FUELING THE SNOWBALL EARTH BIOGEOCHEMICAL RECOVERY: EVIDENCE FOR A BIO-INORGANIC BRIDGE

Organic-rich shales and siltstones interbedded with diamictites from five locations in Tasmania and mainland Australia record marine geochemical conditions during the recovery from the Sturtian (mid-Cryogenian) glaciation. New nitrogen, sulfur and carbon isotope data and Fe-speciation and organic carbon (wt. %) data suggest a dynamic transition from nutrient-rich and sulfidic conditions to a ferruginous, low-sulfate ocean that supported nitrogen fixation-fueled carbon burial. The presence of banded iron formations in glacial strata and Fe-speciation are consistent with anoxic conditions at the end of glaciation. If the whole glacial ocean were anoxic for the duration of the glacial, and sea-air exchange was greatly limited, there would be no clear mechanism for the redox cycling and loss of DIN (dissolved inorganic nitrogen). In this situation, ammonium derived from the slow degradation of organic matter diffusing from sediments could have filled the deep ocean with ammonium. Combined with the high flux of phosphorus from weathering, the post-glacial ocean may have been exceedingly nutrient-rich, resulting in the massive burial of organic carbon inferred by the carbon isotope record. Oxidation would have restarted the ammonium oxidation-nitrate reduction cycle, driving the loss of fixed-N, which is signaled by significant ¹⁵N-enrichment in post-glacial strata from +4 to as high as +9‰. Decreasing fixed-N inventories and a transition to ferruginous conditions would fuel nitrogen fixation and ¹⁵N-depletion as suggested by δ¹⁵N values decreasing to below +2‰ following the post-glacial δ¹⁵N maximum. If the post-Sturtian glacial ocean were indeed metal-rich and sulfate-poor it is possible that N-fixation was not metal limited. Instead, N-fixation may have played an expanded role in supporting carbon burial through the interglacial interval, setting the stage for the Marinoan glaciation.