Oxidation of sedimentary sulfides as negative feedback to ice-sheet expansion: A case study from the Eocene-Oligocene Antarctic glaciation

Abstract:

The Eocene-Oligocene transition (EOT) is characterized by a global cooling trend, falling sea levels, and the onset of Antarctic ice shield formation. Previous studies have extensively investigated the interplay of glaciation, ocean circulation, and weathering during the EOT. Here we report seawater sulfate isotope data, which suggests that this climatic transition coincided with the oxidative weathering of sulfides during the subaerial exposure of shelf sediments. Seawater sulfate δ³⁴S and δ¹⁸O values decline by 0.8‰ and 1.5‰, respectively between 34.5 and 33 Ma. Quantitative modeling demonstrates a transfer of 6,400 Gt sulfur from shelf sediments back into the ocean. The accompanying change in the oxidation state from sulfide to sulfate proceeds through reactions similar to acid mine drainage (4FeS₂ + 15O₂ + 22H₂O + 8CaCO₃ → 4Fe(OH)₃ + 8CaSO_{4 ^•} 2H₂O + 8CO₂), which could release up to 8,800 Gt CO₂. The actual effect depends critically on the rate of carbonate dissolution and how the resulting DIC increase is buffered by increased carbonate precipitation. We note, however, that these numbers are comparable in magnitude to those during the Pleistocene cryosphere expansion (Markovic et al., 2015), and we hypothesize that the oxidation of shelf sediments constitutes may constitute a negative feedback mechanism to ice sheet expansion.