GLACIAL-INTERGLACIAL GRAIN SIZE VARIATIONS ON THE NORTHWIND RIDGE, ARCTIC OCEAN, MEASURED BY LASER PARTICLE SIZE ANALYZER

Grain size analysis of sediment cores from the Arctic Ocean provide information on the intensity of glaciation inferred from changes in ice rafted debris (IRD) or bottom current strength. Core P1-92-AR-P25 was collected by the USGS from the east flank of the Northwind Ridge in the Canada Basin from 1625 m water depth. The grain size trend from the core is compared with a reflectance-based measure of sediment manganese content interpreted to indicate changes in bottom water oxygen. We infer increases in the sediment manganese content correlate with warmer, interglacial periods and less ice cover. Two potential hypotheses could explain cyclic increases in grain size at this location. If the dominant process controlling grain size were bottom current strength, increases in grain size should correlate with high sediment manganese content and presumably warmer, interglacial periods. Alternatively, if ice rafting of glacial debris is the dominant process at this location larger grain size should correlate with low manganese content during times of presumed greater glaciation. The samples collected from the core were first washed and sieved into <63, 63-150 and >150 um size classes. We focus on the <63um size fraction because it includes the “sortable silt” size class which is sensitive to changes in bottom current strength (McCave et al. 2004) This size fraction was treated with acid and base to remove any carbonate and biogenic opal which may be present. The remaining lithic sediment in this size class was run through a Malvern Mastersizer 2000 laser particle size analyzer. In the data collected thus far, there is a general tendency for increased volume concentration of smaller grains during presumed warmer interglacial and larger grains during glacial times. The most likely explanation is that the coarser grains are present in association with IRD in response to greater glaciation during cooler climate intervals. Phillips and Grantz (2001) indicate northwestern Canada as the likely source of the ice that transported the coarse grained material in this area. At present we cannot rule out the possibility that changes in the silt size fraction arise from stronger bottom currents during glacial periods. Additional work will test for correlations between the silt size particles we observed and material in the larger IRD size class.