Paper No. 379-1
Presentation Time: 9:00 AM-6:30 PM
TESTING A QUARTZ GRAIN MICROTEXTURE PROXY FOR PALEO-SEA-ICE IN COASTAL ALASKA
Microtextures of quartz grains are investigated as a paleoenvironmental proxy for ice rafting from coastal Alaska. The proximal location, IODP Site U1421 was drilled 10km from the Bering Trough to record the history of the Bering Ice Stream. We constructed an ice rafted debris mass accumulation rate (IRDMAR) curve from the coarse sand fraction (250 microns-2mm) from 211 samples. We analyzed quartz grains within this IRD population to identify microtextures that have been described in the literature as resulting from transport within a glacier versus those that were modified by other processes such as weathering or fluvial transport. Our hypothesis for Site U1421 is that grains dominated by glacial microtextures were rafted by icebergs and those dominated by other microtextures were passively transported by sea ice. For each sample, 25 quartz grains were imaged using Scanning Electron Microscope (SEM) with EDS. In order to test our glacial transport/sea ice hypothesis we identified: 1) characteristic microtextures typical of glacial crushing that included high relief, conchoidal fracture, fracture faces, arcuate steps, straight steps, and angular (sharp) outlines, and 2) characteristic microtextures identified from grains collected from modern sea-ice floes in the Arctic Ocean (St. John et al., 2015; J. Annals of Glaciology) including medium to low relief, scaling or surface roughness, silica precipitation or solution, etched surfaces, breakage blocks, and smooth edges. Frequency percent of each microtexture was calculated and grains were identified as showing characteristics of glacial transport (i.e., iceberg rafted), sea ice rafted, or mixed (attributes of both modes). Samples were analyzed from the upper 130 m, which has the most complete recovery and is radiocarbon dated. Most samples are dominated by grains with glacial characteristics, up to 80%. However, an interval with reduced IRD from 17.2-17.6 kyr is dominated by a higher percentage of grains that were sea ice rafted. This interval also has a reduced abundance of the foraminifer Elphidium excavatum, an indicator of glacial meltwater discharge suggesting that the sea surface was frozen. Therefore, perennial sea ice cover is suggested to cause suppressed iceberg rafting during this time.