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ABSTRACT: Reliable dating of glaciomarine sediments deposited on the Antarctic shelf since the Last
Glacial Maximum (LGM) is challenging because of the rarity of calcareous (micro-) fossils and the
recycling of fossil organic matter. Consequently, radiocarbon (14C) ages of the acid-insoluble organic
fraction (AIO) of the sediments bear uncertainties that are difficult to quantify. Here we present the
results of three different methods to date a sedimentary unit consisting of diatomaceous ooze and
diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner
shelf in the western Amundsen Sea (West Antarctica). In three cores conventional 14C dating of the AIO
in bulk samples yielded age reversals down-core, but at all sites the AIO 14C ages obtained from
diatomaceous ooze within the diatom-rich unit yielded similar uncorrected 14C ages between
13 517
56 and 11 543
47 years before present (a BP). Correction of these ages by subtracting
the core-top ages, which probably reflect present-day deposition (as indicated by 210Pb dating of the
sediment surface at one core site), yielded ages between ca. 10 500 and 8400 cal. a BP. Correction of
the AIO ages of the diatomaceous ooze by only subtracting the marine reservoir effect (MRE) of 1300 a
indicated deposition of the diatom-rich sediments between 14 100 and 11 900 cal. a BP. Most of these
ages are consistent with age constraints between 13.0 and 8.0 ka for the diatom-rich unit, which we
obtained by correlating the relative palaeomagnetic intensity (RPI) records of three of the sediment
cores with global and regional reference curves. As a third dating technique we applied conventional
radiocarbon dating of the AIO included in acid-cleaned diatom hard parts extracted from the
diatomaceous ooze. This method yielded uncorrected 14C ages of only 5111
38 and 5106
38
a BP, respectively. We reject these young ages, because they are likely to be overprinted by the
adsorption of modern atmospheric carbon dioxide onto the surfaces of the diatom hard parts prior to
sample graphitisation and combustion for 14C dating. The deposition of the diatom-rich unit in the
western Amundsen Sea suggests deglaciation of the inner shelf before ca. 13 ka BP. The deposition of
diatomaceous oozes elsewhere on the Antarctic shelf around the same time, however, seems to be
coincidental rather than directly related. Copyright # 2009 John Wiley & Sons, Ltd.