Carbon cycle feedbacks and the initiation of Antarctic glaciation in the earliest Oligocene

Abstract

The initiation of Antarctic glaciation in the early Oligocene (∼34 Ma) is represented by a distinct positive anomaly in the marine δ 18O record designated Oi-1 and accompanied by positive excursions in the mean δ 13C of oceanic dissolved inorganic carbon and biogenic sediment accumulation rates. Within 400 ky of the onset of Oi-1, the climate system settled into a more moderate but stable “glacial” state. Here, through modeling, we investigate two of the principal biogeochemical processes involved in this response: silicate weathering and marine organic carbon cycling. We initiate the event with a rapid drawdown in atmospheric CO 2 resulting from increased weatherability of the continents associated with Himalayan orogeny. This perturbation triggers the overshoot and adjustment of the δ 18O record because of feedback among ice-sheet coverage, silicate weathering rates, and atmospheric CO 2. The system is a damped oscillator, the strength of which depends on the sensitivity of chemical weathering rates to climate change and climate to changes in atmospheric CO 2. Increased oceanic mixing associated with initial transition into a glacial world accelerates the rates of biological productivity and carbon burial, lowering atmospheric CO 2 and accelerating global cooling and ice-sheet growth, and generating a carbon isotope response that crudely approximates that observed. The Oi-1 overshoot appears to require a rapid (<1 million year) application of the forcing (e.g., tectonic drawdown of atmospheric CO 2). Although further investigation and more sophisticated models ultimately may show that other triggers and feedbacks prevailed during Oi-1, the modeling presented here demonstrates that simple feedbacks in the climate system can explain the overshoot and adjustment response to early Oligocene climate forcing.