ALBERT

Description: 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.

Description: Studies of the sedimentary architecture and characteristics of the Antarctic continental margin provide clues about past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport reflecting the history from pre-glacial times to early glaciation and to the late Pleistocene glacial-interglacial cycles. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between basement cropping out on the inner shelf and buried basement ridge and highs on the outer shelf. A subordinate basin within the large basin on the mid-shelf may be associated with motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata have been eroded from the present inner shelf and coastal hinterland by glacial processes. Six major sedimentary units (ASS-1 to ASS-6) separated by five major erosional unconformities (ASS-u1 to ASS-u5) are distinguished from bottom to top. Unconformity ASS-u4 results from a major truncational event by glacial advance to the middle and outer shelf, which was followed by several episodes of glacial advance and retreat as observed from smaller-scale truncational unconformities within the units above ASS-u4. Some of the eroded sediments were deposited as a progradional wedge that extends the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. We compare the observed seismic characteristics with those of other Antarctic shelf sequences and assign an Early Cretaceous age to bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age to unit ASS-2, an Early to Mid-Miocene age to unit ASS-3, a Mid-Miocene age to unit ASS-4, a Late Miocene to Early Pliocene age to unit ASS-5, and a Pliocene to Pleistocene age to the top unit ASS-6. Buried grounding zone wedges in the upper part of unit ASS-5 on the outer shelf suggest pronounced warming phases and ice sheet retreats during the early Pliocene as observed for the Ross Sea shelf and predicted by paleo-ice sheet models. Our data also reveal that on the middle and outer shelf the flow-path of the Pine Island-Thwaites paleo-ice stream system has remained stationary in the central Pine Island Trough since the earliest glacial advances, which is different from the Ross Sea shelf where glacial troughs shifted more dynamically. This study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to change in the West Antarctic Ice Sheet, such as the contemporary thinning and grounding line retreat in the Amundsen Sea drainage sector.

Description: The Weddell Sea basin is of particular significance for understanding climate processes, including the generation of ocean water masses and their influence on ocean circulation as well as the dynamics of the Antarctic ice sheets. The sedimentary record, preserved below the basin floor, serves as an archive of the pre-glacial to glacial development of these processes, which were accompanied by tectonic processes in its early glacial phase. Three multichannel seismic reflection transects, in total nearly 5000 km long, are used to interpret horizons and define a seismostratigraphic model for the basin. We expand this initial stratigraphic model to the greater Weddell Sea region through a network of more than 200 additional seismic lines. Information from few boreholes is used to constrain sediment ages in this stratigraphy, supported by magnetic anomalies indicating decreasing oceanic basement ages from southeast to northwest. Using these constraints, we calculate grids to depict the depths, thicknesses and sedimentation rates of pre-glacial (145–34 Ma), transitional (34–15 Ma) and full-glacial (15 Ma to present) units. These grids allow us to compare the sedimentary regimes of the pre-glacially dominated and glacially dominated stages of Weddell Sea history. The pre-glacial deposition with thicknesses of up to 5 km was controlled by the tectonic evolution and sea-floor spreading history interacting with terrigenous sediment supply. The transitional unit shows a relatively high sedimentation rate and has thicknesses of up to 3 km, which may attribute to an early formation of the East Antarctic Ice Sheet, which was partly advanced to the coast or even inner shelf. The main deposition center of the full-glacial unit lies in front of the Filchner-Ronne Ice Shelf and has sedimentation rates of up to 140–200 m/Myr, which infers that ice sheets grounded on the middle to the outer shelf and that bottom-water currents strongly impacted the sedimentation. We further calculate paleobathymetric grids at 15 Ma, 34 Ma, and 120 Ma by using a backstripping technique. Our results provide constraints for an improved understanding of the paleo-ice sheet dynamics and paleoclimate conditions of the Weddell Sea region.

Description: The distribution and internal architecture of seismostratigraphic sequences observed on the Antarctic continental slope and rise are results of sediment transport and deposition by bottom currents and ice sheets. Analysis of seismic reflection data allows to reconstruct sediment input and sediment transport patterns and to infer past changes in climate and oceanography. We observe four seismostratigraphic units which show distinct differences in location and shape of their depocentres and which accumulated at variable sedimentation rates. We used an age-depth model based on DSDP Leg 35 Site 324 for the Plio/Pleistocene and a correlation with seismic reflection characteristics from the Ross and Bellingshausen Seas, which unfortunately has large uncertainties. For the period before 21 Ma, we interpret low energy input of detritus via a palaeo-delta originating in an area of the Amundsen Sea shelf, where a palaeo-ice stream trough (Pine Island Trough East, PITE) is located today, and deposition of this material on the continental rise under sea ice coverage. For the period 21-14.1 Ma we postulate glacial erosion for the hinterland of this part of West Antarctica, which resulted in a larger depocentre and an increase in mass transport deposits. Warming during the Mid Miocene Climatic Optimum resulted in a polythermal ice sheet and led to a higher sediment supply along a broad front but with a focus via two palaeo-ice stream troughs, PITE and Abbot Trough (AT). Most of the glaciogenic debris was transported onto the eastern Amundsen Sea rise where it was shaped into levee-drifts by a re-circulating bottom current. A reduced sediment accumulation in the deep-sea subsequent to the onset of climatic cooling after 14 Ma indicates a reduced sediment supply probably in response to a colder and drier ice sheet. A dynamic ice sheet since 4 Ma delivered material offshore mainly via AT and Pine Island Trough West (PITW). Interaction of this glaciogenic detritus with a west-setting bottom current resulted in the continued formation of levee-drifts in the eastern and central Amundsen Sea

Description: 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 3.3–3.9×10^6 km3.