermal submarine vents depositing ferrihydrite. T (oC) pH E.C. (mS/cm) T.D.S. (mg/L) Ca2+ (mg/L) Mg2+ (mg/L) Na+ (mg/L) K+ (mg/L) HCO3- (mg/L) Cl(mg/L) SO42- (mg/L) NO3(mg/L) NH4+ (mg/L) NO2(mg/L) F(mg/L) SiO2- (mg/L) Ag (μg/L) Al (μg/L) As (μg/L) B (μg/L) Ba (μg/L) Be (μg/L) Br (μg/L) Cd (μg/L) Co (μg/L) Cr (μg/L) Cu (μg/L) Fe (μg/L) Hg (μg/L) I (μg/L) Li (μg/L) Mn (μg/L) Mo (μg/L) Ni (μg/L) Pb (μg/L) Rb (μg/L) Sb (μg/L) Se (μg/L) Sr (μg/L) U (μg/L) V (μg/L) Zn (μg/L) Terrestrial Ilia, EuboeaGreece*1 63.5 5.9 34150 23200 1420 190 6940 180 640 12550 810 bdl 1.9 bdl 3.6 100 bdl bdl 250 9700 190 bdl 40300 bdl bdl bdl 70 11300 bdl 560 1900 380 bdl 75 bdl 220 bdl 310 18000 bdl 80 bdl Ilia, EuboeaGreece*1 63.7 6.1 34400 22700 1570 230 6760 210 640 12800 800 bdl 0.6 bdl 3.8 50 bdl bdl 150 9200 210 bdl 38100 bdl bdl bdl 130 13300 bdl 500 3360 500 bdl 80 20 190 bdl 220 24000 bdl 70 bdl ShionahaJapan*2 37.8 6.38 350 34.2 534 90 712 82.1 12100 - Submarine OkuokuhachikurouJapan*3 43.3 6.6 790 150 510 25 1270 770 bdl 51279.021 6800 - Basiluzzo Islet- Italy*4 KolumboGreece*5 5.7 33300 2770 3690 21780 1790 31530 660 53696.43 5 191000 3240000 0.29 (%) 1510 24.2 (%) 0.1 (%) 2.88 (%) - 320000 520000 100000 0.1 (%) 23500 314500 170000 173000 20000 39000 408000 30000 0.57 (%) 0.26 (%) Sea water Tutum BayPapua New Guinea*6 6.1 201 bdl 650 76 840 295 930 231 820 8400 6800 1720 1020 495 350 8.2 6790 - SW, Aegean Sea-Greece*7 25.9 8.18 56350 40900 475 1160 13240 423 168 22270 3257 <5 <0.05 <0.05 2.45 5 105 <10 13800 <10 87300 120 52 3360 <10 30 5800 <30 *1 = Vakalopoulous et al., 2016, *2 = Takashima and Kano, 2005, *3 = Takashima et al., 2008, *4 = Bortoluzzi et al., 2017 (DFS1), *5 = Kilias et al., 2013 (NA014-027 Champagne active mound-2), *6 = Pichler and Veizer, 2004 (Vents 1-3), *7 = Athanasoulis et al., 2016 (seawater analyses from Aegean Sea) This article is protected by copyright. All rights reserved. ccepted Articl Table 3. Whole-rock travertine chemistry of the studied travertines as determined by ICP-MS and ICP-AES analysis and comparison with Ferich travertines elsewhere and submarine Fe-rich hydrothermal depositions. Ilia (Ca-rich part)Greece AedipsosGreece 1 IliaGreece 1 N. CaucassusRussia 2 Savo VolacanoSolomon Islands 3 Tutum Bay (submarine hydrothermal aragonite)Papua New Guinea 4 Tutum Bay (submarine hydroth. Fe (III). Oxyhydroxide dep.)Papua New Guinea 6 KolumboGreece 7 Brothers (Volcanic arcs) 8 Lake Taupo massive sulphides (Volcanic arcs) 9 S. Explorer Ridge (Mid-ocean ridge) 8 Endeavour Segment Main Field (Mid-ocean ridge) 8 Average JADE site1 (Back arc rifts) 10 Izena cauldron (Back arc rifts) 8 Tyrrhenian Sea Panarea (Back arc basins) 8 Tyrrhenian Sea Palinuro (Back arc basins) 8 Submarine Ilia (Fe-rich part)Greece Terrestrial Ca % Fe % 35.3 11 (n=23) 3.2 – 62 0.0049-2.395 (n=3) 10.3-21.9 13.1-28.9 (n=56) 0.4 – 49.3 0.0114-6.83 (n=5) 32-37 0.06-2.29 (n=4) 37.2-38.5 0.049 – 0.126 (up to 29.7)5 (n=7) 1.2-5 31-37.8 (n=14) 16.6 (n=9) 7.4 (n=1) 25.3 (n=51) 25.2 (n=83) 30.1 6.2251 (n=40) 6.2 (n=5) 10.5 (n=15) 15.1 Al mg/Kg As mg/Kg 1340 26120 1850 930 6-1150 66-230 54-150 4895-18300 