Life in the Archean

The study of microbial fossils involves a broad array of disciplines and covers a vast diversity of topics, of which we review a select few, summarizing the state of the art. Microbes are found as body fossils preserved in different modes and have also produced recognizable structures in the rock record (microbialites, microborings). Study of the microbial fossil record and controversies arising from it have provided the impetus for the assembly and refining of powerful sets of criteria for recognition of bona fide microbial fossils. Different types of fossil evidence concur in demonstrating that microbial life was present in the Archean, close to 3.5 billion years ago. Early eukaryotes also fall within the microbial realm and criteria developed for their recognition date the oldest unequivocal evidence close to 2.0 billion years ago (Paleoproterozoic), but Archean microfossils >3 billion years old are strong contenders for earliest eukaryotes. In another dimension of their contribution to the fossil record, microbes play ubiquitous roles in fossil preservation, from facilitating authigenic mineralization to replicating soft tissue with extracellular polymeric substances, forming biofilms that inhibit decay of biological material, or stabilizing sediment interfaces. Finally, studies of the microbial fossil record are relevant to profound, perennial questions that have puzzled humanity and science—they provide the only direct window onto the beginnings and early evolution of life; and the methods and criteria developed for recognizing ancient, inconspicuous traces of life have yielded an approach directly applicable to the search for traces of life on other worlds.

Biological activity fractionates the nitrogen isotopes in a peculiar way, making them a reliable biosignature and an accurate paleoenvironmental proxy. Nitrogen has been ignored for long time, being extremely fragile compared to the more stable graphitic forms of C; however, N has an advantage over other isotopic systems such as those of C and S. The dominant source of N at the surface of the Earth, that is, the atmospheric triple-bonded N 2 , is so stable that only a very limited number of metabolic processes can bridge the abiotic and biotic world. Therefore we can draw relatively simple flux models for N. In this contribution, we review the N isotopic record in the last 4 billions years. Large isotopic shifts recorded by nitrogen are related to specific metabolic changes as a direct response to major environmental stress such as the rise of oxygen in the atmosphere and the evolution of nitrifiers and denitrifiers in the ocean. These isotopic changes are not unique but well correlated with those of C and Fe, indicating that nitrogen can be successfully used for modeling the interplay of changing microbial metabolisms over Earth's history and relate them to precise environmental changes.

Keywords: banded iron formation lamination nitrogen isotope organic productivity iron isotope iron reduction Banded iron formations (BIFs) exhibit alternating silica-and iron-rich laminae, potentially reflecting the dynamics of the paleo-environments in which they were formed, although the exact mechanism remains unclear. Here the formation of a 2.7–2.9 Ga BIF from Dharwar Craton, India, is deciphered by analyzing the inter-band variations of the redox-sensitive isotope biomarkers, 15 N/ 14 N and 56 Fe/ 54 Fe. Organic matter with δ 15 N values as high as +12.0 ± 0.8h appears to be trapped in silica. Iron oxides exhibit systematically positive δ 56 Fe values, ranging between +0.80 ± 0.05h and +1.67 ± 0.02h. Compared to the iron-rich bands, silica-rich bands, which show higher δ 56 Fe values, exhibit an order of magnitude higher concentrations of 15 N-rich organic nitrogen, normalized by the abundances of its host silica. The presence of 15 N-rich organic matter may imply the emergence of a modern-like biological nitrogen cycle that requires the formation of oxidized nitrogen compounds. The higher concentration of 15 N-rich organic nitrogen for the silica-rich bands possibly suggests that the photosynthetic activity was higher during the formation periods of these bands. The heavier iron isotope compositions of the silica-rich bands cannot be explained alone by iron oxidation through probable pathways. The relative 56 Fe-enrichment in silica-rich bands is explained here by the progressive dissolution of iron oxides to the ocean, through iron reduction by 15 N-rich organic matter actively produced at the ocean surface. The formation of iron-rich bands possibly corresponds to periods of reduced biological productivity, when precipitated iron was not effectively dissolved to the ocean. The observed shift in the organic concentration between Fe-and Si-rich bands could be the switch that triggered the BIF laminations. This shift could conceivably represent periodic fluctuations in the oxygen generation, which possibly occurred over periods of millennia, at the dawn of the Great Oxidation Event.

[1] To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460-Myr-old Fe-rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe-and Si-rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe-Mn-oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe-Mn-oxyhydroxides, but a minor fraction is found in K-feldspars and Ba-mica dispersed in the silica matrix. The N and C isotopic composition of Fe-oxides suggests the presence of a unique biogenic source with d 15 N AIR values from +6.0 ± 0.5% to 7.3 ± 1.1% and a d 13 C PDB value of À19.9 ± 0.1%. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion-controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe-isotopic composition of the Marble Bar chert (d 56 Fe = À0.38 ± 0.02%) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact