Preglacial to glacial sediment thickness grids for the Southern Pacific margin of West Antarctica

Circum-Antarctic sediment thickness grids provide constraints for basin evolution and paleotopographic reconstructions, which are important for paleo-ice sheet formation histories. By compiling old and new seismic data, we identify sequences representing pre-glacial, transitional and full glacial deposition processes along the Pacific margin of West Antarctica. The pre-glacial sediment grid depicts 1.3 to 4.0 km thick depocenters, relatively evenly distributed along the margin. The depocenters change markedly in the transitional phase at, or after, the Eocene/Oligocene boundary, when the first major ice sheets reached the shelf. Full glacial sequences, starting in the middle Miocene, indicate new depocenter formation North of the Amundsen Sea Embayment and localized eastward shifts in the Bellingshausen Sea and Antarctic Peninsula basins. Using present-day drainage paths and source areas on the continent, our calculations indicate an estimated observed total sedimentary volume of ~10 x 10**6 km**3 was eroded from West Antarctica since the separation of New Zealand in the Late Cretaceous. Of this 4.9 x 10**6 km**3 predates the onset of glaciation and need to be considered for a paleotopography reconstruction of 34 Ma. Whereas 5.1 x 10**6 km**3 postdate the onset of glaciation, of which 2.5 x 10**6 km**3 were deposited in post mid-Miocene full glacial conditions

Deep sea seismic stratigraphy of the Amundsen Sea and Ross Sea, West Antarctica: Preliminary results of the first linking record

Seamless circum-Antarctic palaeosedimentary and palaeobathymetry maps and grids spanning the Cretaceous to present are important, because sedimentary thickness and sedimentation rates are needed to reconstruct more realistic seafloor geometries, palaeotopography and long term climate change models. To create such a dataset around Antarctica, a complete network of cross-linked deep-sea seismic reflection profiles is necessary. In this context, the multinational Circum-Antarctic Stratigraphy and Palaeobathymetry (CASP) initiative was born to provide constraints for palaeoclimate modelling. Existing international seismic data cover most of Antarcticas continental rise, but from 130°W to 160°W, a completely unsurveyed sector on the western margin of Antarctica, between the Ross Sea and Amundsen Sea, prevents closure of the Circum-Antarctic sedimentary grids, and palaeostratigraphic link. During the RV Polarstern cruise, from January to March 2010, the Alfred Wegener Institute acquired ca. 5000 km high resolution multichannel seismic data on the West Antarctic margin, using a 3 km streamer and 3 GI-guns. Among these profiles, is a ca. 1500 km multichannel seismic reflection profile - the first crossing the previously unsurveyed sector. Preliminary results show minimum sediment thickness of ~0.75s two-way-time (TWT) in the Ross Sea and maximum sediment thickness of ~2.5s TWT in the Amundsen Sea. A clear acoustic basement occurs at ~6 to 8s TWT depth and similar to the internal reflectors, is traceable across the entire length of the profile. The basement topography is slightly rugged at the Ross Sea, becoming undulating and then smooth up to 123°W, where it meets the Mary Byrd Seamount region, and becomes more rugged towards the continental shelf. The continuous seismic reflectors of three distinct sedimentary packages are mostly horizontal and undisturbed, and thus provide the first possibility to link the seismic stratigraphy of pre-glacial, transitional and glacially dominated sedimentary packages in the Ross Sea, with similar stratigraphy in the Amundsen Sea. This data brings us a significant step closer to the circum-Antarctic isopach maps and grids

Stratigraphy of the Pacific margin of West Antarctica: A contribution to the CASP project

Some of the key parameters in paleoclimate modelling of long time-scales are paleobathymetry of the ocean basins and paleotopography of the continents. Sedimentary thicknesses, rates and formation processes are essential ingredients for deriving paleobathymetry. As part of the Circum-Antarctic Stratigraphy and Paleobathymetry (CASP) project, we derive the stratigraphic conditions of the Pacific part of the Southern Ocean along the West Antarctic margin from pre-existing and recently collected multichannel seismic data. We link the network of seismic lines from the Antarctic Peninsula to the Bellingshausen and Amundsen Sea. Some of the seismic lines are re-interpreted, new horizons are identified and mapped. Additional information from ice-sheet models and DSDP/ODP boreholes are used to match the stratigraphy across the regional datasets. The resulting stratigraphic compilation contributes to the CASP mapping project, which focuses on creating a continuous series of seamless circum-Antarctic paleoceanographic and paleosedimentary seafloor maps that span from the Cretaceous to present

Palaeoenvironment of the Weddell Sea and Amundsen Sea – eastern Ross Sea basins, Antarctica: Insights from comprehensive seismostratigraphic analysis

In combining existing multichannel seismic reflection seismic data in the Weddell Sea deep-sea basin, with existing and newly acquired data in the Amundsen Sea and Ross Sea basins, previously unknown sequences representing the pre-glacial to glacial palaeoenvironmental development of the West Antarctic Margin were identified. Pre-glacial sediment deposition centres seemed to have changed near or after the Eocene/Oligocene boundary (~34 Ma) when the first major ice sheets advanced to and across the shelf. The middle Miocene (~16 Ma) full glacial sequences indicate a new depocentre formed North of the Amundsen Sea Embayment. Smaller depocentres in the Bellingshausen Sea and Antarctic Peninsula basins, shifted eastward. Calculations indicate ~4.6 km (~10.2 million km3) of West Antarctica's landmass were eroded since the Late Cretaceous and deposited in the Southern Pacific. This has implications for the palaeotopographic and palaeobathymetric reconstructions, and ice sheet climate models

