DETERMINING THE TIMING AND RATE OF SOUTHEASTERN LAURENTIDE ICE SHEET THINNING USING IN-SITU 10BE DIPSTICKS

In the northeastern United States, there are extensive geochronologic and geomorphic constraints on the deglaciation of the southeastern Laurentide Ice Sheet; thus, it is an ideal area for large-scale ice volume reconstructions and comparison between different ice retreat chronometers. Varve chronologies, lake and bog-bottom radiocarbon ages, and cosmogenic nuclide exposure ages constrain the timing of ice retreat, but the inferred ages exhibit considerable noise and sometimes disagree. Additionally, there are few empirical constraints on ice thinning, forcing ice volume reconstructions to rely on geophysically-based ice thickness models. Here, we aim to improve the understanding of the southeastern Laurentide Ice Sheet recession by (1) adding extensive ice thickness constraints and (2) compiling all available deglacial chronology data in the region to investigate discrepancies between different chronometers.To provide insight about ice sheet thinning history, we collected 120 samples for in-situ 10Be and 10 samples for in-situ 14C cosmogenic exposure dating from various elevations at 13 mountains in the northeastern United States. By calculating ages of exposure at different elevations across this region, we reconstruct paleo-ice surface lowering of the southeastern Laurentide Ice Sheet during deglaciation. Where we suspect that 10Be remains from pre-Last Glacial Maximum periods of exposure, in-situ 14C is used to infer the erosional history and minimum exposure age of samples.Presently, we have measured 10Be in 73 samples. Mountain-top exposure ages located within 150 km of the southeastern Laurentide Ice Sheet terminal moraine indicate that near-margin thinning began early in the deglacial period (~19.5 to 17.5 ka), coincident with the slow initial margin retreat indicated by varve records. Exposure ages from several mountains further inland (>400 km north of terminal moraine) collected over ~1000 m of elevation range record rapid ice thinning between 14.5 and 13 ka. Ages within each of these vertical transects are similar within 1&#963; internal uncertainty, indicating that ice thinned quickly, less than a few hundred years at most. This rapid thinning occurred at about the same time that varve records indicate accelerated ice margin retreat (14.6&#8211;12.9 ka), providing evidence of substantial ice volume loss during the B&#248;lling-Aller&#248;d warm period.Our critical evaluation of deglacial chronometers, including valley-bottom 10Be ages from this project, is intended to constrain ice margin retreat rates and timing in the region. Ultimately, we will integrate our ice thickness over time constraints with the existing network of deglacial ages to create a probabilistic reconstructions of the southeastern Laurentide Ice Sheet volume during its recession through the northeastern United States.

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Cosmogenic nuclide age estimate for Laurentide Ice Sheet recession from the terminal moraine, New Jersey, USA, and constraints on latest Pleistocene ice sheet history

Quaternary Research ◽

10.1017/qua.2017.11 ◽

2017 ◽

Vol 87 (3) ◽

pp. 482-498 ◽

Cited By ~ 7

Author(s):

Lee B. Corbett ◽

Paul R. Bierman ◽

Byron D. Stone ◽

Marc W. Caffee ◽

Patrick L. Larsen

Keyword(s):

New Jersey ◽

Global Climate ◽

Ice Sheet ◽

Laurentide Ice Sheet ◽

Data Set ◽

Terminal Moraine ◽

Cosmogenic Nuclide ◽

Fluvial Incision ◽

Exposure Ages

AbstractThe time at which the Laurentide Ice Sheet reached its maximum extent and subsequently retreated from its terminal moraine in New Jersey has been constrained by bracketing radiocarbon ages on preglacial and postglacial sediments. Here, we present measurements of in situ produced 10Be and 26Al in 16 quartz-bearing samples collected from bedrock outcrops and glacial erratics just north of the terminal moraine in north-central New Jersey; as such, our ages represent a minimum limit on the timing of ice recession from the moraine. The data set includes field and laboratory replicates, as well as replication of the entire data set five years after initial measurement. We find that recession of the Laurentide Ice Sheet from the terminal moraine in New Jersey began before 25.2±2.1 ka (10Be, n=16, average, 1 standard deviation). This cosmogenic nuclide exposure age is consistent with existing limiting radiocarbon ages in the study area and cosmogenic nuclide exposure ages from the terminal moraine on Martha’s Vineyard ~300 km to the northeast. The age we propose for Laurentide Ice Sheet retreat from the New Jersey terminal position is broadly consistent with regional and global climate records of the last glacial maximum termination and records of fluvial incision.

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The first integrated use of in situ U–Pb geochronology and geochemical analyses to determine long-distance transport of glacial erratics from mainland Canada into the western Arctic Archipelago

Canadian Journal of Earth Sciences ◽

10.1139/e09-002 ◽

2009 ◽

Vol 46 (2) ◽

pp. 101-122 ◽

Cited By ~ 5

Author(s):

Chris Doornbos ◽

Larry M. Heaman ◽

Jonathan P. Doupé ◽

John England ◽

Antonio Simonetti ◽

...

Keyword(s):

Inductively Coupled Plasma ◽

Ice Sheet ◽

Laurentide Ice Sheet ◽

Long Distance ◽

Ice Flow ◽

Long Distance Transport ◽

Geochemical Analyses ◽

Distance Transport ◽

East West

Glacial erratics collected on Melville Island, western Canadian Arctic Archipelago, were analyzed to determine their mainland provenance, thereby constraining their long-distance transport by the Laurentide Ice Sheet. These erratics can be broadly subdivided into three main lithologies: granite (n = 15), quartzite (n = 7), and diabase–diorite (n = 3). The granite erratics are most distinctive from a provenance perspective and can be further subdivided into three geochemical groups based on their potassium content: (1) a high-K2O group (K2O > 4.0 wt.%), (2) an intermediate-K2O group (K2O between 2.0 and 4.0 wt.%), and (3) a low-K2O group (K2O < 2.0 wt.%). In situ thin section laser ablation inductively coupled plasma mass spectrometer U–Pb zircon ages obtained for eight granite erratics yielded both Archean (2575 Ma) and a range of Paleoproterozoic (2472–1778 Ma) crystallization ages. In addition, three overprint ages were identified at 1.90, 1.84, and ∼1.0 Ga. The most compelling constraint for a northward regional ice flow originating on the mainland are two high-precision conventional U–Pb zircon ages of 1969.5 ± 1.0 and 2472.3 ± 0.5 Ma, indicating that these granite erratics must have been derived from the 2.0–1.9 Ga Taltson–Thelon Orogen and the nearby 2.5–2.4 Ga Queen Maud Block, respectively. These granite-dominated terranes are located 600 km due south and southeast of the collection area on Melville Island. Although it is unknown whether the final deposition of these erratics on Melville Island involved transport by one or more glaciations, it is apparent that this ice flow cannot be accommodated by the proposed north–south axis of the M’Clintock Ice Divide, the primary topographic feature of the northwestern Laurentide Ice Sheet during the last glacial maximum. The transport of erratics reported here would have required a former ice divide oriented east–west over the mainland, close to that proposed for the Ancestral Keewatin Divide. An east–west ice divide in this region is consistent with previously reported ice-flow indicators that document northward flow from the mainland and recent thermomechanically coupled ice-sheet numerical modeling that indicates former maximum ice thickness on the mainland immediately south of Melville Island.

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