North Atlantic ocean circulation and the onset of Northern Hemisphere glaciation
Bloxsom, Peter G.
Since the early Cenozoic, the Earth’s climate has been gradually cooling. Large ice sheets have expanded on several occasions in the past 5000 kyr (50 Ma), with the last major expansion being the onset of major ice sheets in the Northern Hemisphere between 3600 and 2500 ka. This period was characterised by a prolonged increase in ice volume, modulated by orbital forcing. At the same time, major tectonic changes were taking place, with the closure of the Central American Seaway most significant. \ud \ud This thesis aims to address the issue of what caused the onset of major Northern Hemisphere glaciation through an in depth study of samples from North Atlantic ODP Site 982 (57° 31’ N, 15° 51’ W; 1145 m water depth). Multi-species benthic foraminifera composite trace metal records (Mg/Ca, B/Ca, Li/Ca, Cd/Ca) were produced, and the composite Mg/Ca record was used to develop a new regional temperature calibration for the Pliocene. Paleoceanographic records of bottom water temperature and saturation state were produced, as well as bottom water flow speed records using the Sortable Silt (SS) proxy. Bottom water temperature (BWT) records showed an abrupt, ~2°C cooling at 2800-2700 ka, consistent with other previously published records. \ud These data are combined with previously published benthic stable isotope (δ18Ob) records to reconstruct seawater oxygen isotope reconstructions (δ18Osw) across the period of study, and this is found to predominately reflect changes in global ice volume. A major and prolonged increase in ice volume (3500-3000 ka) is identified, equivalent to up to ~55m sea level decrease. The response of the North Atlantic region to this ice sheet growth was tested using previously published sea surface temperature (SST) and δ18Ob records from ODP Site 982, in addition to the new record from this study. The ice volume was found to have influenced the response of SST, δ18Ob, and SS to orbital forcing from changing insolation. Prior to the ice sheet growth, bottom water flow speed varied proportionally with sea surface temperatures, with increased deep water formation during periods of high seasonality. The growing ice sheets responded more slowly to insolation changes, and the presence of this ice sheet increased deep water formation by further cooling saline surface waters. This resulted in feedback loops that intensified deep water flow, leading to a significant increase in North Atlantic Deep water penetration to the southern ocean. A major decrease in δ18Osw at 2800-2700 ka was interpreted as either a loss of Antarctic ice mass, or a reorganization of North Atlantic water masses.\ud \ud Comparison of ice volume records with orbital spectra and reconstructions of the closure of the Central American Seaway suggest the immediate cause of Northern Hemisphere glaciation was a period of orbital conditions favourable to ice sheet growth. Another underlying cause was a series of tectonic shifts, including the closure of the Central American Seaway from 4200 ka, which changed heat and moisture transport patterns to the high latitudes.
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