2003 Annual Science Report

Harvard University Reporting  |  JUL 2002 – JUN 2003

The Planetary Context of Biological Evolution Subproject: The Proterozoic Oxidiation of the Earth's Surface

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

The history of oxygen in the oceans and atmospheres is thought to have played a key role in Earth’s long term biological evolution. Ongoing research addresses the initial oxygenation of the atmosphere and surface ocean 2.4-2.2 Ga, renewed oxygen influx near the end of the Proterozoic eon, and, increasingly, the nature of the biosphere between those two events.

In Year 5, Ariel Anbar’s lab continued development of the Mo stable isotope system for paleoredox investigations in natural samples and laboratory experiments. The most significant new result was the laboratory determination of the fractionation factor for Mo isotopes during adsorption to Mn oxides (previous lab studies had determined only that such fractionation occurs). Anbar’s results indicate that this is an equilibrium isotope effect and that it dominates Mo isotope fractionation in the oceans. This places his proposed use of Mo isotopes as paleoredox tracers on more solid ground. Anbar and A.H. Knoll also published a paper in Science in which they explored the biological implications of trace metal scarcity in sulfidic Proterozoic oceans.

Anbar’s lab also advanced several Fe isotope projects in collaboration with the PSU Team and with new collaborators in the Center for Environmental Bioinorganic Chemistry at Princeton University. Highlights include: Interpretation of Fe isotope effects associated with mineral weathering (w/PSU); assessment of a new technique (high mass resolution MC-ICP-MS) for Fe isotope studies; and calculation of the equilibrium fractionation factor between Fe(II) and Fe(III) hexaquo complexes using ab initio molecular modeling. The latter study brings theory into agreement with experimental determination of this fractionation factor for the first time.

National Research Council (NRC) postdoctoral fellow Yanan Shen, Andrew Knoll, and Australian collaborator Malcolm Walter added new data to the debate about Proterozoic environments, showing that the 1500-1400 Ma Roper Basin had oxic surface waters but was anoxic at depth. Shen et al. also demonstrated that sulfur isotopic values of syngenetic and early diagenetic pyrite in Roper rocks vary as a strong function of depth in the basin. These findings support the hypothesis that 1500-1400 million years ago sulfate levels remained well below modern values. Knoll’s lab also showed that microfossils in Roper shales preserve diverse and complex ultrastructures (as visualized by transmission electron microscopy (TEM)), recording moderate levels of eukaryotic diversity in coastal environments.

The most exciting new discoveries in H.D. Holland’s lab concern the composition of pyrite in the Timeball Hill and Rooihogte formations in South Africa. Measurements of the isotopic composition of sulfur in these pyrites have shown a considerably larger range in δ34S than in older sulfides, and the absence of a significant signal of mass independent fractionation of the sulfur isotopes. These observations and the presence of hematitic iron ore in the upper part of the Timeball Hill Formation are strong evidence for the presence of oxygen in the atmosphere during the deposition of these sediments. Re-Os dating of the pyrites has yielded an age of 2,322 ± 15 Ma and an initial 187Os/188Os ratio of 0.1087 ± 0.0063. The age shows that the loss of a major Ä33S signal in the atmosphere occurred between 2,450 and 2,322 ± 15 Ma. This indicates that O2 appeared in the atmosphere between these dates.

The initial 187Os/188Os ratio in the pyrites is essentially equal to that of the mantle at 2,322 Ma. This indicates that the input of riverine terrestrial Os was very minor, an inference that is consistent with the absence of Re enrichment in highly carbonaceous shales from this period. The O2 content of the atmosphere was apparently sufficiently high at 2,322 ± 15 Ma to eliminate the MIF signal in sulfur but sufficiently low to prevent a significant amount of oxidative weathering of Re and Os on the continents.

Finally, Anbar remained active as co-chair of the Mission to Early Earth Focus Group. This work included facilitating the development of the “Archean Biosphere Drilling Project” to oversee the Mission to Early Earth’s (MtEE’s) Archean drilling project in Western Australia, now planned for summer, 2004.