Capabilities of the Lamont–Doherty Earth Observatory in situ¹⁴C extraction laboratory updated

We report on the status and capabilities of the Lamont–Doherty Earth Observatory in situ¹⁴C extraction laboratory. In late 2006 we began, in collaboration with the AMS group at the University of Arizona, construction of a new laboratory to extract in situ cosmogenic ¹⁴C from terrestrial silicates. Long-term measurements of the process blank over the last two years give an arithmetic mean and standard deviation of 125 ± 43 × 10³ atoms ¹⁴C (n = 9) and show significant improvement in the number of atoms, as well as stability compared to initial measurements of the process blank. We report long-term measurements of the intercomparison material CRONUS-A, which has been developed as part of the CRONUS-Earth effort to characterize inter- and intra-laboratory variability. We interpret the standard deviation (5%) of six replicate measurements of CRONUS-A as the reproducibility of in situ¹⁴C extractions in our laboratory

The Transport History of Alluvial Fan Sediment Inferred From Multiple Geochronometers

We present a multi-chronometer approach to refine the age of an alluvial fan and to infer sediment transport and deposition history in the Anza Borrego Desert region of Southern California. We measure in situ produced cosmogenic carbon-14 (14C) from boulders on the fan surface and infrared stimulated luminescence (IRSL) ages from single feldspar grains within the alluvium. Our new IRSL age [5.3 ± 0.5 ka (±1σ)] is in excellent agreement with existing uranium-series [U-series; 5.3 ± 0.2 (±2σ)] ages of pedogenic carbonates. The IRSL and U-series ages show that in situ 14C measurements [6.6 ± 1.1 ka (±1σ)] from boulders contain inherited nuclides from prior exposure in the upstream catchment, much like measurements of the longer-lived nuclide, beryllium-10 (10Be). However, in situ 14C ages are closer to the preferred ages inferred from IRSL and U-series and with less scatter than comparative 10Be ages. Our data demonstrate that a multi-geochronometer approach will produce ages of alluvial fan surfaces with the greatest degree of confidence. We then apply the paired 14C and 10Be concentrations to infer the prior exposure and storage duration of the sampled boulders of 3.1 ± 3.2 and 4.6 ± 2.3 Kyr, respectively. A mixture model analysis of the single grain IRSL ages suggests bimodal storage durations prior to remobilization with peaks at ca. 2 and 10 Kyr. We demonstrate that cosmogenic nuclide inheritance and single grain IRSL equivalent dose distributions can provide additional information regarding sediment transport history prior to deposition on the alluvial fan

Holocene dynamics of the Rhone Glacier, Switzerland, deduced from ice flow models and cosmogenic nuclides

We describe efforts to model the Holocene extent of the Rhone Glacier, Switzerland, using four paleoclimate records as templates for paleo-equilibrium line altitude to identify candidate driving mechanisms of glaciers in the Alps. We evaluate the success of each paleoclimate template by comparing cosmogenic 10Be and 14C concentrations in pro-glacial bedrock derived from modeled glacier configurations to measured values. An adequate fit can be obtained using mean summer insolation for 46.5°N. However, use of the Dongee Cave, China, speleothem record yields the best fit by accounting for both sub-millennial (e.g. Little Ice Age and Medieval Warm Period) and multi-millennial climate variations (summer insolation). Our result indicates that glaciers in the Alps primarily responded to changes in insolation during the Holocene were smaller than today during the early Holocene when insolation was relatively high, and became larger during the mid to late Holocene. Superimposed on the first-order insolation response were shorter, sometimes large amplitude, length changes in response to short-lived climate events such as the Medieval Warm Period and the LIA

Do Phreatomagmatic Eruptions at Ubehebe Crater (Death Valley, California) Relate to a Wetter than Present Hydro-Climate?

Phreatomagmatic eruptions occur when rising magma encounters groundwater and/or surface water, causing a steam explosion and the ejection of country rock and pyroclastic material. The predominance of this type of activity at the Ubehebe volcanic field in northern Death Valley, California, is enigmatic owing to the extremely arid climate of the region. A novel application of 10Be surface exposure dating is presented to determine the timing of phreatomagmatic eruptions at Ubehebe Crater and to test the idea that volcanism may relate to a wetter than present hydro-climate. Twelve of the fifteen ages obtained lie between 0.8 and 2.1 ka, while three samples give older, mid-Holocene ages. The cluster between 0.8 and 2.1 ka is interpreted as encompassing the interval of volcanic activity during which Ubehebe Crater was formed. The remaining older ages are inferred to date eruptions at the older neighboring craters. The main and most recent period of activity encompasses the Medieval Warm Period, an interval of prolonged drought in the American southwest, as well as slightly wetter conditions prior to the Medieval Warm Period. Phreatomagmatic activity under varied hydrologic conditions casts doubt on the idea that eruptive timing relates to a wetter hydro-climate. Instead, the presence of a relatively shallow modern water table suggests that sufficient groundwater was generally available for phreatomagmatic eruptions at the Ubehebe site, in spite of prevailing arid conditions. This and the youth of the most recent activity suggest that the Ubehebe volcanic field may constitute a more significant hazard than generally appreciated

