Tephrochronology as a tool to constrain radiocarbon reservoir age in the deglacial Bering Sea

Abstract:

In order to accurately calendar date marine carbon, it is necessary to constrain surface reservoir age, the apparent ¹⁴C age difference between the atmosphere and surface ocean that results from incomplete equilibration of ¹⁴C across the air-sea interface. Surface reservoir age is generally assumed to be constant at the preindustrial value, but evidence suggests it has varied through time by up to 1000 years. In this study we use tephrochronology, a method of correlating tephras across different environments, to identify equivalent strata, as a tool to quantify reservoir age in the Bering Sea during the transition between the Oldest Dryas and Bolling-Allerod (14.7 kcal BP). With frequent volcanic eruptions that allow for possibility of high-resolution reservoir age reconstructions, the Bering Sea/Aleutian Islands region is uniquely positioned to provide insight into the hypothesis that dense water formed in the North Pacific during the last deglaciation. We compare a massive tephra found in three deep-sea sediment cores from Umnak Plateau in the southeast Bering Sea (HLY02-02-55JPC, HLY-02-02-51JPC, and IODP Site U1339) to a tephra dated to 14.8 kcal BP from Deep Lake, Sanak Island in the Eastern Aleutians. For both the Umnak and Sanak tephras, volcanic glass shards are geochemically matched using major and trace elements from electron microprobe and laser-ablation inductively-coupled-plasma mass spectrometry. We compare ¹⁴C ages of foraminiferal species Uvigerina peregrina and Neogloboquadrina pachyderma (sinistral) from just above the tephra in HLY-02-02-51JPC (1467 m) to ¹⁴C age of the corresponding tephra at Sanak Island from terrestrial plant macrofossils. The surface reservoir age found (930 ± 160 ¹⁴C y) is similar to the average preindustrial value in the region (790 ± 130 ¹⁴C y). Benthic-atmosphere age difference (1860 ± 200 ¹⁴C y) is also comparable to the preindustrial value (2030 ± 60 ¹⁴C y). These results and future work on additional tephras from these cores could have implications for evaluating the accuracy of radiocarbon reconstructions of Pacific circulation that assume a constant preindustrial surface reservoir age.