Millennial-scale storminess variability in the northeastern United States during the Holocene epoch

For the purpose of detecting the effects of human activities on climate change, it is important to document natural change in past climate. In this context, it has proved particularly difficult to study the variability in the occurrence of extreme climate events, such as storms with exceptional rainfall. Previous investigations have established storm chronologies using sediment cores from single lakes, but such studies can be susceptible to local environmental bias. Here we date terrigenous inwash layers in cores from 13 lakes, which show that the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr. Our data show four peaks in storminess during the past 14 kyr, approximately 2.6, 5.8, 9.1 and 11.9 kyr ago. This pattern is consistent with long-term changes in the average sign of the Arctic Oscillation, suggesting that modulation of this dominant atmospheric mode may account for a significant fraction of Holocene climate variability in North America and Europe.     

Carbon dioxide release from the North Pacific abyss during the last deglaciation

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that--together with previously published results--show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition approximately 14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.