Glaciological window into the pace of the organic carbon cycle

Abstract

The glaciological record of atmospheric composition suggests O2 has declined over the last 800000 years at an average rate of 0.3 x 1012 mol (Tmol) O2 yr-1. Because the geological carbon cycle regulates long term atmospheric oxygen concentrations, fluctuations in atmospheric O2 are typically attributed to an imbalance between the weathering of organic carbon (OC) and reduced sulfur on land, a sink of atmospheric O2, and the burial of OC and reduced sulfur in marine sediments, a source of O2. Here we compile and confront a database of C, Fe, S and H exchanges between the fluid Earth (atmosphere, ocean, biosphere) and lithosphere (crust and upper mantle) with the record of deoxygenation to quantify organic carbon sources and sinks in the Cenozoic. We show that the subduction of oxidized oceanic lithosphere and degassing of reduced gas from the Earth's interior is a sink of oxygen, and that this sink significantly exceeds the rate of atmospheric deoxygenation in the Pleistocene. A relative constancy of atmospheric O2 in the Cenozoic requires that the organic carbon cycle was a net source of O2 and sink of CO2, photosynthesis outpaced respiration by an average of ca 40 MtC yr-1 over the last 50 million years. The cost for the relatively invariant atmospheric oxygen concentration is the coexistence of two photosynthetically-driven imbalances in the cycles of iron and carbon that offset each other to near perfection. The weak escape of OC from continents and oceans to the lithosphere is more intriguing. It demonstrates that the organic carbon cycle remains under surprisingly strong kinetic control, despite evolution/optimization of respiratory metabolisms and rising atmospheric O2 for more than 2.4 billion years.