Reconciling the Apparent Absence of a Last Glacial Maximum Alpine Glacial Advance, Yukon Territory, Canada, through Cosmogenic Beryllium-10 and Carbon-14 Measurements

We present a new in situ produced cosmogenic beryllium-10 and carbon-14 nuclide chronology from two sets (outer and inner) of alpine glacier moraines from the Grey Hunter massif of southern Yukon Territory, Canada. The chronology potential of moraines deposited by alpine glaciers outside the limits of the Last Glacial Maximum (LGM) ice sheets potentially provide a less-ambiguous archive of mass balance, and hence climate than can be inferred from the extents of ice sheets themselves. Results for both nuclides are inconclusive for the outer moraines, with evidence for pre-LGM deposition (beryllium-10) and Holocene deposition (carbon-14). Beryllium-10 results from the inner moraine are suggestive of canonical LGM deposition, but with relatively high scatter. Conversely, in situ carbon-14 results from the inner moraines are tightly clustered and suggestive of terminal Younger Dryas deposition. We explore plausible scenarios leading to the observed differences between nuclides and find that the most parsimonious explanation for the outer moraines is that of pre-LGM deposition, but many of the sampled boulder surfaces were not exhumed from within the moraine until the Holocene. Our results thus imply that the inner and outer moraines sampled pre- and post-date the canonical LGM and that moraines dating to the LGM are lacking likely due to overriding by the subsequent Late Glacial/earliest Holocene advance.

Late Cenozoic (13-0 Myr) Glacimarine Sedimentology, Facies Analysis, and Sequence Stratigraphy from the Western Ross Embayment, Antarctica: Implications for the Variability of the Antarctic Ice Sheets

<p>Sedimentary processes related to oscillations of the marine-based sector of Antarctic Ice Sheet (AIS) in the Ross Embayment over the past 13 Myr are examined at various timescales from stratigraphic records of glacial advance and retreat obtained from the McMurdo Sound region. An initial sedimentary model was developed from short (<2 m) sediment cores collected from beneath the present-day McMurdo Ice Shelf and seasonally open water in the Ross Island region. These cores document sedimentary processes associated with subglacial, ice shelf and open marine environments since the Last Glacial Maximum (LGM) in the Ross Sea Embayment. A radiocarbon chronology from these short cores implies that lift-off of grounded ice in the 900 m-deep marine basins surrounding Ross Island occurred by ~10,100 14C yr BP. Following lift-off, the ice shelf calving line retreated toward its present position. By ~8,900 14C yr BP, seasonally open marine conditions extended as far south as Ross Island. Glacial retreat was rapid and preceded the timing of Meltwater Pulse 1B. Since 8,900 14C yr BP, the calving line of the Ross Ice Shelf has remained pinned to Ross Island despite warmer-than-present temperatures during the mid-Holocene. Depositional models developed for the LGM to recent sediments were then applied to the interpretation of the 1284-m-long ANDRILL McMurdo Ice Shelf core (AND-1B) to documenting oscillations of the AIS in the Ross Embayment over the past 13 Myr. A sequence stratigraphic framework for grounding-line fluctuations of under a variety of glacial regimes, with three distinct types of glacimarine cycle (sequence motif) identified. Motif 1 (Pleistocene and Mid to early Late Miocene) is dominated by thick sub-glacial diamictite, deposited during glacial advance, with occasional thin interbeds of sparsely- to non-fossiliferous mudstone that marks an ice shelf setting during interglacial maxima. Motif 2 (Pliocene) comprises subglacial to glacimarine diamictite overlain by thin, proglacial deposits and capped with substantial beds of diatom-bearing mudstone or diatomite formed under open-marine conditions. Motif 3 (Late Miocene) extends from subglacial diamictite into a thick proglacial succession that includes a combination of stratified diamictite, graded sandstone, conglomerate, and rhythmically-stratified mudstone. The differences in these facies successions (motifs) are associated with the long-term evolution of the AIS in the Ross Embayment from a cold glacial regime with limited volumes of subglacial meltwater (Motif 1) to warmer styles (Motifs 2 and 3) of glaciation with increased subglacial meltwater discharge, before passing back to the cold style of glaciation that characterises the present-day AIS (i.e., limited subglacial meltwater). Each motif was interpreted on the basis of modern analogues of glacimarine sedimentation from a range of climatic/glacial settings, recording a fundamental change in the mass balance for the AIS in the Ross Embayment. For cold glacial regimes similar to the present day Antarctic Ice Sheets, ablation was largely controlled by calving at the marine margin and the melting of the underside of ice shelves by oceanic processes. For warmer regimes, in particular for Motif 3, ablation by melting was a significant influence on mass balance. This sedimentary model was then applied in detail to interpret the Pleistocene section of AND-1B (upper 150 m) with a chronostratigraphic interpretation constrained by sequence stratigraphy, 40Ar/39Ar dating of volcanic ashes, and magneto-stratigraphy. The glacimarine sequences in AND-1B drill core correlate one-to-one with cycles in the benthic delta 18 O record for the past ~0.8 Myr (Marine Isotope Stages 20-2), and are interpreted as recording fluctuations of the AIS in the Ross Embayment with a 100-kyr cyclicity. In this "100-kyr world", the AIS is relatively stable, with subglacial to grounding-zone sedimentation dominating at the AND-1B drill site, with only thin intervals of ice-shelf sedimentation during interglacials and little evidence for open-marine conditions during the Late Pleistocene "super-interglacials". An unconformity spans (~200 kyr) most of the Mid-Pleistocene Transition and is inferred to represent large scale expansion of AIS at ~0.8 Myr. Prior to this, Early Pleistocene glacial/interglacial cycles had a 40-kyr frequency, with interglacial periods characterised by open water deposits that contain volcanoclastic debris and diatomaceous sediments. This upper 150 m of AND-1B provides clear evidence for both a change in the frequency (40- to 100-kyr cycles), and a reduction in the sensitivity of a cooler marine-based AIS in the Ross Embayment.</p>

