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2010 Annual Science Report

Arizona State University Reporting  |  SEP 2009 – AUG 2010

Stoichiometry of Life, Task 3a: Ancient Records - Geologic

Project Summary

We are analyzing, at high resolution, Mid-Proterozoic drill core and outcrop samples in an effort to fingerprint the evolving redox state of the atmosphere and ocean at critical intervals in Earth history. This refined view of biospheric oxygenation provides the key backdrop for measuring and inferring abundances of diverse bioessential elements. Within this context we can better understand the distribution and evolution of early eukaryotic organisms at a variety of spatial and temporal scales.

4 Institutions
3 Teams
17 Publications
0 Field Sites
Field Sites

Project Progress

To set the stage for 2010, efforts during 2009 centered on collection of new samples (McArthur Basin, Australia, and North China) keyed to questions of mid-Proterozoic biospheric evolution, by Tim Lyons, Gordon Love and Postdoctoral Fellows Amy Kelly and Chao Li. Sample acquisition was the critical first step as we began our study of mid-Proterozoic ocean chemistry and associated organic biomarker trends. Efforts during 2010 have emphasized extensive, lab-intensive sample processing and analysis. The priority for the organic work is patterns of prokaryotic diversity, including a search for photic euxinia and ecological relationships in light of independent, inorganic constraints on paleoredox. Preliminary work is suggesting episodes of ferruginous conditions in the deep ocean. Second, we are searching, within a tight facies framework, for molecular evidence of eukaryotes in strata roughly coeval with the oldest robust fossil records of these organisms.

Substantial analytical efforts during 2010 are summarized below:

2010 Efforts, McArthur Basin, Australia (Amy Kelly, Postdoctoral Fellow, Project Leader)
Sulfur isotopes and weight percent total carbon, inorganic carbon, sulfur, pyrite iron, HCl extractable iron and highly reactive iron species have been measured on 109 GR-10 samples spanning the Barney Creek including the upper units, HYC Shale and W-Fold Shale. The bitumens from 24 of the GR-10 samples have been studied, and over 100 biomarkers were quantified. Organic analyses have been completed on 13 samples of the 82/3 core, spanning the Abner Sandstone, Crawford Fm. and Mainoru Fm. (shallow facies of the Roper Group).

Efforts on North China Samples (Chao Li, Postdoctoral Fellow, Project Leader)
Inorganic analyses were begun or completed on 65 total samples from Chuanlinggou Formation, Jixian area (deep paleodepths). Analyses included total carbon and total sulfur contents. Total Inorganic Carbon analyses (with Total Organic Carbon obtained by difference) is ongoing. These samples were also digested for trace element analyses, and Fe speciation was studied by sequential extraction. Chromium reduction analyses for Fepy and S-isotopes are ongoing. Organic analyses focused on 11 large samples: Jixian deep-water area (3 samples); Xuanhua shallow-water area (3 samples); Tuanshanzi Formation (2 samples from Jixian area); Hongshuizhuang Formation (1 sample from Jixian area); Tieling Formation (2 samples from Jixian area). Organic extractions from these samples are ongoing.
In addition to this focused Proterozoic project, Anbar, Lyons, Love, Postdoctoral Fellows Tais Dahl, Brian Kendall, Chao Li, Graduate Students Greg Brennecka, Yun Duan, Noah Planavsky, Steven Romaniello and others collaborated on a number of studies related to unraveling the tempo of ocean redox evolution through Earth history. These studies centered in particular on analyses of metal concentrations and isotopes in Archean sediments from Western Australian and South Africa, published in Nature – Geoscience and EPSL, and on the development and application of the uranium isotope system to ocean redox evolution in the Archean and Paleozoic (including one paper in Geology).

Graduate Student Michael Sheehan, working with collaborator Paul Knauth, successfully established correction factors for analyzing D/H ratio variations in small amounts of water in hydrous minerals using continuous flow on an isotope ratio mass spectrometer. Previously published attempts to make such analyses have been fraught with difficulties and errors, but an exact technique has now been established. The first suite of samples to be successfully analyzed were from a vertical sequence of seafloor basalts and silicified basalts from the 3.5 Ga Barberton Greenstone Belt and also samples from the Buck Reef Chert. A report by Stanford researches of very low D/H in the chert was not confirmed and, instead, was clearly due to faulty sample preparation procedures and incorrect reduction of raw mass spectrometer data. Both basalts and chert yield D/H ratios consistent with high Archean temperatures as deduced from previously published studies of O isotopes in cherts.

Kendall et al. (2010) provided new evidence of mild ocean oxygenation in sedimentary rocks deposited 2.6 – 2.5 billion years ago. a) Simplified relative water-depth profile. b) Ocean redox conditions on the platform and slope. O2 accumulation was confined to the shallow waters (up to several hundred metres) along the ocean margin where rates of photosynthetic O2 production were high. At mid-water depths, locally high organic carbon and sulphate fluxes promoted extensive microbial H2S production, leading to euxinic conditions. Otherwise, the deep (and open) ocean was anoxic and ferruginous. During upper Nauga time, the water column on the platform slope was mildly oxygenated, but drowning of the carbonate platform (Klein Naute Formation) resulted in replacement of the oxic waters with deeper anoxic waters. The low-O2 atmosphere permitted both mass-independent fractionation of S isotopes and oxidative mobilization of sulphate, Re and Mo from weathering of crustal sulphide minerals.