Age determinations and stable isotope records of four sediment profiles off Nova Scotia

2019-11-22T00:39:18Z (GMT) by Lloyd D Keigwin Glenn A Jones
Continental margin sediments off Nova Scotia accumulate at high rates (up to 360 cm/kyr) and contain a history of millennial-scale environmental changes which are dominated by the proximity of the Laurentide ice sheet during the latest Quaternary. Using stable isotope ratios of oxygen, accelerator mass spectrometer radiocarbon dating, micropaleontology, and sedimentology, we document these changes in six piston cores ranging in water depth from ab. 450 to ab. 4300 m. We find that maximum d18O in N. pachyderma occurred about 15 ka and preceded the maximum abundance of this species in these cores by ab. 1000 years. Between 13 and 14 ka we find a second peak in abundance of N. pachyderma, minimum d18O, and two pulses of ice rafting. The sediment lithology supports terrestrial studies which indicate that there was a general withdrawal of ice beyond the upper Paleozoic and Mesozoic red beds by 14 ka in southeastern Canada, so the ice rafting events between 13 and 14 ka probably reflect ice stream activity in the St. Lawrence valley. The Younger Dryas event is recognized as a peak in abundance of N. pachyderma and ice rafting (dated as ab. 11.3 ka), but meltwater discharge to the Gulf of St. Lawrence was either too small or occurred over too long a time to leave a distinct d18O minimum off Nova Scotia. At 7.1 ka, in the middle of Holocene warming, we find a third peak in abundance of N. pachyderma and another d18O minimum but no ice rafting. We interpret these data as evidence of a late-occurring meltwater event which, if correct, could have originated in the Great Lakes, in the Labrador-Ungava region, or in both. The final millennial-scale phenomenon off Nova Scotia is the onset of “Neoglaciation,” marked by increased ice rafting and increased % N. pachyderma beginning about 5 kyr ago.



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