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As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5øC air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50øN. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation. INTRODUCTIONOne of the most puzzling features of the ice core records from Greenland is the series of large and rather abrupt changes in oxygen isotope composition during the last period of glaciation [Dansgaard et al., 1971[Dansgaard et al., , 1982[Dansgaard et al., , 1989]. Based on the correlation between the b180 in present-day precipitation and mean annual temperature for high-latitude stations, these changes translate into 5øC jumps in air temperature. Correlating with the oxygen isotope changes are severalfold changes in the dust content of the ice (see Figure 1), Copyfight 1990 by the American Geophysical Union. Paper number 90PA01194. 0883-8305/90/90PA-01194 $10.00 50 ppm changes in the CO2 content of the air trapped in the ice [Stauffer et al., 1984], changes in the SO4 concentration in the ice [Finkle and Langway, 1985] and changes in 10Be content of the ice [Beer et al., 1984]. As the 180 signature for these events has now been found in three Greenland ice cores (Camp Century [Dansgaard et al., 1971], Dye 3 [Dansgaard et al., 1982], and Renland (W. Dansgaard personal communication, 1990)), they must be real climatic events. However, the validity of the CO2 signal as a record of atmospheric composition remains in limbo. The reason is that similar events were not found in a study of an Antarctic ice core [Neftel et al., 1988]. As discussed by Dansgaard and Oeschger [1989], this absence could reflect either a smoothing of the record in low deposition Antarctic record or meltwater layers in the Dye 3 ice. Oeschger et al. [1984], fin'st suggested that the Greenland events constitute jumps between two modes of operation of the climate system. Broecker et al. [1985] went a step further and postulated that Oeschger's modes involved a turning on and off of the Atlanfic's conveyor circulation system. Rather than pursuing the cause of the entire series of oscillations, recent attention has been focused on a single event, the Younger Dryas, which followed upon the heels of the last glacial termination. This event has roughly the same shape, amplitude, and duration as those during the glacial period. The lure of the Younger Dryas is that in contrast to the others it is well documented in pollen records from the lands surrounding the northern Atlantic [W...

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It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep‐sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.

Accelerator mass spectrometric radiocarbon measurements on benthic foraminifera shells, picked from samples on which concordant ages were obtained on the shells of two species of planktonic foraminifera, reveal that the age of deep water in the equatorial Atlantic during glacial time was 675±80 years (compared to today's age of 350 years) and that the age of deep water in the South China Sea was 1670±105 years (compared to today's value of 1600 years). These results demonstrate that the 1.3 to 1.5 times higher radiocarbon content of carbon in glacial surface waters of the Caribbean Sea reconstructed by Bard et al. [1990] was primarily the result of a higher global inventory of radiocarbon rather than a decrease in rate of mixing between surface and deep waters of the ocean. The results are also consistent with the conclusion by Boyle and Keigwin [1987] that the flow of North Atlantic Deep Water was considerably weakened during glacial time, allowing deep waters of Antarctic origin to push much further north into the Atlantic than they do today.

Concentrations of U, Th, 231pa and 1°Be were measured in Holocene sediments from two cores collected off the west coast of South America, two cores from the East Pacific Rise, two from the equatorial Pacific and one from the south Pacific central gyre. Our results, together with data from 5 cores reported in the literature, show that boundary scavenging plays a major role in the removal of 1°Be from the Pacific Ocean. Deposition rates of 1°Be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central Pacific. Deposition of 231pa is 4 to 5-fold greater at the margin sites. The residence time of 1°Be with respect to chemical scavenging, defined as its inventory in the water column divided by its rate of removal to the sediments, varies regionally from > 1000 years at the red-clay sites in the deep central Pacific to -100 years at the margin sites. Different factors control boundary scavenging of Pa and Be. For example, scavenging of 231pa is enhanced by metal-oxide coatings of particles, whereas this seems to have little influence on the scavenging of l°Be.

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