A geological constraint on relative sea level in Marine Isotope Stage 3 in the Larsemann Hills, Lambert Glacier region, East Antarctica (31 366–33 228 cal yr BP)

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Abstract

In this paper we present geological evidence from the Larsemann Hills (Lambert Glacier – Amery Ice Shelf region, East Antarctica) of marine sediments at an altitude of c. 8 m a.s.l., as revealed by diatom, pigment and geochemical proxies in a lake sediment core. The sediments yielded radiocarbon dates between c. 26 650 and 28 750 14C yr BP (31 366–33 228 cal yr BP). This information can be used to constrain relative sea level adjacent to the Lambert Glacier at the end of Marine Isotope Stage 3. These data are compared with the age and altitude of Marine Isotope Stage 3 marine deposits elsewhere in East Antarctica and discussed with reference to late Quaternary ice sheet history and eustatic sea-level change.

Introduction

Observations of relative sea-level (RSL) changes constitute one of the primary data sets for constraining the geometry, volume and melt history of past and present glacial masses. These data can be used to determine the loading and rebound of continental plates in response to changes in the weight of a growing or shrinking ice sheet and can be used by modellers to infer, indirectly, the thickness of the overlying ice through time (Lambeck et al., 1998, Lambeck and Chappell, 2001). RSL is the relative vertical displacement between the sea surface and the land over a period of time. It is influenced by changes in both global ocean volume (eustatic changes in sea level) and local vertical movements and deformation of the solid Earth (isostatic changes) that are caused by changes in the mass of overlying ice and water (Agostinetti et al., 2004). In regions of Antarctica where good RSL field data are available, these can be used to constrain both local ice and earth parameters and, to a lesser extent, global meltwater influx (Ivins and James, 2005, Bassett et al., 2007). A number of relative sea-level curves that span the Holocene are now available for Antarctica (Zwartz et al., 1998, Hall et al., 2003, Bentley et al., 2005, Verleyen et al., 2005).

In contrast, there is little published geological evidence of RSL that can be used to constrain Antarctic Ice Sheet thickness prior to the Last Glacial Maximum (LGM) (Huybrechts, 2002). This has meant that most estimates of pre-LGM Antarctic ice volume are derived from global models where the Antarctic contribution is largely unconstrained. Instead it is calculated by subtracting the volumes of other global ice sheets (e.g. Greenland) from the global eustatic total (Andrews, 1992). This lack of geological evidence for glacial isostatic adjustment in Antarctica has partly resulted from the assumption that most of the Antarctic coastline was overridden by the Antarctic Ice Sheet at the LGM and therefore the pre-LGM geological evidence was removed by glacial erosion.

The best pre-LGM RSL evidence is in East Antarctica and consists of raised beaches along the margins of the Syowa Coast in Lützow-Holm Bay (Miura et al., 1998a, Nakada et al., 2000, Fig. 1; Anderson et al., 2002). There AMS radiocarbon dating of over 180 in situ fossil molluscs (Laternula elliptica) yields two groups of dates. One group is approximately Holocene in age (c.1030–10 590 14C yr BP) and a second group is mostly older than 30 000 14C yr BP (ranging from c.22 800 to 46 420 14C yr BP) (Miura et al., 1998a). The latter are interpreted as indicating ice-free conditions and a higher relative sea level in Marine Isotope Stage 3 (MIS 3) reaching a maximum during the last interstadial prior to the LGM (Yoshida, 1983, Miura et al., 1998a). Observations of the MIS 3 shell material in the vicinity of East Ongul Island and northern Langhovde show that it is embedded in marine matrix and is neither reworked nor broken (Yoshida, 1983, Igarashi et al., 1995, Miura et al., 1998a). However, there is some debate that the MIS 3 dates might relate to older material being incorporated into the deposits of the LGM (Colhoun, 1991, Denton et al., 1991). Independent tests of the MIS 3 radiocarbon dates using electron spin resonance (ESR) showed that 2 out of 6 samples tested had minimum ages (within the large ESR error) that were within MIS 3, whilst the remaining 4 yielded ages up to MIS 7 (Takada et al., 2003). The ESR dating did not test any of the c. 20 14C AMS dates between 22 800 and 34 650 14C yr BP.

