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.

Acknowledgments

We would like to thank the members of the R/V Alpha Helix and R/V Maurice Ewing cruises who collected the sediment cores from DB. This material is based upon work supported by the National Science Foundation, Arctic Natural Sciences under Grant no. OPP-0327106 (Ellen A. Cowan, Appalachian State University), OPP-0326926 (John M. Jaeger, University of Florida), and OPP-0327107 (Ross D. Powell, Northern Illinois University), as well as Ocean Sciences under Grant no. OCE-0351062 (Cowan), OCE-0351043 (Jaeger), and OCE-0351089 (Powell).

S. Zellers would like to thank Dulce Cruz for sample preparation, ExxonMobil for access to Gulf of Alaska collections, and the University of Central Missouri Research Council. Finally, we appreciate the helpful instruction at the Smithsonian Institution foraminifera collection provided by Dr. Martin Buzas, as well as the assistance of Dr. Jason Curtis of the University of Florida in producing carbon and nitrogen chemistry data for multicore 76MC.

The authors would also like to acknowledge the thoughtful reviews and insight provided by three anonymous reviewers.