SEISMIC REFLECTION CHARACTERISTICS OF TEMPERATE GLACIMARINE VARVES: AN EXAMPLE FROM DISENCHANTMENT BAY, ALASKA

Disenchantment Bay is located at the head of Yakutat Bay along the southern coast of Alaska. The bay receives sediment from several tidewater glaciers, although dominantly from the actively advancing Hubbard Glacier, the longest tidewater glacier in North America. Deposition of sediment from submarine outwash discharge during summer and iceberg rafting during winter form temperate glacimarine varves. These varves are observed within piston and gravity cores as summer-winter sediment couplets composed of laminated mud with dropstones and diamicton, respectively. The textural difference between the two lithofacies forms by the relative decrease of fine-grained silt and clay in sediment laden plume waters during winter as subglacial conduit flow wanes significantly. Although cores are most critical for producing a time series of inferred annual accumulation rates, lateral and vertical varve variability is of particular interest when correlating between core sites. Here we show that high-resolution seismic reflection profiles image the varved sequence, as well as individual, correlative winter diamictons within the proximal portion of the Bay where varve thickness exceeds 30 cm. These results support earlier sedimentological investigations regarding annual accumulation rates inferred from varve thickness and also provide a more encompassing understanding of varve variability that allows a more accurate determination of annual sediment accumulation volumes. The recognition of dynamic patterns of fjordal sedimentation inferred from varve variability is especially critical in locations where annual layers are difficult to correlate between cores, such as the eastern half of the fjord where reflection profiles reveal an increased frequency of sediment gravity flow strata that reduces varve preservation due to local erosion and redeposition. Collectively, these results show that individual varves are in fact correlative throughout the fjord, and also that seismic data can be invaluable when calculating annual sediment accumulation volumes, which if correlated to changes in a glacier's meltwater discharge, may provide insight into glacier and/or climate dynamics, such as ablation or precipitation.