Glacial-Holocene stratigraphy, chronology, and paleoceanographic observations on some North Atlantic sediment drifts

Several sediment drifts in the deep North Atlantic have been studied using oxygen isotopes, percent CaCO ₃, clay mineralogy, tephra content and Accelerator Mass Spectrometer (AMS) radiocarbon dating. Because enhanced deposition of fine-grained terrigenous sediment increases rates of sedimentation by as much as 100 times the regional mean, sediment drifts contain high-resolution records of climate and oceanographic change. The major results of this study are that: (1) patterns of sedimentation rate are regionally variable. In the Western North Atlantic (Bermuda Rise, Blake-Bahama Outer Ridges), rates of sedimentation have been relatively low during the Holocene, but were as high as 200 cm/1000 years during the latest glacial episode, reflecting a greater supply of sediment due to glaciation and sea level lowering. Deep circulation patterns were probably similar to today. In the northern North Atlantic (Gardar Drift), which is dominated today By Norwegian Sea Overflow Water (NSOW), sedimentation rates were lowest during the glacial, despite the availability of sediment, because NSOW production was stopped. Rates peaked during the early Holocene as NSOW resumed. Glacial and Holocene rates of sedimentation were roughly comparable in the northeast Atlantic (Feni Drift) because sedimentation at that location is influenced less by NSOW than by Northeast Atlantic Deep Water (NEADW), which has a large southern-source component. (2) Stable isotopic events of deglacial age are preserved with unprecedented clarity in the sediment of North Atlantic drifts. Where the Younger Dryas cooling and the Vedde Ash occurred together, at 56°N on Feni Drift, direct AMS dating suggests that the radiocarbon age difference between the surface ocean and atmosphere reservoirs in the northeast Atlantic was the same 10,500 years ago as it is today (about 400 years). A δ¹⁸O maximum of Younger Dryas age indicates cooler or more saline surface waters above the Bermuda Rise at 33°N and as far south as 28°N on the Bahama Outer Ridge. Two δ¹⁸O minima at 12,000 and 13,500 radiocarbon years BP on the Bahama Outer Ridge and the Bermuda Rise probably resulted from lowered surface water salinity. These low salinity events most likely originated as meltwater discharge down the Mississippi and into the Gulf of Mexico, followed by advection into the open North Atlantic via the Gulf Stream.