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Marine sedimentary rocks are logical targets for extracting geochemical proxies for the ancient surface environments of the Earth. Many workers would agree that shallow-water, Ca-Mg carbonates may be suitable for understanding processes that occurred in the photic zone of Archean and Proterozoic oceans. For Ca-Mg carbonates that are carefully selected to avoid alteration and diagenetic effects, their isotopic compositions are likely to reflect those of the ancient shallow oceans. This in turn allows inferences to be made of some, but not all, components of the biogeochemical cycles that operated on the surface of the Earth in the past. Because, however, Ca-Mg carbonates have low Fe contents, they do not directly record microbial metabolisms that are associated with the Fe cycle; insights into the Fe cycle must come from direct study of Fe-rich rocks such as banded iron formations (BIFs).

In this study, C, O, S, Fe, Mo, and Sr isotopes from 2.6-2.5 Ga Campbellrand carbonate platform (South Africa) were combined into an integrated model for the Neoarchean oceans. The ancient marine record is best studied by dividing the record into Fe-rich and Fe-poor lithologies. The Fe-poor record recorded in Ca-Mg carbonates probably reflects removal of Fe from the shallow oceans via oxidation of hydrothermally sourced Fe(II)_aq from reduced, deeper portions of the oceans, followed by precipitation as ferric oxides/hydroxides that settled back to the deep ocean. This follows common models of the Archean oceans that have deep, Fe(II)-rich zones (e.g., Holland, 1984). Although it has been proposed that oxidation of Fe(II)_aq could have occurred in the absence of biology by UV-photo oxidation (e.g., Braterman and Cairnssmith 1987), this mechanism is considered less likely based on experiments using seawater analogs (Konhauser et al. 2007), leaving us with photosynthetic pathways for oxidation. Oxidation may have occurred by anaerobic photosynthetic Fe(II) oxidation (e.g., Canfield 2005; Olson 2006; Widdel et al. 1993), or oxygenic photosynthesis; a preponderance of the data suggests that oxygenic photosynthesis developed in the Paleoarchean based on morphological, phylogenetic, molecular biomarker, and C isotope data (see review by Farquhar et al. 2011). It is anticipated that extensive Fe(II) oxidation would precede rise of free oxygen in the atmosphere, since only after all sinks for oxygen were exhausted could a significant rise in O₂ occur (e.g., Claire et al. 2006; Goldblatt et al. 2006; Kump and Barley 2007). Given the great abundance of Fe(II) in hydrothermal fluids and igneous and metamorphic rocks, Fe(II) was likely a very important sink for oxygen during the Archean.

The above discussion suggests that marine sedimentary rocks that reflect accumulation of Fe, such as Fe-rich shales and iron formations, reflect a complementary side of surface biogeochemical pathways that is not represented by low-Fe, Ca-Mg carbonates. The C, O, Fe, and Sr isotope compositions of iron formation carbonates of the Campbellrand carbonate platform indicate that these minerals did not form in equilibrium with seawater. Given the large ferric oxide/hydroxide and organic carbon fluxes to the seafloor that would accompany oxygenic photosynthesis in the Neoarchean, conditions would have been ideal to support dissimilatory iron reduction (DIR). DIR is phylogenetically diverse, deeply rooted in the Bacteria and Archaea, including hyperthermophiles, sulfate reducers, nitrate reducers, and methanogens, involving a wide variety of electron donors (e.g., Lovley et al. 2004), and is considered to be one of the earliest microbial metabolisms on Earth (Lovley 2004; Vargas et al. 1998). Although bacterial sulfate reduction (BSR) has figured prominently in the geochemical literature for several decades, the importance of DIR as an ancient biogeochemical process is only now being widely recognized in this literature. Nevertheless, it is important to note that Walker (1984) proposed that Fe(III) was likely the most important electron acceptor in environments of BIF deposition. It is therefore concluded that the isotopic compositions of the Neoarchean and Paleoproterozoic iron formations discussed here were largely controlled by microbial processes in the soft sediment prior to lithification, and therefore cannot be used as a paleo proxy for deep ocean water.

When the broad temporal variations in isotopic compositions of marine sedimentary rocks of Archean and Proterozoic age are simultaneously viewed from the perspective of paleoenvironmental proxies and microbial cycling, many previously puzzling data, as well as new data, begin to fall into place. For example, Johnson et al. (2008) proposed that expansion of DIR as an important microbial metabolism was most likely when marine sulfate contents were low, which would produce low levels of sulfide production by BSR, allowing reactive Fe(III) oxides/hydroxides to be utilized by DIR. In addition to Fe isotopes, C, O, S, and Sr isotope variations in detailed studies of Neoarchean and Paleoproterozoic basins (Figure 1) support such a model. The results of this study demonstrate that the question “do Archean and Proterozoic isotopic records reflect paleo-ocean proxies or microbial cycling?” does not have a single answer. Full understanding of the environmental and biological information contained in the isotopic compositions of ancient rocks requires looking at many isotopic systems at once, in addition, of course, to a firm geologic understanding of the samples.

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