9 – 7475 - 0.6-626 bdl-5500 4.9-6.2 (%) 3810 2155 588 575 356 17500 17500 505 3400 Ba mg/Kg Cd mg/Kg Co μg/Kg Cr μg/Kg Cu mg/Kg K mg/Kg Li μg/Kg Mg mg/Kg Mn mg/Kg Na mg/Kg Ni μg/Kg Pb mg/Kg Rb μg/Kg S mg/Kg Sc μg/Kg Si mg/Kg Th μg/Kg U μg/Kg V μg/Kg Y μg/Kg Zn mg/Kg 117 1.1 705 16830 6 9960 6640 1915 98 40480 7635 4.7 1460 897 73445 720 240 3970 322790 63 88 bdl 2450 26800 8.4 10750 8565 1650 330 810 15860 2.9 1290 2720 4985 30350 450 290 19700 48 3-145 0.001-0.025 bdl bdl 3.6-14.7 36-1090 140-3900 87-2990 1-1984. 700-33100 bdl 0.348-10.6 417-690 230-67800 130-630 12-950 bdl-150 10-91. 140-2560 80-7980 2.52-25.9 80-90 0.015-0.018 bdl bdl bdl 335-510 1530-1940 545-690 74-195 3280-4960 bdl 0.72-2.21 520-790 650-2740 2560-8870 86-340 9-41. 75-200 1040-2610 71100-258900 16-24.3 4-10890 bdl -48200 bdl-29000 bdl - 58.5 bdl - 1270 bdl - 88600 bld-41800 5 - 18000 bdl-6870 bdl – 48200 bdl – 34.6 16 - 12400 30 – 6650 27 – 8140 bdl - 3700 bdl - 6500 bdl - 28800 61-86000 bdl -162 23-167 0.05-0.36 100-2900 1700 0.150-0.780 bdl-100 900-4300 1260-7065 140-630 1500-4000 0.03-1.25 530-1800 (up to 33200) 5 bdl-47 bdl -0.4 bdl bdl bdl-18 bdl 240-540 77-540 bdl-590 bdl 7-24. 430-49000 bdl bdl bdl bdl-970 30-1100 bdl 18000-90000 bdl 70-220 bld 16000-45000 13-160 996-2739 4400-7720 542-851 3560-8080 80-550 14-49 3800-13000 bdl 5.7-9.77 (%) 340-920 bdl-80000 8100-21000 24-56 276 1640 35000 14100 10200 105 28000 2000 33000 4.2 8.2 62.5 200 32000 1000 44409 54000 250 27000 5000 42539 64000 794 33000 110800 19634 200200 820 33000 118000 47214 202000 255 500 28000 118735 62000 4000 86000 179000 1500-10500 100-400 bdl-200 0.3-21 This article is protected by copyright. All rights reserved. ccepted Articl Zr μg/Kg - - 29-2080 290-1070 bdl - 338000 - 700-49000 - - - - - - - - - - La μg/Kg Ce μg/Kg Pr μg/Kg Nd μg/Kg Sm μg/Kg Eu μg/Kg Gd μg/Kg Tb μg/Kg Dy μg/Kg Ho μg/Kg Er μg/Kg Tm μg/Kg Yb μg/Kg Lu μg/Kg 54760 157070 21030 96070 30520 10290 49910 9450 62920 13330 35700 4310 24160 2960 470 3920 500 2120 620 205 1035 220 1565 380 1190 160 875 130 180-1100 19-1660 2-220 6-1110 3-390 1-150 6-770 bdl-140 1-950 bdl-190 7-1140 bdl-60 2-300 bdl-38 14300-57300 27500-119300 3640-16900 16700-76500 5280-24400 1850-8300 8830-38400 1750-7580 12700-51300 2710-10800 7570-29700 940-3760 5230-19800 650-2410 bdl-32100 60-78200 7-8400. 57-31000 bdl-7200 bdl-1700 22-8700 bdl-1600 bdl-11900 bdl-2800 10-9800. bdl-1700 bdl – 11000 bdl-1800 700-2700 - - bdl-18000 - - - - - - - - - - 1=Kanellopoulos et al., 2017, 2=Lavrushin et al., 2006, 3=Smith, 2008. 4=hydrothermal aragonite, Pichler and Veizer, 2004, 5=sediment cores, Price and Pichler, 2005, 6=hydrothermal Fe III. oxyhydroxide precipitates, Pichler and Veizer, 1999, 7=Kilias et al., 2013, 8=Hannington et al., 2005, 9=De Ronde et al., 2002, 10=Glasby et al., 2008, bdl=below detection limit. This article is protected by copyright. All rights reserved. Figure Legends Accepted Article Figure 1. Simplified geological Map of Northern Euboea Island, Greece (after Kanellopoulos et al., 2018). Figure 2. Paired views of Ilia’s hot spring pictures and corresponding thermal images of