POTENTIAL FOR TEPHROCHRONOLOGY OF MARINE SEDIMENT CORES FROM BRANSFIELD STRAIT AND THE NORTHWESTERN WEDDELL SEA

For marine sediments around the Antarctic Peninsula, there is a critical need for chronostratigraphic tools alternate to 14-C dating. In the South Shetland Islands and Bransfield Strait (northern Antarctic Peninsula) there are at least four subaerial volcanic centers that are capable of producing Quaternary ash layers. We have undertaken a reconnaissance electron microprobe survey of individual shards in ash layers in sediment cores from Bransfield Strait to assess the potential for tephrochronology of the cores and to document Quaternary volcanic activity in the area. We are concentrating on ash layers that primarily consist of glass shards and angular mineral fragments, and avoiding those that contain evidence for substantial disturbance or reworking, such as abundant microfossils and rounded lithic fragments.

Almost all of the analyzed ash layers match rock compositions on Deception Island, a caldera-topped volcano that is the largest in the region, and the only one known to have been historically active. At least one of the Deception ashes extends as far as 190 km to the ESE, where it is found in a core from the Vega Drift (northern Prince Gustav Channel, Weddell Sea). That Vega Drift ash layer has an approximate age of 6500 years before present, based upon interpolation between radiocarbon dates of fossil shells found elsewhere in the same core (see Domack et al., this session). This widespread ash layer may be useful for stratigraphic correlation throughout the northern Antarctic Peninsula region.

Two ash layers in a core in northeastern Bransfield Strait are distinctly different in composition from the Deception Island ashes, but are similar to published analyses from the Penguin Island and Melville Peak volcanoes, both located about 30 km northwest of the core location. These two ash layers are the first known evidence for the timing of eruptions at Penguin and Melville. We do not have radiocarbon dates from this core, but based upon preliminary correlation with the dated core from the Vega Drift, the Penguin and Melville eruptions may have been on the order of several thousand years ago, and may provide useful stratigraphic markers on a more local scale.