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1. Glaciogenic sedimentary infill of late Paleozoic glacial paleovalleys of the Kaokoveld, Northwest Namibia

   https://doi.org/10.1130/abs/2019AM-340908  

   Rosa, E.L. ; Isbell, J. L. ( September 2019 , Geological Society of America Abstracts with Programs)

   In the Kaokoveld (NW Namibia), several modern river valleys are exhuming late Paleozoic glacial valleys cut onto Precambrian fold belts. They represent one of the most prominent late Paleozoic exhumed glacial landscapes and are widely considered to have been carved by outlet glaciers that drained the Windhoek Ice Sheet and fed marginal lobes that flowed into the Paraná Basin, southern Brazil. No detailed research exists on the glacial sedimentary fill of these valleys. Two study sites in the Khumib and Kunene rivers catchment were analyzed for depositional environments, glacial cyclicity, and relative timing of deposition recorded in the Dwyka Group. The Dwyka strata are confined within these valleys and dip up to 30 degrees outward away from the valley walls becoming horizontal near the axis of the valleys. Sedimentary units include: 1) thick successions of diamictite- and conglomerate-bearing clinoforms containing boulders up to 2 m in diameter generated by sediment-laden meltwater, sediment gravity flows and iceberg rainout with intraformational grooved surfaces generated by coeval iceberg scour; 2) laminated, fine-grained sandstone/mudstone rhythmites with dropstones, dump structures, interbedded rainout diamictites and sole mark-bearing finegrained massive to current-rippled sandstones (turbidites). These units were deposited in the distal zones of a subaqueous outwash system; 3) folded and sheared intervals of the above facies interpreted as having been deformed subglacially and in ice-marginal settings during ice advance. Ice advance is indicated by the occurrence of overlying erosional based conglomerates interpreted as outwash deposits; and 4) a capping succession of fine-grained massive, horizontally laminated, and current-rippled sandstones with sole marks and laminated rhythmites with convolute bedding interpreted as turbidity flow deposits generated following glaciers retreat. The stacking of these units is consistent with the occurrence of oscillating margins of temperate, tidewater termini of fast flowing ice with deposition occurring in morainal banks or grounding-zone wedges during at least two glacial advance-retread cycles. The morphology of the valleys and their sedimentary infill suggest that they were shaped by ice streams during the Late Paleozoic Ice Age. 

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2. Glaciation During the Late Paleozoic Ice Age

   Isbell, J. L. ( January 2018 , Revista del Museo de La Plata)

   The Late Paleozoic Ice Age (LPIA) was one of Earth’s most extreme climatic events where sea level and biotic restructuring were driven by linked oscillations in the climate system. Despite an evolving understanding of the ice age, the size, distribution, paleogeography, timing, depositional settings, and possible bipolarity of the glaciation remains unresolved. However, new and refined radioisotopic age dates are revising the timing and extent of individual stages of the ice age. Recent studies suggest numerous, ice centers fluctuated diachronously as glaciation shifted across Gondwana. The LPIA began in the Famennian in northern South America and Africa and ended in eastern Australia during the Wuchiapingian. Although glaciation was widespread, numerous ice-free areas occurred adjacent to major glacial centers. Deglaciation was also diachronous beginning in the Bashkirian in western Argentina, shifting to the Paraná Basin by the end of the Pennsylvanian, with deglaciation of the South Polar Region occurring during the late Early Permian. Deglaciation culminated in eastern Australia with the disappearance of high, mid-latitude, alpine glaciers during the Wuchiapingian at a time when Polar Gondwana was ice-free. Recent work on diamictites in northeastern Russia indicates that these strata were not glacigenic but instead were deposited as volcanic debris flows and slides/slumps associated with concurrent activity in the Okhotsk-Taigonos volcanic arc. Therefore, bipolar glaciation cannot be confirmed. Although fluctuations in greenhouse gases were a major driver of climate, paleogeography, tectonism, and other minor drivers also played a role in the nucleation and disappearance of LPIA glaciers. 

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3. Detrital zircon geochronology and provenance of glaciogenic strata of the Middle Carboniferous San Eduardo Formation, Calingasta-Uspallata Basin, NW Argentina

   https://doi.org/10.1130/abs/2019AM-337940  

   Malone, J. E. ( September 2019 , Geological Society of America Abstracts with Programs)

