Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and ... more Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143 Nd/ 144 Nd ratios (ε Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ε Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site ...
The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are... more The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are sensitive to seawater temperature. Corals, as cosmopolitan marine taxa with carbonate skeletons that can be precisely dated, represent ideal hosts for these geochemical proxies. However, efforts to calibrate and refine temperature proxies in cold-water corals (<20 • C) remain limited. Here we present skeletal Li/Mg, Sr/Ca, δ 18 O and carbon isotope (δ 13 C) data from live-collected specimens of aragonitic scleractinian corals (Balanophyllia, Caryophyllia, Desmophyllum, Enallopsammia, Flabellum, Lophelia, and Vaughanella), both aragonitic and high-Mg calcitic stylasterid genera (Stylaster and Errina), and shallow-water high-Mg calcite crustose coralline algae (Lithophyllum, Hydrolithon, and Neogoniolithon). We interpret these data in conjunction with results from previously explored taxa including aragonitic zooxanthellate scleractinia and foraminifera, and high-Mg calcite octocorals. We show that Li/Mg ratios covary most strongly with seawater temperature, both for aragonitic and high-Mg calcitic taxa, making for reliable and universal seawater temperature proxies. Combining all of our biogenic aragonitic Li/Mg data with previous calibration efforts we report a refined relationship to temperature: Li/Mg All Aragonite = 5.42 exp(−0.050 × T (• C)) (R 2 = 0.97). This calibration now permits paleo-temperature reconstruction to better than ±3.4 • C (95% prediction intervals) across biogenic aragonites, regardless of taxon, from 0 to 30 • C. For taxa in this study, aragonitic stylasterid Li/Mg offers the most robust temperature proxy (Li/Mg Stylasterid (Arag) = 5.64 exp(−0.046 × T (• C)) (R 2 = 0.95)) with a reproducibility of ±2.3 • C. For the first time, we show that high-Mg calcites have a similar exponential relationship with temperature, but with a lower intercept value (Li/Mg = 0.63 exp(−0.050 × T (• C) (R 2 = 0.92)). This calibration opens the possibility of temperature reconstruction using high-Mg calcite corals and coralline algae. The commonality in the relationship between Li/Mg and temperature transcends phylogeny and suggests a similar abiogenic trace metal incorporation mechanism.
Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and ar... more Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and are commonly associated with high human population density, yet little is known about the disease pathology or calcification behavior. SEM imagery, skeletal trace elements and boron isotopes (δ 11 B) have been combined as a novel approach to study coral disease. Low Mg/Ca, and high U/Ca, Mo/Ca, and V/Ca potentially suggest a decreased abundance of "centers of calcification" and nitrogen-fixation in GAs. estimates of carbonate system parameters from δ 11 B and B/Ca measurements indicate reduced pH (−0.05 units) and [CO 3 2− ] within GA calcifying fluid. We theorize GAs re-allocate resources away from internal pH upregulation to sustain elevated tissue growth, resulting in a porous and fragile skeleton. Our findings show that dystrophic calcification processes could explain structural differences seen in GA skeletons and highlight the use of skeletal geochemistry to shed light on disease pathophysiology in corals.
Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical c... more Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical cycling in the oceans. Chromium isotopes are used as a proxy to infer changes in past oceanic redox state. Chromium isotopes in carbonates, including the prime proxy carrier foraminifera, were initially thought to record the seawater composition during crystallisation. However, the uptake of Cr into foraminiferal tests and carbonates is still poorly understood and recent studies question this assumption. We assess whether Cr in foraminiferal calcite is taken up during biomineralisation, has a post-depositional origin or is a combination of the two. Laser Ablation-MC-ICP-MS analyses and NanoSIMS imaging of individual tests were used to characterise the distribution of Cr in both planktic and benthic foraminifera. Foraminifera in sediment core-top samples have up to two orders of magnitude more Cr than sediment trap, plankton net, and culture samples. In cultured specimens, Cr is incorporated in foraminiferal tests at low concentrations (0.04–0.13 ppm) with a distribution coefficient of ∼250 ± 43 (2SE) which is an upper estimate due to substantial loss of dissolved Cr during the experiment. Part of the Cr signal in sedimentary foraminifera may be primary, but this primary signal is likely often overprinted by the uptake of Cr from bottom and pore waters. In sediment samples, there is no significant isotopic offset between individual species and bulk foraminiferal calcite from the same size fraction. The >500 μm fraction has a heavier isotopic composition than the smaller 250–500 μm fraction with an offset of −0.3 to −0.5‰ due to an increase in surface area to volume. We propose that Cr in foraminifera is predominantly post-depositional and records bottom/pore water signals. This is contrary to current interpretations of the foraminiferal Cr isotope proxy as a surface seawater redox proxy.
The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be ... more The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.
