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1. 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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2. Coupled stratigraphic and U-Pb zircon age constraints on the late Paleozoic icehouse-to-greenhouse turnover in south-central Gondwana

   https://doi.org/10.1130/G46740.1  

   Griffis, Neil Patrick; Montañez, Isabel Patricia; Mundil, Roland; Richey, Jon; Isbell, John; Fedorchuk, Nick; Linol, Bastien; Iannuzzi, Roberto; Vesely, Fernando; Mottin, Thammy; et al (October 2019, Geology)

   The demise of the Late Paleozoic Ice Age has been hypothesized as diachronous, occurring first in western South America and progressing eastward across Africa and culminating in Australia over an ~60 m.y. period, suggesting tectonic forcing mechanisms that operate on time scales of 106 yr or longer. We test this diachronous deglaciation hypothesis for southwestern and south-central Gondwana with new single crystal U-Pb zircon chemical abrasion thermal ionizing mass spectrometry (CA-TIMS) ages from volcaniclastic deposits in the Paraná (Brazil) and Karoo (South Africa) Basins that span the terminal deglaciation through the early postglacial period. Intrabasinal stratigraphic correlations permitted by the new high-resolution radioisotope ages indicate that deglaciation across the south to southeast Paraná Basin was synchronous, with glaciation constrained to the Carboniferous. Cross-basin correlation reveals two additional glacial-deglacial cycles in the Karoo Basin after the terminal deglaciation in the Paraná Basin. South African glaciations were penecontemporaneous (within U-Pb age uncertainties) with third-order sequence boundaries (i.e., inferred base-level falls) in the Paraná Basin. Synchroneity between early Permian glacial-deglacial events in southwestern to south-central Gondwana and pCO2 fluctuations suggest a primary CO2 control on ice thresholds. The occurrence of renewed glaciation in the Karoo Basin, after terminal deglaciation in the Paraná Basin, reflects the secondary influences of regional paleogeography, topography, and moisture sources. 

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3. 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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4. A lithofacies analysis of a South Polar glaciation in the Early Permian: Pagoda Formation, Shackleton Glacier region, Antarctica

   https://doi.org/10.2110/jsr.2021.004  

   Ives, Libby R.W.; Isbell, John L. (June 2021, Journal of Sedimentary Research)

   ABSTRACT The currently favored hypothesis for Late Paleozoic Ice Age glaciations is that multiple ice centers were distributed across Gondwana and that these ice centers grew and shank asynchronously. Recent work has suggested that the Transantarctic Basin has glaciogenic deposits and erosional features from two different ice centers, one centered on the Antarctic Craton and another located over Marie Byrd Land. To work towards an understanding of LPIA glaciation that can be tied to global trends, these successions must be understood on a local level before they can be correlated to basinal, regional, or global patterns. This study evaluates the sedimentology, stratigraphy, and flow directions of the glaciogenic, Asselian–Sakmarian (Early Permian) Pagoda Formation from four localities in the Shackleton Glacier region of the Transantarctic Basin to characterize Late Paleozoic Ice Age glaciation in a South Polar, basin-marginal setting. These analyses show that the massive, sandy, clast-poor diamictites of the Pagoda Fm were deposited in a basin-marginal subaqueous setting through a variety of glaciogenic and glacially influenced mechanisms in a depositional environment with depths below normal wave base. Current-transported sands and stratified diamictites that occur at the top of the Pagoda Fm were deposited as part of grounding-line fan systems. Up to at least 100 m of topographic relief on the erosional surface underlying the Pagoda Fm strongly influenced the thickness and transport directions in the Pagoda Fm. Uniform subglacial striae orientations across 100 m of paleotopographic relief suggest that the glacier was significantly thick to “overtop” the paleotopography in the Shackleton Glacier region. This pattern suggests that the glacier was likely not alpine, but rather an ice cap or ice sheet. The greater part of the Pagoda Fm in the Shackleton Glacier region was deposited during a single retreat phase. This retreat phase is represented by a single glacial depositional sequence that is characteristic of a glacier with a temperate or mild subpolar thermal regime and significant meltwater discharge. The position of the glacier margin likely experienced minor fluctuations (readvances) during this retreat. Though the sediment in the Shackleton Glacier region was deposited during a single glacier retreat phase, evidence from this study does not preclude earlier or later glacier advance–retreat cycles preserved elsewhere in the basin. Ice flow directions indicate that the glacier responsible for this sedimentation was likely flowing off of an upland on the side of the Transantarctic Basin closer to the Panthalassan–Gondwanide margin (Marie Byrd Land), which supports the hypothesis that two different ice centers contributed glaciogenic sediments to the Transantarctic Basin. Together, these observations and interpretations provide a detailed local description of Asselian–Sakmarian glaciation in a South Polar setting that can be used to understand larger-scale patterns of regional and global climate change during the Late Paleozoic Ice Age. 

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5. A cosmogenic 10Be moraine chronology of arid, alpine Late Pleistocene glaciation in the Pioneer Mountains of Montana, USA

   https://doi.org/10.1016/j.quascirev.2023.108283  

   Schoenemann, Spruce W.; Bryant, Mana M.; Larson, Will B.; Corbett, Lee B.; Bierman, Paul R. (October 2023, Quaternary Science Reviews)

   We test the hypothesis that glacier systems, located in continental regions proximal to the Laurentide Ice Sheet (LIS), had local ice maxima considerably earlier than the LIS maximum and thus before the insolation minima at ~21 ka. Ranges located in the northwest US exhibit earlier deglaciation timing between ~23 and 22 ka, except for the Yellowstone region where younger time-transgressive ages complicate regional interpretations and the northern Montana ice cap where late glacial ages have recently been produced. Constraining the glacial history of more ice sheet-proximal alpine glaciers provides insight into whether the contrasting maximum-ice times in the northern Rocky Mountains were caused by regional climatic differences, such as anticyclonic wind patterns driven by the presence of the LIS. In the Pioneer Mountains of Montana, we measured in situ cosmogenic 10Be in 35 boulders on moraines marking the maximum Late Pleistocene positions of alpine glaciers from three valleys. The 10Be samples produced a range of ages, spanning pre Bull Lake to the last glaciation (i.e., Pinedale/Marine Isotope Stage (MIS) 2). We find an average exposure age for initial deglaciation of 18.2 ±0.9 during the local Last Glacial Maximum, indicative of synchronous retreat in the Pioneer Mountains. The similarity of initial deglaciation timing of the Pioneer Mountain glaciers with the northwestern Yellowstone glacial system and northern MT ice cap suggests that topography more proximal to the LIS margin maintained full ice extent longer. Our findings, in context of previous work, suggest that in the case of the Pioneer Mountains their more proximal location to the ice margin may have delayed onset of deglaciation by greater exposure to local cooling from katabatic winds and/or additional moisture sourced from large ice-marginal glacial lakes, hence the lack of earlier deglacial ages like those found further to the west and east of the northern Rocky Mountain cordillera. 

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