Export productivity and carbonate accumulation in the Pacific Basin at the transition from a greenhouse to icehouse climate (late Eocene to early Oligocene)

摘要

The late Eocene through earliest Oligocene (40-32 Ma) spans a major transition from greenhouse to icehouse climate, with net cooling and expansion of Antarctic glaciation shortly after the Eocene/Oligocene (E/O) boundary. We investigated the response of the oceanic biosphere to these changes by reconstructing barite and CaCO_3 accumulation rates in sediments from the equatorial and North Pacific Ocean. These data allow us to evaluate temporal and geographical variability in export production and CaCO_3 preservation. Barite accumulation rates were on average higher in the warmer late Eocene than in the colder early Oligocene, but cool periods within the Eocene were characterized by peaks in both barite and CaCO_3 accumulation in the equatorial region. We infer that climatic changes not only affected deep ocean ventilation and chemistry, but also had profound effects on surface water characteristics influencing export productivity. The ratio of CaCO_3 to barite accumulation rates, representing the ratio of particulate inorganic C accumulation to C_(org) export, increased dramatically at the E/O boundary. This suggests that long-term drawdown of atmospheric CO_2 due to organic carbon deposition to the seafloor decreased, potentially offsetting decreasing pCO_2 levels and associated cooling. The relatively larger increase in CaCO_3 accumulation compared to export production at the E/O suggests that the permanent deepening of the calcite compensation depth (CCD) at that time stems primarily from changes in deep water chemistry and not from increased carbonate production.