Biological productivity and ocean circulation are both important oceanographic variables that con... more Biological productivity and ocean circulation are both important oceanographic variables that control the distribution of dissolved barium in the ocean interior ([Ba]sw). The ability to accurately reconstruct [Ba]sw will provide key constraints on these processes in the past. The geochemistry of cold-water corals has the potential to unlock paleoceanographic records at spatial and temporal resolutions not available using other sedimentary archives. Previous studies have suggested that the Ba/Ca ratio of coral skeletons is linearly related to [Ba]sw. However, these efforts have used a limited number of species, sparse global seawater databases, or have not explicitly measured the Ba/Ca ratio. Here we investigate the Ba/Ca ratio in a well-constrained set of cold-water scleractinian (aragonitic) corals as a proxy for [Ba]sw, using 58 specimens from 7 coral genera along with co-located measurements of [Ba]sw. We find that traditional chemical cleaning procedures do not significantly affect the Ba/Ca ratio of cold-water coral skeletons, allowing rapid sample throughput. We also determine that intra-sample variation in Ba/Ca ratios can be reduced by using larger sample sizes (e.g. 20 mg). By combining our results with existing data, we find that cold-water coral Ba/Ca is linearly related to [Ba]sw according to the relationship: Ba/Ca μmol/mol = [0.15 ± 0.02] [Basw nmol/kg] + [2.5 ± 1.4], (R2 = 0.7). We observe no species-specific ‘vital effects’ in cold-water coral Ba/Ca ratios, but site-specific effects could be a factor. Nevertheless, our results highlight the potential of Ba/Ca in cold-water corals to reconstruct biological and physical changes in the ocean interior.
The increase in atmospheric carbon dioxide (CO 2) observed since the industrial revolution has re... more The increase in atmospheric carbon dioxide (CO 2) observed since the industrial revolution has reduced surface ocean pH by $0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pH sw) such as this due to the associated changes in the oceanic car-bonate system. The boron isotopic composition (d 11 B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pH cf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pH sw on this process. Motivated by these investigations, this study examines the d 11 B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pH cf , and investigates how this taxon's pH cf is impacted by ocean acidification. d 11 B was measured in multiple algal replicates (n = 4–5) cultured at four different pCO 2 scenarios – averaging (±1r) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) latm, corresponding to average pH sw (±1r) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal d 11 B is elevated relative to the d 11 B of seawater borate at all pH sw treatments by up to 18‰. Although substantial variability in d 11 B exists between replicate samples cultured at a given pH sw (smallest range = 2.32‰ at pH sw 8.19, largest range = 6.08‰ at pH sw 7.91), strong correlations are identified between d 11 B and pH sw (R 2 = 0.72, p < 0.0001, n = 16) and between d 11 B and B/Ca (R 2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal d 11 B reflects pH cf as previously observed for scleractinian corals, the average pH cf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pH sw , with the magnitude of this offset varying parabolically with decreasing pH sw , with a maximum difference between pH sw and pH cf at a pH sw of 7.91. Observed relationships between pH sw and calcification rate, and between pH sw and pH cf , suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pH cf relative to pH sw in a similar manner to scleractinian corals. However, these results also indicate that pH cf cannot be sufficiently increased by algae exposed to a larger reduction in pH sw , adversely impacting calcification rates of coralline red algae.
Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers... more Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient " Mi-1 " glaciation event at the Oligocene-Miocene transition (OMT; ~23 Ma). However, detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ 18 O, δ 13 C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate an ~20% increase in silicate weathering rates, which would have lowered atmospheric CO 2 , potentially forcing the Antarctic glaciation ~ 23 Ma. δ 13 C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ 13 C across the OMT and during the Mi-1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ 13 C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi-1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca) and enhanced respiratory dissolution of carbonates, characterized by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering-induced CO 2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT, and increased burial of organic carbon acted as a feedback that intensified cooling at this time.
The response of the marine carbon cycle to changes in atmospheric CO 2 concentrations will be det... more The response of the marine carbon cycle to changes in atmospheric CO 2 concentrations will be determined , in part, by the relative response of calcifying and non-calcifying organisms to global change. Planktonic foraminifera are responsible for a quarter or more of global carbonate production, therefore understanding the sensitivity of calcification in these organisms to environmental change is critical. Despite this, there remains little consensus as to whether, or to what extent, chemical and physical factors affect foraminiferal calcification. To address this, we directly test the effect of multiple controls on calcification in culture experiments and core-top measurements of Globigeri-noides ruber. We find that two factors, body size and the carbonate system, strongly influence calcification intensity in life, but that exposure to corrosive bottom waters can overprint this signal post mortem. Using a simple model for the addition of calcite through ontogeny, we show that variable body size between and within datasets could complicate studies that examine environmental controls on foraminiferal shell weight. In addition, we suggest that size could ultimately play a role in determining whether calcification will increase or decrease with acidification. Our models highlight that knowledge of the specific morphological and physiological mechanisms driving ontogenetic change in calcification in different species will be critical in predicting the response of foraminiferal calcification to future change in atmospheric pCO 2 .
