HOLOCENE GLACIER LENGTH VARIATIONS ALONG THE AMERICAN CORDILLERAS FROM PAIRED 14C-10BE MEASUREMENTS

Rising temperatures in the industrial era have led to the smallest observed global glacier lengths since record keeping began roughly a century ago. The decline of worldwide alpine glaciation poses a significant societal challenge, most notably in locations reliant on summer glacier melt for agriculture and drinking water. The degree to which modern glacial positions across the globe are anomalous relative to the Holocene, however, remains largely unknown despite a geologic perspective providing insight into future changes. Here, we present 65 paired cosmogenic in situ 14C and 10Be measurements in recently exposed proglacial bedrock for 11 glaciers spanning the length of the American Cordilleras to assess Holocene alpine glacier lengths.

We collected 5-10 bedrock samples at or near the modern front of each glacier. 10Be exposure ages are inferred to represent cumulative exposure durations during the Holocene, while 14C is used to infer burial, as 14C will experience significant decay relative to 10Be when the bedrock is shielded from nuclide production by overlying ice. Because nuclide concentrations can only provide cumulative exposure and burial durations, observed 14C/10Be ratios were compared to outputs from 100,000 randomized exposure-burial histories through a Monte-Carlo-based model to determine when glaciers may have advanced or retreated during the Holocene. Nuclide concentrations are near zero for all four tropical glaciers, a glacier in the Sierra Nevada, California, and a glacier in Patagonia, Chile, providing evidence that these glaciers have retreated to their smallest positions of the Holocene. Other sites in the Northern and Southern hemispheres (30-60º) exhibit a more complex pattern of glacial position through the Holocene, likely reflecting both local and global scale climate forcings.