Search

The Svalbard-Barents ice sheet was predominantly a marine-based ice sheet and reconstructing the timing and rate of its decay during the last deglaciation can inform predictions of future decay of marine-based ice sheets (e.g. West Antarctica). Records of ice-sheet change are now routinely built with cosmogenic surface exposure ages, but in some regions, this method is complicated by the presence of isotopic inheritance yielding artificially old and erroneous exposure ages. Here, we present forty-six 1025 Be ages from bedrock (n = 38) and erratic boulders (n = 8) in southwestern Spitsbergen that, when paired with in situ 1427 C measurements (n = 5), constrain the timing of coastal deglaciation following the last glacial maximum. 10Be and 1428 C measurements from bedrock along a ~400 m elevation transect reveal inheritance-skewed 10Be ages, whereas 1429 C measurements constrain 400 m of ice-sheet thinning and coastal deglaciation at 17.4 ± 1.5 ka. Combined with three additional 10Be-dated coastal sites, we show that the southwestern margin of the Svalbard-Barents ice sheet retreated out of Norwegian Sea between ~18-16 ka. In situ 1432 C measurements can provide key chronological information on ice-sheet response to the last termination in cases where measurements of long-lived nuclides are compromised by isotopic inheritance.

Calculation of surface exposure ages, using in situ cosmogenic nuclides, requires accurate knowledge of local production rates. Here, we report the first attempt to calibrate cosmogenic 3He production in the south-west Pacific region. We present a new radiocarbon chronology that precisely dates the emplacement of the Murimotu Formation, a large debris avalanche deposit in central North Island, New Zealand (ca. 830 m asl; 39°S), which occurred 10.4–10.6 cal ka BP. Measurements of cosmogenic 3He in pyroxene separated from large andesitic blocks exposed during this event yield a sea-level high-latitude production rate of 120 ± 12 atoms g−1 a−1 (‘Lm’ scaling). This is consistent with a recent global compilation, comprised predominantly of calibration sites located in the Northern Hemisphere. Thus, we conclude that the globally compiled cosmogenic 3He production rate is valid in the south-west Pacific. Using independent, proximal calibrations of cosmogenic isotopes 10Be and 14C from quartz in New Zealand, we derive cosmogenic 3He/10Be and 3He/14C production ratios of 32.2 ± 3.2 and 10.6 ± 1.6, respectively.

The in situ cosmogenic nuclide 14 C is unique compared with other nuclides because of its short half- life, and when combined with longer-lived isotopes (e.g. 10 Be), in situ 14 C can be a powerful tool for deciphering recent and complex surface exposure histories. Like all in situ cosmogenic nuclides, quantifying earth surface processes with in situ 14 C requires a well-constrained in situ 14 C production rate. We present a production-rate calibration from an independently dated moraine in West Greenland, previously used as an in situ 10 Be production-rate calibration site. The local in situ 14 C production rate is 22.8 � 1.4 atoms g � 1 a � 1 (69.28uN, 50.76u W; 350m asl) and when scaled to sea level/high latitude using time-dependent Lal/Stone scaling (Lm), we calculate a spallation-only in situ 14 C production rate of 12.0 � 0.9 atoms g � 1 a � 1 and a 14 C/ 10 Be production rate ratio of 3.1 � 0.2. The West Greenland in situ 14 C production rate is indistinguishable from the New Zealand, Promontory Point and Scottish Highlands in situ 14 C production rates. When combined, we calculate a global production rate of 12.1 � 0.5 atoms g � 1 a � 1 (Lm). Copyright # 2014 John Wiley & Sons, Ltd.

The abundant production of in situ cosmogenic 36Cl from potassium renders 36Cl measurements in K-rich rocks or minerals, such as K-feldspars, potentially useful for precisely dating rock surfaces, either in single-nuclide or in multi-nuclide studies, for example combined with 10Be measurements in quartz. However, significant discrepancies in experimentally calibrated 36Cl production rates from spallation of potassium (36PK-sp), referenced to sea-level/high-latitude (SLHL), limit the accuracy of 36Cl dating from K-rich lithologies. We present a new 36Cl calibration using K-feldspars, in which K-spallation is the most dominant 36Cl production pathway (>92% of total 36Cl), thus minimizing uncertainties from the complex multi-pathway 36Cl production systematics. The samples are derived from boulders of an ∼13.4 ka-old landslide in the Swiss Alps (∼820 m, 46.43°N, 8.85°E). We obtain a local 36PK-sp of 306 ± 16 atoms 36Cl (g K)−1 a−1 and an SLHL 36PK-sp of 145.5 ± 7.7 atoms 36Cl (g K)−1 a−1, when scaled with a standard scaling protocol (‘Lm’). Applying this SLHL 36PK-sp to determine 36Cl exposure ages of K-feldspars from 10Be-dated moraine boulders yields excellent agreement, confirming the validity of the new SLHL 36PK-sp for surface exposure studies, involving 36Cl in K-feldspars, in the Alps.