The isotopic composition of boron (δ 11 B) in marine carbonates is well established as a proxy fo... more The isotopic composition of boron (δ 11 B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal δ 11 B measurements, likely resulting from microstructural heterogeneities in δ 11 B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus δ 11 B-pH calibration, we present a new δ 11 B calibration of micro-sampled fibrous aragonite from this species. Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the δ 11 B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises δ 11 B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral δ 11 B; δ 11 B fibre = (0.93 ± 0.17) × δ 11 B borate + (12.02 ± 2.63). We suggest that the majority of the variability in measured δ 11 B between replicate bands of fibrous arago-nite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (±0.07 pH units), provided that suitable sampling strategies are applied. Published by Elsevier B.V.
Caribbean sea surface temperatures (SSTs) have increased at a rate of 0.2°C per decade since 1971... more Caribbean sea surface temperatures (SSTs) have increased at a rate of 0.2°C per decade since 1971, a rate double that of the mean global change. Recent investigations of the coral Siderastrea siderea on the Belize Mesoamerican Barrier Reef System (MBRS) have demonstrated that warming over the last 30 years has had a detrimental impact on calcification. Instrumental temperature records in this region are sparse, making it necessary to reconstruct longer SST records indirectly through geochemical temperature proxies. Here we investigate the skeletal Sr/Ca and Li/Mg ratios of S. siderea from two distinct reef zones (forereef and backreef) of the MBRS. Our field calibrations of S. siderea show that Li/Mg and Sr/Ca ratios are well correlated with temperature, although both ratios are 3 times more sensitive to temperature change in the forereef than in the backreef. These differences suggest that a secondary parameter also influences these SST proxies, highlighting the importance for site-and species-specific SST calibrations. Application of these paleothermometers to downcore samples reveals highly uncertain reconstructed temperatures in backreef coral, but well-matched reconstructed temperatures in forereef coral, both between Sr/Ca-SSTs and Li/Mg-SSTs, and in comparison to the Hadley Centre Sea Ice and Sea Surface Temperature record. Reconstructions generated from a combined Sr/Ca and Li/Mg multiproxy calibration improve the precision of these SST reconstructions. This result confirms that there are circumstances in which both Li/Mg and Sr/Ca are reliable as stand-alone and combined proxies of sea surface temperature. However, the results also highlight that high-precision, site-specific calibrations remain critical for reconstructing accurate SSTs from coral-based elemental proxies.
Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and ... more Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143 Nd/ 144 Nd ratios (ε Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ε Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene–Miocene transition (OMT; ∼23 Ma). We compare our ε Nd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. ε Nd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (ε Nd ≈ −15); much lower than the ε Nd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (ε Nd ≈ −10) and South Atlantic (ε Nd ≈ −8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (ε Nd ≈ −18) strongly influences the ε Nd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect ε Nd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.
Coral skeletons archive past climate variability with unrivaled temporal resolution. However, ext... more Coral skeletons archive past climate variability with unrivaled temporal resolution. However, extraction of accurate temperature information from coral skeletons has been limited by " vital effects, " which confound, and sometimes override, the temperature dependence of geochemical proxies. We present a new approach to coral paleothermometry based on results of abiogenic precipitation experiments interpreted within a framework provided by a quantitative model of the coral biomineralization process. DeCarlo et al. (2015a) investigated temperature and carbonate chemistry controls on abiogenic partitioning of Sr/Ca and U/Ca between aragonite and seawater and modeled the sensitivity of skeletal composition to processes occurring at the site of calcification. The model predicts that temperature can be accurately reconstructed from coral skeleton by combining Sr/Ca and U/Ca ratios into a new proxy, which we refer to hereafter as the Sr-U thermometer. Here we test the model predictions with measured Sr/Ca and U/Ca ratios of 14 Porites sp. corals collected from the tropical Pacific Ocean and the Red Sea, with a subset also analyzed using the boron isotope (δ 11 B) pH proxy. Observed relationships among Sr/Ca, U/Ca, and δ 11 B agree with model predictions, indicating that the model accounts for the key features of the coral biomineralization process. By calibrating to instrumental temperature records, we show that Sr-U captures 93% of mean annual temperature variability (26–30°C) and has a standard deviation of prediction of 0.5°C, compared to 1°C using Sr/Ca alone. The Sr-U thermometer may offer significantly improved reliability for reconstructing past ocean temperatures from coral skeletons.
The boron isotopic composition (δ 11 B) of coral skeleton is a proxy for seawater pH. However, δ ... more The boron isotopic composition (δ 11 B) of coral skeleton is a proxy for seawater pH. However, δ 11 B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ΔpH. We report that skeletal δ 11 B and ΔpH are related to the skeletal carbon isotopic composition (δ 13 C) in four genera of deep-sea corals collected across a natural pH range of 7.89–8.09, with ΔpH related to δ 13 C by ΔpH = 0.029 × δ 13 C + 0.929, r 2 = 0.717. Seawater pH can be reconstructed by determining ΔpH from δ 13 C and subtracting it from the δ 11 B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing δ 11 B versus seawater pH calibration with deep-sea scleractinian corals.
Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely dis... more Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely distributed Neoproterozoic carbonates associated with the Marinoan "snowball Earth" event are interpreted to represent considerable perturbations of the carbon cycle and the accompanying reduction in global ocean pH. Yet this interpretation is predicated on these isotopic signals being primary in origin. Recent studies of Pleistocene carbonate platform sediments from the Great Bahama Bank (western Atlantic Ocean; Clino core, drilled by the Bahamas Drilling Project) and elsewhere demonstrate that δ13C excursions of similar magnitude and global distribution to the snowball Earth excursions are formed following eustatic sea-level fall and exposure of shelf carbonates to meteoric diagenesis. Here we present δ11B and trace element data (B/Ca, Na/Ca, Mg/Ca, and Sr/Ca) from the same Clino core carbonate sediments in order to explore the influence of this diagenetic process on the boron system. We find that within the interval of meteoric diagenesis the δ11B of bulk carbonate is reduced by ∼6‰ in conjunction with a drop in the B/Ca ratio of 90%. Our results clearly demonstrate that the boron system is impacted by meteoric diagenesis, implying that a rigorous assessment of the diagenetic history of all ancient carbonates is required to ensure any paleoceanographic interpretation based on δ11B and/or B/Ca are robust.
To better understand the links between the carbon cycle and changes in past climate over tectonic... more To better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates. A number of proxies are under development, but some of the most promising (e.g. palaeoseawater records of Li and Nd isotope change) can only be employed on such large samples of mono-specific foraminifera¬ that application to the deep sea sediment core archive becomes highly problematic. “Dentoglobigerina” venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large (>355 μm diameter), abundant and cosmopolitan planktic foraminifer that ranges from the early Oligocene to early Pliocene. Yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed. Here, we assess the taxonomy, palaeoecology, and ontogeny of “D.” venezuelana using stable isotope (oxygen and carbon) and Mg/Ca data measured in tests of late Oligocene to early Miocene age from Ocean Drilling Program (ODP) Site 925, on Ceara Rise, in the western equatorial Atlantic. To help constrain the depth habitat of “D.” venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within Globigerina, Globigerinoides, Paragloborotalia and Catapsydrax. We define three morphotypes of “D.” venezuelana based on the morphology of the final chamber and aperture architecture. We determine the trace element and stable isotope composition of each morphotype for different size fractions, to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat. Our data indicate that “D.” venezuelana maintains a lower thermocline depth habitat at Ceara Rise between 24 and 21 Ma. Comparing our results to published data sets we conclude that this lower thermocline depth ecology for the Oligo-Miocene is part of an Eocene-to-Pliocene evolution of depth habitat from surface to sub-thermocline for “D.” venezuelana. Our size fraction data advocate the absence of photosymbionts in “D.” venezuelana and suggest that juveniles calcify higher in the water column, descending into slightly deeper water during the later stages of its life cycle. Our morphotype data show that δ18O and δ13C variation between morphotypes is no greater than within-morphotype variability. This finding will permit future pooling of morphotypes in the generation of the “sample hungry” palaeoceanographic records. Keywords: Planktic foraminifera, Dentoglobigerina venezuelana, morphotype, palaeoecology, ontogeny, taxonomy
Changes in global silicate weathering have a profound effect on the global carbon cycle and Earth... more Changes in global silicate weathering have a profound effect on the global carbon cycle and Earth’s climate on multi-million year timescales. They may also be associated with short-term (<10^5 yrs) climatic aberrations: for example, temperature anomalies observed across the Oligocene-Miocene (O/M) boundary (~23 Ma) have been linked to changes in silicate rock exposure on Antarctica. To explore this idea further, we present trace element data along with lithium and neodymium isotope data measured in the carbonate tests of O/M-aged planktic foraminifera. These temporal records of the Li and Nd isotopic composition (δ7Li and εNd) of seawater generated from deep-sea sediment core material (ODP Site 926, Ceara Rise) require large samples of mono-specific foraminifera (~20 mg). Therefore we first assess the geochemical utility of the large, abundant taxa, Dentoglobigerina venezuelana, for the purposes of δ7Li and εNd analysis. Three morphotypes of D. venezuelana are defined based on the morphology of the final chamber and aperture architecture. We find that the palaeoecology of these morphotypes based on their Mg/Ca, δ18O, and δ13C compositions is suitably similar to allow them to be grouped for the purpose of generating “sample-hungry” continental weathering records. Because the δ7Li and εNd of seawater are influenced by changes in continent-derived fluvial input to the oceans, records of seawater δ7Li and εNd have the potential to help constrain past changes in continental weathering. Li isotopes fractionate strongly during weathering processes, with δ7Li values becoming lower as weathering reactions tend towards completion. Nd sourced from ancient continental material typically possesses distinctively unradiogenic compositions (low εNd) relative to younger, mantle-derived sources. Finally, local vs. global signals of weathering input can be evaluated through utilisation of the differing oceanic residence times of Li (~1 Myrs) and Nd (~1 kyrs). To this end, we present a 4 Myr isotopic and trace element record for the O/M boundary, and we use these new data to better constrain the links between weathering and climate during this interval of significant climate change.
Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and ... more Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143 Nd/ 144 Nd ratios (ε Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ε Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site ...
The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are... more The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are sensitive to seawater temperature. Corals, as cosmopolitan marine taxa with carbonate skeletons that can be precisely dated, represent ideal hosts for these geochemical proxies. However, efforts to calibrate and refine temperature proxies in cold-water corals (<20 • C) remain limited. Here we present skeletal Li/Mg, Sr/Ca, δ 18 O and carbon isotope (δ 13 C) data from live-collected specimens of aragonitic scleractinian corals (Balanophyllia, Caryophyllia, Desmophyllum, Enallopsammia, Flabellum, Lophelia, and Vaughanella), both aragonitic and high-Mg calcitic stylasterid genera (Stylaster and Errina), and shallow-water high-Mg calcite crustose coralline algae (Lithophyllum, Hydrolithon, and Neogoniolithon). We interpret these data in conjunction with results from previously explored taxa including aragonitic zooxanthellate scleractinia and foraminifera, and high-Mg calcite octocorals. We show that Li/Mg ratios covary most strongly with seawater temperature, both for aragonitic and high-Mg calcitic taxa, making for reliable and universal seawater temperature proxies. Combining all of our biogenic aragonitic Li/Mg data with previous calibration efforts we report a refined relationship to temperature: Li/Mg All Aragonite = 5.42 exp(−0.050 × T (• C)) (R 2 = 0.97). This calibration now permits paleo-temperature reconstruction to better than ±3.4 • C (95% prediction intervals) across biogenic aragonites, regardless of taxon, from 0 to 30 • C. For taxa in this study, aragonitic stylasterid Li/Mg offers the most robust temperature proxy (Li/Mg Stylasterid (Arag) = 5.64 exp(−0.046 × T (• C)) (R 2 = 0.95)) with a reproducibility of ±2.3 • C. For the first time, we show that high-Mg calcites have a similar exponential relationship with temperature, but with a lower intercept value (Li/Mg = 0.63 exp(−0.050 × T (• C) (R 2 = 0.92)). This calibration opens the possibility of temperature reconstruction using high-Mg calcite corals and coralline algae. The commonality in the relationship between Li/Mg and temperature transcends phylogeny and suggests a similar abiogenic trace metal incorporation mechanism.
Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and ar... more Coral growth anomalies (GAs) are tumor-like lesions that are detrimental to colony fitness and are commonly associated with high human population density, yet little is known about the disease pathology or calcification behavior. SEM imagery, skeletal trace elements and boron isotopes (δ 11 B) have been combined as a novel approach to study coral disease. Low Mg/Ca, and high U/Ca, Mo/Ca, and V/Ca potentially suggest a decreased abundance of "centers of calcification" and nitrogen-fixation in GAs. estimates of carbonate system parameters from δ 11 B and B/Ca measurements indicate reduced pH (−0.05 units) and [CO 3 2− ] within GA calcifying fluid. We theorize GAs re-allocate resources away from internal pH upregulation to sustain elevated tissue growth, resulting in a porous and fragile skeleton. Our findings show that dystrophic calcification processes could explain structural differences seen in GA skeletons and highlight the use of skeletal geochemistry to shed light on disease pathophysiology in corals.
Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical c... more Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical cycling in the oceans. Chromium isotopes are used as a proxy to infer changes in past oceanic redox state. Chromium isotopes in carbonates, including the prime proxy carrier foraminifera, were initially thought to record the seawater composition during crystallisation. However, the uptake of Cr into foraminiferal tests and carbonates is still poorly understood and recent studies question this assumption. We assess whether Cr in foraminiferal calcite is taken up during biomineralisation, has a post-depositional origin or is a combination of the two. Laser Ablation-MC-ICP-MS analyses and NanoSIMS imaging of individual tests were used to characterise the distribution of Cr in both planktic and benthic foraminifera. Foraminifera in sediment core-top samples have up to two orders of magnitude more Cr than sediment trap, plankton net, and culture samples. In cultured specimens, Cr is incorporated in foraminiferal tests at low concentrations (0.04–0.13 ppm) with a distribution coefficient of ∼250 ± 43 (2SE) which is an upper estimate due to substantial loss of dissolved Cr during the experiment. Part of the Cr signal in sedimentary foraminifera may be primary, but this primary signal is likely often overprinted by the uptake of Cr from bottom and pore waters. In sediment samples, there is no significant isotopic offset between individual species and bulk foraminiferal calcite from the same size fraction. The >500 μm fraction has a heavier isotopic composition than the smaller 250–500 μm fraction with an offset of −0.3 to −0.5‰ due to an increase in surface area to volume. We propose that Cr in foraminifera is predominantly post-depositional and records bottom/pore water signals. This is contrary to current interpretations of the foraminiferal Cr isotope proxy as a surface seawater redox proxy.
The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be ... more The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.
