ALBERT

Description: We have recently suggested that microbial carbonates, including stromatolites, could potentially serve as an Fe isotope proxy for seawater. Amongst the evidence provided was the observation that modern microbial carbonates contain virtually unfractionated Fe. New data published for dissolved Fe in Atlantic seawater also appear to indicate unfractionated modern seawater Fe. Microbial carbonate offers considerable advantages over more widely used archives to date, which record bottom water (BIF, shales) or diagenesis (sulphides). Microbial carbonates are possibly the only widely available archives of truly open ocean water and their major and trace element chemistry as well as Sr-isotope record can be used to screen against diagenetic overprint and to verify precipitation in unrestricted open ocean water. Our initial survey of microbial carbonates throughout Earth history shows an Fe isotope evolution that mimics that of many other proxies: namely, Neoarchean stromatolitic carbonates from the Hamersley Group, Western Australia, and of the Campbellrand Formation, Kapvaal craton, contain uniquely light Fe (d56Fe as low as -2.5‰), while older Archean stromatolites and Paleozoic as well as Mesozoic microbial limestones mostly range between 0 and -1‰ d56Fe. The sum of these new records and the chemical sediments published to date provides evidence for the presence of a prominent reservoir of light seawater iron in the ferrous form at the termination of the Archean. The light Fe is likely a residue from large-scale oxidation of ferrous iron, precipitated into BIF or pelagic clay. A diagenetic origin of the light Fe is unlikely in light of preserved seawater and drothermal REE patterns. In the post 2.4 Ga era, microbial carbonates could be overwhelmed by interstitial ferric Fe from (hydr)oxides. Quantitative oxidation under excess O2 conditions led to unfractionated conditions.

Description: In a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve Fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation. It was found that δ56Fe values of Archean stromatolites vary between −2.1‰ and −0.5‰, Devonian and Carboniferous microbiallites vary between 0‰ and −1‰, and modern microbialites from the Great Barrier Reef show a range from −0.12‰ to +0.15‰. Five lines of evidence support the possibility that these compositions are potentially pristine. 1) Fe concentrations and Fe isotope ratios are not correlated. 2) The concentrations of other elements that are potentially mobile during carbonate diagenesis (Mg, Sr) do not correlate with δ56Fe. 3) Mn concentrations are inversely correlated with δ56Fe, which might hint at meteoric alteration. However, if Mn concentrations and Fe isotope signatures were pristine features, they might serve as a record of the reduction of these two metals in seawater. 4) Dolomitisation of two Devonian limestones does not shift their Fe isotope compositions. 5) Rare earth element and yttrium (REY) patterns in all limestones are similar to seawater REY sources. This suggests that the bulk of the Fe in the samples occurs in hydrogenous carbonates, not in primary or secondary Fe and Mn hydroxides, because the latter have different REY patterns. The modern microbialites are unfractionated in Fe relative to crustal rocks. Thus, these hydrogenous precipitates either incorporated unfractionated Fe of the ambient seawater composition, or they modified the Fe composition upon incorporation from a fractionated ocean Fe reservoir. In either case, provided diagenetic alteration has not modified the Fe isotope composition in amajor way, a potential open ocean Fe isotope proxy may exist that may allow reconstruction of secular variations in seawater Fe isotope compositions. Our preliminary results suggest that Neoarchean microbial carbonate Fe may have been lighter than today's microbialite Fe, and that the carbonates' Fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time.