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.