Biological productivity and ocean circulation are both important oceanographic variables that con... more Biological productivity and ocean circulation are both important oceanographic variables that control the distribution of dissolved barium in the ocean interior ([Ba]sw). The ability to accurately reconstruct [Ba]sw will provide key constraints on these processes in the past. The geochemistry of cold-water corals has the potential to unlock paleoceanographic records at spatial and temporal resolutions not available using other sedimentary archives. Previous studies have suggested that the Ba/Ca ratio of coral skeletons is linearly related to [Ba]sw. However, these efforts have used a limited number of species, sparse global seawater databases, or have not explicitly measured the Ba/Ca ratio. Here we investigate the Ba/Ca ratio in a well-constrained set of cold-water scleractinian (aragonitic) corals as a proxy for [Ba]sw, using 58 specimens from 7 coral genera along with co-located measurements of [Ba]sw. We find that traditional chemical cleaning procedures do not significantly affect the Ba/Ca ratio of cold-water coral skeletons, allowing rapid sample throughput. We also determine that intra-sample variation in Ba/Ca ratios can be reduced by using larger sample sizes (e.g. 20 mg). By combining our results with existing data, we find that cold-water coral Ba/Ca is linearly related to [Ba]sw according to the relationship: Ba/Ca μmol/mol = [0.15 ± 0.02] [Basw nmol/kg] + [2.5 ± 1.4], (R2 = 0.7). We observe no species-specific ‘vital effects’ in cold-water coral Ba/Ca ratios, but site-specific effects could be a factor. Nevertheless, our results highlight the potential of Ba/Ca in cold-water corals to reconstruct biological and physical changes in the ocean interior.
The increase in atmospheric carbon dioxide (CO 2) observed since the industrial revolution has re... more The increase in atmospheric carbon dioxide (CO 2) observed since the industrial revolution has reduced surface ocean pH by $0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pH sw) such as this due to the associated changes in the oceanic car-bonate system. The boron isotopic composition (d 11 B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pH cf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pH sw on this process. Motivated by these investigations, this study examines the d 11 B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pH cf , and investigates how this taxon's pH cf is impacted by ocean acidification. d 11 B was measured in multiple algal replicates (n = 4–5) cultured at four different pCO 2 scenarios – averaging (±1r) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) latm, corresponding to average pH sw (±1r) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal d 11 B is elevated relative to the d 11 B of seawater borate at all pH sw treatments by up to 18‰. Although substantial variability in d 11 B exists between replicate samples cultured at a given pH sw (smallest range = 2.32‰ at pH sw 8.19, largest range = 6.08‰ at pH sw 7.91), strong correlations are identified between d 11 B and pH sw (R 2 = 0.72, p < 0.0001, n = 16) and between d 11 B and B/Ca (R 2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal d 11 B reflects pH cf as previously observed for scleractinian corals, the average pH cf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pH sw , with the magnitude of this offset varying parabolically with decreasing pH sw , with a maximum difference between pH sw and pH cf at a pH sw of 7.91. Observed relationships between pH sw and calcification rate, and between pH sw and pH cf , suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pH cf relative to pH sw in a similar manner to scleractinian corals. However, these results also indicate that pH cf cannot be sufficiently increased by algae exposed to a larger reduction in pH sw , adversely impacting calcification rates of coralline red algae.
Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers... more Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient " Mi-1 " glaciation event at the Oligocene-Miocene transition (OMT; ~23 Ma). However, detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ 18 O, δ 13 C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate an ~20% increase in silicate weathering rates, which would have lowered atmospheric CO 2 , potentially forcing the Antarctic glaciation ~ 23 Ma. δ 13 C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ 13 C across the OMT and during the Mi-1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ 13 C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi-1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca) and enhanced respiratory dissolution of carbonates, characterized by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering-induced CO 2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT, and increased burial of organic carbon acted as a feedback that intensified cooling at this time.
The response of the marine carbon cycle to changes in atmospheric CO 2 concentrations will be det... more The response of the marine carbon cycle to changes in atmospheric CO 2 concentrations will be determined , in part, by the relative response of calcifying and non-calcifying organisms to global change. Planktonic foraminifera are responsible for a quarter or more of global carbonate production, therefore understanding the sensitivity of calcification in these organisms to environmental change is critical. Despite this, there remains little consensus as to whether, or to what extent, chemical and physical factors affect foraminiferal calcification. To address this, we directly test the effect of multiple controls on calcification in culture experiments and core-top measurements of Globigeri-noides ruber. We find that two factors, body size and the carbonate system, strongly influence calcification intensity in life, but that exposure to corrosive bottom waters can overprint this signal post mortem. Using a simple model for the addition of calcite through ontogeny, we show that variable body size between and within datasets could complicate studies that examine environmental controls on foraminiferal shell weight. In addition, we suggest that size could ultimately play a role in determining whether calcification will increase or decrease with acidification. Our models highlight that knowledge of the specific morphological and physiological mechanisms driving ontogenetic change in calcification in different species will be critical in predicting the response of foraminiferal calcification to future change in atmospheric pCO 2 .
The isotopic composition of boron (δ 11 B) in marine carbonates is well established as a proxy fo... more The isotopic composition of boron (δ 11 B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal δ 11 B measurements, likely resulting from microstructural heterogeneities in δ 11 B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus δ 11 B-pH calibration, we present a new δ 11 B calibration of micro-sampled fibrous aragonite from this species. Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the δ 11 B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises δ 11 B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral δ 11 B; δ 11 B fibre = (0.93 ± 0.17) × δ 11 B borate + (12.02 ± 2.63). We suggest that the majority of the variability in measured δ 11 B between replicate bands of fibrous arago-nite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (±0.07 pH units), provided that suitable sampling strategies are applied. Published by Elsevier B.V.
