Biological carbon precursor to diagenetic siderite with spherical structures in iron formations

Abstract

During deposition of Precambrian iron formation, the combined sedimentation of ferrihydrite and phytoplankton biomass should have facilitated Fe(III) reduction during diagenesis. However, the only evidence for this reaction in iron formations is the iron and carbon isotope values preserved in the authigenic ferrous iron-containing minerals. Here we show experimentally that spheroidal siderite, which is preserved in many iron formation and could have been precursor to rhombohedral or massive siderite, forms by reacting ferrihydrite with glucose (a proxy for microbial biomass) at pressure and temperature conditions typical of diagenesis (170°C and 1.2 kbar). Depending on the abundance of siderite, we found that it is also possible to draw conclusions about the Fe(III):C ratio of the initial ferrihydrite-biomass sediment. Our results suggest that spherical to rhombohedral siderite structures in deepwater, Fe-oxide iron formation can be used as a biosignature for photoferrotrophy, whereas massive siderite reflects high cyanobacterial biomass loading in highly productive shallowwaters.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…For instance, the spheroidal forms of siderite found in several IFs are similar to those of the siderite grains formed experimentally when ferrihydrite is reacted with organic carbon at pressure and temperature conditions commensurate with IF diagenesis (Köhler et al, 2013). Likewise, the large magnetite grains found in many IFs can be replicated experimentally through a three-stage sequence, beginning with DIR of an initial ferric 42 iron-rich sediment coupled to the oxidation of dead phytoplankton biomass, followed by magnetite crystal aging, and ultimately by pressure-and temperature-induced abiotic alteration of the biogenic magnetite during metamorphism (Y.-L. Li et al, 2013 (Fig.…”

“…This progression was also found in the Dales Gorge Member, where siderite spheres and rhombohedra occur near each other. Such features suggest that some siderite formed during the diagenetic (biological or non-biological), and subsequent metamorphic, oxidation of microbial biomass to CO 2 /bicarbonate coupled to the reduction of Fe(III), followed by reaction of the product Fe 2+ aq with sediment pore-water bicarbonate (Johnson et al, 2008;Heimann et al, 2010;Köhler et al, 2013;Halama et al, 2016). Importantly, in most cases, the C, O, Fe, and Sr isotope compositions of IF carbonates clearly preclude formation in equilibrium with seawater .…”

Limited Zn and Ni mobility during simulated iron formation diagenesis

“…Capsules were subjected for 14 days at 170¼C and 1.2 kbar, approximately the diagenetic pressure and temperature (P-T) conditions of one of the largest and most studied iron formations on Earth, the Transvaal Supergroup BIF (Miyano and Klein, 1984;Miyano, 1987). Furthermore, these conditions mimic previous diagenetic capsule experiments (Posth et al, 2013b;Kšhler et al, 2013), and therefore, facilitate a ready comparison between experimental conditions. Our experimental duration and P-T parameters are obviously much abbreviated compared to natural burial diagenesis and metamorphism that takes thousands to millions of years; and are less than experienced by some IF which were subjected to amphibolite facies.…”

“…The potential presence of siderite in the post-diagenetic sample ZnFh-2 (see also Kšhler et al, 2013;Posth et al, 2013b) would indicate that both Fe(III) and Fe(II) are present for the diagenetic formation of more crystalline mineral phases.…”

“…However, the extent to which Zn and Ni are mobilized when the precursor ferrihydrite underwent reduction and dehydration during diagenesis remains unclear. Our work builds on methodologies of previous simulated IF diagenesis experiments that focused on Fe-mineral transformations (Posth et al, 2013b;Kšhler et al, 2013), in order to evaluate trace element mobilization during diagenesis and the robustness of previous paleomarine Zn and Ni estimates. Additionally, due to the role of organic matter as a diagenetic reductant, we consider reactions between organic carbon and Fe(III) in our diagenetic experiments; with Zn or Ni sorbed to either synthetic 2-line ferrihydrite with added glucose in the capsule or sorbed directly to biogenic ferrihydrite.…”