Late Pleistocene changes in northern component water: Inferences from geochemical and sedimentological records from Gardar Drift.

摘要

This dissertation reconstructs late Pleistocene oceanic circulation variability within the North Atlantic, a critical region of deep-water formation, using proxies that reconstruct surface and deepwater changes. Unlike other studies that examine North Atlantic circulation as a whole, my study focuses on changes in Iceland Scotland Overflow Water (ISOW), one of the largest contributors to Northern Component Water (NCW). Each NCW component reflects the regional climate within its formation region thus, different climates may produce different deepwater states by changing the relative contribution from each component. Southern Gardar Drift is bathed by ISOW, thus the accumulating sediments are ideal for examining ISOW.A high-resolution record of the Younger Dryas cold event provides an analog for abrupt climate events. The benthic foraminiferal delta 13C record from core 11JPC (2707m) on Gardar Drift reveals NCW shoaled during the early and late Younger Dryas. These reductions are coincident with increased meltwater from Northern Hemisphere ice sheets, linking surface freshening to NCW production changes on short-timescales.On longer time-scales, benthic foraminiferal delta13C records from Gardar Drift show ISOW density was paced by northern high-latitude summer insolation, particularly within the precessional band. Uniform benthic foraminiferal delta13C values on Gardar Drift indicate that the mixing zone between NCW and Southern Component Water (SCW) was positioned to the south of Gardar Drift during interglacial periods. Conversely a large north-south gradient in benthic foraminiferal delta13C values during glacial periods indicates that ISOW shoaled, allowing SCW to bathe southern Gardar Drift. High-frequency ISOW variability caused by surface freshening during intermediate climate states is superimposed on the orbitally paced variations.A study of the trace metal compositions in Krithe carapaces found in core top samples demonstrates that calcification temperature is the dominant control on magnesium incorporation. Carbonate ion concentration is a secondary control on magnesium to calcium (Mg/Ca) ratios at low temperatures (3°C). Correcting for carbonate ion effects results in a linear paleotemperature equation with higher temperature sensitivity in the lower temperature range than previously published equations. This study indicates that Krithe magnesium to calcium ratios are reliable in reconstructing paleotemperatures.