SEASONAL VARIATIONS IN BENTHIC FORAMINIFERAL ASSEMBLAGES IN DISENCHANTMENT BAY, SOUTHERN ALASKA

Previous studies have shown a recurring pattern in temporal variation of sedimentary lithofacies in Disenchantment Bay, a glacial fjord in southern coastal Alaska. Here, sediment consists predominantly of regularly alternating laminated mud, deposited by high meltwater discharge from Hubbard Glacier in the summer months, and diamicton beds, consisting of iceberg-rafted debris deposited during the winter months. In June and September 2004, short (40 cm) multicores and long (6 to 10 m) piston cores, respectively, were collected in order to obtain the annual meltwater discharge record over the last 20 years. From these cores, discrete samples were collected for foraminiferal analyses. Samples from multicores taken from a deep basin (~250 m) influenced by the tidewater Hubbard Glacier show abundant benthic foraminifera just above winter diamicton layers. Predominant taxa include Elphidium spp., Quinqueloculina spp., Buccella frigida, and Spiroplectammina earlandi. Within the spring-summer mud layer, abundance and diversity of foraminiferal assemblages decreases upcore until the next winter diamicton layer. We hypothesize that this pattern of seasonal variation may be related to decreased resources resulting from turbidity associated with increasing meltwater discharge. Preliminary analyses indicate that this pattern occurs at depth within piston cores from Disenchantment Bay. We are further testing this possible pattern of seasonal variation in benthic foraminifera with discrete facies samples collected from the mud intervals of selected piston cores taken within 11 km of the terminus of Hubbard Glacier. These studies may provide insight into the annual response of benthic foraminifera to meltwater fluctuations and can be used as proxies for understanding environmental variability in similar glacial fjords where lithologic boundaries are less distinct.