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

摘要

The initiation of Antarctic glaciation in the early Oligocene (～34 Ma) is represented by a distinct positive anomaly in the marine δ~(18)O record designated Oi-1 and accompanied by positive excursions in the mean δ~(13)C 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~(18)O 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.