27 GSA: Bering Glacier to Glacier Bay—From Tectonics to Ice: Session in Honor of Austin Post

Tuesday, 9 May 2006
1:15 PM - 5:00 PM

WHAT ARE CLAMS DOING IN THE BERING GLACIER?

Paper number: 27-5
Program: THEME SESSIONS
Time: 2:20 PM – 2:40 PM

Authors

Abstract

Not only clams, but crabs, worms and snails as well are coming out of the Bering Glacier. These marine invertebrates in the Holocene glacial deposits indicate that the Gulf of Alaska shoreline was several kilometers north of its present position during the Late Pleistocene/Early Holocene. The invertebrates were deposited by melt water at the face of the ice, and were clearly transported in portions of the Bering Glacier flowing southward. Much of the piedmont lobe and the lower part of the valley portion lies below sea level. Conventional radiocarbon dates of 9 bivalves from 4 localities range in age from 7,190 +/- 140 to 13,050 +/- 70 years BP bracket the age range of that shoreline. Therefore, the Bering Glacier must have been in a retracted position 7 – 13,000 years ago and marine conditions prevailed 30 or more kilometers north of the present coastline. The topographic break between the base of the coastal mountains and the present forelands would be a logical position for such an ancient shoreline, and the present forelands must have been formed within the past 7,000 years.

The unusual preservation of delicate invertebrate skeletons during transport in glacial ice is difficult to explain. Sediment preserved in the interior of some mollusk shells is a sticky silt. If the invertebrates were encased in this material and it was ripped up from the sea floor in a frozen state, skeletons could have been protected by this sediment during transport in ice and released intact when the silt blocks thawed during deposition on outwash. Fragments of shells are ubiquitous throughout the forelands south of the active glacier ice front, either in stratified outwash or the overlying till. However, unique conditions existed at particular sites where deposition of entire valves and other fragile skeletal remains occurred.

A total of 100 species representing 8 major taxa were identified. Intertidal and shallow subtidal species dominate the collection indicating an origin close to a shoreline. It consists mainly of mollusk species: gastropods (43%) and bivalves (36%). Other taxa present are arthropods (7%), bryozoans (6%), polychaetes (2%), echinoderms (1%), chitons (1%) and a single foraminifera (1%). They invite comparison to the contemporary fauna and habitats in the Gulf of Alaska.