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1. 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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2. 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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3. 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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4. A Carboniferous apex for the late Paleozoic icehouse

   https://doi.org/10.1144/sp535-2022-256  

   Griffis, N.; Mundil, R.; Montañez, I.; Le Heron, D.; Dietrich, P.; Iannuzzi, R. (October 2022, Geological Society, London, Special Publications)

   Abstract Icehouse climate systems occur across an abbreviated portion of Earth history, constitutingc.25% of the Phanerozoic record. The Late Paleozoic Ice Age (LPIA) was the most extreme and longest lasting glaciation of the Phanerozoic and is characterized by periods of acute continental-scale glaciation, separated by periods of ice minima or ice-free conditions on the order of <10⁶years. The late Paleozoic glaciogenic record of the Paraná and Kalahari basins of southern Gondwana form one of the largest, best-preserved and well-calibrated records of this glaciation. In the Carboniferous, the eastern and southern margins of the Paraná Basin and the Kalahari Basin were characterized by subglacial conditions, with evidence for continental and upland glaciers. In the latest Carboniferous, these basins transitioned from subglacial reservoirs to ice-free or ‘ice distal‘ conditions evidenced by the widespread deposition of marine deposits juxtaposed on subglacial bedforms. High-precision U–Pb zircon chemical abrasion thermal ionization mass spectrometry geochronological constraints from volcanic ash deposits in the deglacial marine black shales of the Kalahari Basin and from fluvial and coal successions, which overlie marine deposits in the Paraná Basin, indicate subglacial evidence in these regions is constrained to the Carboniferous. The loss of ice in these regions is congruent with a late Carboniferous peak inpCO₂and widespread marine anoxia in the late Carboniferous. The permeant retreat of glaciers in basinal settings, despite an early PermianpCO₂nadir, highlights the influence of short-term perturbations on the longer-term CO₂record and suggests an ice threshold had been crossed in the latest Carboniferous. A definitive driver for greenhouse gases in the LPIA, such as abundant and sustained volcanic activity or an increased biological pump driven by ocean fertilization, is unresolved for this period. Lastly, the proposed Carboniferous apex for the high-latitude LPIA record is incongruent with observations from the low-latitude tropics where an early Permian peak is proposed. 

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5. Holocene thinning of Darwin and Hatherton glaciers, Antarctica, and implications for grounding-line retreat in the Ross Sea

   https://doi.org/10.5194/tc-15-3329-2021  

   Hillebrand, Trevor R.; Stone, John O.; Koutnik, Michelle; King, Courtney; Conway, Howard; Hall, Brenda; Nichols, Keir; Goehring, Brent; Gillespie, Mette K. (January 2021, The Cryosphere)

   Abstract. Chronologies of glacier deposits in the Transantarctic Mountains provide important constraints on grounding-line retreat during the last deglaciation in the Ross Sea. However, between Beardmore Glacier and Ross Island – a distance of some 600 km – the existing chronologies are generally sparse and far from the modern grounding line, leaving the past dynamics of this vast region largely unconstrained. We present exposure ages of glacial deposits at three locations alongside the Darwin–Hatherton Glacier System – including within 10 km of the modern grounding line – that record several hundred meters of Late Pleistocene to Early Holocene thickening relative to present. As the ice sheet grounding line in the Ross Sea retreated, Hatherton Glacier thinned steadily from about 9 until about 3 ka. Our data are equivocal about the maximum thickness and Mid-Holocene to Early Holocene history at the mouth of Darwin Glacier, allowing for two conflicting deglaciation scenarios: (1) ∼500 m of thinning from 9 to 3 ka, similar to Hatherton Glacier, or (2) ∼950 m of thinning, with a rapid pulse of ∼600 m thinning at around 5 ka. We test these two scenarios using a 1.5-dimensional flowband model, forced by ice thickness changes at the mouth of Darwin Glacier and evaluated by fit to the chronology of deposits at Hatherton Glacier. The constraints from Hatherton Glacier are consistent with the interpretation that the mouth of Darwin Glacier thinned steadily by ∼500 m from 9 to 3 ka. Rapid pulses of thinning at the mouth of Darwin Glacier are ruled out by the data at Hatherton Glacier. This contrasts with some of the available records from the mouths of other outlet glaciers in the Transantarctic Mountains, many of which thinned by hundreds of meters over roughly a 1000-year period in the Early Holocene. The deglaciation histories of Darwin and Hatherton glaciers are best matched by a steady decrease in catchment area through the Holocene, suggesting that Byrd and/or Mulock glaciers may have captured roughly half of the catchment area of Darwin and Hatherton glaciers during the last deglaciation. An ensemble of three-dimensional ice sheet model simulations suggest that Darwin and Hatherton glaciers are strongly buttressed by convergent flow with ice from neighboring Byrd and Mulock glaciers, and by lateral drag past Minna Bluff, which could have led to a pattern of retreat distinct from other glaciers throughout the Transantarctic Mountains. 

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