Paired 14C-10Be exposure ages from Mount Murphy, West Antarctica: Implications for accurate and precise deglacial chronologies

Cosmogenic-nuclide surface exposure ages provide empirical data for validating models simulating the timing and pace of ice-sheet response to a warming climate. Increasing emphasis is being placed on obtaining exposure ages that both accurately constrain Holocene deglaciation and are precise enough to capture ice sheet change at the sub-millennial scale. However, longer-lived nuclides such as 10Be are susceptible to cosmogenic nuclide inheritance often persisting through multiple periods of exposure and burial, which can impact the accuracy of the most recent Holocene exposure history. Shorter-lived in situ cosmogenic 14C (in situ 14C) is largely insensitive to nuclide inheritance pre-dating the last glacial maximum (LGM), and when combined with longer-lived nuclides can be used to constrain complex ice sheet histories over Holocene timescales. Here, we present new in situ 14C exposure ages from nine erratic cobbles from Mount Murphy, West Antarctica. Six of these suggest Mt Murphy deglaciated from 5-3 ka; this is inconsistent with previously measured 10Be ages of the same samples that place deglaciation from 8-6 ka. We investigate potential explanations for the conflicting exposure histories by analysing paired 14C-10Be data of Holocene age presently archived in the informal cosmogenic-nuclide exposure-age database (ICE-D, https://version2.ice-d.org/, last access: 29 March 2024). Our analysis reveals that neither variations in geologic setting nor modelled scenarios of subsurface nuclide production can explain the conflicting Mt Murphy ages. However, replicate in situ 14C measurements indicate that initial in situ 14C concentrations used to calculate the youngest exposure ages (5-3 ka) do not reproduce within stated 2 sigma uncertainty, whereas measurements used to calculate the older ages (8-6 ka) are reproducible. Furthermore, we observe that in situ 14C concentrations measured in 15 of 31 samples taken from ICE-D do not replicate within their nominal 2 sigma analytical uncertainty. Together, these results suggest that analytical uncertainty for in situ 14C measurements may currently be underestimated. We provide recommendations for improving measurement precision that will benefit future Holocene deglaciation studies, including analysis and publication of more replicate measurements and the continuation of efforts to quantify and minimise sources of scatter in blank measurements.