Caribbean sea surface temperatures (SSTs) have increased at a rate of 0.2°C per decade since 1971... more Caribbean sea surface temperatures (SSTs) have increased at a rate of 0.2°C per decade since 1971, a rate double that of the mean global change. Recent investigations of the coral Siderastrea siderea on the Belize Mesoamerican Barrier Reef System (MBRS) have demonstrated that warming over the last 30 years has had a detrimental impact on calcification. Instrumental temperature records in this region are sparse, making it necessary to reconstruct longer SST records indirectly through geochemical temperature proxies. Here we investigate the skeletal Sr/Ca and Li/Mg ratios of S. siderea from two distinct reef zones (forereef and backreef) of the MBRS. Our field calibrations of S. siderea show that Li/Mg and Sr/Ca ratios are well correlated with temperature, although both ratios are 3 times more sensitive to temperature change in the forereef than in the backreef. These differences suggest that a secondary parameter also influences these SST proxies, highlighting the importance for site-and species-specific SST calibrations. Application of these paleothermometers to downcore samples reveals highly uncertain reconstructed temperatures in backreef coral, but well-matched reconstructed temperatures in forereef coral, both between Sr/Ca-SSTs and Li/Mg-SSTs, and in comparison to the Hadley Centre Sea Ice and Sea Surface Temperature record. Reconstructions generated from a combined Sr/Ca and Li/Mg multiproxy calibration improve the precision of these SST reconstructions. This result confirms that there are circumstances in which both Li/Mg and Sr/Ca are reliable as stand-alone and combined proxies of sea surface temperature. However, the results also highlight that high-precision, site-specific calibrations remain critical for reconstructing accurate SSTs from coral-based elemental proxies.
Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and ... more Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143 Nd/ 144 Nd ratios (ε Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ε Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene–Miocene transition (OMT; ∼23 Ma). We compare our ε Nd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. ε Nd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (ε Nd ≈ −15); much lower than the ε Nd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (ε Nd ≈ −10) and South Atlantic (ε Nd ≈ −8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (ε Nd ≈ −18) strongly influences the ε Nd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect ε Nd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.
Coral skeletons archive past climate variability with unrivaled temporal resolution. However, ext... more Coral skeletons archive past climate variability with unrivaled temporal resolution. However, extraction of accurate temperature information from coral skeletons has been limited by " vital effects, " which confound, and sometimes override, the temperature dependence of geochemical proxies. We present a new approach to coral paleothermometry based on results of abiogenic precipitation experiments interpreted within a framework provided by a quantitative model of the coral biomineralization process. DeCarlo et al. (2015a) investigated temperature and carbonate chemistry controls on abiogenic partitioning of Sr/Ca and U/Ca between aragonite and seawater and modeled the sensitivity of skeletal composition to processes occurring at the site of calcification. The model predicts that temperature can be accurately reconstructed from coral skeleton by combining Sr/Ca and U/Ca ratios into a new proxy, which we refer to hereafter as the Sr-U thermometer. Here we test the model predictions with measured Sr/Ca and U/Ca ratios of 14 Porites sp. corals collected from the tropical Pacific Ocean and the Red Sea, with a subset also analyzed using the boron isotope (δ 11 B) pH proxy. Observed relationships among Sr/Ca, U/Ca, and δ 11 B agree with model predictions, indicating that the model accounts for the key features of the coral biomineralization process. By calibrating to instrumental temperature records, we show that Sr-U captures 93% of mean annual temperature variability (26–30°C) and has a standard deviation of prediction of 0.5°C, compared to 1°C using Sr/Ca alone. The Sr-U thermometer may offer significantly improved reliability for reconstructing past ocean temperatures from coral skeletons.
The boron isotopic composition (δ 11 B) of coral skeleton is a proxy for seawater pH. However, δ ... more The boron isotopic composition (δ 11 B) of coral skeleton is a proxy for seawater pH. However, δ 11 B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ΔpH. We report that skeletal δ 11 B and ΔpH are related to the skeletal carbon isotopic composition (δ 13 C) in four genera of deep-sea corals collected across a natural pH range of 7.89–8.09, with ΔpH related to δ 13 C by ΔpH = 0.029 × δ 13 C + 0.929, r 2 = 0.717. Seawater pH can be reconstructed by determining ΔpH from δ 13 C and subtracting it from the δ 11 B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing δ 11 B versus seawater pH calibration with deep-sea scleractinian corals.
Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely dis... more Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely distributed Neoproterozoic carbonates associated with the Marinoan "snowball Earth" event are interpreted to represent considerable perturbations of the carbon cycle and the accompanying reduction in global ocean pH. Yet this interpretation is predicated on these isotopic signals being primary in origin. Recent studies of Pleistocene carbonate platform sediments from the Great Bahama Bank (western Atlantic Ocean; Clino core, drilled by the Bahamas Drilling Project) and elsewhere demonstrate that δ13C excursions of similar magnitude and global distribution to the snowball Earth excursions are formed following eustatic sea-level fall and exposure of shelf carbonates to meteoric diagenesis. Here we present δ11B and trace element data (B/Ca, Na/Ca, Mg/Ca, and Sr/Ca) from the same Clino core carbonate sediments in order to explore the influence of this diagenetic process on the boron system. We find that within the interval of meteoric diagenesis the δ11B of bulk carbonate is reduced by ∼6‰ in conjunction with a drop in the B/Ca ratio of 90%. Our results clearly demonstrate that the boron system is impacted by meteoric diagenesis, implying that a rigorous assessment of the diagenetic history of all ancient carbonates is required to ensure any paleoceanographic interpretation based on δ11B and/or B/Ca are robust.
To better understand the links between the carbon cycle and changes in past climate over tectonic... more To better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates. A number of proxies are under development, but some of the most promising (e.g. palaeoseawater records of Li and Nd isotope change) can only be employed on such large samples of mono-specific foraminifera¬ that application to the deep sea sediment core archive becomes highly problematic. “Dentoglobigerina” venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large (>355 μm diameter), abundant and cosmopolitan planktic foraminifer that ranges from the early Oligocene to early Pliocene. Yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed. Here, we assess the taxonomy, palaeoecology, and ontogeny of “D.” venezuelana using stable isotope (oxygen and carbon) and Mg/Ca data measured in tests of late Oligocene to early Miocene age from Ocean Drilling Program (ODP) Site 925, on Ceara Rise, in the western equatorial Atlantic. To help constrain the depth habitat of “D.” venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within Globigerina, Globigerinoides, Paragloborotalia and Catapsydrax. We define three morphotypes of “D.” venezuelana based on the morphology of the final chamber and aperture architecture. We determine the trace element and stable isotope composition of each morphotype for different size fractions, to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat. Our data indicate that “D.” venezuelana maintains a lower thermocline depth habitat at Ceara Rise between 24 and 21 Ma. Comparing our results to published data sets we conclude that this lower thermocline depth ecology for the Oligo-Miocene is part of an Eocene-to-Pliocene evolution of depth habitat from surface to sub-thermocline for “D.” venezuelana. Our size fraction data advocate the absence of photosymbionts in “D.” venezuelana and suggest that juveniles calcify higher in the water column, descending into slightly deeper water during the later stages of its life cycle. Our morphotype data show that δ18O and δ13C variation between morphotypes is no greater than within-morphotype variability. This finding will permit future pooling of morphotypes in the generation of the “sample hungry” palaeoceanographic records. Keywords: Planktic foraminifera, Dentoglobigerina venezuelana, morphotype, palaeoecology, ontogeny, taxonomy
Changes in global silicate weathering have a profound effect on the global carbon cycle and Earth... more Changes in global silicate weathering have a profound effect on the global carbon cycle and Earth’s climate on multi-million year timescales. They may also be associated with short-term (<10^5 yrs) climatic aberrations: for example, temperature anomalies observed across the Oligocene-Miocene (O/M) boundary (~23 Ma) have been linked to changes in silicate rock exposure on Antarctica. To explore this idea further, we present trace element data along with lithium and neodymium isotope data measured in the carbonate tests of O/M-aged planktic foraminifera. These temporal records of the Li and Nd isotopic composition (δ7Li and εNd) of seawater generated from deep-sea sediment core material (ODP Site 926, Ceara Rise) require large samples of mono-specific foraminifera (~20 mg). Therefore we first assess the geochemical utility of the large, abundant taxa, Dentoglobigerina venezuelana, for the purposes of δ7Li and εNd analysis. Three morphotypes of D. venezuelana are defined based on the morphology of the final chamber and aperture architecture. We find that the palaeoecology of these morphotypes based on their Mg/Ca, δ18O, and δ13C compositions is suitably similar to allow them to be grouped for the purpose of generating “sample-hungry” continental weathering records. Because the δ7Li and εNd of seawater are influenced by changes in continent-derived fluvial input to the oceans, records of seawater δ7Li and εNd have the potential to help constrain past changes in continental weathering. Li isotopes fractionate strongly during weathering processes, with δ7Li values becoming lower as weathering reactions tend towards completion. Nd sourced from ancient continental material typically possesses distinctively unradiogenic compositions (low εNd) relative to younger, mantle-derived sources. Finally, local vs. global signals of weathering input can be evaluated through utilisation of the differing oceanic residence times of Li (~1 Myrs) and Nd (~1 kyrs). To this end, we present a 4 Myr isotopic and trace element record for the O/M boundary, and we use these new data to better constrain the links between weathering and climate during this interval of significant climate change.
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Keywords: Planktic foraminifera, Dentoglobigerina venezuelana, morphotype, palaeoecology, ontogeny, taxonomy
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Keywords: Planktic foraminifera, Dentoglobigerina venezuelana, morphotype, palaeoecology, ontogeny, taxonomy