Consistent CO2 release by pyrite oxidation on continental shelves prior to glacial terminations

Abstract

Previous evidence suggests enhanced pyrite oxidation on exposed continental shelves during glacial phases of low sea level. While pyrite oxidation directly consumes atmospheric oxygen, acid generated by this reaction should increase the release of CO2 through carbonate dissolution. This scenario represents a climate control loop that could temper or even prevent glacials because increasing CO2 triggers warming and rising sea level. However, the amplitudes of sea-level changes increased over the Quaternary, and CO2 concentrations co-varied with sea level throughout most of the past 800,000 years. Only during peak glacial conditions did CO2 levels reach an apparent lower threshold independent of falling sea level. Here we suggest that during the last nine glacial–interglacial cycles, pyrite-oxidation-driven release of CO2 and consumption of O2 occurred during 10 kyr to 40 kyr periods preceding glacial terminations. We demonstrate that repeated sea-level lowstands force pyrite oxidation to ever-greater depths in exposed shelf sediments and cause CO2 release that could explain the glacial CO2 threshold. When the duration of interglacials with high sea level is insufficient to restock the shelf pyrite inventory, this CO2-releasing process represents a discharging ‘acid capacitor’. Minimum atmospheric CO2 levels during glacial intervals were set, in part, by repeated CO2 release from pyrite oxidation on exposed continental shelves, according to a geochemical model of the past 3 Myr.