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

Abstract

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 past (Holocene) and corresponds to a low-depletion glacial scenario; while an upper bound representing high-depletion, ΔR_20^GS, 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.

A RESPONSE TO COMMUNITY QUESTIONS ON THE MARINE20 RADIOCARBON AGE CALIBRATION CURVE: MARINE RESERVOIR AGES AND THE CALIBRATION OF 14C SAMPLES FROM THE OCEANS

“…The use of Marine20, or any of the earlier Marine calibration curves, using a constant (Holocene-based) Δ R is likely to significantly underestimate the calendar age uncertainty of glacial-period polar oceanic 14 C samples and introduce biases, providing calendar age estimates that may be substantially older than the true age of the 14 C sample. See Section 2.1.6 and Heaton et al (2022) for further advice on calibration of polar marine 14 C samples.…”

“…The Marine20 group have proposed that a user calibrate polar samples against latitudinal-specific maximum-depletion and minimum-depletion curves separately and use two resultant ages to inform a bracketing which it is hoped encompasses the true calendar age (Heaton et al 2022). These bracketing calendar age intervals are however wide—with differences of up to 1500 calendar years between the maximum - and minimum-depletion calibration scenarios.…”

A Greenland-wide empirical reconstruction of paleo ice-sheet retreat informed by ice extent markers: PaleoGrIS version 1.0

“…We note these ΔR values mostly overlap with the newest Greenland-specific marine reservoir age correction assessment of Pearce et al (in review, submitted after our compiled ages were re-calculated). We acknowledge that for polar latitudes (>50ºN), calibrating marine radiocarbon ages against the Marine20 curve may be problematic, due to greater variability in ocean ventilation and air-sea gas exchange caused by fluctuations in sea ice extent and wind strength, leading to increased and more time-variable marine reservoir effects (Butzin et al, 2005;Heaton et al, 2022). However, this is more likely to be problematic during glacial periods (Reimer et al, 2020).…”

“…For polar samples dating to the Holocene (11.5-0 kyr BP), Heaton et al (2022) recommend calibrating directly against Marine20. Since the PaleoGrIS 1.0 reconstruction spans the Late-glacial and early-to-mid Holocene (14-6.5 kyr BP) period, and because 90% of the calibrated radiocarbon ages compiled in our database are younger than 11 kyr BP, we choose to treat all samples the same, for consistency.…”

“…7). This is in part explained by numerous event ages resulting from a single marine radiocarbon age, here attributed a low confidence rating due to large uncertainties in past marine radiocarbon reservoir effects at high latitudes (Heaton et al, 2022) (see criteria list: Table 1). However, we stress that event ages described as "low confidence" may still closely estimate the true timing of margin retreat and are by no means excluded from our isochrone reconstruction.…”