Abrupt mid-Holocene ice loss in the western Weddell Sea Embayment of Antarctica

The glacial history of the westernmost Weddell Sea sector of Antarctica since the Last Glacial Maximum is virtually unknown, and yet it has been identified as critical for improving reliability of glacio-isostatic adjustment models that are required to correct satellite-derived estimates of ice sheet mass balance. Better knowledge of the glacial history of this region is also important for validating ice sheet models that are used to predict future contribution of the Antarctic ice sheet to sea level rise. Here we present a new Holocene deglacial chronology from a site on the Lassiter Coast of the Antarctic Peninsula, which is situated in the western Weddell Sea sector. Samples from 12 erratic cobbles and 18 bedrock surfaces from a series of presently-exposed ridges were analysed for cosmogenic 10Be exposure dating, and a smaller suite of 7 bedrock samples for in situ 14C dating. The resulting 10Be ages are predominantly in the range 80–690 ka, whereas bedrock yielded much younger in situ 14C ages, in the range 6.0–7.5 ka for samples collected from 138–385 m above the modern ice surface. From these we infer that the ice sheet experienced a period of abrupt thinning over a short time interval (no more than 2700 years) in the mid-Holocene, resulting in lowering of its surface by at least 250 m. Any late Holocene change in ice sheet thickness — such as re-advance, postulated by several modelling studies — must lie below the present ice sheet surface. The substantial difference in exposure ages derived from 10Be and 14C dating for the same samples additionally implies ubiquitous 10Be inheritance acquired during ice-free periods prior to the last deglaciation, an interpretation that is consistent with our glacial-geomorphological field observations for former cold-based ice cover. The results of this study provide evidence for an episode of abrupt ice sheet surface lowering in the mid-Holocene, similar in rate, timing and magnitude to at least two other locations in Antarctica

New Last Glacial Maximum Ice Thickness constraints for the Weddell Sea Embayment, Antarctica

We describe new Last Glacial Maximum (LGM) ice thickness constraints for three locations spanning the Weddell Sea Embayment (WSE) of Antarctica. Samples collected from the Shackleton Range, Pensacola Mountains, and the Lassiter Coast constrain the LGM thickness of the Slessor Glacier, Foundation Ice Stream, and grounded ice proximal to the modern Ronne Ice Shelf edge on the Antarctic Peninsula, respectively. Previous attempts to reconstruct LGM-to-present ice thickness changes around the WSE used measurements of long-lived cosmogenic nuclides, primarily Be-10. An absence of post-LGM apparent exposure ages at many sites led to LGM thickness reconstructions that were spatially highly variable and inconsistent with flow line modelling. Estimates for the contribution of the ice sheet occupying the WSE at the LGM to global sea level since deglaciation vary by an order of magnitude, from 1.4 to 14.1m of sea level equivalent. Here we use a short-lived cosmogenic nuclide, in situ-produced C-14, which is less susceptible to inheritance problems than Be-10 and other long-lived nuclides. We use in situ C-14 to evaluate the possibility that sites with no post-LGM exposure ages are biased by cosmogenic nuclide inheritance due to surface preservation by cold-based ice and non-deposition of LGM-aged drift. Our measurements show that the Slessor Glacier was between 310 and up to 655m thicker than present at the LGM. The Foundation Ice Stream was at least 800m thicker, and ice on the Lassiter Coast was at least 385m thicker than present at the LGM. With evidence for LGM thickening at all of our study sites, our in situ C-14 measurements indicate that the long-lived nuclide measurements of previous studies were influenced by cosmogenic nuclide inheritance. Our inferred LGM configuration, which is primarily based on minimum ice thickness constraints and thus does not constrain an upper limit, indicates a relatively modest contribution to sea level rise since the LGM of < 4.6 m, and possibly as little as < 1.5 m

Measuring multiple cosmogenic nuclides in glacial cobbles sheds light on Greenland Ice Sheet processes