We present a Baffin Bay 10 Be production-rate calibration derived from glacial deposits in western Greenland and Baffin Island, and test our results against published 10 Be calibration datasets to develop an Arctic 10 Be production rate. Our calibration comprises: (i) 10 Be measurements from moraine boulders linked to a 14 C- dated moraine at Jakobshavn Isfjord in western Greenland, (ii) an independent and previously published 10 Be production rate at Jakobshavn Isfjord and (iii) re-measured 10 Be concentrations from a Baffin Island calibration site that is included in the north-eastern North America dataset. Combined, we calculate a sea-level/high-latitude 10 Be production rate for the Baffin Bay region of 3.96 � 0.07 atoms g � 1 a � 1 (Lal/Stone scaling model). After testing the Baffin Bay rate against calibration sites in Norway and north-eastern North America, we calculate a more conservative Arctic production rate of 3.96 � 0.15 atoms g � 1 a � 1 . The Baffin Bay and Arctic 10 Be production rates are indistinguishable from the north-eastern North America 10 Be production rate (3.91 � 0.19 atoms g � 1 a � 1 ) and yield overall uncertainties of

The absence of a production rate calibration experiment on Greenland has limited the ability to link 10Be exposure dating chronologies of ice-margin change to independent records of rapid climate change. We use radiocarbon age control on Holocene glacial features near Jakobshavn Isbrae, western Greenland, to investigate 10Be production rates. The radiocarbon chronology is inconsistent with the 10Be age calculations based on the current globally averaged 10Be production rate calibration data set, but is consistent with the 10Be production rate calibration data set from north-eastern North America, which includes a calibration site nearby on north-eastern Baffin Island. Based on the best-dated feature available from the Jakobshavn Isbrae forefield, we derive a 10Be production rate value of 3.98 ± 0.24 atoms g a−1, using the ‘St’ scaling scheme, which overlaps with recently published reference 10Be production rates. We suggest that these 10Be production rate data, or the very similar data from north-eastern North America, are used on Greenland. Copyright © 2011 John Wiley & Sons, Ltd.

The occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes. Accurate constraint of both the timing and magnitude of events, such as the Younger Dryas and Antarctic Cold Reversal, is vital in order to test different hypotheses for the causes and propagation of abrupt climate change. However, in contrast to higher-latitude regions, well-dated records from the Tropics are rare and the structure of late-glacial tropical climate remains uncertain. As a step toward addressing this problem, we present an in situ cosmogenic 3 He surface exposure chronology from Nevado Coropuna, southern Peru, documenting a significant fluctuation of the ice margin during the late-glacial period. Ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the Last Glacial Maximum terminus range from 11.9 to 13.9ka and give an arithmetic mean age of 12.8! 0.7ka (1s). These data constitute direct evidence for a readvance, or prolonged stillstand, of glaciers in the arid Andes of southwestern Peru. Copyright # 2011 John Wiley & Sons, Ltd.

Here we combine 10Be depth profile techniques applied to late glacial ice-contact marine and lacustrine deltas, as well as boulder exposure dating of associated features in the Scoresby Sound region, east Greenland, to determine both the surface age and the magnitude of cosmogenic nuclide inheritance. Boulder ages from an ice-contact delta in northern Scoresby Sund show scatter typical of polar regions and yield an average age of 12.8 ± 0.5 ka – about 2 ka older than both our average profile surface age of 10.9 ± 0.7 ka from three depth profiles and a radiocarbon-based estimate. On the other hand, boulder exposure ages from a set of moraines in southern Scoresby Sund show excellent internal consistency for polar regions and yield an average age of 11.6 ± 0.2 ka. The profile surface age from a corresponding ice-contact delta is 8.1 ± 0.9 ka, while a second delta yields an age of 10.0 ± 0.4 ka. Measured 10Be inheritance concentrations from all depth profiles are internally consistent and are between 10% and 20% of the surface concentrations, suggesting a regional cosmogenic inheritance signal for the Scoresby Sound landscape. Based on the profile inheritance concentrations, we explore the first-order catchment-averaged bedrock erosion under the Greenland ice sheet, yielding estimates of total erosion during the last glacial cycle of the order of 2–30 m. Copyright © 2010 John Wiley & Sons, Ltd.

(1) Calculation of the sea level high latitude production rate based on the samples from the Herdla-Halsnoy Younger Dryas end moraine required accounting for the isostatic rebound of the site following deglaciation (Lohne et al., 2007). In the process of calculating exposure ages of samples from nearby sites, it was realized that the original calibration of the production rates from Halsnoy were calculated using a correction based on the relative sea level history (70m maximum) plus the eustatic sea level history (50m at maximum), i.e. that the amount of isostatic depression had been over corrected. Sea level high latitude Be production rates based on the correct relative sea level history are presented below in a corrected version of Table 2. (2) In the original publication, the ‘Combined’ production rates were determined using a chi-squared minimization based on the data from both calibration sites. However, because the two studied sites have significantly different ages (Halsnoy 11650 100 and Oldedalen 6070 111), and therefore integrate production over different lengths of time, it is inappropriate to determine a sea level high latitude production rate for the entire data set, using the chi-squared minimization. Instead, the arithmetic mean of the production rates from the two sites is the more appropriate average production rate. We present in the corrected Table 2 below both the updated production rate values for the samples from Halsnoy and average production rates using the arithmetic mean and standard deviation of the production rates from the two sites. We apologize for any inconvenience caused. References