Glacial and glaciomarine environments in the southern Bellingshausen Sea since the last glacial maximum a reconstruction based on the sedimentary record


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hgrobe [ at ] awi-bremerhaven.de

Abstract

Multibeam swath bathymetric data and sub-bottom acoustic profiles collected during RRS James Clark Ross Cruise JR104 revealed the existence of a major glacial trough (Belgica Trough) on the West Antarctic continental shelf in the southern Bellingshausen Sea (75°-90°W, 69°-73°S), and of a trough mouth fan on the adjacent slope. Distinct seabed morphological features, such as mega-scale glacial lineations, drumlins and grounding-zone wedges, indicate that Belgica Trough represents the former pathway of a grounded ice stream, which had advanced to the shelf break during the last glacial maximum (LGM). Moreover, the orientation of the subglacial bedforms suggests that the ice stream was fed by grounded ice draining both through Eltanin Bay and Ronne Entrance, located to the south and south-east of Belgica Trough, respectively. These results give evidence that, in contrast to the present drainage pattern of the West Antarctic Ice Sheet, ice drainage to the southern Bellingshausen Sea played a significant role during the LGM.Sedimentary sequences recovered with gravity cores in the southern Bellingshausen Sea document a whole suite of (sub-)glacial and glaciomarine environmental settings. The lowermost units in the cores from the slope, the outer shelf and from Ronne Entrance consist of grey, massive, lithogenic diamictons. The diamictons on the slope represent glaciogenic debris flow deposits (GDFs), consisting of the detritus initially delivered by the grounded ice stream to the shelf edge during the LGM. In contrast, the lithologically very similar diamictons on the shelf are interpreted as deformation tills deposited directly by the grounded ice, and as glaciomarine tills deposited during the subsequent deglaciation phase, respectively. The glaciomarine tills may comprise both sub-ice shelf tills as well as iceberg-rafted tills and thus reflect the retreat of grounded ice. The lowermost sediments in cores recovered from Eltanin Bay consist of grey-olive, lithogenic, massive to stratified gravelly muddy sands, characterized by extensive deposition of iceberg-rafted debris (IRD) in a depositional setting distant from the grounding line. This facies type is also observed at other shelf sites, overlying the glaciomarine till as thin-bedded unit. The GDFs on the slope are capped by lithogenic muds bearing silty and sandy layers, which are interpreted as distal turbidites. Like the gravelly muddy sands, the deposition of the turbidites may originate in the transition from a depositional environment characterized by grounded ice located close to the shelf break to a glaciomarine setting influenced by the presence of ice shelves on the shelf and/or long-term sea-ice coverage, hampering significant biological production in the surface waters, and melting icebergs. During the present interglacial period brown, bioturbated foraminifer-bearing muds were deposited on the middle to outer shelf and on the slope. Manganese-coated gravelly IRD clasts are often scattered on the sediment surface and point to low sedimentation rates (<1 cm/kyr). A seaward increase in sand contents and AMS 14C ages of core tops is likely to result from current-induced winnowing of the fine fraction. Olive to brown, bioturbated diatomaceous muds were found on the inner shelf. IRD concentrations in these surface sediments are low, and Holocene sedimentation rates there are assumed to be significantly higher than farther offshore. Both the foraminifer- and the diatom-bearing sediments document plankton productivity in a seasonally open-marine setting.



Item Type
Conference (Talk)
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Not peer-reviewed
Publication Status
Published
Event Details
Conference on "Glacial Sedimentary Processes and Products", Aberystwyth, UK, August 2005..
Eprint ID
12148
Cite as
Hillenbrand, C. D. , Ó Cofaigh, C. , Larter, R. , Dowdeswell, J. , Pudsey, C. , Ehrmann, W. and Grobe, H. (2005): Glacial and glaciomarine environments in the southern Bellingshausen Sea since the last glacial maximum a reconstruction based on the sedimentary record , Conference on "Glacial Sedimentary Processes and Products", Aberystwyth, UK, August 2005. .


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