Cosmogenic Nuclide Systematics and the CRONUScalc Program

As cosmogenic nuclide applications continue to expand, the need for a common basis for calculation becomes increasingly important. In order to accurately compare between results from different nuclides, a single method of calculation is necessary. Calculators exist in numerous forms with none matching the needs of the CRONUS-Earth project to provide a simple and consistent method to interpret data from most commonly used cosmogenic nuclides. A new program written for this purpose, CRONUScalc, is presented here. This unified code presents a method applicable to 10Be, 26Al, 36Cl, 3He, and 14C, with 21Ne in testing. The base code predicts the concentration of a sample at a particular depth for a particular time in the past, which can be used for many applications. The multi-purpose code already includes functions for performing production rate calibrations as well as calculating erosion rates and surface exposure ages for single samples and depth profiles. The code is available under the GNU General Public License agreement and can be downloaded and modified to deal with specific atypical scenarios

Sedimentological characterization of Antarctic moraines using UAVs and Structure-from-Motion photogrammetry

In glacial environments particle-size analysis of moraines provides insights into clast origin, transport history, depositional mechanism and processes of reworking. Traditional methods for grain-size classification are labour-intensive, physically intrusive and are limited to patch-scale (1m2) observation. We develop emerging, high-resolution ground- and unmanned aerial vehicle-based ‘Structure-from-Motion’ (UAV-SfM) photogrammetry to recover grain-size information across an moraine surface in the Heritage Range, Antarctica. SfM data products were benchmarked against equivalent datasets acquired using terrestrial laser scanning, and were found to be accurate to within 1.7 and 50mm for patch- and site-scale modelling, respectively. Grain-size distributions were obtained through digital grain classification, or ‘photo-sieving’, of patch-scale SfM orthoimagery. Photo-sieved distributions were accurate to <2mm compared to control distributions derived from dry sieving. A relationship between patch-scale median grain size and the standard deviation of local surface elevations was applied to a site-scale UAV-SfM model to facilitate upscaling and the production of a spatially continuous map of the median grain size across a 0.3 km2 area of moraine. This highly automated workflow for site scale sedimentological characterization eliminates much of the subjectivity associated with traditional methods and forms a sound basis for subsequent glaciological process interpretation and analysis

Evaluating the size and extent of paleolakes in central Tibetduring the late Pleistocene

Subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleolake 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 and 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.Published versio

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]

No abstract available

Late Glacial-Holocene cirque glacier chronology on sub-Antarctic Kerguelen Archipelago (49°S) based on cosmogenic 36Cl exposure dating

Sub-Antarctic glacier chronologies can provide valuable information about the past variability of climate dynamics in the Southern Ocean region. The Kerguelen Archipelago (49°S) is advantageously located under the influence of the Southern Hemisphere's westerly wind belt, thus fluctuations of climate-sensitive glaciers on Kerguelen can provide a baseline for understanding the behavior of this atmospheric regime in response to climatic forcings. We present 17 36Cl exposure ages of moraine and erratic boulders to provide chronological constraints to paleoglacier extents of the Guynemer cirque glacier, located just north of the Cook Ice Cap. Erratic boulders show ice thinning in the Guynemer region started to occur in the Late Glacial by 13.5 ka and continued past 12.4 ka. Ice retreat was punctuated by the formation of two moraine stages, the outermost at 11.5 ± 0.4 ka followed by another at 10.4 ± 1.2 ka, which are indicative of Early Holocene glacier advances/standstills. A glacial advance occurred at 1.4 ± 0.3 ka, which corroborates other Late Holocene re-advances elsewhere on the archipelago. Finally, three undated moraine stages are found between 1.4 ka and the 1960s. The lack of moraines after 10.4 ka and through the Mid-Holocene suggests that the Guynemer glacier was significantly smaller during this extended period of the Holocene compared to its Early Holocene as well as its Late Holocene limits. The Guynemer glacier history provides unique evidence of Early Holocene moraines on Kerguelen, which have not been discovered thus far on the archipelago. Similar to glaciers in Patagonia, New Zealand and South Georgia, the Guynemer glacier was at its largest Holocene extent in the Early Holocene. However, while other southern mid-latitude glacier chronologies show progressively smaller glacial extents throughout the Mid- to Late Holocene, the Late Holocene re-advance of the Guynemer glacier, like other Kerguelen glaciers, likely exceeded its Mid-Holocene extent

Radar‐detected englacial debris in the West Antarctic Ice Sheet

Structural glaci‐geological processes can entrain basal sediment into ice, leading to its transportation and deposition downstream. Sediments potentially rich in essential nutrients, like silica and iron, can thus be transferred from continental sources to the ocean, where deposition could enhance marine primary productivity. However, a lack of data has limited our knowledge of sediment entrainment, transfer and distribution in Antarctica, until now. We use ice‐penetrating radar to examine englacial sediments in the Weddell Sea sector of the West Antarctic Ice Sheet. Radargrams reveal englacial reflectors on the lee side of nunataks and subglacial highlands, where Mie scattering analysis of the reflectors suggests particle sizes consistent with surface moraine sediments. We hypothesize that these sediments aare entrained at the thermal boundary between cold and warm‐based ice. Conservative estimates of >130 x109 kg of englacial sediment in Horseshoe Valley alone suggest that the ice sheet has significant entrainment potential unappreciated previously

