CLIMATE MECHANISMS CONTROLLING HOLOCENE VARIATIONS IN NORTH ATLANTIC ICE-RAFTED SEDIMENT

Records of Holocene IRD (ice rafted debris) in the subpolar North Atlantic have recently been tied to variations in the cosmogenic nuclides beryllium-10 and carbon-14, implying a close connection between variations in solar output and changes in the amounts and trajectories of ice drifting in the high latitude North Atlantic. That finding underscores the importance of understanding how coarse (sand-sized) lithic sediment in those records is linked to drift ice and to climate.

Analyses of lithic grains in sediment trap samples from south of Fram Strait demonstrate that large numbers of sand sized grains are released from drifting ice (mostly sea ice) during all months of the year. Fluxes of those grains are higher by factors of 3 to 4 than those estimated at the sites of the Holocene records, consistent with those locations in warmer waters to the south. In addition, 15 to 20 percent of the sediment trap lithic grains have hematite staining, which is a robust petrologic tracer of drift ice trajectories during the Holocene. The sediment trap results thus confirm what has been inferred previously for drift ice trajectories based solely on abundances of hematite-stained grains in core top sediment.

New evidence from NCEP-NCAR reanalysis of observational climatologies demonstrates that anomalous north to northwesterly winds in the eastern North Atlantic, which are required to explain the record of hematite-stained grains in that region during the Holocene, occurred at rather specific times during the last several decades. Those times correspond to increases in extreme drift ice sightings and to a 1 to 2 degree cooling of the ocean surface. The most persistent north to northwesterly winds occurred during spring and summer and at times when sea level pressure increased in both the Azores and Hawaiian highs. A link therefore appears to exist between the Holocene shifts in North Atlantic drift ice and subtropical atmospheric circulation, which in turn, based on results of recent GCM models, may be influenced by solar variability.