Age assignment of a diatomaceous ooze deposited in the western Amundsen Sea Embayment after the Last Glacial Maximum

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 ( 14 C) 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 14 C dating of the AIO in bulk samples yielded age reversals down-core, but at all sites the AIO 14 C ages obtained from diatomaceous ooze within the diatom-rich unit yielded similar uncorrected 14 C ages between 13 517 AE 56 and 11 543 AE 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 210 Pb 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 14 C ages of only 5111 AE 38 and 5106 AE 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 14 C 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.

New constraints on the timing of West Antarctic Ice Sheet retreat in the eastern Amundsen Sea since the Last Glacial Maximum

“…From this, three main facies have been identified which are described and interpreted below and illustrated in Figure 2 and supplementary Figure S1. The facies succession broadly follows that documented for sediments recovered in the western ASE Hillenbrand et al, 2010b) and elsewhere on the Antarctic continental shelf (Domack et al, 1999Licht et al, 1999;Evans et al, 2005;Anderson, 2005, 2007;Ó Cofaigh et al, 2005;Hillenbrand et al, 2005Hillenbrand et al, , 2010aHillenbrand et al, ,b, 2013Pudsey et al, 2006;Heroy et al, 2008;McKay et al, 2008) with a muddy diamicton at the base of the sequence, overlain by a transitional muddy sand/sandy-gravel which in turn is capped by a terrigenous to biogenic mud. The primary purpose of our facies approach is to establish the major depositional environments (subglacial, glacier proximal and open marine) to inform and direct our dating strategy.…”

“…Andrews et al, 1999), alongside sufficient down-core dates to identify major steps in 14 C age progression (so-called 14 C dog-legs). In the western ASE the validity of this approach has been verified by other independent dating methods (relative palaeomagnetic intensity (RPI) and paired carbonate-AIO dating (e.g., Hillenbrand et al, 2010b;Smith et al, 2011) illustrating that with careful sample selection, reliable deglacial chronologies can be obtained even when biogenic carbonate is sparse.…”

The sedimentary legacy of a palaeo-ice stream on the shelf of the southern Bellingshausen Sea: Clues to West Antarctic glacial history during the Late Quaternary

“…With the exception of site GC359, the down-core variability of the d 13 C org values at each individual core site is 1.1& (Table 1), which is comparable or less than at core sites from other parts of the Antarctic shelf (e.g. Harden et al, 1992;Domack et al, 1998Domack et al, , 1999Domack et al, , 2001Licht and Andrews, 2002;Ó Cofaigh et al, 2005a;Pudsey et al, 2006;Hemer et al, 2007;McKay et al, 2008;Hillenbrand et al, 2010). The d 13 C org composition of marine particulate organic substance typically ranges from À18& to À27& (e.g.…”

“…The LCO is caused by the mixing of fresh, recently formed organic matter (formed mainly by diatoms) with reworked, fossil organic matter (e.g. Licht et al, 1996;Andrews et al, 1999;Pudsey et al, 2006;Ohkouchi and Eglinton, 2008;Rosenheim et al, 2008;Hillenbrand et al, 2010). At core sites, for which 14 C dates from calcareous material were unavailable, we assumed that the surface sediments are of modern age.…”

“…This is because the underlying terrigenous transitional sediments deposited more proximal to the retreating grounding line usually lack calcareous microfossils, while the organic carbon contents in these transitional sediments are subject to significant contamination with subglacially reworked, fossil organic matter (e.g. Domack, 1992;Domack et al, 1999;Heroy and Anderson, 2007;Rosenheim et al, 2008;Hillenbrand et al, 2010). In ageedepth profiles for sediment cores, this higher degree of down-core contamination is visible as a pronounced kink, sometimes referred to as a "dog leg" (Fig.…”