Shasta M. Marrero

Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present.

Mid-Holocene pulse of thinning in the Weddell Sea sector of the West Antarctic ice sheet

Establishing the trajectory of thinning of the West Antarctic ice sheet (WAIS) since the last glacial maximum (LGM) is important for addressing questions concerning ice sheet (in)stability and changes in global sea level. Here we present detailed geomorphological and cosmogenic nuclide data from the southern Ellsworth Mountains in the heart of the Weddell Sea embayment that suggest the ice sheet, nourished by increased snowfall until the early Holocene, was close to its LGM thickness at 10 ka. A pulse of rapid thinning caused the ice elevation to fall ∼400 m to the present level at 6.5–3.5 ka, and could have contributed 1.4–2 m to global sea-level rise. These results imply that the Weddell Sea sector of the WAIS contributed little to late-glacial pulses in sea-level rise but was involved in mid-Holocene rises. The stepped decline is argued to reflect marine downdraw triggered by grounding line retreat into Hercules Inlet.

Relationship of channel steepness to channel incision rate from a tilted and progressively exposed unconformity surface

We examine the relationship of channel steepness to incision rate from channels eroding into a previously tilted, planar, and progressively exhumed unconformity surface. Channel and unconformity slopes are measured from a suite of channels developed on erosionally resistant Paleozoic limestone exhumed by the removal of Cenozoic sediments from the Baybeiche Range bordering the Naryn basin in the western Tian Shan. The compiled data set, sampling 5 orders of magnitude of upstream drainage area (0.03 to 227 km2), is used to derive the exponent, n, relating channel steepness to channel incision rate and the ratio, K/V, of the rate constant for channel incision of the resistant substrate, K, to the erosion rate, V, of the cover strata. We show that for a typical value of intrinsic concavity (slope-area exponent, θ=0.5), erosion rates that are proportional to specific stream power (n=1) satisfy the data set. However, valley width data suggest that the intrinsic concavity is higher (θ=0.8) and that incision data can also be fit if erosion is proportional to basal shear stress (n=2/3). Our results do not support values of n significantly greater than one. Using36Cl exposure age dating of the unconformity surface, we independently demonstrate that the Cenozoic cover strata have been progressively stripped downward from the unconformity surface at a vertical rate of 1 to 2 m/kyr. Using V=1 m/kyr, we constrain the rate constant, K, to between 6 ± 1 and 9 ± 2 ×10 -4kyr-1 for incision of resistant limestone bedrock in this field setting. Key Points Channel incision relation to steepness calibrated from tilted bedrock surface Chlorine-36 exposure age-dating shows bedrock surface exhumed at 1 to 2 m/kyr Low n values, consistent with shear stress (n=2/3) satisfy field data ©2014. American Geophysical Union. All Rights Reserved.

Assessing the continuity of the blue ice climate record at Patriot Hills, Horseshoe Valley, West Antarctica

We use high resolution Ground Penetrating Radar (GPR) to assess the continuity of the Blue Ice Area (BIA) horizontal climate record at Patriot Hills, Horseshoe Valley, West Antarctica. The sequence contains three pronounced changes in deuterium isotopic values at ~18 cal ka, ~12 cal ka and ~8 cal ka. GPR surveys along the climate sequence reveal continuous, conformable dipping isochrones, separated by two unconformities in the isochrone layers, which correlate with the two older deuterium shifts. We interpret these incursions as discontinuities in the sequence, rather than direct measures of climate change. Ice-sheet models and Internal Layer Continuity Index plots suggest that the unconformities represent periods of erosion occurring as the former ice surface was scoured by katabatic winds in front of mountains at the head of Horseshoe Valley. This study demonstrates the importance of high resolution GPR surveys for investigating both paleo-flow dynamics and interpreting BIA climate records.

Evaluating the size and extent of paleolakes in central Tibet during the late Pleistocene

Subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleo-lake in central Tibet, the East Qiangtang Lake (EQL), during the last interglacial period (Marine Isotope Stage [MIS] 5e). Reconstructions based on relict lake deposits suggest that the EQL would have been ~400 m deep and over ~66,000 km2. Models of flexural rebound driven by lake recession predict that shorelines near the EQL center, at the present-day location of Siling Co, would have rebounded 60-90 m above their initial elevation. New 36Cl chronology of the highest relict shorelines around Siling Co indicates that they reflect lake levels between 110-190 ka. These shorelines, however, are presently >300 m below their predicted elevations, implying a substantially smaller water load. Our results reveal that the expansion of Tibetan lakes during MIS 5e was relatively limited. Instead, individual lakes were supplied by river networks, much as they are today.

Corrigendum to “The million-year evolution of the glacial trimline in the southernmost Ellsworth Mountains, Antarctica” [Earth and Planetary Science Letters 469 (2017) 42–52]

This corrigendum fixes an error in the reporting of 21Ne concentrations, which affected one batch of samples that included the bedrock depth profile from which cosmogenic 10Be, 26Al and 21Ne were modelled to constrain the age and exposure history of the Patriot Hills (Fig. 8 in the manuscript). Re-modelling the cosmogenic nuclide data using the corrected 21Ne data yields an apparent exposure age of 3.5–5.1 Ma. This corrects an age published as 2.1–2.6 Ma in Sugden et al. (2017), and reinforces the conclusion of the original paper that the glacial trimline is pre-Quaternary and that the climatic conditions necessary for its erosion last occurred in the Mid-Miocene. The revised Supplementary Table 1 has been updated with corrected 21Ne concentrations and consistent reporting of 10Be concentrations. The revised Supplementary Table 2 has been updated with 21Ne exposure ages for the affected batch of samples. Below, we describe the revised model results and present a revised Fig. 8. Tables 1 and 2, Fig. 8 and its caption replace those in the original paper. The corrections reinforce the conclusions of the original paper.