ALBERT

Description: The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope--a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory--conditions that result in low organoclastic sulfate reduction rates. Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur. Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulfide phases meters below the SMT demonstrates that in sulfide-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths.

Description: Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations ([O~2~]) around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine to calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in atmospheric _p_O~2~ at ~400 million years ago (Ma), and reveals a step-change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma.

Description: The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere-ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere-ocean system decreased considerably after ca. 2.06 billion years ago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85 Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06 Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88 Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93 Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90 million years before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

Description: In order to advance our understanding of the emerging chromium (Cr) isotope system as a paleoredox proxy, we measured the 53Cr/52Cr of black shales deposited before, during, and after Cretaceous Oceanic Anoxic Event 2 (OAE2). We observed a 〉 1 per mil coherent negative 53Cr/52Cr excursion and significant drawdown in Cr enrichments during OAE2 coincident with a large positive carbon isotope excursion. Our observed negative 53Cr/52Cr excursion during OAE2 is most easily linked to an increase in the ratio of euxinic to reducing conditions. Additional work on other OAE sections is needed to determine the spatial significance of this Cr isotope trend.