Amundsen Sea sediment drifts: Archives of modifications in oceanographic and climatic conditions

Abstract

Drift deposits document stages of particular dynamic bottom-currents and associated sedimentary transport activities. The analysis of seismic reflection data from the Amundsen Sea, southern Pacific Ocean, reveals sediment drift formation already in Eocene/Oligocene times. This observation indicates bottom current activity and hence a cold climate for the late Palaeogene in an area, which today lies under the influence of Antarctic Bottom Water (AABW) originating in the Ross Sea. The generation of sediment drifts is accompanied by the occurrence of mass transport deposits leading to the identification of a phase of strong ice sheet expansion (15–4 Ma), which due to a change in ice regime from wet- to dry-based was followed by less material input during the last ~ 4 Ma.

Introduction

During the last years the West Antarctic Ice Sheet (WAIS) has been reported to undergo significant changes, especially in the area of Pine Island Bay, Amundsen Sea Embayment, where large glaciers draining the ice sheet have shown rapid flow acceleration, increased thinning, and grounding line retreat (Rignot and Jacobs, 2002, Shepherd et al., 2004, Thomas et al., 2004). In order to better understand the dynamics of the WAIS and be able to provide reliable constraints for numerical simulations of a possible future behaviour of the ice sheet, information on the development of the WAIS is needed. This, especially for the early phase, is still under debate and not known in much detail both chronologically and regionally. While good progress has been made over the last years in reconstructing the development of the ice sheet since the late Miocene for the western Antarctic Peninsula and Bellingshausen Sea, the Pliocene for the Ross Sea area and the late Quaternary for Pine Island Bay, little is known from the greater Amundsen Sea, which lies between the Ross and Bellingshausen Seas under the east setting Antarctic Circumpolar Current (ACC) (Orsi et al., 1995, Carter et al., 2009).

The Amundsen Sea is located along the southern Pacific margin of West Antarctica (Fig. 1). This part of the southern Pacific was created by rift and breakup processes and the formation of the earliest oceanic crust between Campbell Plateau/Chatham Rise and Marie Byrd Land/Thurston Island Block at 90–80 Ma (Eagles et al., 2004, Gohl et al., 2007, and references therein). The emplacement of the Marie Byrd Seamounts (MBS) was a result of magmatic activity at 65–55 Ma or older (Kipf et al., 2008, Kipf et al., submitted for publication). Up to 4 km of sediment have been deposited on top of basement. These sediment deposits have been subject to extensive reworking by oceanic currents and, on the shelf, advance-retreat cycles of the ice sheet.

Presently, the Amundsen Sea lies under the flow of Antarctic Bottom Water (AABW), which here is formed mainly in the western Ross Sea (Gordon et al., 2009, Orsi and Wiederwohl, 2009) and participates in the cyclonic circulation of the Subpolar gyres once it has reached the oceanic domain (Orsi et al., 1999). Circumpolar Deep Water (CDW) is observed to flow onto the shelf areas of the Amundsen Sea via bathymetric troughs (Jacobs et al., 1996, Thoma et al., 2008). In the Bellingshausen Sea, a southwest setting flow of bottom water has been observed close to the continental slope as far west as 83°W (see Hillenbrand et al. (2008) for a detailed presentation). Based on the study of sediment drifts, Scheuer et al. (2006b) and Hillenbrand et al. (2003) inferred that this bottom water flow extends even farther west to 93°W.

The work presented here will provide insight on the palaeoceanographic evolution of the greater Amundsen Sea, which represents one of the outflow regions of the Ross Sea gyre into the deep Pacific–Antarctic Basin (Orsi, 2010) and into which the glaciers of the Pine Island Bay area drain, by analysing sedimentary features observed in seismic reflection data.