   The Calingasta-Uspallata Basin preserves a near continuous sequence of glaciomarine deposition from the middle to late Carboniferous, represented by five separate formations. Correlation between these formations have been achieved using index marine invertebrates, which also provides some implications for max-depositional ages. However, no isotopic dating analyses have been sought in this basin to further constrain the age of deposition or provide a source of provenance for sediments. The San Eduardo formation near the El Leoncito Astronomical Complex, San Juan Province, Argentina, was deposited within the Calingasta—Upsallata Basin on the western margin of the Proto-precordillera during the late Mississippian to early Pennsylvanian. This succession preserves a complete sequence of proximal glaciomarine, nearshore, and fluvial systems deposited at the beginning of the late Paleozoic ice age. Samples were collected from various stages throughout the sequence for detrital zircon U-Pb geochronology to determine sediment provenance as a way of isolating different glacier sources. Results indicate multiple stages of glaciation, with at least three distinct source areas. The lowermost stage includes locally sourced basement and recycled underlying Silurian, represented by similar Famatinian (500-460 mya) and Grenville peaks (1250-1000 mya) peaks, where the Grenville source likely originating from the Western Sierras Pampeans, which would represent a breaching of the Proto-precordillera from the east. The middle stage shows a population distinct unto itself, with a peak during the Mississippian (330-360 mya). A volcanic island arc was situated along the Andean margin during the late Paleozoic, likely resulting in the influx of Carboniferous aged volcanic sediments. The lower most stage shows relations based on K-S results to formations within the Paganzo basin to the northeast, likely serving as the outwash of these distant glaciers through braided fluvial systems. This study will expand upon current chronologic knowledge within the Calingasta-Uspallata basin and will be supported by sandstone petrology and mineralogic composition, pebble counts and composition of dropstones. 

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4. Amundsen Sea West Antarctic Ice Sheet History

   https://doi.org/10.14379/iodp.proc.379.2021  

   Gohl, K. ; Wellner, J.S. ; Klaus, A. ( February 2021 , Proceedings of the International Ocean Discovery Program)

   The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet (WAIS) because of the great water depth at the grounding line, a subglacial bed seafloor deepening toward the interior of the continent, and the absence of substantial ice shelves. Glaciers in this configuration are thought to be susceptible to rapid or runaway retreat. Ice flowing into the Amundsen Sea Embayment is undergoing the most rapid changes of any sector of the Antarctic ice sheets outside the Antarctic Peninsula, including substantial grounding-line retreat over recent decades, as observed from satellite data. Recent models suggest that a threshold leading to the collapse of WAIS in this sector may have been already crossed and that much of the ice sheet could be lost even under relatively moderate greenhouse gas emission scenarios. Drill cores from the Amundsen Sea provide tests of several key questions about controls on ice sheet stability. The cores offer a direct offshore record of glacial history in a sector that is exclusively influenced by ice draining the WAIS, which allows clear comparisons between the WAIS history and low-latitude climate records. Today, relatively warm (modified) Circumpolar Deep Water (CDW) is impinging onto the Amundsen Sea shelf and causing melting under ice shelves and at the grounding line of the WAIS in most places. Reconstructions of past CDW intrusions can assess the ties between warm water upwelling and large-scale changes in past grounding-line positions. Carrying out these reconstructions offshore from the drainage basin that currently has the most substantial negative mass balance of ice anywhere in Antarctica is thus of prime interest to future predictions. The scientific objectives for this expedition are built on hypotheses about WAIS dynamics and related paleoenvironmental and paleoclimatic conditions. The main objectives are: 1. To test the hypothesis that WAIS collapses occurred during the Neogene and Quaternary and, if so, when and under which environmental conditions; 2. To obtain ice-proximal records of ice sheet dynamics in the Amundsen Sea that correlate with global records of ice-volume changes and proxy records for atmospheric and ocean temperatures; 3. To study the stability of a marine-based WAIS margin and how warm deepwater incursions control its position on the shelf; 4. To find evidence for the earliest major grounded WAIS advances onto the middle and outer shelf; 5. To test the hypothesis that the first major WAIS growth was related to the uplift of the Marie Byrd Land dome. International Ocean Discovery Program (IODP) Expedition 379 completed two very successful drill sites on the continental rise of the Amundsen Sea. Site U1532 is located on a large sediment drift, now called the Resolution Drift, and it penetrated to 794 m with 90% recovery. We collected almost-continuous cores from recent age through the Pleistocene and Pliocene and into the upper Miocene. At Site U1533, we drilled 383 m (70% recovery) into the more condensed sequence at the lower flank of the same sediment drift. The cores of both sites contain unique records that will enable study of the cyclicity of ice sheet advance and retreat processes as well as ocean-bottom water circulation and water mass changes. In particular, Site U1532 revealed a sequence of Pliocene sediments with an excellent paleomagnetic record for high-resolution climate change studies of the previously sparsely sampled Pacific sector of the West Antarctic margin. Despite the drilling success at these sites, the overall expedition experienced three unexpected difficulties that affected many of the scientific objectives: 1. The extensive sea ice on the continental shelf prevented us from drilling any of the proposed shelf sites. 2. The drill sites on the continental rise were in the path of numerous icebergs of various sizes that frequently forced us to pause drilling or leave the hole entirely as they approached the ship. The overall downtime caused by approaching icebergs was 50% of our time spent on site. 3. A medical evacuation cut the expedition short by 1 week. Recovery of core on the continental rise at Sites U1532 and U1533 cannot be used to indicate the extent of grounded ice on the shelf or, thus, of its retreat directly. However, the sediments contained in these cores offer a range of clues about past WAIS extent and retreat. At Sites U1532 and U1533, coarse-grained sediments interpreted to be ice-rafted debris (IRD) were identified throughout all recovered time periods. A dominant feature of the cores is recorded by lithofacies cyclicity, which is interpreted to represent relatively warmer periods variably characterized by sediments with higher microfossil abundance, greater bioturbation, and higher IRD concentrations alternating with colder periods characterized by dominantly gray laminated terrigenous muds. Initial comparison of these cycles to published late Quaternary records from the region suggests that the units interpreted to be records of warmer time intervals in the core tie to global interglacial periods and the units interpreted to be deposits of colder periods tie to global glacial periods. Cores from the two drill sites recovered sediments of dominantly terrigenous origin intercalated or mixed with pelagic or hemipelagic deposits. In particular, Site U1533, which is located near a deep-sea channel originating from the continental slope, contains graded silts, sands, and gravels transported downslope from the shelf to the rise. The channel is likely the pathway of these sediments transported by turbidity currents and other gravitational downslope processes. The association of lithologic facies at both sites predominantly reflects the interplay of downslope and contouritic sediment supply with occasional input of more pelagic sediment. Despite the lack of cores from the shelf, our records from the continental rise reveal the timing of glacial advances across the shelf and thus the existence of a continent-wide ice sheet in West Antarctica during longer time periods since at least the late Miocene. Cores from both sites contain abundant coarse-grained sediments and clasts of plutonic origin transported either by downslope processes or by ice rafting. If detailed provenance studies confirm our preliminary assessment that the origin of these samples is from the plutonic bedrock of Marie Byrd Land, their thermochronological record will potentially reveal timing and rates of denudation and erosion linked to crustal uplift. The chronostratigraphy of both sites enables the generation of a seismic sequence stratigraphy for the entire Amundsen Sea continental rise, spanning the area offshore from the Amundsen Sea Embayment westward along the Marie Byrd Land margin to the easternmost Ross Sea through a connecting network of seismic lines. 