Late Cenozoic (13-0 Myr) Glacimarine Sedimentology, Facies Analysis, and Sequence Stratigraphy from the Western Ross Embayment, Antarctica: Implications for the Variability of the Antarctic Ice Sheets

<p>Sedimentary processes related to oscillations of the marine-based sector of Antarctic Ice Sheet (AIS) in the Ross Embayment over the past 13 Myr are examined at various timescales from stratigraphic records of glacial advance and retreat obtained from the McMurdo Sound region. An initial sedimentary model was developed from short (<2 m) sediment cores collected from beneath the present-day McMurdo Ice Shelf and seasonally open water in the Ross Island region. These cores document sedimentary processes associated with subglacial, ice shelf and open marine environments since the Last Glacial Maximum (LGM) in the Ross Sea Embayment. A radiocarbon chronology from these short cores implies that lift-off of grounded ice in the 900 m-deep marine basins surrounding Ross Island occurred by ~10,100 14C yr BP. Following lift-off, the ice shelf calving line retreated toward its present position. By ~8,900 14C yr BP, seasonally open marine conditions extended as far south as Ross Island. Glacial retreat was rapid and preceded the timing of Meltwater Pulse 1B. Since 8,900 14C yr BP, the calving line of the Ross Ice Shelf has remained pinned to Ross Island despite warmer-than-present temperatures during the mid-Holocene. Depositional models developed for the LGM to recent sediments were then applied to the interpretation of the 1284-m-long ANDRILL McMurdo Ice Shelf core (AND-1B) to documenting oscillations of the AIS in the Ross Embayment over the past 13 Myr. A sequence stratigraphic framework for grounding-line fluctuations of under a variety of glacial regimes, with three distinct types of glacimarine cycle (sequence motif) identified. Motif 1 (Pleistocene and Mid to early Late Miocene) is dominated by thick sub-glacial diamictite, deposited during glacial advance, with occasional thin interbeds of sparsely- to non-fossiliferous mudstone that marks an ice shelf setting during interglacial maxima. Motif 2 (Pliocene) comprises subglacial to glacimarine diamictite overlain by thin, proglacial deposits and capped with substantial beds of diatom-bearing mudstone or diatomite formed under open-marine conditions. Motif 3 (Late Miocene) extends from subglacial diamictite into a thick proglacial succession that includes a combination of stratified diamictite, graded sandstone, conglomerate, and rhythmically-stratified mudstone. The differences in these facies successions (motifs) are associated with the long-term evolution of the AIS in the Ross Embayment from a cold glacial regime with limited volumes of subglacial meltwater (Motif 1) to warmer styles (Motifs 2 and 3) of glaciation with increased subglacial meltwater discharge, before passing back to the cold style of glaciation that characterises the present-day AIS (i.e., limited subglacial meltwater). Each motif was interpreted on the basis of modern analogues of glacimarine sedimentation from a range of climatic/glacial settings, recording a fundamental change in the mass balance for the AIS in the Ross Embayment. For cold glacial regimes similar to the present day Antarctic Ice Sheets, ablation was largely controlled by calving at