Marine Radiocarbon Calibration in Polar Regions: A Simple Approximate Approach using Marine20

The Marine20 radiocarbon (14C) age calibration curve, and all earlier marine radiocarbon calibration curves from the IntCal group, must be used extremely cautiously for the calibration of marine 14C samples from polar regions (outside ~ 40ºS – 40ºN) during glacial periods. Calibrating polar 14C marine samples from glacial periods against any Marine calibration curve (Marine20 or any earlier product) using an estimate of ΔR, the regional 14C depletion adjustment, that has been obtained from samples in the recent (non-glacial) past is likely to lead to bias and overconfidence in the calibrated age. We propose an approach to calibration that aims to address this by accounting for the possibility of additional, localized, glacial 14C depletion in polar oceans. We suggest, for a specific polar location, bounds on the value of ΔR_20 (θ) during a glacial period. The lower bound ΔR_20^Hol may be based on 14C samples from the recent non-glacial (Holocene) past and corresponds to a low-depletion glacial scenario. The upper bound, ΔR_20^GS, representing a high-depletion scenario is found by increasing ΔR_20^Hol according to the latitude of the 14C sample to be calibrated. The suggested increases to obtain ΔR_20^GS are based upon simulations of the Hamburg Large Scale Geostrophic Ocean General Circulation Model (LSG OGCM). Calibrating against the Marine20 curve using the upper and lower ΔR_20 bounds provide estimates of calibrated ages for glacial 14C samples in high- and low-depletion scenarios which should bracket the true calendar age of the sample. In some circumstances, users may be able to determine which depletion scenario is more appropriate using independent paleoclimatic or proxy evidence.