Arctic sedimentation on interannual and million year time scales: Laminations and ice rafted debris
This thesis examines arctic sedimentation in two distal subarctic marginal seas. Laminations are preserved across the North Pacific and its marginal seas at several glacial terminations. Lamination deposition in marine sediments is favored by anoxic bottom water conditions that limit secondary productivity and bioturbation and instead preserve a high-resolution climatic signal. Anoxia is caused by a variety of factors including excessively high primary productivity or expansion of the oxygen minimum zone. However, a detailed analysis of pre-Last Glacial Maximum Bering Sea laminae composition has not yet been produced. Here I examine a 3.5 m long deposit from the shelf-slope break of the Bering Sea, IODP Site U1345, dated to the beginning of Marine Isotope Stage (MIS) 11 (425-413 ka). I test the hypothesis that these laminations are sedimentary varves showing annual to seasonal variability or longer-scale alternations of high productivity events. This thesis also investigates the contribution of iceberg vs. sea ice-rafted sediment at ODP Site 918, located 130 km from the SE Greenland margin, within the present limit of iceberg discharge. Its off-shelf location allowed for a continuous record of ice-rafted debris (IRD) deposition in distal to proximal glaciomarine facies from the late Miocene to present. Prior research shows five significant episodes of large ice-rafting events since ~4.7 Ma; these occurred at times of climatic transition, presumably triggered by major calving events from the SE to East-Central Greenland coast. However, the IRD used was assumed to be deposited by icebergs without accounting for sea ice transport. Here I aim to determine the contribution of iceberg vs. sea ice-rafted sediment at peak and background deposition times at ODP Site 918.
Results from multiple methodologies including laminae counting, diatom counting, and SEM analysis indicate that the Bering Sea laminae are deposited in couplets every four years, representative of longer-scale climatic changes. These couplets are composed of a terrigenous lamina and a diatomaceous lamina, which alternate consistently. However, seasonal signals are preserved in the diatom assemblages, contradicting the lamination counts. Sedimentation rate was set to be constant through MIS 11 due to the poorly constrained age model, but it is plausible that it would change during times of lamination deposition. Results from SEM analysis of quartz grain microfeatures show that both background and peak IRD events in the Irminger Basin are dominated by grains that are subangular, with medium relief, breakage blocks, fractures, and some degree of dissolution and microlayering. Discriminant analysis showed a significant difference (p<0.01) between these two populations with 64% correctly reclassified; however, it also demonstrates the similarity between the populations and points to mixed IRD flux during both peak and background times. Results suggest that iceberg transport dominated along the SE Greenland margin since the late Miocene, with relative increases in sea ice transport during periods of low IRD abundance. This depositional environment appears better suited to capture the paleo-glacial record rather than the sea ice record, with calving overwhelming most of the sea ice processes at the time of deposition.