the marine margin and the melting of the underside of ice shelves by oceanic processes. For warmer regimes, in particular for Motif 3, ablation by melting was a significant influence on mass balance. This sedimentary model was then applied in detail to interpret the Pleistocene section of AND-1B (upper 150 m) with a chronostratigraphic interpretation constrained by sequence stratigraphy, 40Ar/39Ar dating of volcanic ashes, and magneto-stratigraphy. The glacimarine sequences in AND-1B drill core correlate one-to-one with cycles in the benthic delta 18 O record for the past ~0.8 Myr (Marine Isotope Stages 20-2), and are interpreted as recording fluctuations of the AIS in the Ross Embayment with a 100-kyr cyclicity. In this "100-kyr world", the AIS is relatively stable, with subglacial to grounding-zone sedimentation dominating at the AND-1B drill site, with only thin intervals of ice-shelf sedimentation during interglacials and little evidence for open-marine conditions during the Late Pleistocene "super-interglacials". An unconformity spans (~200 kyr) most of the Mid-Pleistocene Transition and is inferred to represent large scale expansion of AIS at ~0.8 Myr. Prior to this, Early Pleistocene glacial/interglacial cycles had a 40-kyr frequency, with interglacial periods characterised by open water deposits that contain volcanoclastic debris and diatomaceous sediments. This upper 150 m of AND-1B provides clear evidence for both a change in the frequency (40- to 100-kyr cycles), and a reduction in the sensitivity of a cooler marine-based AIS in the Ross Embayment.</p>

Maximum Glacier Extent of the Penultimate Glacial Cycle in the Upper Garonne Basin (Pyrenees): New Chronological Evidence

The Upper Garonne Basin included the longest glacier in the Pyrenees during the Late Pleistocene. During major glacial advances, the Garonne palaeoglacier flowed northwards along ~ 90 km from peaks of the axial Pyrenees exceeding 2,800-3,000 m until the foreland of this mountain range at the Loures-Barouse-Barbazan basin, at only 420–440 m. Here, the palaeoglacier formed a terminal moraine complex that is examined in this work. Based on geomorphological observations and a 12-sample dataset of 10Be Cosmic-Ray Exposure (CRE) ages, we have constrained the timing of the maximum glacial extent as well as the onset of the deglaciation from the end of the Last Glacial Cycle (LGC). Chronological data shows evidence that the external moraines in this basin were abandoned by the ice at the end of the Penultimate Glacial Cycle (PGC) and the onset of the Eemian Interglacial, at ~ 129 ka. No evidence of subsequent glacial advances or standstills occurred during the LGC in this basin were found, as the few existing datable boulders provided in the internal moraine showed inconsistent ages, thus probably being affected by post-glacial processes. The terminal basin was already deglaciated during the global Last Glacial Maximum at 24 − 21 ka, as revealed by exposure ages of polished surfaces at the confluence of the Garonne-La Pique valleys, 13 km south of the entrance of the Loures-Barousse-Barbazan basin. This study introduces the first CRE ages in the Pyrenees for the glacial advance occurred during the PGC and provides also new evidence that glaciers had already significantly shrunk during the LGM.

