Detrital Thermochronometry Reveals That the Topography Along the Antarctic Peninsula is Not a Pleistocene Landscape

Using offshore detrital apatite (U‐Th)/He thermochronometry and 3D thermo‐kinematic modeling of the catchment topography, we constrain the timing of major topographic change at Bourgeois Fjord, Antarctic Peninsula (AP). While many mid‐latitude glacial landscapes developed primarily in response to global cooling over the last ~2.6 Ma, we find that kilometer‐scale landscape evolution at Bourgeois Fjord began ~30–12 Ma ago and <2 km of valley incision has occurred since ~16 Ma. This early onset of major topographic change occurred following the initiation of alpine glaciation at this location and prior to the development of a regional polythermal ice sheet inferred from sedimentary evidence offshore of the AP. We hypothesize that topographic change relates to (i) feedbacks between an evolving topography and glacial erosion processes, (ii) effects of glacial‐interglacial variability, and (iii) the prevalence of subglacial meltwater. The timing and inferred spatial patterns of long‐term exhumation at Bourgeois Fjord are consistent with a hypothesis that glacial erosion processes were suppressed at the AP during global Plio‐Pleistocene cooling, rather than enhanced. Our study examines the long‐term consequences of glacial processes on catchment‐wide erosion as the local climate cooled. Our findings support the hypothesis that landscapes at different latitudes had different responses to global cooling. Our results also suggest that erosion is enhanced along the plateau flanks of Bourgeois Fjord today, which may be due to periglacial processes or mantling via subglacial till. If regional warming persists and meltwater becomes more pronounced, we predict that enhanced erosion along the plateau flank will accelerate topographic change

Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10Be fallout, but 9Be and 10Be concentrations considered together show that the majority of 10Be is present in glacial sediment when it enters the lake, and only a minority of total 10Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10Be fallout that have similar properties as coeval 10Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10Be fallout variations. We attempted to match centennial-period variations in reconstructed 10Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10Be measurements on targeted sections of the NAVC

Cosmogenic-nuclide data from Antarctic nunataks can constrain past ice sheet instabilities

We apply geologic evidence from ice-free areas in Antarctica to evaluate model simulations of ice sheet response to warm climates. This is important because such simulations are used to predict ice sheet behaviour in future warm climates, but geologic evidence of smaller-than-present past ice sheets is buried under the present ice sheet and therefore generally unavailable for model benchmarking. We leverage an alternative accessible geologic dataset for this purpose: cosmogenic-nuclide concentrations in bedrock surfaces of interior nunataks. These data produce a frequency distribution of ice thickness over multimillion-year periods, which is also simulated by ice sheet modelling. End-member transient models, parameterized with strong and weak marine ice sheet instability processes and ocean temperature forcings, simulate large and small sea-level impacts during warm periods and also predict contrasting and distinct frequency distributions of ice thickness. We identify regions of Antarctica where predicted frequency distributions reveal differences in end-member ice sheet behaviour. We then demonstrate that a single comprehensive dataset from one bedrock site in West Antarctica is sufficiently detailed to show that the data are consistent only with a weak marine ice sheet instability end-member, but other less extensive datasets are insufficient and/or ambiguous. Finally, we highlight locations where collecting additional data could constrain the amplitude of past and therefore future response to warm climates.</p

Cosmogenic-nuclide data from Antarctic nunataks can constrain past ice sheet instabilities

We apply geologic evidence from ice-free areas in Antarctica to evaluate model simulations of ice sheet response to warm climates. This is important because such simulations are used to predict ice sheet behaviour in future warm climates, but geologic evidence of smaller-than-present past ice sheets is buried under the present ice sheet and therefore generally unavailable for model benchmarking. We leverage an alternative accessible geologic dataset for this purpose: cosmogenic-nuclide concentrations in bedrock surfaces of interior nunataks. These data produce a frequency distribution of ice thickness over multimillion-year periods, which is also simulated by ice sheet modelling. End-member transient models, parameterized with strong and weak marine ice sheet instability processes and ocean temperature forcings, simulate large and small sea-level impacts during warm periods and also predict contrasting and distinct frequency distributions of ice thickness. We identify regions of Antarctica where predicted frequency distributions reveal differences in end-member ice sheet behaviour. We then demonstrate that a single comprehensive dataset from one bedrock site in West Antarctica is sufficiently detailed to show that the data are consistent only with a weak marine ice sheet instability end-member, but other less extensive datasets are insufficient and/or ambiguous. Finally, we highlight locations where collecting additional data could constrain the amplitude of past and therefore future response to warm climates.</p

