A Foraminiferal Record of Ice-Shelf Decay, Larsen Ice Shelf-A, Eastern Antarctic Peninsula

Dr. Scott E. Ishman, Southern Illinois University
Department of Geology
Carbondale, IL 62901-4324
E-mail: sishman@geo.siu.edu

The analysis of modern benthic and planktonic foraminiferal distributions in surface sediment samples collected from the former Larsen Ice Shelf-A (LIS-A) and the Prince Gustav Channel (PGC), Eastern Antarctic Peninsula (EAP) show associations with specific environmental conditions. Cluster analyses were used to determine species distribution in these areas. Distinct trends in foraminiferal assemblages are noted from the Larsen Ice Shelf-B (LIS-B) edge northward to the PGC. Four distinct assemblages were defined. Foraminiferal analyses of kasten core sediments collected from the Greenpeace Trough show distinct patterns of foraminiferal biofacies transition associated with the late ice shelf history and changes in paleoproductivity of the EAP.

Common taxa within the modern foraminiferal biofacies include the agglutinated foraminifera Adercotryma glomerata, Reophax spp., Portatrochammina spp., Textularia wiesneri, and T. antarctica. The Greenpeace Trough and its periphery comprise an assemblage defined by abundant Nonionella spp., Globocassidulina biora, and Epistominella exigua. An assemblage defined by Trifarina spp. and an abundance of planktonic foraminifers, including Neogloboquadrina pachyderma, is closely associated with the northern margin of the LIS-B. These two assemblages overlap near the southern boundary of the Greenpeace Trough. A primarily agglutinated assemblage containing a lower diversity of calcareous species than other LIS-A regions defines the northern extent of the former LIS-A region. Portatrochammina spp., Textularia wiesneri, and Epistominella exigua are prevalent in this assemblage. Agglutinated foraminifers, including Textularia wiesneri, Portatrochammina spp., Miliammina spp., and Reophax spp., define the PGC assemblage. Planktonic foraminifers were sparse in the PGC samples in comparison to the LIS-A samples; but showed increased abundance in PGC areas influenced by outlet glaciers. Data from the PGC also support benthic species distributions controlled by glacial and bathymetric conditions.

Foraminiferal analyses of kasten core sediments from the Greenpeace Trough show a distinct biofacies transition associated with the decay of the LIS-A. An initial ice-shelf dominated assemblage composed of agglutinated benthic foraminifera represents the onset of this transition. A period of fluctuating ice-shelf and oceanographic conditions are indicated by fluctuations between agglutinated and Miliammina spp. assemblages. The late Holocene is represented by open marine conditions and high biological productivity, indicated by the appearance of planktonic and abundant calcareous benthic assemblages in the upper kasten core samples. Results from stable isotopic analyses will provide additional information regarding oceanographic changes on the EAP shelf associated with the collapse of the LIS-A.