The behavior of the Greenland Ice Sheet during the Pleistocene remains uncertain due to the paucity of evidence predating the Last Glacial Maximum. Here, we employ a novel approach, cosmogenic nuclide analysis of individual subglacially-derived cobbles, which allows us to make inferences about ice sheet processes and subglacial erosion. From three locations in western Greenland, we collected 86 cobbles from the current ice sheet margin and nine cobbles exposed on the modern proglacial land surface. We measured the concentration of in situ 10Be in all cobbles (n = 95) and 26Al and 14C in a subset (n = 14). Cobbles deposited during Holocene retreat have 10Be exposure ages generally consistent with the timing of ice retreat determined by other methods. Conversely, most of the 86 subglacial cobbles contain very low concentrations of 10Be (median 1.0×10 3 atoms g −1), although several have ∼10 4 and one has ∼10 5 atoms g −1. The low concentrations of 10Be in most subglacial cobbles imply that their source areas under the Greenland Ice Sheet are deeply eroded, preserving minimal evidence of surface or near-surface exposure. The presence of measurable 14C in ten of the cobbles requires that they experienced cosmogenic nuclide production within the past ∼30 ka; however, 14C/ 10Be ratios of ∼6 suggest that nuclide production occurred during shielding by overlying material. Only two of the 86 subglacial cobbles definitively have cosmogenic nuclide concentrations consistent with prior surface exposure. Overall, isotopic analysis of subglacial cobbles indicates that much of western Greenland's subglacial landscape is characterized by deep erosion and minimal subaerial exposure

The last glaciation of Bear Peninsula, central Amundsen Sea Embayment of Antarctica: Constraints on timing and duration revealed by in situ cosmogenic 14C and 10Be dating

Ice streams in the Pine Island-Thwaites region of West Antarctica currently dominate contributions to sea level rise from the Antarctic ice sheet. Predictions of future ice-mass loss from this area rely on physical models that are validated with geological constraints on past extent, thickness and timing of ice cover. However, terrestrial records of ice sheet history from the region remain sparse, resulting in significant model uncertainties. We report glacial-geological evidence for the duration and timing of the last glaciation of Hunt Bluff, in the central Amundsen Sea Embayment. A multi-nuclide approach was used, measuring cosmogenic 10Be and in situ14C in bedrock surfaces and a perched erratic cobble. Bedrock 10Be ages (118–144 ka) reflect multiple periods of exposure and ice-cover, not continuous exposure since the last interglacial as had previously been hypothesized. In situ14C dating suggests that the last glaciation of Hunt Bluff did not start until 21.1 ± 5.8 ka – probably during the Last Glacial Maximum – and finished by 9.6 ± 0.9 ka, at the same time as ice sheet retreat from the continental shelf was complete. Thickening of ice at Hunt Bluff most likely post-dated the maximum extent of grounded ice on the outer continental shelf. Flow re-organisation provides a possible explanation for this, with the date for onset of ice-cover at Hunt Bluff providing a minimum age for the timing of convergence of the Dotson and Getz tributaries to form a single palaeo-ice stream. This is the first time that timing of onset of ice cover has been constrained in the Amundsen Sea Embayment

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

Geological calibration of spallation production rates in the CRONUS-Earth project

Models of the production of cosmogenic nuclides typically incorporate an adjustable production rate parameter that is scaled for variations in production with latitude and altitude. In practice, this production rate parameter is set by calibration of the model using cosmogenic nuclide data from sites with independent age constraints. In this paper, we describe a calibration procedure developed during the Cosmic-Ray Produced Nuclide Systematics on Earth (CRONUS-Earth) project and its application to an extensive data set that included both new CRONUS-Earth samples and samples from previously published studies. We considered seven frameworks for elevation and latitude scaling and five commonly used cosmogenic nuclides, 3He, 10Be, 14C, 26Al, and 36Cl. In general, the results show that the calibrated production rates fail statistical tests of goodness-of-fit. One conclusion from the calibration results is that two newly developed scaling frameworks and the widely used Lal scaling framework provide qualitatively similar fits to the data, while neutron-monitor based scaling frameworks have much poorer fit to the data. To further test the fitted models, we computed site ages for a number of secondary sites not included in the primary calibration data set. The root-mean-square percent differences between the median computed ages for these secondary sites and independent ages range from 7.1% to 27.1%, differences that are much larger than the typical uncertainties in the site ages. The results indicate that there are substantial unresolved difficulties in modeling cosmogenic nuclide production and the calibration of production rates