NUMERICAL MODELING OF ARCHEAN UPWELLING

Euxinia, which is defined as a water column characterized by anoxia and accumulation of free H₂S, is generally considered an unlikely chemical feature of the Archean ocean. This assumption hangs on arguments about the balance between Fe²⁺ and SO₄^2-, namely (1) an abundance of dissolved Fe²⁺ derived from hydrothermal weathering of oceanic basalt and (2) subaerial weathering under a largely reducing atmosphere such that continental sources of solutes to the ocean generally favored Fe²⁺ over SO₄²⁺. Such a picture is consistent with many lines of evidence, including the abundance and distribution of Fe-rich chemical sediments; the loss of Fe from ancient weathering profiles; and sulfur isotope systematics in marine sulfides that point to low sulfate concentrations. A growing body of data, however, suggests this picture is incomplete.

Evidence for euxinia during the Archean has been found in multiple locations, both with and without other indications tying the euxinia to changes in Earth surface oxidation. The latter, which decouples euxinia from atmospheric oxygenation, implies that accumulation of water column H₂S was not only possible but was perhaps unremarkable throughout the Archean. We have examined this notion by constructing a simple box model meant to capture the first order biogeochemical features of an Archean coastal upwelling system. Initial results indicate that dissolved sulfide will accumulate in the water column given even slight variability in export production and deep ocean nutrient status and that euxinia in such a setting can be well accommodated by known constraints on deep ocean [Fe²⁺] and [SO₄^2-]. However, the microbial consumption of SO₄^2- (and thus production of H₂S) will only scale directly with export production if [SO₄^2-] is sufficiently high.

Although the model does not, at present, adequately treat water column Fe reduction or denitrification, these refinements are not likely to alter our primary conclusion that Archean euxinia should have been common and at times widespread. Our results are broadly relevant to Precambrian ocean chemistry and ecology, as it is now becoming clear that a complex spatial and temporal fabric of Fe²⁺-rich and H₂S-rich conditions was a pervasive feature of ocean chemistry during large periods of Earth’s history.