The nitrogen cycle in the aftermath of the mid-Cryogenian snowball Earth glaciation.

Abstract:

The geochemical record through the mid-Cryogenian (Sturtian) glaciation suggests an extraordinary resumption of productivity following snowball glaciation. Here we present new nitrogen and carbon isotopic data from five sites in the Centralian Superbasin of Australia, and Tasmania. Our data suggest that post-glacial marine conditions were anoxic and nutrient replete, which helped fuel the biological recovery after snowball glaciation. Nitrogen isotope values increase from +5‰ to +9‰ at the glacial termination and decrease to +2‰ through the carbon isotope increase that characterizes the post-glacial interval. Oxidation of the ocean following glaciation fostered loss of DIN via ammonia oxidation coupled with nitrate reduction in addition to drawdown of DIN by high productivity. We interpret the decrease in δ¹⁵N as an increase in the relative importance of nitrogen fixation as compensatory response to DIN loss. The long-term low in δ¹⁵N values suggests that nitrogen fixation played an important role in maintaining high fractional burial of organic carbon as indicated by the carbon isotope record. Site-to-site differences in the δ¹³Corg records suggest a strong role for local carbon cycling, vertical mixing, and are consistent with a large surface-to-deep gradient in the δ¹³C of DIC. If this interpretation is correct, our results are consistent with high phosphate concentrations and a vigorous biological pump in the post-snowball world.