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Project Progress

The Proterozoic Oxidiation of the Earth’s Surface (dm)

The Harvard team’s research focuses on major transitions in the evolution of life on Earth. Currently we have six active subprojects that address this theme:

- The oxidation of the Earth’s surface, especially events that took place 2400-2200 million years ago and, again, near the end of the Proterozoic Eon

- Intense glaciation, biogeochemical change and the radiation of animals 600-525 million years ago

- Mass extinction at the Permo-Triassic boundary 250 million years ago.

- Isotopic characterization of molecular biosignatures, with the goal of understanding the functional as well as systematic relationships of microorganisms in natural ecosystems

- Geobiology of sedimentary iron deposits, with goals that include understanding terrestrial analogs of the aqueous hematite terrain likely to be chosen as a principal landing site in the Mars MER 2003 mission.

- Evogenomics: collaborative focus group research on molecular phylogeny.

Our results includie the following:

In a reconnaissance study, Mo isotopic compositions were characterized in Devonian black shales, Black Sea sediments, Pacific and Atlantic ferromanganese nodules, continental molybdenites, and seawater. The largest variation, between Mo in ferromanganese and carbonaceous sediments, suggest Mo isotope fractionation related to environmental redox conditions. The isotopic composition of Mo in seawater is similar to that of carbonaceous sediments, consistent with high efficiency Mo scavenging in sulfidic environments. The data suggest that fractionation occurs during scavenging of Mo onto ferromanganese particles. Isotopic mass balance indicates that the observations are consistent with the proportions of Mo removal to oxic and sulfidic marine sediments today. If so, the Mo isotopes in seawater may reflect changes in the relative proportions of oxic to sulfidic seafloor, and the isotopic composition of Mo in black shales may constitute a paleoredox record.

Geochemical study of mid-Proterozoic black shales deposited during maximum flooding of the McArthur Basin, northern Australia, indicates sulfidic deep water, as proposed in 1998 by Canfield. Both ca. 1730 and 1636. S-isotopes also suggest substantial sulfate depletion in these waters, consistent with an oceanic sulfate inventory much lower than today’s.

Canfield proposed that the deep sea was sulfidic, rather than oxic, from 1850 Ma (when BIF stopped forming) until at least 1200-1000 Ma ago. This period of time is also characterized by unusual stability in the carbonate carbon isotope record, by unexplained delay in the diversification of eukaryotes, and by unusual onshore-offshore decrease in diversity of fossils of eukaryotic origin. We propose that these observations are related by the extreme redox sensitivity of Fe and Mo- both metals critical to the biological N cycle.

We have discovered abundant and well-preserved microfossil assemblages in the siliciclastic Roper Group, northern Australia. The Roper Group is well characterized in terms of age (1492+/-3 SHRIMP age on zircon in a tuff low in the succession) and sequence architecture. It is also fossiliferous throughout, enabling us to relate fossil assemblages to facies. Eukaryotic fossils decrease in abundance and diversity from marginal marine to basinal shales, but increase in equability. This ecological pattern is predicted by the trace element model. Process-bearing acritarchs provide the first evidence for modern cytoskeletal architecture in 1500 Ma protists.