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Stephen E. Kesler, Hiroshi Ohmoto, 2006. "Preface", Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits, Stephen E. Kesler, Hiroshi Ohmoto
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This volume was stimulated by a Pardee Symposium titled “Evolution of the Early Atmosphere, Hydrosphere, and Biosphere: Constraints from Ore Deposits,” which we convened in 2002 at the national meeting of the Geological Society of America. The history of Earth's early atmosphere, hydrosphere, and biosphere, from Hadean through Archean and into Proterozoic time, is one of the enduring puzzles in the geological sciences. When did the oceans appear, and did they remain liquid throughout Earth's history? What was the composition of the early atmosphere, and how did it affect climate? How did the atmosphere and ocean compositions change through time, and why? When, where, and how did life emerge on Earth? When did cyanobacteria, sulfate reducers, methanogens, and eukarya appear, and how did they affect their geologic environments? How did changes in the atmosphere, hydrosphere, and biosphere affect the lithosphere, and vice versa?
One of the most prominent aspects of this puzzle is when Earth's atmosphere became oxic. Controversy today focuses on two possibilities: that the atmosphere has been oxic since early Archean time (ca. 3.8 Ga) or that it gained oxygen between about 2.3 and 2.1 Ga, an event recently termed the Great Oxidation Event (GOE). The availability of free oxygen would have had a strong effect on the (bio)geochemical cycles of elements that exist in more than one oxidation state in nature. Of the major rock-forming elements, only iron does this. In contrast, multiple oxidation states are common in nature for many of the trace elements that are concentrated in some ore deposits, such as manganese, molybdenum, uranium, and vanadium. The atmospheric concentrations of CO2 and CH4 are also of concern as potential greenhouse gases to resolve the faint young sun problem, as the sources and products of biological activities, and as the source of acid rain for weathering the early continents. Finally, the concentrations of various forms of sulfur species in the oceans, especially H2S and SO42–, are of interest because they are linked to the atmospheric pO2 history, the evolution of a variety of sulfur-utilizing microbes (e.g., sulfate-reducing bacteria and sulfide-oxidizing bacteria), and the origins of a variety of mineral deposits, including banded iron-formations, volcanogenic massive sulfide, sedimentary exhalative, and Mississippi Valley–type (MVT) deposits. The abundance ratios of many of the elements in ore deposits respond to the oxygen-carbon-sulfur geochemistry of the atmosphere and oceans, making ore deposits particularly good indicators of their geochemical environment.
Historically, the first ore deposit types linked to atmosphere-hydrosphere compositions were uranium paleoplacers and banded iron formations. Later work has expanded the list to include other types of uranium deposits, sedimentary manganese deposits, laterites, and sedimentary exhalative and MVT lead-zinc deposits. Papers in this volume deal with most of these deposit types and include new research results, as well as summaries of the current opinions on how they relate to proposed histories of Earth's early atmosphere, hydrosphere, and biosphere.
SUMMARY OF THE VOLUME
This volume includes papers based on some of the presentations at the symposium, as well as additional papers. It starts with a section dealing with the biosphere and the origin of life. Here, Russell and Hall show that low-temperature hydrothermal systems could have constituted simple reactors in which H2 and CO2 formed acetate, complex organic molecules and even cells (i.e., the emergence of life) using a froth-like substrate of iron-sulfides. Next, Grassineau et al. use sulfur and carbon isotope geochemistry of Archean rocks and ore deposits to trace the metabolic evolution of life from 3.8 to 2.7 Ga. The second section focuses on development of the early continents and begins with the study by Thiart and de Wit demonstrating that extraction of ore elements from the mantle has become less efficient through time. This is followed by a study by Ishihara et al. showing that the range in oxidation states of Archean granitoid intrusions is essentially the same as that of younger granitoids, and a final study by Kerrich et al. indicating that atmospheric N2 has been drawn into ore deposits and other parts of the lithosphere through geologic time by N-fixing microorganisms.
