Olivier J Rouxel

7.10 Chemical Characteristics of Sediments and Seawater

The transition from an anoxic to oxygenated atmosphere was arguably the most dramatic change in the history of the Earth. This “Great Oxidation Event” (Holland 2006) transformed the biogeochemical cycles of the elements by imposing an oxidative step in the cycles, creating strong redox gradients in the terrestrial and marine realms that energised microbial metabolism. Although much past research was focused on establishing when the rise of atmospheric oxygen took place, recognition that substantial mass-independent fraction (MIF) of the sulphur isotopes is restricted to the time interval before 2.45 Ga and requires an anoxic atmosphere (Farquhar et al. 2000, 2007; Mojzsis et al. 2003; Ono et al. 2003; Bekker et al. 2004) argues the atmosphere became permanently oxygenated at this time (Pavlov and Kasting 2002). A false-start to the modern aerobic biosphere and a “whiff” of atmospheric oxygen (Anbar et al. 2007) may have occurred in the latest Archaean, as reflected in a transient enrichment in the redox-sensitive element molybdenum in marine shales and a reduction in the extent of MIF precisely coincident with the peak in Mo and FeS2 enrichment (Kaufman et al. 2007). Geochemical proxies are imperfect, and an earlier (c. 3 Ga) appearance of atmospheric oxygen is possible (Ohmoto et al. 2006) but disputed (Farquhar et al. 2007; Buick 2008).Ancient rocks record the redox conditions of the oceanatmosphere system through the distribution of iron (Fe) between oxidised and reduced minerals, which can be formulated into a suite of Fe palaeoredox proxies. The balance between Fe and S in a given system reflects the variance in a range of highand low-temperature sources and sinks. Iron can be delivered by hydrothermal, diagenetic or clastic fluxes and can be buried and removed as Fe-oxide phases, Febearing carbonates such as siderite or ankerite, relatively unreactive silicate phases, which often pass through the system in detrital form, or as a constituent of pyrite (FeS2) using sulphide sourced by sulphate reduction. Sulphate is delivered to the ocean primarily from continental weathering, which requires that a surface oxidative cycle exists, and rates of sulphate delivery and Fe removal as pyrite should thus depend on ocean-atmosphere redox. Among other successes, the iron proxies discussed here have proven their value in studies of the 2.5 Ga Mt. McRae Formation and specifically in delineating subtle increases in atmospheric oxygen prior to the Great Oxidation Event, or ‘GOE’. (Anbar et al. 2007; Kaufman et al. 2007; Reinhard et al. 2009). These Fe proxies are our most effective inorganic proxy for ancient euxinia (anoxic and H2S-rich conditions) on the local scale and are an essential independent backdrop for meaningful application of Mo isotopes to address extents of euxinia on ocean scales (Arnold et al. 2004; Gordon et al. 2009). Thus, in addition to being informative on their own, Fe-based palaeoredox indicators are a crucial component of multi-proxy approaches for distinguishing among oxic, anoxic and Fe (II)-rich (ferruginous), and euxinic depositional conditions. The quantity and speciation of highly reactive iron (FeHR) in sediments and sedimentary rocks can provide crucial insight into the redox state of the local depositional environment. The total pool of FeHR consists of mineral phases that have the potential to react with dissolved H2S when exposed on short timescales (within the water column or during earliest diagenesis) plus Fe that has already reacted and is present as FeS2 (Raiswell and Canfield 1998). Such minerals include ferrous carbonates (siderite, FeCO3; ankerite, Ca(Fe,Mg,Mn)(CO3)2), crystalline ferric oxides (haematite, Fe2O3; goethite, FeOOH), and the mixed-valence Fe oxide magnetite (Fe3O4). These phases are separated by means of a well-calibrated sequential extraction scheme described in detail elsewhere (Poulton et al. 2004; Poulton and Canfield 2005; Reinhard et al. 2009). Briefly, ~100 mg of sample powder is first treated with a buffered sodium acetate solution for 48 h to mobilise ferrous carbonate phases. A split of the extract is removed for analysis, the sample is centrifuged, and the remaining supernatant is discarded. The sample is then treated with a sodium dithionite solution for 2 h to dissolve crystalline ferric oxides and processed as before. Finally, the sample is treated with an ammonium oxalate solution for 6 h to mobilise magnetite. All extractions are performed at room temperature in 15 mL centrifuge tubes under constant agitation. The sequential extracts are analysed on an Agilent 7500ce ICP-MS after 100-fold dilution in trace-metal grade HNO3 (2 %). Pyrite iron is calculated separately based on weight percent pyrite sulphur extracted during a 2-h, hot chromous chloride distillation followed by iodometric titration (Canfield et al. 1986), assuming a stoichiometry of FeS2. For measurement of total Fe (FeT), sample powders are ashed overnight at 450 C (in order to remove organic matter but preserve volatile metals, such as rhenium) and digested using sequential HNO3-HFHCl acid treatments (see, for example, Kendall et al. 2009). After digestion, samples are reconstituted in trace-metal grade HNO3 (2 %), diluted, and analysed by ICP-MS In modern oxic sediments deposited across a wide range of environments, FeHR comprises 6–38 % of total sedimentary Fe (i.e. FeHR/FeT 1⁄4 0.06–0.38), with an average value for FeHR/FeT of 0.26 0.08 defining the modern siliciclastic baseline (Raiswell and Canfield 1998). Enrichments in FeHR that are in excess of this detrital background ratio indicate a source of reactive Fe that is decoupled from the siliciclastic flux and thus reflect the transport, scavenging and enrichment (see below) of Fe within an anoxic basin (Canfield et al. 1996; Wijsman et al. 2001). In this context, ratios of FeHR/FeT exceeding the siliciclastic range point to anoxic deposition, and the ratio FePY/FeHR can then be used to establish whether the system was Fe(II)or H2S-buffered. An anoxic system with a relatively small amount of FeHR converted to pyrite indicates a depositional environment in which reactive Fe supply was greater than the titrating capacity of available H2S produced microbially by sulphate reduction, and thus no dissolved H2S was accumulating in pore fluids or the water column. Importantly, this is true even if microbial sulphate reduction and pyrite formation was occurring in the system (Canfield 1989) because the preponderance of Fe precludes the accumulation of free H2S. In contrast, if the vast majority of FeHR is present as pyrite in an anoxic system, euxinic depositional conditions are indicated – a consequence of the C.T. Reinhard (*) Department of Earth Sciences, University of California, Riverside, CA 92521, USA 10 7.10 Chemical Characteristics of Sediments and Seawater 1483