Meltwater channel scars and the extent of Mid-Pleistocene glaciation in central Pennsylvania
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Abstract The glacial development of the lower Borgarfjördur region, western Iceland, was investigated with regard to morphology, lithostratigraphy and chronology of glacial events. The maximum glacial situation is outlined, and a synthesis of all available evidence on the deglaciation is proposed. It is concluded that after an initial deglaciation of the coastal lowlands, some time prior to 12,500 BP, glaciers again advanced to the outer coastal areas between 12,000 and 11,700 BP, and, after a minor retreat between 11,700 and 11,000 BP, retained nearly their former positions between 11,000 and 10,300 BP. The marine maximum limit, at 80–90 m a.s.l., was reached in connection with the former advance, and the regional marine limit, at 60–70 m, at the end of the latter advance. A raised beach at 40 m a.s.l. possibly relates to an Early Flandrian glacial episode. These results imply a more extensive glaciation in coastal western Iceland at the end of the Late Weichselian than hitherto assumed. Key Words: Glacial geologychronologyradiocarbon dateslithostratigraphyLate Weichseliandeglaciationsea level changesBorgarfjördurHvalfjördurIceland
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Freshwater pulses from melting ice sheets are thought to be important for driving deglacial climate variability. This study investigates challenges in simulating and understanding deglacial climate evolution within this framework, with emphasis on uncertainties in the ocean overturning sensitivity to meltwater inputs. The response of an intermediate complexity model to a single Northern Hemisphere meltwater pulse is familiar: a weakening of the ocean overturning circulation in conjunction with an expansion of sea ice cover and a meridional temperature seesaw. Nonlinear processes are vital in shaping this response and are found to have a decisive influence when more complex scenarios with a history of pulses are involved. A meltwater history for the last deglaciation (21–9 ka) was computed from the ICE‐5G ice sheet reconstruction, and the meltwater was routed into the ocean through idealized ice sheet drainages. Forced with this meltwater history, model configurations with altered freshwater sensitivity produce a range of outcomes for the deglaciation, from those in which there is a complete collapse of the overturning circulation to those in which the overturning circulation weakens slightly. The different outcomes are interpreted in terms of the changing hysteresis behavior of the overturning circulation (i.e., non‐stationary freshwater sensitivity) as the background climate warms through the course of the deglaciation. The study illustrates that current uncertainties in model sensitivity are limiting in efforts to forward‐model deglacial climate variability. Furthermore, ice sheet reconstructions are shown to provide poor constraints on meltwater forcing for simulating the deglaciation.
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Surface exposure dating
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the glacial features line layer describes linear features associated with surficial geology these glacial features include but are not limited to wisconsinan synglacial sea extents meltwater channels meltwater spillways ice margins and eskers this layer also includes non glacial exposures such as gravel pit extents an associated dbase table glac_desc184l dbf is available with glacial feature descriptions
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During the last glacial maximum, the British–Irish Ice Sheet (BIIS) extended to the shelf edge in the Malin Sea between Ireland and Scotland, delivering sediments to the Donegal Barra Fan (DBF). Analysis of well-preserved, glacially derived sediment in the DBF provides new insights on the character of the BIIS final deglaciation and palaeoenvironmental conditions at the Younger Dryas. Chaotic/laminated muds, ice-rafted debris (IRD)-rich layers and laminated sand–mud couplets are interpreted as respectively mass transport deposits, plumites and turbidites of BIIS-transported sediments. Peaks in IRD, constrained by radiocarbon dating to after 18 cal ka BP, indicate discrete intervals of iceberg calving during the last stages of deglaciation. Glacially derived sedimentation on the slope occurred until c . 16.9 cal ka BP. This is interpreted as the last time the ice sheet was present on to the shelf, allowing glacial meltwater to reach the fan. Bioturbated and foraminifera-rich muds above glaciomarine sediments are interpreted as interglacial hemipelagites and contourites, with the presence of Zoophycos suggesting restoration of bottom currents at the transition between stadial and interstadial conditions. During the Younger Dryas, Neogloboquadrina pachyderma sinistral abundances and an isolated peak in IRD indicate the temporary restoration of cold conditions and the presence of icebergs in the region. Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research
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