Seismic stratigraphy and paleo-glaciology of Sabrina Coast, East Antarctica and Bering Trough, Gulf of Alaska




Montelli, Aleksandr

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Examination of the sedimentary record in areas occupied by fast flowing outlet glaciers and ice streams is critical for understanding ice sheet dynamics on millennial timescales. We use high-resolution seismic data together with log data and foraminiferal- based radiocarbon and isotope data from Integrated Ocean Drilling Program (IODP) Expedition 341 cores to examine the evolution of the Cordilleran Ice Sheet (CIS) in southeast Alaska. Evidence for at least seven advances of the Bering Glacier to the shelf break since the end of Middle Pleistocene Transition (MPT) are interpreted from the seismic data. Seismic data demonstrate two distinctive patterns of slope sedimentation, which are: (1) dominated by sediments bypassing a steep paleo-slope or (2) dominated by glacigenic debris flows (GDFs) that form a trough mouth fan (TMF) on a lower slope. Integration of seismic, well, and age data suggests that the TMF was formed as a result of three advances to the shelf break since ca. 140 ka, implying extreme (average > 4m/ka) rates of sediment delivery. Extraordinarily high sediment flux played a key role in the development of the margin architecture resulting from a temperate, highly aggressive Bering Glacial System and abundant source of erodible bedrock onshore (St. Elias orogeny). We use the first high-resolution seismic data acquired on the Sabrina Coast, East Antarctica, together with core data, to examine major transitions in glacial regime of East Antarctic Ice Sheet. Three large-scale megasequences represent changes in the dominant pattern of sedimentation: Megasequence I shows signs of Eocene fluvial and possibly glacio-fluvial sedimentation; Megasequence II provides evidence of Oligocene-Miocene polythermal glacial sedimentation with significant amount of meltwater; Megasequence III overlies the regional erosional surface that marks major Miocene ice sheet expansion and transition into polar (cold- based) ice sheet glacial regime with no significant evidence of subglacial meltwater preserved. Megasequence II exhibits preserved record of EAIS evolution starting from its first expansion. Seismic stratigraphic analysis suggests a dynamic glacial late Paleogene-early Neogene evolution including expansions of the EAIS across the shelf at least eight times, which is marked by erosional surfaces and chaotic acoustic character of overlying strata. We report on finding the first conclusive seismic evidence of deep, extensive tunnel valley systems incised into sedimentary substrate from Antarctic continental margins. The Sabrina Coast tunnel valley system was presumably formed during Oligocene as a result of the second major EAIS shelf expansion. Shallower erosion events observed in the upper part of Megasequence II suggest more extensive glaciations in the Oligocene compared to the Miocene.


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