Ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (d 13 C, d 18 O, and d 34 S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically influenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 lm and bulk radiocarbon ages decrease by 360 yr westward along the~20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion-in favor of net growth-as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric.

Ocean Anoxic Event 2 (OAE2) was a period of dramatic disruption to the global carbon cycle when massive amounts of organic matter (OM) were buried in marine sediments via complex and controversial mechanisms. Here we investigate the role of OM sulfurization, which makes OM less available for microbial respiration, in driving variable OM preservation in OAE2 sedimentary strata from Pont d’Issole (France). We find correlations between the concentration, S:C ratio, S-isotope composition, and sulfur speciation of OM suggesting that sulfurization facilitated changes in carbon burial at this site as the chemocline moved in and out of the sediments during deposition. These patterns are reproduced by a simple model, suggesting that small changes in primary productivity could drive large changes in local OM burial in environments poised near a critical redox threshold. This amplifying mechanism may be central to understanding the magnitude of global carbon cycle response to environmental perturbations.

No abstract

The sulfur (S) isotope difference between sedimentary sulfate and sulfi de phases preserved in sedimentary rocks (Δ 34 S) has been utilized to reconstruct marine sulfate concentrations and inferentially the redox evolution of Earth's surface. These interpretations are largely based on experimental studies that indicate that microbial sulfate reduction is accompanied by a substantial kinetic isotope effect (up to 66‰), but only at sulfate concentrations >~200 µM. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (~100-350 μM) and fi nd that the calculated kinetic isotope effect associated with microbial sulfate reduction (ε 34 S) is relatively large (~23.5‰), but preserved Δ 34 S values are considerably smaller (4.7‰-9.9‰). Δ 34 S values in this system are controlled by the fraction of the sulfate reservoir that is consumed during sulfate reduction and the location of pyrite formation. This reservoir effect strongly infl uences the S isotope composition of sulfi de preserved in the rock record such that Δ 34 S values increase as a function of sulfate levels, even when sulfate concentrations are >200 µM and the kinetic isotope effect is expressed. These fi ndings have important implications for reconstructing the chemical evolution of the ocean-atmosphere system throughout Earth history-not just for the Precambrian.

The sulfur (S) isotope composition of pyrite in the sedimentary record has played an important part in our understanding of the evolution of biogeochemical cycles throughout Earth history. However, the kinetics of pyritization are complex and depend strongly on the reactivity and mineralogy of available iron. As a second major sink for sulfide in anoxic sediments, organic matter (OM) provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide. To first order, roughly parallel pyrite and OM d 34 S profiles reflect changes in sulfide, while independent patterns require alternative explanations, including changes in iron availability or OM characteristics. We apply this framework to Ocean Anoxic Event 2 (OAE-2, ~94 Mya), a period of enhanced burial of reduced C and S (in OM and pyrite) that has been associated with an expansion of reducing marine conditions. We present paired S-isotope records for pyrite and OM along with profiles of OM S:C ratio and S redox speciation from four wellcharacterized lithologic sections with a range of depositional environments (Pont d'Issole, Cismon, Tarfaya Basin, and Demerara Rise) to reconstruct both local redox structure and global mechanisms impacting the C, S and Fe cycles around OAE-2.

Biogeochemical sulfur cycling has varied widely over geologic time, mainly in response to changes in primary productivity and organic carbon burial, volcanism, weathering, and evaporite deposition. Several of these processes are explicitly linked to discreet (<1.2 Ma) intervals of widespread organic carbon burial, termed oceanic anoxic events (OAEs). During the Cretaceous, there is a highly distinctive ~4‰ negative excursion in the sulfur isotope composition of seawater sulfate (δ34SSO4) that is bracketed by the two most prominent OAEs (OAE1a and OAE2). This excursion lasted for ~25 Ma and has been variously attributed to enhanced volcanism, changes in weathering, evaporite burial, and/or changes in modes of organic carbon remineralization. We present new high‐resolution carbon and sulfur isotope records from carbonate‐associated sulfate and pyrite for OAE1a and OAE2. OAE1a is characterized by a monotonic decrease in δ34SSO4 values. Both negative and positive δ34SSO4 excursions are associated with OAE2. To refine hypotheses for the observed changes in biogeochemical sulfur cycling associated with these events, we use a simple sulfur isotope box model. Both empirical and modeling results indicate that δ34SSO4 variability was dominated by input fluxes during OAE1a, whereas enhanced volcanism, weathering, and pyrite burial controlled δ34SSO4 records during OAE2. Our analysis supports the conclusion that Cretaceous marine sulfate concentrations were much lower than modern concentrations and indicates that increases in marine sulfate occurred at the onset of both events. We conclude that increases in marine sulfate from low background concentrations, in conjunction with other environmental characteristics, contributed to the development of OAEs.

Rationale Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10 μm) grains or for detecting intra‐grain variability. Methods Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20–130 μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 × 128 pixel grids) for between 20 and 960 cycles. Results The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32S in 1–4 μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ ±2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤ ±2‰, 1σ). In contrast, grain‐specific 34S/32S ratios in modern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. These data allow past local environmental conditions to be inferred.

No abstract