Palaeoumwelt des Weddellmeer und Amundsenmeer - östlichen Rossmeer Beckens, Antarktis : Erkenntnisse aus umfassenden seismischen stratigraphische Analyse

In combining existing multichannel seismic reflection seismic data in the Weddell Sea deep-sea basin, with existing and newly acquired data in the Amundsen Sea and Ross Sea basins, previously unknown sequences representing the pre-glacial to glacial palaeoenvironmental development of the West Antarctic Margin were identified. Pre-glacial sediment deposition centres seemed to have changed near or after the Eocene/Oligocene boundary (~34 Ma) when the first major ice sheets advanced to and across the shelf. The middle Miocene (~16 Ma) full glacial sequences indicate a new depocentre formed North of the Amundsen Sea Embayment. Smaller depocentres in the Bellingshausen Sea and Antarctic Peninsula basins, shifted eastward. Calculations indicate ~4.6 km (~10.2 million km3) of West Antarctica's landmass were eroded since the Late Cretaceous and deposited in the Southern Pacific. This has implications for the palaeotopographic and palaeobathymetric reconstructions, and ice sheet climate models

Anomalous South Pacific lithosphere dynamics derived from new total sediment thickness estimates off the West Antarctic margin

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

Revealing the Beattie Magnetic Anomaly and the anatomy of the crust of southernmost Africa: Geophysics and deep sub-surface geology where the Cape Fold Belt and Karroo Basin meet

The deep crust of the southernmost margin of Africa contains unresolved tectonic features such as the Paleozoic Cape Fold Belt (CFB), the Paleozoic-Mesozoic Karroo Basin and the largest terrestrial magnetic anomaly, the Beattie Magnetic Anomaly (BMA). Without resolving these structures, our understanding of the evolution of the southern margin will be incomplete and limited. Under the auspices of the Inkaba yeAfrica framework, several geophysical datasets were acquired from 2004 to 2007, along two transects, across the margin and its unique tectonic features. This research presents a tectonic model and crustal geometry, at the centre 100 km of the western transect. The model is derived from the joint interpretation of: surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions, impedance spectroscopy measurements on borehole samples, near vertical reflection seismic data (NVR), shallow P- and S-wave velocity data, wide angle refraction data and magnetotelluric data. The model differentiates a four component ~42 to 45 km thick crust and constrains the two part BMA to a ~10 to 12 km wide northern zone, and a ~5 to 7 km wide southern zone, both at ~7 to 8 km below surface, continuing for a depth of ~5km, and, contained in the Mesoproterozoic Namaqua-Natal mid-crust. The BMA source is interpreted to be a Namaqua-like massive to disseminated, deformed/metamorphosed strataform sulphide ore body. The model presents evidence in support of a thin-skinned tectonic thrust model for the evolution of the flat-based CFB and shows no significant fore-deep stratigraphic thickening in the Karroo Basin towards the CFB front. The tectonic model suggests a Meso-Proterozoic collision orogen setting, overprinted by a mid-Phanerozoic thin-skinned fold and thrust belt coupled to the far-field accretion/subduction margin to the south

Seismic stratigraphy along the Amundsen Sea to Ross Sea continental rise: A cross-regional record of pre-glacial to glacial processes of the West Antarctic margin

AbstractThe seismic sediment record of the Amundsen Sea continental rise provides insight into the sedimentation processes from pre-glacial to glacial times, variations in ocean-bottom circulation, early ice sheet growth, and intensification towards the present icehouse regime. Seismic reflection data acquired during the 2010 RV Polarstern and the 2006 RV Tangaroa expeditions, created a >2000km long continuous Amundsen Sea to Ross Sea seismic transect. Pre-existing lines linked to this transect, connect key seismic stratigraphy horizons from the Ross Sea shelf to the rise and farther along the West Antarctic margin, up to the Amundsen Sea Embayment. Seismic units AS-1 to AS-3 constitute the Cretaceous to Eocene pre-glacial (PG) sequence (79–34Ma), units AS-4 to AS-6 the Eocene to mid-Miocene transitional (T) sequence (34–15.5Ma), and units AS-7 to AS-11 the mid-Miocene to Quaternary full glacial (FG) climate sequence (15.5–0Ma). The top PG sequence boundary horizon AS-u3/uPG-T, links to unconformity RSU6 of the Ross Sea shelf and to the base of Unit II of the eastern Amundsen Sea, and is interpreted as the first arrival of grounded ice on the shelf. The top T sequence boundary AS-u6/uT-FG, links to RSU4 and the base of Unit III, and is interpreted as the onset of the FG regime with intensified ice sheet advances onto the outer shelves. The Amundsen Sea basin accumulated up to 3.9km thick sediments in its centre. Seismic facies geometry analysis suggests Paleocene–Eocene bottom-current activity, late Eocene shelf grounding of the West Antarctic Ice Sheet, and no apparent difference in the deep-sea sediment transport processes or temporal shift in deposition between the Amundsen Sea and Ross Sea. Implications for a Marie Byrd Land uplift starting at ~30Ma are observed by a progressive change in horizon dip in the central Amundsen Sea seismic sequences