Intra-annual Variability in Benthic Foraminiferal Abundance in Sediments of Disenchantment Bay, an Alaskan Glacial Fjord

Abstract

Sediment deposited from turbid meltwater from Hubbard Glacier dominates the benthic environment in Disenchantment Bay, a glacial fjord in southern Alaska. Sedimentation rates during the meltwater season average 22 cm yr−1 at a station 12 km from the glacier. Samples were collected for foraminiferal analyses from multicores and a piston core. Samples from multicores show annual trends in abundance of Elphidium spp. and Textularia earlandi. Lithofacies consist of couplets of laminated mud deposited by meltwater discharge in summer and diamicton beds deposited as ice-rafted debris during winter. Within mud layers, counts of Elphidium spp. and T. earlandi increase upcore until the winter diamicton layer, where they are absent to rare. Evidence for this variation in abundance can be seen at depth in the piston core. High C∶N ratios (30–90) indicate that carbon flux is from refractory, terrestrial sources rather than labile organic matter. We hypothesize that the pattern of seasonal variation is related to vertical migration of foraminifera toward a preferred sediment depth, then death following reproduction in late summer. Episodic events reduce total abundance, impacting the intra-annual pattern. The high resolution record from an Alaskan glacial fjord can be used to better understand in situ foraminiferal ecology.