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5. Isotopes to ice: Constraining provenance of glacial deposits and ice centers in west-central Gondwana

   Griffis, N.P. ; Montañez, I.P. ; Fedorchuk, N. ; Isbell, J. ; Mundil, R. ; Vesely, F. ; Weinshultz, L. ; Iannuzzi, R. ; Gulbranson, E. ; Taboada, A. ; et al ( April 2018 , Palaeogeography, palaeoclimatology, palaeoecology)

   The timing and geographic distribution of glaciers in high-latitude southern Gondwana during the Late Paleozoic Ice Age remain poorly constrained, ultimately precluding our ability to estimate ice volume and associated climate teleconnections and feedbacks during Earth's penultimate icehouse. Current aerial extents of glaciers, constrained by sedimentary flow directions, near exclusively infer paleo-glaciation to be highland-driven and may underestimate potential ice sources in continental regions from which ice sheets may have emanated. Here, we report new U-Pb ages and Hf isotope compositions of detrital zircons recovered from diamictites in two key mid- to high-latitude Gondwanan basins (Paraná, Brazil and Tepuel, Argentine Patagonia). The results indicate regional sediment sources for both basins during the early period of late Paleozoic glaciation evolving into more distal sources during the final deglaciation along southern and western Gondwana. Similar age sediment sourced from diamictites in the Congo Basin, that require an ice center in eastern Africa suggest the possibility of a large ice sheet in this area of Africa proximal to the Carboniferous-Permian boundary, which may have sourced sediments to the Paraná Basin. An inferred distal southern source of glacial sediment for the Tepuel Basin argues for the presence of an ice sheet(s) in the Ellsworth Block of Antarctica towards the end of the glaciation history in Patagonia. These findings indicate an evolution during the Late Paleozoic Ice Age from proximally to extrabasinally sourced sediment reflecting continental-scale glaciation and subsequent drainage from the Windhoek Highlands, Ellsworth Block and an east African source in west-central Gondwana. Coincidence with a long-term fall in atmospheric pCO2 during the Pennsylvanian to a minimum across the Carboniferous-Permian boundary and a subsequent rise in the early Permian suggests a primary CO2-driver for deglaciation in the Paraná Basin. Additional boundary conditions including availability of moisture and paleogeography likely further contributed to the timing of nucleation, growth and demise of these Gondwanan glaciers. 

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