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Project Progress

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Evolution of Atmospheric O2, Climate, and the Terrestrial Biosphere (dm)

The Rise of Oxygen: A model for the rise of atmospheric oxygen approximately 2.3 Ga indicates that the interval of intense mantle plume activity in the latest Archean not only stimulated the widespread deposition of banded iron formation, but also created more oxidized source regions for volcanic emissions in the upper mantle, reducing an important Archean oxygen sink, and thus allowing for the establishment of an oxygen-rich atmosphere. A consequence of this change in atmospheric composition was the first Snowball Earth event of the earliest Paleoproterozoic.

Ge/Si Ratio in Precambrian Cherts: Germanium behaves in a geochemically similar pattern to silicon, but has a more important seafloor hydrothermal source. Thus, Ge/Si ratios in the ocean represent a balance between riverine and hydrothermal inputs. In addition, the Ge/Si ratio of the riverine source depends on the intensity of chemical weathering on land. Cherts precipitated from the ocean potentially record the Ge/Si ratio of ancient oceans, thus providing insights into the nature of elemental cycling of Precambrian oceans. We have analyzed a number of Precambrian cherts from the Gunflint (2.0 Ga) and Overnwacht (3.2 Ga) formations. We find that the Ge/Si ratio in these rocks is 2-9x higher than modern cherts, suggesting an overall increased influence of hydrothermal inputs and/or intensity of chemical weathering, both of which can be supported theoretically. Interestingly, iron-formation-associated cherts have the highest ratios, suggesting a hydrothermal source for the iron in banded iron formations (BIFs).

Oceanic Modeling: We have developed a biogeochemical module for the GFDL Modular Ocean Model that is unique in its ability to simulate biogeochemical cycles in anoxic world oceans. We have applied the model to the problem of Late Permian anoxia and mass extinction to test the idea that hypercapnia and H2S toxicity contributed to the extinction.

Models of Microbial Mat Biogeochemistry: We have generated a 1-dimensional numerical model of the biogeochemistry of microbial mats that considers interactions among cyanobacteria, purple, and colorless sulfur bacteria. Our ultimate goal is to explore the sensitivity of mat structure and function under a variety of boundary conditions appropriate for modern and ancient environments, looking for characteristics that might leave a geological record and indicate whether, for example, the overlying waters were oxygen or hydrogen-sulfide rich.