Recently, geomorphological and palaeolimnological data have shown that parts of the Larsemann Hills were ice-free during the LGM (Burgess et al., 1997, Hodgson et al., 2001, Hodgson et al., 2005, Hodgson et al., 2006, Fig. 1). This raised the possibility that there might be pre-LGM marine deposits in isolation basins there. These are coastal lakes cut off from the sea as RSL falls, or flooded by the sea as RSL rises. Dating the transitions between marine and lacustrine sediments and measuring the height of the sill that formerly connected the lake to the ocean enables an accurate determination of mean sea level at the time that the basin became isolated (Bassett et al., 2007). For example, a 158 cm sediment core from Kirisjes Pond studied by Verleyen et al., 2004b, Verleyen et al., 2005; Fig. 2) was divided into six stratigraphic zones based on the analysis of sedimentary diatoms and examined for marine and freshwater transitions (Table 1). This revealed that the basal zones of the core (KPI and KPII) were likely to have been deposited under marine conditions in MIS 3. At the time these papers were published there was insufficient microfossil evidence to assign, unequivocally, a marine origin to zones KPI and KP II. The age of KPII was also only constrained by two thermoluminescence (TL) dates of 24 000–35 000 TL yr BP (148–150 cm) and 30 000–43 000 TL yr BP (152–154 cm) (Hodgson et al., 2001). In the discussion of their paper Verleyen et al. (2004b) noted that if this deposit were of marine origin then it would correspond to the MIS 3 age of the marine shells deposited on the Syowa Coast. In order to test this hypothesis we have re-examined zones KPI and KP II (158–144 cm) in the Kirisjes Pond core at higher resolution than Verleyen et al. (2005), applying new chronological control and a series of biological and biogeochemical analyses to determine if the sediment was deposited in a marine or freshwater environment.

The Larsemann Hills is an ice-free polar oasis in Eastern Antarctica, located approximately midway between the eastern extremity of the Amery Ice Shelf and the southern boundary of the Vestfold Hills (Fig. 1). At 50 km2, it is one of the larger ice-free oases found along East Antarctica's 5000 km of coastline. More than 150 freshwater lakes are found in the hills, ranging from small ephemeral ponds to large water bodies such as Progress Lake (10 ha and 38 m deep). Many of the lakes contain sediments of up to 3 m thickness, comprising finely layered remains of freshwater benthic microbial mats, interspersed in some lower altitude coastal lakes by marine sediments resulting from marine transgressions. These sediments are particularly well suited to palaeolimnological studies as there is no significant bioturbation, a limited season of open water when wind-induced mixing might encourage resuspension, and microfossils and biogeochemical markers that are remarkably well-preserved. Kirisjes Pond (69°22′15.83″S, 76°08′34.60″E, Fig. 2) is a 9 m deep freshwater lake located on Kollöy, with a sill height of 8 m above sea level (a.s.l.), based on Australian Antarctic Data Centre GIS (spot-height elevation accuracy of <±0.5 m).

Section snippets

Methods

Zones KPI and KP II were dated at 150, 155 and 156 cm sediment depth by Accelerator Mass Spectrometry radiocarbon analysis at Beta Analytic, Florida. These supplemented the existing chronological control for the core which consisted of 8 radiocarbon dates and 2 TL dates (Hodgson et al., 2001, Verleyen et al., 2004a). Calibration of conventional radiocarbon ages to calendar years was undertaken using the calibration data set CalPal-2007Hulu (Weninger and Jöris, 2008) using CalPal-2007online (

Results

Three new AMS radiocarbon dates for zones KPI and KP II constrain the age of this part of the core (150–156 cm) to between 26 650 ± 220 and 28 750 ± 300 14C yr BP (31 366–33 228 cal yr BP; Table 2). The dates are in stratigraphic order both within KPI and KPII and when compared with the previous radiocarbon and TL dates for the core, which are also of late MIS 3 age. It is, therefore, likely that this sediment was deposited in situ and is not reworked.

Diatom species composition in KPI and KPII shows that

Discussion

The results presented here are consistent with sediments between 154 and 158 in zones KPI and KPII being laid down under marine conditions with a marine–freshwater transition complete by c. 148–146 cm. In order for this zone to be used as a relative sea-level constraint it is essential to demonstrate that these sediment layers are unequivocally marine, that the stratigraphy is undisturbed, and that the material is not a reworked fossil deposit.

Fossil evidence that the deposit is marine includes

Conclusions

A glacio marine sediment from 154 to 158 cm depth in a sediment core from Kirisjes Pond provides evidence for a MIS 3 marine transgression of at least 8 m in the Larsemann Hills, and a constraint on relative sea-level change in this region of Antarctica.

The timing of deposition of the marine sediment is constrained by three AMS radiocarbon dates between 150 and 156 cm to between 26 650 ± 220 and 28 750 ± 300 14C yr BP (31 366–33 228 cal yr BP), and by TL dates of late MIS 3 age, and the marine origin is

Acknowledgements

This paper contributes to the CACHE-PEP and HOLANT projects led by DH and WV respectively. EV is a post-doctoral research fellow with the Fund for Scientific Research, Flanders. We thank the UK Natural Environment Research Council, British Antarctic Survey, the Belgian Federal office for Scientific, Technical and Cultural Affairs and the European Commission for research funding and the Australian Antarctic Science Advisory Committee for logistic support. Glenn Milne, University of Ottawa, is

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