the same area. In the right side of each thermal picture, a color column shows the temperature scale corresponding to the colors. (A and B) General view of the hot water venting out from the pipe. (C and D) Close view of the pipe end point. (E) Venting point of the hot water and associated deposition. Orange-yellowish color component is Ca-rich and brownish-metallic color component is Fe-rich. At close-up image (F) could be observed that inside the Ca-rich component are developing Fe-rich bands (see red arrows). Figure 3. Position of the different samples used for microbiological study. (A) Artesian borehole and locations of the ID samples. (B) Ferrihydrite encrusted leaf, from the borehole. (C) Pipe outflow where IP samples were taken. (D) Localization of IP2 and IP3 underneath the pipe opening. Figure 4. Different macro- and microfacies of Ila Fe-rich travertine deposit. (A) Spicular, knob-like Fe-rich thermogenic travertine deposition from inside the borehole (upper part). (B) Dense Fe-rich travertine deposition. (E-F) Fe-rich laminae/bands with varying thickness from few mm up to few cm, alternating with Ca-rich laminae/bands. Figure 5. XRD pattern of Fe-travertine from Ilia. (A) Combined XRD patterns of selected material from pure Fe-rich (with black color) and Ca-rich (with blue color) bands. It is distinct the presence of the two characteristic broad peaks at around 35o and 62o in 2θ, which This article is protected by copyright. All rights reserved. are typical of poorly crystalline two-line ferrihydrite and the sharp peaks corresponding to the Accepted Article other mineral phases. (B) XRD pattern of selected material from pure Fe-rich bands (red color = aragonite). (C) XRD pattern of selected material from pure Ca-rich bands (red color = aragonite, blue color = calcite, green color = halite). Figure 6. Microscopic features of the travertines, under transmitted-light microscope, with parallel nicols. (A) Well developed and separated Ca- and Fe-rich components. (B) Close-up of the Ca-rich component composed of numerous growth laminae. (C) Ca-rich laminas separated by laminae of silt-sized micritic aggregates. (D) Ca-rich component develops isolated in a Fe-rich- component. Fe-rich component, even if no crystalline shape could be identified under the optical microscope, it seems that it is also developing in layers/ laminas. (E and F) Laterally aligned shrub-like structures creating dendritic fabric extending and branching upwards. At (G) aragonitic laminas has filled up the empty spaces between the Ferich shrub-like structures and at the centers of the shrubs could be distinguished filaments often show dark-tinted axes, likely consisting of organic substances, and are encased in ferrihydrite. (H) Fe-rich band consisted of ferrihydrite, even if no crystalline shape could be identified under the optical microscope, it seems that it is also developing in layers/ laminas. Figure 7. Back-scattered electron images (BSEI: A-D) of laminated travertines from Ilia. Light areas have a higher average atomic weight (Fe-rich) than the dark areas (Ca-rich). And SEM photomicrograph (E-H) of Fe-rich travertines fragments from Ilia (Arg = Aragonite and Fh = Ferrihydrite). (A-B). Numerous growth laminae which have different average atomic weights (Fe-rich and Ca-rich). At image (B) could be seem laterally aligned Fe-rich shrublike structures extending and branching upwards, creating dendritic fabric and composed by spherical nodules aggregates of about 250 nm. (C) Dense, well developed and separated This article is protected by copyright. All rights reserved. laminas of the two phases. Only, the Fe-rich area present some empty spaces. Additionally, Accepted Article (C) is presenting the transition from one lamina to the other. The Fe-rich lamina expands (shrubs) in to the Ca-rich lamina. (D) Close-up of image. (E) Transition from ferrihydrite laminae to aragonite laminae. The aragonite laminae are composed by needle-like crystals (radiating arrays). The crystals are elongated parallel to the c-axis, exhibiting straight extinction and creating dense laminae. The ferrihydrite have been developed filling the empty spaces of the top part of the aragonite crystals, no empty space could be observed. The ferrihydrite laminae are usually also very dense, without any crystallographic habit. Although, in some cases empty spaces between the dendrite aggregations were observed inside the laminas (see arrows). (F) Spherical nodules aggregates of ferrihydrite co-existing with aragonite crystals at the surface of a sample. The aragonite crystals have been developing by following -covering the empty spaces of the ferrihydrite aggregates shrubs (see arrows). (G) Encrusted filaments and EPS with spherical nodules aggregates of ferrihydrite. The aragonite crystals are developing in a way to fill up the empty spaces. (H) Dendritic branches of encrusted filaments with spherical nodules aggregates of ferrihydrite. Figure 8. Bulk geochemical analysis of travertine normalized against average Upper Continental Crust (Rudnick & Gao, 2003) and Post-Archaean Australian Shale (PAAS, Taylor and McLennan, 1985). (A and B) Major and trace elements. (C and D) REE. Figure 9. Microbial assemblages (16S rRNA gene sequences) of the Ilia travertine. (A) Composition of the different samples at the phylum level as a function of the water flow exposure. (B) Principal component analysis of the microbial community composition at the phylum level. This article is protected by copyright. All rights reserved. Figure 10. Model for the formation of banded iron travertine of Ilia. Switching from one Accepted Article mode to another might be controlled by variations in the flow of venting water, potentially controlled by volcanic pulses and whatever controls the fluid circulation in the hydrothermal system. This article is protected by copyright. All rights reserved. Accepted Article This article is protected by copyright. All rights reserved. Accepted Article This article is protected by copyright. All rights reserved. Accepted Article This article is protected by copyright. All rights reserved. Accepted Article This article is protected by copyright. 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