PHOTONIC-DATING TESTS OF GLACIMARINE SEDIMENTS AROUND THE ANTARCTIC PENINSULA

Numeric chronologies for the waxing and waning of antarctic Quaternary ice sheets as recorded in the glacimarine sediments surrounding the continent have been difficult to obtain for the time range of ¹⁴C dating (<35 ka), and nearly impossible for deposits >30-40 ka. For Holocene antarctic glacimarine sediments, the reservoir effect and sediment reworking lead to large ¹⁴C age overestimates. AMS ¹⁴C ages for surface (0-10 cm) glacimarine sediment near the Larsen Ice Shelf are in the range 9-10 ka B.P. (for acid insoluble organic matter) and 1.1 to 0.9 ka (for modern biogenic calcite).

We are testing the accuracy of photon-stimulated-luminescence (PSL) dating methods for glacimarine sediments from around the Antarctic Peninsula. We report here results from the use of the multi-aliquot, fine-silt, IR-PSL method on 15 samples from 9 box-core sites from 6 areas: Admiralty Bay (King George Island, north of the Peninsula); Gerlache Strait (western side of the Peninsula); Larsen-A Ice Shelf, Larsen-B Ice Shelf, James Ross Island, Erebus-Terror Gulf (all on the eastern side).

Have detrital fine-silt feldspar grains been exposed to sufficient daylight to "zero" the light-sensitive IR-PSL clock? Only one (1-3 cm depth) of the Larsen-B samples (all collected within a few 100 m of the ice shelf) could be dated, yielding age overestimates of 38-90 ka (depending on laboratory bleaching conditions): the other 4 samples showed near-saturation signals, indicating no effective clock zeroing. All 5 Larsen-A samples (up to 30 km from the ice shelf), and the single sample from near James Ross Island, revealed little clock re-setting. Four samples from the Erebus-Terror Gulf yielded apparent ages from 10 ± 1 ka to 23 ± 4 ka as a function of location, sample depth and laboratory bleaching conditions. Similarly, the sample from the Gerlache Strait provided apparent ages from 13 to 22 ka. The Admiralty Bay box core yielded an apparent age reversal: 16 ± 2 to 25 ± 2 ka for the 1-2.5 cm sample depth and 3.0 ± 0.5 to 6.9 ± 0.7 ka for the 8.5-10 cm sample depth. These results reveal suggestive intra-area and inter-area differences. Perhaps the PSL method can provide more information about the sediment transport history than the numeric age. Even this information is extremely important as we try to unravel the history of ice-shelf fluctuation from the sediment record.