Abstract
Iron isotope compositions of sedimentary pyrites (FeS2) are used to constrain the redox evolution of the Precambrian ocean and early Fe-based metabolisms such as Dissimilatory Iron Reduction (DIR). Sedimentary pyrites can record biotic and abiotic iron reduction, which have similar ranges of Fe isotopic fractionation, as well as post-depositional histories and metamorphic overprints that can modify Fe isotope compositions. However, some exceptionally well-preserved sedimentary records, such as the stromatolite-bearing Tumbiana Formation (ca. 2.7 Ga, Western Australia) have been proven to retain primary information on Early Neoarchean microbial ecosystems and associated metabolic pathways. Here, we present in situ Fe isotope measurements of micropyrites included in four stromatolites from the Tumbiana Formation in order to assess iron respiration metabolism using Fe isotope signatures. A set of 142 micropyrites has been analyzed in three lamina types, i.e. micritic, organic-rich and fenestral laminae, by Secondary Ion Mass Spectrometry (SIMS), using a Hyperion radio-frequency plasma source. The diversity of laminae is attributed to specific depositional environments, leading to the formation of Type 1 (micritic laminae) and Type 2 (organic-rich laminae) and early diagenetic effects (Type 3, fenestral laminae). Type 1 and 2 laminae preserved comparable δ56Fe ranges, respectively from −1.76‰ to +4.15‰ and from −1.54‰ to +4.44‰. Type 3 laminae recorded a similar range, although slightly more negative δ56Fe values between −2.20‰ and +2.65‰. Globally, our data show a large range of δ56Fe values, from −2.20‰ to +4.44‰, with a unimodal distribution that differs from the bimodal distribution previously reported in the Tumbiana stromatolites. Such a large range and unimodal distribution cannot be explained by a unique process (e.g., biotic/abiotic Fe reduction or pyrite formation only controlled by the precipitation rate). It rather could reflect a two-step iron cycling process in the sediment pore water including i) partial Fe oxidation forming Fe(OH)3 with positive δ56Fe values followed by ii) partial, possibly microbially induced, Fe reduction leading to Fe2+ availability for pyrite formation by sulfate reducers carrying both negative δ56Fe and δ34S signatures. In this model, the buildup and subsequent reduction through time of a residual Fe(OH)3 reservoir arising from the activity of methanotrophs, can explain the strongly positive δ56FeFe(OH)3 values up to 4‰. These results indicate that Archean microbial mats have been the site of the interaction of several closely linked biogeochemical cycles involving Fe, S and C.
Highlights
Stromatolites are laminated organo-sedimentary structures, formed by microbial activities in response to environmental and sediment dynamics, and are considered to be the most undisputable evidence of early life on Earth (Allwood et al, 2006; Awramik, 2006; Schopf et al, 2007)
Other biological pathways reported in these stromatolites include Microbial Sulfate Reduction (Thomazo et al, 2009; Marin-Carbonne et al, 2018), Dissimilatory Iron Reduction (Yoshiya et al, 2012), arsenic cycling involving both As(III) oxidation and As(V) reduction (Sforna et al, 2014), microbially-induced ammonia oxidation leading to one of the most extreme isotopic 15N enrichment (d15N up to +50‰) ever recorded on Earth (Thomazo et al, 2011; Stueken et al, 2015b), and microbially-derived nanoaragonite precipitates associated with organic globules containing aromatic, aliphatic and carboxyl groups (Lepot et al, 2008; Lepot et al, 2009)
DsFhaigata.pse7e.tdPimsrtoreiabbnaubtviiolaintlyuecdeenanntesdirteyrddoitsnhter+ibs1ut‰atnio.dnaTsrhdceadlrceeudvliaacttueidorvnfe;orrbe)ap)rPetrsheeevniteosnuttsihreeputdhb5e6lFiosreheetddiacatdal5s6GeFtae(ubsvlsaaicaluknescfuufrnovcret,iosttnhroisgmisvateutnodlybit)yesfhsaoðxmwÞip1⁄4nlgesrÃap1dffi2ceffipffiosÃncrteiiÀbnð2xiuÀrn2logÞ2uswa, igtbhaimulsosdtiahanel distribution; c) Cumulative function of the entire dataset and Gaussian fit and d) Cumulative function associated with Nishizawa et al (2010) and Yoshiya et al (2012) data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Summary
Stromatolites are laminated organo-sedimentary structures, formed by microbial activities in response to environmental and sediment dynamics, and are considered to be the most undisputable evidence of early life on Earth (Allwood et al, 2006; Awramik, 2006; Schopf et al, 2007). Sulfato-reducing metabolizers reduce sulfate into dissolved S species (e.g., H2S, HS-, S0), a process accompanied by a large fractionation of sulfur isotopes between sulfide and sulfate of ~70‰ (e.g., Johnston, 2011; Sim et al, 2011). This metabolic effect has been identified in the Tumbiana Formation by bulk rock (Thomazo et al, 2009) and in situ (Marin-Carbonne et al, 2018) S-
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