The third section of the volume, which focuses on uranium deposits and their relation to atmospheric evolution, starts with two contrasting views of the famous Mesoarchean Witwatersrand Supergroup in South Africa that contains Earth's largest gold-uranium deposits. These deposits are hosted by quartz-pebble conglomerates, and controversy centers on whether they formed as paleoplacers. In the first paper, Minter shows that the sedimentary setting of gold, pyrite, and uraninite favors a detrital origin and their deposition under a reducing atmosphere. In the second paper, Law and Phillips describe geologic and geochemical features indicating that the gold, uranium, and sulfur were deposited by hydrothermal fluids after deposition of the conglomerates and conclude that these deposits do not provide evidence for a reducing Archean atmosphere. In a third paper, Yamaguchi and Ohmoto argue against a paleoplacer origin for pyrite in the slightly younger Paleoproterozoic Huronian Supergroup in Canada, which also contains quartz-pebble conglomerates that host only uraninite and pyrite, but no gold. In the final paper, Gauthier-Lafaye shows that the well-known Oklo sandstone-type uranium deposits of Gabon, which formed at ca. 2.1 Ga, are best explained if they formed when oxygenated water was available to transport dissolved uranium.
The fourth section of the volume concerns lead-zinc and manganese deposits and the information that they provide about the evolution of sulfur in seawater. The first two papers by Lyons et al. and Kesler and Reich show that the temporal history of sedimentary exhalative (sedex) and MVT lead-zinc deposits, respectively, is best explained by an ocean that lacked widespread, abundant sulfate until at least middle Proterozoic time. Liu et al. show that Neoproterozoic Mn-carbonate deposits in China resulted from low levels of sulfate in the ocean caused by an anomalous influx of Fe from lateritic soils rather than from a “snowball Earth.” The final section of the volume deals with banded iron formation (BIF) deposits and the insights that they provide about the geochemistry of iron, oxygen, sulfur, and carbon and the nature of organisms in the oceans. The paper by Raiswell deals with the relative importance of weathering, diagenetic, and exhalative processes as sources of iron. Brown discusses the problem of depositing iron and concludes from experiments and energy considerations that microbes could have accounted for large volumes of iron carbonate and oxide precipitates, and Gutzmer et al. show that high-grade hematite BIF deposits formed under conditions similar to those of today. In the final two papers, Kato et al. and Ohmoto et al. suggest that BIFs formed throughout geologic history by the mixing of locally discharged hydrothermal fluids with ambient seawater and show that this is consistent with an ocean that has been oxygen- and sulfate rich since ca. 3.8 Ga.
It will be apparent to even the most casual reader that authors who have contributed to this volume hold a wide range of views about the composition of the early Earth atmosphere, hydrosphere, and biosphere and the constraints offered by ore deposits, and we regard this as one of the major contributions of the volume. Our sincere hope is that the range of views propounded in this volume will help students and researchers to realize the nature of the remaining controversies and the sorts of data that are required to resolve them. For example, why do Lyons et al. and Kesler and Reich conclude that the early ocean was low in sulfate whereas Ohmoto et al. conclude that it was high in sulfate? What sort of evidence do they have, how did they interpret this evidence, and what sorts of new studies are necessary to test their assumptions and conclusions? What new methods can be used to resolve these controversies?
Our understanding of early Earth conditions has evolved tremendously over the past few decades, and we expect that this will continue in the future, spurred in part by studies inspired by this volume. As this research continues, three points are apparent. First, in the final analysis, our conclusions are based largely on the rock record, and that record is complex. Mineralogical and textural features of undisturbed rocks are difficult enough to understand, but things get much harder with older rocks because in general, they have been subjected to higher degrees of metamorphism and/or to longer periods of weathering. As geochemical studies become more and more sophisticated, it is increasingly important that the utmost care be taken to assure that samples truly represent the event or feature of interest and to document the textures and features that prove it. Second, the abundance of Precambrian rocks, especially those formed in near-surface and active tectonic settings, decreases with increasing geologic age because most of them were eroded or destroyed by later tectonic events, but also perhaps because the earlier continents were smaller. Therefore, more accurate reconstructions of paleogeographic and tectonic settings of the major Archean and Proterozoic terrains are necessary in order to relate the temporal change in the preserved number/size of a specific type of ore deposit to the chemical and biological evolution of early Earth. Third, there is growing evidence that Earth's ore deposit history rarely reflects a single dominant control and instead is result of complex interaction among many factors. These can be classified in many ways, but are perhaps best viewed as the tectonic, hydrologic, and biologic cycles. Our growing understanding of earth system science shows how intimately these cycles are related and it comes as no surprise that ore deposits reflect this interplay. Hopefully, when the successor to this volume is compiled, continuing research will have brought us closer to an understanding of the constraints that ore deposits provide on early Earth's atmosphere, hydrosphere, and biosphere.
The symposium that stimulated this volume was supported generously by the Geological Society of America, the Society of Economic Geologists and the NASA Astrobiology Institute. We are grateful to all of these for their interest in this important subject.