Cryogenian glaciostatic and eustatic fluctuations and massive Marinoan-related deposition of Fe and Mn in the Urucum District, Brazil

Global Neoproterozoic glaciations are related to extreme environmental changes and the reprise of iron formation in the rock record. However, the lack of narrow age constraints on Cryogenian successions bearing iron-formation deposits prevents correlation and understanding of these deposits on a global scale. Our new multiproxy data reveal a long Cryogenian record for the Jacadigo Group (Urucum District, Brazil) spanning the Sturtian and Marinoan ice ages. Deposition of the basal sequence of the Urucum Formation was influenced by Sturtian continental glaciation and was followed by a transgressive interglacial record of &gt;600 m of carbonates that terminates in a glacioeustatic unconformity. Overlying this, there are up to 500 m of shale and sandstone interpreted as coeval to global Marinoan glacial advance. Glacial outwash delta deposits at the top of the formation correlate with diamictite-filled paleovalleys and are covered by massive Fe and Mn deposits of the Santa Cruz Formation and local carbonate. This second transgression is related to Marinoan deglaciation. Detrital zircon provenance supports glaciostatic control on Cryogenian sedimentary yield at the margins of the Amazon craton. These findings reveal the sedimentary response to two marked events of glacioeustatic incision and transgression, culminating in massive banded iron deposition during the Marinoan cryochron.

Marine record of late-glacial readvance and last recession of Laurentide ice, inner Frobisher Bay, Baffin Island, Arctic Canada

The Cockburn Substage readvance marks the last major late-glacial advance of the northeast sector of the Laurentide Ice Sheet on Baffin Island. The causes of this abrupt, late reversal of retreat are still unclear, but greater chronological control may provide some insight. To date, the literature has focused on the large terminal moraines in the region, providing a date of readvance (c. 9.5-8.5 ka cal BP). In Frobisher Bay, the Cockburn Substage readvance and recession onshore are marked by a series of moraines spread over ~20 km along the inner bay. Acoustic marine mapping reveals five distinct transverse ridges, morphologically suggestive of grounding-zone wedges, and two later fields of DeGeer moraines on the floor of the inner bay. These indicate that the style of ice retreat (beginning no later than 8.5 ka cal BP) changed over time from punctuated recession of a floating ice-front (20 km over >680 years, with four pauses) to more regular tidewater ice-front retreat, reaching the head of the bay 900 years or more after withdrawal from the outer Cockburn limit. The established chronology for final recession in the region is based largely on radiocarbon dating of bulk shell samples and single shells of deposit-feeding molluscs, notably <i>Portlandia arctica</i>, affected by old carbon from carbonate-rich sediments. Sedimentary analysis and judicious sampling for ¹⁴C dating of glaciomarine and marine facies in seabed sediment cores enables development of a late- and postglacial lithostratigraphy that indicates final withdrawal of ice from the drainage basin by 7 ka cal BP.

Holocene glacier history of northeastern Cordillera Darwin, southernmost South America (55°S)

In the Cordillera Darwin, southernmost South America, we used 10Be and 14C dating, dendrochronology, and historical observations to reconstruct the glacial history of the Dalla Vedova valley from deglacial time to the present. After deglacial recession into northeastern Darwin and Dalla Vedova, by ~16 ka, evidence indicates a glacial advance at ~13 ka coeval with the Antarctic Cold Reversal. The next robustly dated glacial expansion occurred at 870 ± 60 calendar yr ago (approximately AD 1150), followed by less-extensive dendrochronologically constrained advances from shortly before AD 1836 to the mid-twentieth century. Our record is consistent with most studies within the Cordillera Darwin that show that the Holocene glacial maximum occurred during the last millennium. This pattern contrasts with the extensive early- and mid-Holocene glacier expansions farther north in Patagonia; furthermore, an advance at 870 ± 60 yr ago may suggest out-of-phase glacial advances occurred within the Cordillera Darwin relative to Patagonia. We speculate that a southward shift of westerlies and associated climate regimes toward the southernmost tip of the continent, about 900–800 yr ago, provides a mechanism by which some glaciers advanced in the Cordillera Darwin during what is generally considered a warm and dry period to the north in Patagonia.