Search

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.

An objective of terrestrial in situ cosmogenic nuclide research is to obtain precise and accurate production-rate estimates on the basis of geological calibration sites from a diverse range of latitudes and altitudes. However, a challenge has been to establish production rates on the basis of landforms for which independent ages have been determined directly using absolute isotopic dating techniques. Here we present a 10Be production-rate calibration from a recessional moraine belt located in Rannoch Moor, central Scottish Highlands (56.63°N, 4.77°W; ∼310 330 m a.s.l.). This moraine belt was deposited at the margin of the disintegrating late-glacial West Highland ice field (WHIF) during the final stages of deglaciation. Minimum-limiting 14C dates on macrofossils of the earliest terrestrial vegetation to arrive on the landscape place the timing of moraine abandonment, and hence exposure of morainal boulder surfaces to the cosmic-ray flux, to no later than 12,480 ± 100 calendar years before C.E. 1950 (cal yrs BP). Maximum-limiting 14C dates on marine shells incorporated into basal tills deposited during expansion of the WHIF to its full late-glacial extent place the onset of deglaciation, and thus deglaciation of Rannoch Moor, to no earlier than 12,700 ± 100 cal yrs BP. After removal of a single high-concentration outlier, surface 10Be concentrations of 11 boulders rooted in two sub-parallel moraine ridges exhibit a high degree of internal consistency and affords an arithmetic mean of 6.93 ± 0.24 [x104] atoms g−1 (1σ). This data set yields a site-specific 10Be production rate of 5.50 ± 0.18 at g−1 yr−1, based on the midpoint age 12,590 ± 140 cal yrs BP of the bracketing 14C chronology. Transforming this result to sea-level/high-latitude (SLHL) neutron-spallation 10Be production-rate values using Version 3 of the University of Washington (UW) Online Production-Rate Calculator yields upper and lower bounds, and a mid-point rate. Maximum-limiting SLHL 10Be production rates, based on minimum-limiting 14C age control, are 3.95 ± 0.11 (2.7%) at g−1 yr−1 for the commonly used Lm and St scaling protocols. The corresponding (non-dimensional) correction factor for a reference production rate determined by the LSDn scaling model is 0.79 ± 0.02 (2.7%). Minimum-limiting SLHL reference 10Be production rates, based on maximum-limiting 14C age control, are 3.88 ± 0.11 (2.7%) at g−1 yr−1 (St) and 3.89 ± 0.11 (2.7%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.77 ± 0.02 (2.7%). SLHL reference production-rate values based on a midpoint age of 12,590 ± 140 yrs are 3.91 ± 0.11 (2.8%) at g−1 yr−1 (St) and 3.92 ± 0.11 (2.8%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.78 ± 0.02. The production-rate calibration data set presented here for Scotland yields SLHL values that agree with those determined from calibration data sets based on directly dated landforms from northeastern North America, the Arctic, the Swiss Alps, the Southern Hemisphere middle latitudes, and from the high tropical Andes. We suggest that this production-rate calibration data set from the central Scottish Highlands, used together with the UW online calculators, will produce accurate 10Be surface-exposure ages in the British Isles. We acknowledge support from the Dan and Betty Churchill Exploration Fund and the Lamont-Doherty Earth Observatory (LDEO) Climate Center. A.E.P and G.R.M.B. each acknowledge support from the LDEO Postdoctoral Fellowship. A.E.P. acknowledges the Lenfest Foundation, the Comer Family Foundation, and the Quesada Family Fund. J.M.S. acknowledges support from the Lamont Climate Center. We thank R. Schwartz and J. Frisch for assistance in the laboratory, as well as D. Duerden, E. Watson, and H. Senn for their help during field work at Rannoch Moor. This is LDEO contribution number 8267. peer-reviewed 2020-11-23

Interhemispheric differences in the timing of pauses or reversals in the temperature rise at the end of the last ice age can help to clarify the mechanisms that influence glacial terminations. Our beryllium-10 (10Be) surface-exposure chronology for the moraines of the upper Rakaia valley of New Zealand's Southern Alps, combined with glaciological modeling, show that late-glacial temperature change in the atmosphere over the Southern Alps exhibited an Antarctic-like pattern. During the Antarctic Cold Reversal, the upper Rakaia glacier built two well-defined, closely-spaced moraines on Reischek knob at 13,900 ± 120 [1σ; ± 310 yrs when including a 2.1% production-rate (PR) uncertainty] and 13,140 ± 250 (±370) yrs ago, in positions consistent with mean annual temperature approximately 2 °C cooler than modern values. The formation of distinct, widely-spaced moraines at 12,140 ± 200 (±320) and 11,620 ± 160 (±290) yrs ago on Meins Knob, 2 km up-valley from the Reischek knob moraines, indicates that the glacier thinned by ∼250 m during Heinrich Stadial 0 (HS 0, coeval with the Younger Dryas 12,900 to 11,600 yrs ago). The glacier-inferred temperature rise in the upper Rakaia valley during HS 0 was about 1 °C. Because a similar pattern is documented by well-dated glacial geomorphologic records from the Andes of South America, the implication is that this late-glacial atmospheric climate signal extended from 79°S north to at least 36°S, and thus was a major feature of Southern Hemisphere paleoclimate during the last glacial termination.

The progress in the method of cosmogenic nuclide surface exposure dating has drastically accelerated over the last years, in part fueled by the National Science Foundation (NSF)-funded Cosmic-Ray Produced Nuclide Systematics on Earth (CRONUS-Earth) initiative. This progress has affected the sensitivity of the method, its overall precision and accuracy, and its robustness, and opened up new scientific fields in the Earth sciences and beyond. This article attempts to highlight some of the most recent and important areas of progress to give an overview of the state of the method and to outline the next line of progress. While the technique is clearly surging ahead, cosmogenic nuclide surface exposure dating is far from reaching its limits.