Description: Paleotopographic models of the West Antarctic margin, which are essential for robust simulations of paleoclimate scenarios, lack information on sediment thickness and geodynamic conditions, resulting in large uncertainties. A new total sediment thickness grid spanning the Ross Sea–Amundsen Sea–Bellingshausen Sea basins is presented and is based on all the available seismic reflection, borehole, and gravity modeling data offshore West Antarctica. This grid was combined with NGDC's global 5 arc minute grid of ocean sediment thickness (Whittaker et al., 2013) and extends the NGDC grid further to the south. Sediment thickness along the West Antarctic margin tends to be 3–4 km larger than previously assumed. The sediment volume in the Bellingshausen, Amundsen, and Ross Sea basins amounts to 3.61, 3.58, and 2.78 million km3, respectively. The residual basement topography of the South Pacific has been revised and the new data show an asymmetric trend over the Pacific–Antarctic Ridge. Values are anomalously high south of the spreading ridge and in the Ross Sea area, where the topography seems to be affected by persistent mantle processes. In contrast, the basement topography offshore Marie Byrd Land cannot be attributed to dynamic topography, but rather to crustal thickening due to intraplate volcanism. Present-day dynamic topography models disagree with the presented revised basement topography of the South Pacific, rendering paleotopographic reconstructions with such a limited dataset still fairly uncertain.

Description: Multichannel seismic reflection lines collected in the western Amundsen Sea Embayment (ASE) provide an insight into the sedimentary cover on the shelf, which documents glacial processes. Numerous columnar, reflection-poor structures penetrating the sedimentary sequences on the middle shelf form the focus of this study. The features range between 50 to 500 m in width, and from a few metres up to 500 m in height. The columns originate and end at different depths, but do not seem to penetrate to the seafloor. They show well-defined vertical boundaries, and reflection signals can be identified below them. Hence, we exclude gas-bearing chimneys. Based on the general seismic reflection characteristics we suggest that the columns originate from dewatering processes which occur close to glaciated areas where fluids are pressed out of rapidly loaded sediments. Likely several mud-diapirs rise from water-rich mud layers within a mixed sedimentary succession and penetrate overlying denser and coarse-grained sediment strata. The presence of fluid-escape veins indicates a glacial origin and overprinting of the older sedimentary sequences on the ASE. The locations of the structures indicate that grounded ice sheets reached at least onto the middle shelf during former glacial periods.

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 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. 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 river estuary 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 21-14.1 Ma we postulate intense 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 led to a wet-based ice sheet and a higher sediment supply along a broad front but with a focus via two palaeo-ice stream troughs. 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. 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: The Amundsen Sea Embayment of West Antarctica represents a key component in the tectonic history of Antarctic-New Zealand continental breakup. The region played a major role in the plate-kinematic development of the southern Pacific from the inferred collision of the Hikurangi Plateau with the Gondwana subduction margin at approximately 110-100 Ma to the evolution of the West Antarctic Rift System. However, little is known about the crustal architecture and the tectonic processes creating the embayment. During two RV Polarstern expeditions in 2006 and 2010 a large geophysical dataset was collected consisting of seismic-refraction and reflection data, ship-borne gravity and helicopter-borne magnetic measurements. Two P-wave velocity-depth models based on forward travel-time modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the Amundsen Sea Embayment. Seismic-reflection data image the sedimentary architecture and the top-of-basement. The seismic data provide constraints for 2-D gravity modelling, which supports and complements P-wave modelling. Our final model shows 10 - 14 km thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf. The homogenous crustal architecture of the continental rise, including horst and graben structures are interpreted as indicating that wide-mode rifting affected the entire region. We observe a high-velocity layer of variable thickness beneath the margin and related it, contrary to other “normal volcanic type margins”, to a proposed magma flow along the base of the crust from beneath eastern Marie Byrd Land – West Antarctica to the Marie Byrd Seamount province. Furthermore, we discuss the possibility of upper mantle serpentinization by seawater penetration at the Marie Byrd Seamount province. Hints of seaward-dipping reflectors indicate some degree of volcanism in the area after break-up. A set of gravity anomaly data indicate several phases of fully developed and failed rift systems, including a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment.

Description: The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica’s continental ice volume. Over the last few decades, studies on glacialegeological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23e19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on theWeddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different icesheet models have reconstructed conflicting LGM ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the LGM sealevel low-stand of w130 m vary considerably. In this paper, we summarise and review the geological records of past ice-sheet margins and past icesheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the LGM ice-sheet configuration, including time-slice reconstructions for post- LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim of our study is to provide two alternative interpretations of glacialegeological datasets on Antarctic Ice- Sheet History for the Weddell Sea sector, which can be utilised to test and improve numerical icesheet models

Description: The Davis Strait is located between Canada and Greenland and connects the Labrador Sea and the Baffin Bay basins. Both basins formed in Cretaceous to Eocene time and were connected by a transform fault system in the Davis Strait. Whether the crust in the central Davis Strait is oceanic or continental has been disputed. This information is needed to understand the evolution of this transform margin during the separation of the North American plate and Greenland. We here present a 315-km-long east–west-oriented profile that crosses the Davis Strait and two major transform fault systems—the Ungava Fault Complex and the Hudson Fracture Zone. By forward modelling of data from 12 ocean bottom seismographs, we develop a P-wave velocity model.We compare thismodel with a density model from ship-borne gravity data. Seismic reflection and magnetic anomaly data support and complement the interpretation. Most of the crust is covered by basalt flows that indicate extensive volcanism in the Davis Strait. While the upper crust is uniform, the middle and lower crust are characterized by higher P-wave velocities and densities at the location of the Ungava Fault Complex. Here, P-wave velocities of the middle crust are 6.6 km s−1 and of the lower crust are 7.1 km s−1 compared to 6.3 and 6.8 km s−1 outside this area; densities are 2850 and 3050 kg m−3 compared to 2800 and 2900 kg m−3. We here interpret a 45-km-long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern Davis Strait. A high-velocity lower crust (6.9–7.3 km s−1) indicates a high content of mafic material. This mantle-derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the Iceland mantle plume. With plate kinematic modelling, we can demonstrate the importance of two transform fault systems in the Davis Strait: the Ungava Fault Complex with transpression and the Hudson Fracture Zone with pure strike-slip motion. We show that with recent poles of rotation, most of the relative motion between the North American plate and Greenland took place along the Hudson Fracture Zone.

Description: Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity, and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.8 - 4.0 km s−1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 - 6.4 km s−1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.4 - 7.2 km s−1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle, and lower crust with P-wave velocities from 5.3 - 7.0 km s−1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transi- tion zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.