Deep-sea pre-glacial to glacial sedimentation in the Weddell Sea and southern Scotia Sea from a cross-basin seismic transect

Abstract

Identification of the pre-glacial, transitional and full glacial components in the deep-sea sedimentary record is necessary to understand the ice sheet development of Antarctica and to build circum-Antarctic sediment thickness grids for palaeotopography/-bathymetry reconstructions, which constrain palaeoclimate models. A ~ 3300 km long Weddell Sea to Scotia Sea multichannel seismic reflection data transect was constructed to define the first basin-wide seismostratigraphy and to identify the pre-glacial to glacial components. Seven main seismic units were mapped: Of these, WS-S1, WS-S2 and WS-S3 comprise the inferred Cretaceous–Palaeocene pre-glacial regime (> 27 Ma in our age model), WS-S4 the Eocene–Oligocene transitional regime (27–11 Ma) and WS-S5, WS-S6, WS-S7 the Miocene–Pleistocene full glacial climate regime (11–1 Ma). Sparse borehole data from ODP Leg 113 and SHALDRIL constrain the ages of the upper three seismic units and seafloor spreading magnetic anomalies compiled from literature constrain the basement ages in the presented age model. The new horizons and stratigraphy often contradict local studies and show an increase in age from southeast to the northwest. The up to 1130 m thick pre-glacial seismic units form a mound in the central Weddell Sea basin and in conjunction with the eroded flank geometry, allow the interpretation of a Cretaceous proto-Weddell Gyre bottom current. The base reflector of the transitional seismic unit has a model age of 26.6–15.5 Ma from southeast to northwest, suggesting similar southeast to northwest initial ice sheet propagation to the outer shelf. We interpret an Eocene East Antarctic Ice Sheet expansion, Oligocene grounding of the West Antarctic Ice Sheet and Early Miocene grounding of the Antarctic Peninsula Ice Sheet. The transitional regime sedimentation rates in the central and northwestern Weddell Sea (6–10 cm/ky) are higher than in the pre-glacial (1–3 cm/ky) and full glacial regimes (4–8 cm/ky). The pre-glacial to glacial rates are highest in the Jane- and Powell Basins (10–12 cm/ky). Total sediment volume in the Weddell Sea deep-sea basin is estimated at 3.3–3.9 × 10⁶ km³.

Introduction

Deep-sea sediment thicknesses, distribution patterns and deposition characteristics reveal the erosional, transport and deposition processes that were active during Antarctica's transition from a warm, pre-glacial to a cold, glacial climate. The geometry, distribution and thickness of sediment sequences produced by these processes can provide insight into the ice sheet development and palaeocirculation of the Weddell Sea. Additionally, sediment thickness grids are needed for palaeotopography (Lythe et al., 2001, Le Brocq et al., 2010, Wilson et al., 2011) and palaeobathymetry (Brown et al., 2006, Hayes et al., 2009) reconstructions at epochs with similar or higher atmospheric pCO₂ than today, like the Eocene, Miocene, Pliocene and Pleistocene (Pagani et al., 2005, Tripati et al., 2009, Tripati et al., 2011). These palaeo-surface reconstructions provide boundary conditions for palaeoclimate models (e.g. Pollard and DeConto, 2009), which focus on predicting ice sheet behaviour under continued increase of pCO₂ levels.

Identification of these pre-glacial to glacial components in the deep-sea seismic sedimentary records is largely unresolved for the Weddell Sea basin and cross-regional stratigraphic grids for the West Antarctic margin are still absent. As a result, sediment thickness is largely omitted in palaeobathymetry reconstructions (e.g. Brown et al., 2006), or if considered, contain data from the 1970's (Hayes and La Brecque, 1991, Hayes et al., 2009) and few data points (Laske and Masters, 1997), which distort the grids. Tracing continuous horizons over large (> 500 km) distances in seamless seismic data are thus needed to develop a basin-wide stratigraphy, identify the pre-glacial to glacial components in the deep-sea sedimentary record and estimate sediment thickness and volume.

Previous seismic reflection studies presented seismostratigraphy models for the southern Scotia Sea (e.g. Maldonado et al., 1998, Maldonado et al., 2003, Maldonado et al., 2005; Fig. 1), the Antarctic Peninsula (e.g. Larter and Barker, 1989, Rebesco and Camerlenghi, 2008, Smith and Anderson, 2010), the Jane and Powell Basins in the northwestern Weddell Sea (e.g. Coren et al., 1997, Bohoyo et al., 2002, Bohoyo, 2004; Fig. 1), and the southeastern Weddell Sea basin (Hinz and Kristoffersen, 1987, Miller et al., 1990, Rogenhagen and Jokat, 2000, Rogenhagen et al., 2004; Fig. 1). These identified three pre-glacial seismic stratigraphic units in the Weddell Sea and one in the Scotia Sea (Pw5, SH5, Sh5, SOM-C, fourth column in Table 1 and references in footnote) span the Jurassic to the end of the Oligocene. Three glacial regime units in the Weddell Sea and four in the Scotia Sea, Jane and Powell Basins were also identified. In contrast to the pre-glacial units, the glacial units were deposited over a comparatively short period of time (~ 21 Ma) during the Miocene to late Pleistocene. These studies are however local scaled, stratigraphically disconnected and use different nomenclatures, making it difficult to construct regional and cross-regional sediment thickness grids.

Rooted in a ~ 3300 km long transect, hereafter referred to as the Weddell Sea–Scotia Sea (WS–SS) seismic transect, this study aims to: (i) define a basin-wide seismic stratigraphic model for the Weddell Sea that is correlated to the southern Scotia Sea stratigraphy and tested against local studies and sparse boreholes; (ii) identify the pre-glacial (PG), transitional (T) and full glacial (FG) components in the deep-sea sediment record; (iii) quantify the sediment thicknesses, lateral age variation and tentative sedimentation rates of these components; (iv) consider the implications the findings might have for understanding the pre-glacial to glacial development of Antarctica amidst changes in climate, tectonics, and ocean circulation.

We define the pre-glacial regime as warm, predominantly ice sheet free conditions and open-ocean. The transitional regime describes a colder alpine-type climate and periods of ephemeral continental scale ice sheets that initially grounded on the outer shelf after multiple cycles of advance and retreat. The full glacial regime denotes a cold polar climate and the expansion of the ice sheets to the coast that permanently grounded on the outer shelf. Smaller advance and retreat cycles occurred, but the ice sheets remain grounded.

The WS–SS seismic transect focuses on the deep-sea sedimentary record because there the reflections are less disturbed or influenced by changes in sea level and glacial processes, making it easier to trace horizons over long distances. The thickness and geometry of the seismic sequences can give an indication of high sediment influx to the deep-sea, triggered for example by expanding ice sheets pushing sediments off the outer shelf, onto the slope and rise. The proposed age model provides a working hypothesis for further unravelling of the past ice-sheet dynamics and ocean circulation in the Weddell Sea that can be tested by future deep-sea drilling.