New 10Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica

Evidence for the timing and pace of past grounding line retreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE) of Antarctica provides constraints for models that are used to predict the future trajectory of the West Antarctic Ice Sheet (WAIS). Existing cosmogenic nuclide surface exposure ages suggest that Pope Glacier, a former tributary of Thwaites Glacier, experienced rapid thinning in the early to mid-Holocene. There are relatively few exposure ages from the lower ice-free sections of Mount Murphy (< 300 m asl) that are uncomplicated by either nuclide inheritance or scattering due to localised topographic complexities; this makes the trajectory for the latter stages of deglaciation uncertain. This paper presents 12 new 10Be exposure ages from erratic cobbles collected from the western flank of Mt Murphy, within 160 m of the modern ice surface and 1 km from the present grounding line. The ages comprise two tightly clustered populations with mean deglaciation ages of 7.1 ± 0.1 ka and 6.4 ± 0.1 ka (1SE). Linear regression analysis applied to the age-elevation array of all available exposure ages from Mt Murphy indicates that the median rate of thinning of Pope Glacier was 0.27 m yr-1 between 8.1–6.3 ka, occurring 1.5 times faster than previously thought. Furthermore, this analysis better constrains the uncertainty (95 % confidence interval) in the timing of deglaciation at the base of the Mt Murphy vertical profile (~80 m above the modern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 ka to 6.3 ± 0.4 ka). Taken together, the results presented here suggest that early–mid Holocene thinning of Pope Glacier occurred over a shorter interval than previously assumed and permit a longer duration over which subsequent late Holocene rethickening could have occurred

Offshore-onshore record of Last Glacial Maximum-to-present grounding line retreat at Pine Island Glacier, Antarctica

Pine Island Glacier, West Antarctica, is the largest Antarctic contributor to global sea-level rise and is vulnerable to rapid retreat, yet our knowledge of its deglacial history since the Last Glacial Maximum is based largely on marine sediments that record retreat to ~120 km downstream of the modern grounding line by the early Holocene. We show, with a suite of 10Be exposure ages from onshore glacial deposits directly adjacent to Pine Island Glacier, that this major glacier thinned rapidly in the early- to mid-Holocene. Our results indicate that Pine Island Glacier was at least 690 m thicker than present prior to ~8 ka. We infer that the rapid thinning detected at the site furthest downstream records the arrival and stabilization of the retreating grounding line at that site by 8-6 ka. By combining our exposure ages and the marine record, we extend knowledge of Pine Island Glacier retreat both spatially and temporally: to 50 km from the modern grounding line and to the mid-Holocene, providing a dataset that is important for future numerical ice sheet model validation

Validation of earthquake ground-motion models in southern California, USA, using precariously balanced rocks

Accurate estimates of earthquake ground shaking rely on uncertain ground-motion models derived from limited instrumental recordings of historical earthquakes. A critical issue is that there is currently no method to empirically validate the resultant ground-motion estimates of these models at the timescale of rare, large earthquakes; this lack of validation causes great uncertainty in ground-motion estimates. Here, we address this issue and validate ground-motion estimates for southern California utilizing the unexceeded ground motions recorded by 20 precariously balanced rocks. We used cosmogenic 10Be exposure dating to model the age of the precariously balanced rocks, which ranged from ca. 1 ka to ca. 50 ka, and calculated their probability of toppling at different ground-motion levels. With this rock data, we then validated the earthquake ground motions estimated by the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) seismic-source characterization and the Next Generation Attenuation (NGA)-West2 ground-motion models. We found that no ground-motion model estimated levels of earthquake ground shaking consistent with the observed continued existence of all 20 precariously balanced rocks. The ground-motion model I14 estimated ground-motion levels that were inconsistent with the most rocks; therefore, I14 was invalidated and removed. At a 2475 year mean return period, the removal of this invalid ground-motion model resulted in a 2−7% reduction in the mean and a 10−36% reduction in the 5th−95th fractile uncertainty of the ground-motion estimates. Our findings demonstrate the value of empirical data from precariously balanced rocks as a validation tool for removing invalid ground-motion models and, in turn, reducing the uncertainty in earthquake ground-motion estimates

