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Project Progress

Our efforts over the last year included continued analysis of mid-Proterozoic samples from Australia—emphasizing sulfur isotope systematics, trace metal geochemistry, and organic biomarkers. This work builds on the previous sample collection and analyses of NAI postdoc Amy Kelly, while at ASU and UCR. Kelly’s results will soon be summarized and submitted in a paper. Our most recent analyses define the core of an M.S. thesis at UCR (Kevin Nguyen), which is arguably the first systematic search for eukaryotic biomarkers within a tight, independent paleo-environmental context based on inorganic proxies. There has been substantial energy devoted to the preparation and publication of the manuscript. Paleoredox proxy development and application centered on uranium isotope systematics, led by former ASU postdoc (now faculty at University of Waterloo) Brian Kendall, and former ASU lecturer (now faculty at Louisiana State University) Steven Romaniello. A large number of other manuscripts emerged from ASU and UCR centered on task themes and supported in various ways by the ASU-led Follow the Elements NAI team, involving in particular recent UCR Ph.D. graduates Noah Planavky and Chris Reinhard (now faculty at Yale University and Georgia Institute of Technology, respectively).

Highlights from Our New Views on the Proterozoic Biosphere:

Reassessing Biomarker Evidence for a Mid-Proterozoic Purple Ocean

Recent arguments for temporally and spatially extensive anoxic and hydrogen-sulphide-containing (euxinic) conditions in the mid-Proterozoic (surface) ocean rely heavily on biomarker data from the 1.64 billion-year-old (Ga) Barney Creek Formation (BCF) in Australia. The presence of aromatic hydrocarbon biomarkers diagnostic of light- and H2S-dependent purple photosynthetic bacteria (Chromatiaceae) has been cited as evidence for widespread euxinic conditions high within the photic zone and helped shape the prevailing view of extremely reducing conditions throughout the Proterozoic ocean (Brocks et al., 2005, Nature; Johnston et al., 2009, PNAS). However, we found the most abundant, and best structurally preserved aromatic carotenoid markers diagnostic of Chromatiaceae in turbidite intervals of the BCF. These layers also contain a range of saturated hydrocarbon molecular markers that suggest saline, likely shallow benthic microbial mats. Thus, instead of living in a planktonic marine setting beneath an extremely shallow H₂S-interface, which would be unsustainable in the face of ocean mixing, the Chromatiaceae grew in benthic mats on the shallow seafloor likely beneath oxygenated waters. The mats were reworked and transported from the shelf into the basin via downslope, mass-flow processes. Evidence for planktonic green sulphur bacteria nevertheless points to free sulfide at least episodically within the deepest photic zone (potentially as deep as 100 m). Given our results, there is no compelling evidence for very shallow euxinia (<20 m deep) anywhere in the mid-Proterozoic ocean, forcing a re-evaluation of the redox structure of the water column and particularly the relative importance of anoxygenic primary production by bacteria and their role in large-scale carbon and oxygen cycling.

Mid-Proterozoic Ocean Chemistry Recorded in the 1.4 to 1.5-Billion-Year-Old Roper Group, Australia

Understanding the nature of mid-Proterozoic oceanic redox is vital to unraveling the apparent biogeochemical stasis that defines an estimated one-billion-year interval and its relationship to the early evolution of eukaryotic life. It is widely accepted that the chemistry of the early ocean and its life co-evolved. However, the true geochemical nature of this ‘boring billion’ remains poorly constrained, and much of the best data has come from the McArthur basin, Northern Territory, Australia. Specifically, black shales of the Roper Group have emerged as one of our best windows to mid-Proterozoic ocean redox because they are well dated, minimally metamorphosed, and lie in the middle of this key interval. Most importantly, paleo-geographic data suggest relative strong connection to the open ocean, and therefore conditions in the Roper basin may reflect the redox state of the broader margin of the open ocean.

Despite this potential, a high-resolution, fully integrated, large-scale geochemical study of the Roper Group has not been undertaken. This data provides an essential backdrop for our ongoing organic geochemical study and specifically our research for the early record of eukaryotic organisms and their paleo-environmental context. The primary aim is a high-resolution geochemical history captured in samples from the Roper Group obtained by drill core. The emphasis is on redox proxies, specifically TOC, pyrite-S isotopes, Fe speciation, and trace metal concentrations, all measured to provide a high-resolution perspective on the degree of oxygenation and its temporal variability, ultimately with an eye on the broad coeval water-column redox structure through analysis of other cores. Also among the targets are degrees of basin restriction and, overall, the conditions for shallow and deep life. Uncertainties about global versus local redox conditions remain paramount and are often difficult to resolve. Nevertheless, preliminary data suggests that the Roper, in contrast to previous claims, was not widely sulfidic—consistent with an emerging view that limits the spatial extent of mid-Proterozoic euxinia. These results will factor prominently in our overarching view of nutrient availability during this important time period and related drivers and feedbacks for the oxygen landscape of the mid-Proterozoic ocean.

Paleoredox Proxy Development

Uranium isotopes in black (carbonaceous) shales and in carbonates have emerged as a promising new paleoredox proxy, based in part on work done in Year 4, published in Brennecka et al. (2011). In Year 5, we published our findings on applying this proxy to the 2.5 billion year old Mt. McRae shale (Kendall et al., 2013). This carbonaceous shale has been the focus of extensive paleoredox reconstruction documenting evidence of a “whiff of oxygen” before the Great Oxidation Event. We find instances where the U isotope composition in the upper Mt. McRae Shale (δ²³⁸U = −0.2 to 0.0‰ relative to standard SRM950a) is isotopically heavier than average upper crust (δ²³⁸U = −0.31 ± 0.14 [2SD] based on granitoids and basalts). The high δ²³⁸U values point to U isotope fractionation in the late Archean marine environment and hence indicate the presence of a small amount of dissolved U in seawater and authigenic U in the Mt. McRae Shale. The supracrustal δ²³⁸U signatures are associated with some of the highest Mo and Re enrichments in the Mt. McRae Shale as well as distinctive Mo, S and N isotope signatures that are indicative of mild environmental oxygenation. Hence, our findings suggest that small amounts of U were oxidatively mobilized from the upper crust at 2.50 Ga.

At the same time, in Romaniello et al. (2013) we published cautionary data for the use of this proxy in carbonates. In this study, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1 ppm) than in primary precipitates (avg. 1.5 ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H₂S, and thus indirectly to organic matter content. δ²³⁸/²³⁵U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, δ²³⁸/²³⁵U of bulk sediments from the push cores was 0.2–0.4‰ heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and ²³⁸U/²³⁵U ratios. Clearly, careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of ²³⁸U/²³⁵U data from ancient carbonate sediments.