| Summary: | A meltwater stream in the Marshall Valley, southern Victoria Land, Antarctica has revealed a stratigraphy of glacial drift and lacustrine lake beds. The sequence has been differentiated by U/Th dating into the Ross Sea I Glaciation (15-30,000 years B.P.), represented by a gypsum bed and the Ross Sea II(?) Glaciation (130-180,000 years B.P.), represented by three calcium carbonate and gypsum beds. The three Ross Sea II(?) lake beds may be differentiated from one another by an increasing calcite content with age, and a systematic change in the crystal type and morphology of the carbonates. Chemical analysis of the lake beds reveals that their components have differing sources. The freezing on of seawater and/or frozen out seawater precipitates can quantitatively provide the salts required. However U isotope studies shows that a weathered crustal component is present in the lake beds. Stable isotope analysis indicates that the meltwaters were formed at high altitudes of 1,900 m (Ross Sea I) and 2,500 m (Ross Sea II(?)), and originated as snow, precipitated approximately 5,000 km to the south. Ice sheet incursion into the valley created a proglacial lake. Calcium carbonate and gypsum were precipitated due to increased lake saturation conditions. Stable isotope studies show that the calcium carbonate deposition was triggered biologically by phytoplankton. The morphology of the calcium carbonate and the presence of gypsum indicates that evaporitic conditions were also necessary for lake bed deposition, with the distribution of gypsum and calcium carbonate within the lake being controlled by depth of water. Correlation of lake bed geochronology and geochemistry, together with the stratigraphy of the stream sections provides a glacial history of Marshall Valley for the last 180,000 years B.P. The last two Ross Sea glacial sequences rest upon an unconformably eroded glacial surface. The Ross Sea II(?) Glaciation commenced at 180,000 years with a glacial ice incursion into the valley, a stillstand, proglacial lake occupation and deposition of a lake bed. The ice front then advance farther up the valley, depositing large terminal loop moraines between Sections III and IV. The ice then retreated at least 2 km. A second glacial advance commenced prior to 160,000 years, stillstanded and produced the 160,000 year lake bed, before retreating. A minor fluctuation and readvance, deposited another lake bed (160,000 years) or reworked the previous lake bed into the overlying stratigraphy, before the ice retreat. The deposition of the 130,000 year lake bed was also a the result of a stillstand, before the retreat and cessation of the Ross Sea II(?) glaciation. The Ross Sea I Glaciation involved the advance of a glacier front from an ice sheet occupying the McMurdo Sound and the deposition of a gypsum lake bed (15-30,000 years B.P.). With the retreat of the ice front, downwasting of ice-cored moraines in the mouth of the valley began, and currently continues. The geochronology of the Ross Sea II(?) Glaciation correlates with eustatic sea level lowering in the Illinoian (Isotope Stage 6). Subsurface drilling indicates that the calcium carbonate deposition was a unique event but at least 8 earlier glacial incursions had occurred in the Marshall Valley. The geochronology of the Ross Sea I Glaciation correlates with the eustatic sea level lowering in the Wisconsin (Isotope Stage 2). The chronology and isotopic composition of the proglacial lake deposits can be matched to those in other ice-free valleys exposed to McMurdo Sound.
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