Numerical investigations of apatite ^4He/^3He thermochronometry

Apatite ^4He/^3He thermochronometry has the potential to constrain cooling histories for individual samples provided that several presently untested assumptions are valid. Here we simulate the sensitivity of ^4He/^3He spectra to assumptions regarding geometric model, crystallographic anisotropy, broken grain terminations, parent nuclide zonation, and the accuracy of results obtained from analyses of aggregates of multiple crystals. We find that ^4He/^3He spectra obtained from a cylinder with isotropic diffusion are almost indistinguishable from those obtained from an equivalent sphere with an equivalent initial ^4He distribution. Under similar conditions anisotropic diffusion from the cylinder can greatly bias ^4He/^3He spectra, but only if diffusion is >10 times faster in the axial than the radial direction. Existing data argue against anisotropy of this magnitude. We find that analysis of apatites with broken terminations will also bias ^4He/^3He spectra, but not greatly so. In contrast, we find that zonation of a factor of 3 in parent nuclide concentration produces ^4He/^3He spectra that deviate substantially from the homogeneous model. When parent nuclides are highly concentrated near the grain rim and/or cooling is fast, the resulting ^4He/^3He spectra will be readily identified as aberrant. However, more subtle zonation, higher concentrations in the grain interior, or samples that have cooled slowly regardless of zonation style can yield ^4He/^3He spectra that look acceptable but will lead to inaccurate thermochronometric interpretation if parent homogeneity is assumed. Finally, we find that analysis of an aggregate of crystals with identical ^4He distributions can yield ^4He/^3He spectra (and diffusion Arrhenius arrays) that are very different from those that would be obtained on the individual crystals if even small variations in He diffusion exist among the grains. Overall, our observations suggest that modeling tools that assume spherical geometry and isotropic diffusion are appropriate for interpreting apatite ^4He/^3He spectra. However, it is essential to analyze only individual crystals and to assess the degree of parent nuclide zonation in those crystals

#35 - Enamel isotopes reveal late Pleistocene ecosystem dynamics in southeastern North America

The end of the late Pleistocene (~10,000 years ago) witnessed the extinction of over seventy percent of North America’s megafaunal genera. Although this pattern has been extensively investigated, its causal mechanisms remain elusive. Much of this difficulty is related to the spatial and temporal discontinuity of sites dating to the period leading up to the extinctions. Due to its removal from glacial conditions, southeastern North America provides a unique window into ecosystem dynamics just prior to human arrival in the region. In this study, we present new stable carbon and oxygen isotope data from Mammuthus columbiand Bison latifronsteeth collected from a well-dated Last Glacial Maximum (~20,000 rcybp) locality called Clark Quarry in coastal Georgia, USA. We compare these data to those from similarly aged (middle and late Rancholabrean) localities from Florida and demonstrate the presence of a vegetation gradient with elevated levels of C3vegetation at higher latitudes. We hypothesize that this pattern may have contributed to previously described migratory patterns of mastodon (Mammut) populations in southeastern North America. Serially-sampled δ13C and δ18O values suggest that Clark Quarry Mammuthus and Bison changed their diet seasonally with the incorporation of elevated quantities of C4vegetation during warmer periods. Our data indicate more exaggerated seasonal dietary variability in these taxa at Clark Quarry relative to those collected from the interglacial locality of Waccasassa River in Florida, providing additional evidence for the significant influence of glacial dynamics in structuring North American ecosystems

Holocene Deglaciation of Marie Byrd Land, West Antarctica

Surface exposure ages of glacial deposits in the Ford Ranges of western Marie Byrd Land indicate continuous thinning of the West Antarctic Ice Sheet by more than 700 meters near the coast throughout the past 10,000 years. Deglaciation lagged the disappearance of ice sheets in the Northern Hemisphere by thousands of years and may still be under way. These results provide further evidence that parts of the West Antarctic Ice Sheet are on a long-term trajectory of decline. West Antarctic melting contributed water to the oceans in the late Holocene and may continue to do so in the future