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2011 Annual Science Report

Arizona State University Reporting  |  SEP 2010 – AUG 2011

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

Project Summary

We have generated and are interpreting a wide range of geochemical data from rocks that are over 1.5 billion years old. The data indicate that the ancient ocean was very different than today and had regions that were full of toxic hydrogen sulfide. These extreme conditions in the ocean were the backdrop against which early organisms appeared and evolved—and perhaps struggled.

4 Institutions
3 Teams
13 Publications
0 Field Sites
Field Sites

Project Progress

The centerpiece of this effort is a coupled study of mid-Proterozoic ocean chemistry and ecology as revealed by inorganic and molecular geochemical proxies, spearheaded by Postdoc Kelly. We have completed iron speciation and trace metal analysis for 109 samples from the mid-Proterozoic Barney Creek Formation in north-central Australia. These data reveal a significant extent of euxinia in the Glyde Sub-Basin. Twelve of these samples were selected for iron isotope measurements, with values ranging from -0.01 to 0.53 per mil. These values are fairly heavy and are inconsistent with the pathways of iron enrichment seen in many other euxinic basins of diverse ages. We are now working to understand the importance of this difference. Twenty-three samples were selected for uranium isotope measurements, with values ranging from -0.02 to -0.44 per mil, which are all quite light. The sample with the most authigenic signal, known from iron speciation to be from a euxinic environment, has a value of -0.07 per mil. If the current 0.6 per mil offset between the euxinic sediments of the Black Sea and the ocean today was the same through Earth history, this would put the U isotope value for the mid-Proterozoic ocean at -0.7, three per mil lighter than today. Reducing environments are expected to have enriched uranium, so expanded zones of euxinia compared to today would leave the global oceans depleted in 238U. These are the only whole rock iron and uranium isotope measurements for this time period. One paper resulting from this study is near submission. That paper focuses on the extent of very shallow euxinia in the McArthur basin and more broadly, and the implications extend to models for organic productivity in the early ocean.

Additional efforts have centered on extensive analysis of mid-Proterozoic shales from north China. These samples confirm the widespread occurrence of anoxic and iron-rich conditions in the ocean at this time, which contrasts dramatically with conventional arguments. It is likely that euxinia was limited to productive continental margins and restricted settings, such as the Glyde Sub-Basin. These data play a central role in a recent paper published in Nature that argues for ferruginous conditions over much of the mid-Proterozoic. These chemical conditions have important implications for life and specifically the availability of essential micronutrients.

NAI support under this heading also facilitated a number of publications by the Lyons and Anbar groups featured in the publications list.