Research paperIn situ cosmogenic 10Be production rate in the High Tropical Andes

- Dating of a glaciogenic fan in the Cerro Azanaques massif (Bolivia, 18.91°S - 66.76°W - 3800 masl).
- Stratigraphic correlation and 14C dating yields a fan age of 16.07 ± 0.64 ka BP.
- Mean in situ-produced 10Be concentrations of fan boulders 4.92 ± 0.05 × 105 at g−1.
- Local (18.91°S - 66.76°W - 3800 masl) 10Be production rate is 30.8 ± 1.3 at g−1 yr−1 (SLHL P10 = 3.76 ± 0.15 at g−1 yr−1 Lm).
- An internally-consistent reference calibration dataset combining three High Tropical Andes calibration studies is proposed.

Continental climate change during the late glacial period has now been widely documented thanks to Cosmic-Ray Exposure (CRE) dating of glacial features. The accuracy of these CRE ages mainly relies on a priori knowledge of the production rate of the cosmogenic nuclide that has accumulated in a specific mineral. To produce unequivocal and accurate chronologies of glacier fluctuations during the late glacial period, it is crucial that the cosmogenic nuclide production rates are better constrained, particularly in the high tropics where existing spatial and temporal scaling models show significant discrepancies. Here we report a new production rate established at low latitude (19°S) and high elevation (3800 masl) on the Challapata fan-delta, at the edge of the Paleolake Tauca, on the flank of Cerro Azanaques (Bolivia). Sedimentological evidence for synchronicity with the Tauca Lake highstand along with U-Th and 14C measurements established that the fan-delta is 16.07 ± 0.64 kyr BP old. In situ-produced 10Be concentrations measured in 15 boulders lying on the fan-delta yield a mean 10Be concentration of 4.92 ± 0.05 × 105 at g−1. A local in situ 10Be production rate of 30.8 ± 1.3 at g−1 yr−1 is thus obtained at 3800 masl and 19°S. Application of the “Lal-modified” scaling scheme to this Azanaques production rate, using a standard atmosphere and the Muscheler et al. (2005) geomagnetic reconstruction, leads to a Sea Level High Latitude (SLHL) in situ 10Be production rate of 3.76 ± 0.15 at g−1 yr−1 (1σ uncertainty). In addition, we propose a reference in situ 10Be calibration dataset for the region that combines the production rates of this study with those of Blard et al. (2013b) and Kelly et al. (2015). This dataset of three calibration sites shows a good consistency and yields a regional in situ 10Be production rate of 3.74 ± 0.09 at g−1 yr−1 using the same scaling.