COMPARING THE RELATIVE CONTRIBUTION OF ICEBERG VS. SEA ICE-RAFTED SEDIMENT DURING TIMES OF HIGH AND LOW IRD ACCUMULATION AT SITE 918, SE GREENLAND MARGIN

ODP Site 918 lies 130km from the SE Greenland margin within the present limit of iceberg discharge. Its off-shelf location allowed for a continuous record of IRD deposition in distal to proximal glaciomarine facies from the late Miocene to present. Five episodes of large ice-rafting events have occurred since ~4.7 Ma; these occurred at times of climatic transition presumably triggering major calving events from the SE to central east Greenland coast. Here we aim to better understand the relative contribution of iceberg vs. sea ice-rafted sediment at peak and background times of IRD input at Site 918. 18 samples of 20 quartz grains each from IRD peak and background horizons are studied using a SEM to observe textural features that serve as indicators of glacial ice or sea ice activity. Prior work has established that glacially-transported grains display physical weathering characteristics, including angularity, high relief, fractures, and striations/gouges. In contrast, sea ice-rafted grains tend to contain more chemical weathering features; they are more subrounded, with medium relief, pervasive silica dissolution and microlayering.

Preliminary results show that both background and peak IRD events are dominated by grains that are subangular, with medium relief, breakage blocks, fractures, and some degree of dissolution and microlayering. However, peak vs. background differences also exist: peak IRD events tend to display more glacial features overall, with 75% of the grains being angular or subangular; this decreases to 66% at times of background levels of IRD input. Similarly, 96% of grains in peak IRD events and 86% of grains in low IRD periods have breakage blocks. Step fractures, isolated cusps, and microlayering all show similar trends. Additionally, 12% of grains rafted at background times have pervasive silica dissolution, while no grains in the peak events displayed this much dissolution. These results suggest that iceberg transport dominates overall, but sea ice increases in importance as a transport mechanism during periods of low IRD abundance, assuming a similar provenance at IRD peak vs. background times and turbidite influence on grain features is minimal. This implies an environment better suited to capture the glacial record, with calving overwhelming most of the sea ice processes.