In situ cosmogenic nuclide production rate calibration for the CRONUS-Earth project from Lake Bonneville, Utah, shoreline features

Well-dated bedrock surfaces associated with the highstand and subsequent catastrophic draining of Pleistocene Lake Bonneville, Utah, during the Bonneville flood are excellent locations for in situ cosmogenic nuclide production rate calibration. The CRONUS-Earth project sampled wave-polished bedrock and boulders on an extensive wave-cut bench formed during the Bonneville-level highstand that was abandoned almost instantaneously during the Bonneville flood. CRONUS-Earth also sampled the Tabernacle Hill basalt flow that erupted into Lake Bonneville soon after its stabilization at the Provo level, following the flood. New radiocarbon dating results from tufa at the margins of Tabernacle Hill as part of this study have solidified key aspects of the exposure history at both sites. Both sites have well-constrained exposure histories in which factors such as potential prior exposure, erosion, and shielding are either demonstrably negligible or quantifiable. Multi-nuclide analyses from multiple labs serve as an ad hoc inter-laboratory comparison that supplements and expands on the formalized CRONUS-Earth and CRONUS-EU inter-laboratory comparisons (Blard et al., 2015; Jull et al., 2015; Vermeesch et al., 2015). Results from 10Be, 26Al, and 14C all exhibit scatter comparable to that observed in the CRONUS-Earth effort. Although a 36Cl inter-laboratory comparison was not completed for Jull et al. (2015), 36Cl from plagioclase mineral separates exhibits comparable reproducibility. Site production rates derived from these measurements provide valuable input to the global production rate calibration described by Borchers et al. (2015). Whole-rock 36Cl concentrations, however, exhibit inter-laboratory variation exceeding analytical uncertainty and outside the ranges observed for the other nuclides (Jull et al., 2015). A rigorous inter-laboratory comparison studying the systematics of whole-rock 36Cl extraction techniques is currently underway with the goals of delineating the source(s) of this discrepancy and standardizing these procedures going forward

Sedimentological characterisation of Antarctic moraines using UAVs and Structure-from-Motion photogrammetry

In glacial environments particle-size analysis of moraines provides insights into clast origin, transport history, depositional mechanism and processes of reworking. Traditional methods for grain-size classification are labour-intensive, physically intrusive and are limited to patch-scale (1m2) observation. We develop emerging, high-resolution ground- and unmanned aerial vehicle-based ‘Structure-from-Motion’ (UAV-SfM) photogrammetry to recover grain-size information across an moraine surface in the Heritage Range, Antarctica. SfM data products were benchmarked against equivalent datasets acquired using terrestrial laser scanning, and were found to be accurate to within 1.7 and 50mm for patch- and site-scale modelling, respectively. Grain-size distributions were obtained through digital grain classification, or ‘photo-sieving’, of patch-scale SfM orthoimagery. Photo-sieved distributions were accurate to <2mm compared to control distributions derived from dry sieving. A relationship between patch-scale median grain size and the standard deviation of local surface elevations was applied to a site-scale UAV-SfM model to facilitate upscaling and the production of a spatially continuous map of the median grain size across a 0.3 km2 area of moraine. This highly automated workflow for site scale sedimentological characterization eliminates much of the subjectivity associated with traditional methods and forms a sound basis for subsequent glaciological process interpretation and analysis

A new Holocene eruptive history of Erebus volcano, Antarctica using cosmogenic 3He and 36Cl exposure ages

AbstractThe ages of recent effusive eruptions on Erebus volcano, Antarctica are poorly known. Published 40Ar/39Ar ages of the 10 youngest “post-caldera” lava flows are unreliable because of the young ages of the flows (<10 ka) and the presence of excess 40Ar. Here we use cosmogenic 3He and 36Cl to provide new ages for the 10 youngest flows and 3 older summit flows, including a newly recognized flow distinguished by its exposure age. Estimated eruption ages of the post-caldera flows, assuming no erosion or prior snow cover, range from 4.52 ± 0.08 ka to 8.50 ± 0.19 ka, using Lifton et al. (2014) to scale cosmogenic production rates. If the older Lal (1991)/Stone (2000) model is used to scale production rates, calculated ages are older by 16–25%. Helium-3 and chlorine-36 exposure ages measured on the same samples show excellent agreement. Helium-3 ages measured on clinopyroxene and olivine from the same samples are discordant, probably due in part to lower-than-expected 3He production rates in the Fe-rich olivine. Close agreement of multiple clinopyroxene 3He ages from each flow indicates that the effects of past snow coverage on the exposure ages have been minimal.The new cosmogenic ages differ considerably from published 40Ar/39Ar and 36Cl ages and reveal that the post-caldera flows were erupted during relatively brief periods of effusive activity spread over an interval of ∼4 ka. The average eruption rate over this interval is estimated to be 0.01 km3/ka. Because the last eruption was at least 4 ka ago, and the longest repose interval between the 10 youngest eruptions is ∼1 ka, we consider the most recent period of effusive activity to have ended