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

Abstract

Radiocarbon (14C) concentrations in the oceans are different from those in the atmosphere. Understanding these ocean-atmospheric 14C differences is important both to estimate the calendar ages of samples which obtained their 14C in the marine environment, and to investigate the carbon cycle. The Marine20 radiocarbon age calibration curve is created to address these dual aims by providing a global-scale surface ocean record of radiocarbon from 55,000–0 cal yr BP that accounts for the smoothed response of the ocean to variations in atmospheric 14C production rates and factors out the effect of known changes in global-scale palaeoclimatic variables. The curve also serves as a baseline to study regional oceanic 14C variation. Marine20 offers substantial improvements over the previous Marine13 curve. In response to community questions, we provide a short intuitive guide, intended for the lay-reader, on the construction and use of the Marine20 calibration curve. We describe the choices behind the making of Marine20, as well as the similarities and differences compared with the earlier Marine calibration curves. We also describe how to use the Marine20 curve for calibration and how to estimate ΔR—the localized variation in the oceanic 14C levels due to regional factors which are not incorporated in the global-scale Marine20 curve. To aid understanding, illustrative worked examples are provided.

Holocene history of the 79° N ice shelf reconstructed from epishelf lake and uplifted glaciomarine sediments

“…relative palaeointensity, it remains difficult to fully quantify this. For example, variations in sea-ice cover, different water masses, or input of glacial meltwater all have the potential to impact the local marine reservoir age (see O'Regan et al, 2021;Pados-Dibattista et al, 2022;Hansen et al, 2022;and Heaton et al, 2022, for further discussion).…”

The impact of Holocene deglaciation and glacial dynamics on the landscapes and geomorphology of Potter Peninsula, King George Island (Isla 25 Mayo), NW Antarctic Peninsula

“…AMS and 13 C/ 12 C isotope ratio measurements undertaken at ETH Zürich, the NERC Radiocarbon Laboratory, East Kilbride, and by Beta Analytical, Miami, were used to calculate conventional radiocarbon ages following established procedures ( 14 C years; Table 2; Supplementary Material for details). Calibration of marine sample radiocarbon ages (marine shells, penguin bones, bones from undetermined species, and seaweed) was undertaken in Oxcal v. 4.4 using the Marine20 calibration curve (Gordon and Harkness, 1992;Bronk Ramsey, 2009;Heaton et al, 2020Heaton et al, , 2022. We used a newly recalculated local marine reservoir age offset (ΔR) of 666 ± 76 14 C years that represents the weighted mean ΔR of the four closest and most applicable radiocarbon-dated marine samples collected before 1950 CE from the northern AP and Signy Island in the online Marine20 database (http://calib.org/marine/) (Table 2; Supplementary Material for details).…”

Holocene deglaciation and glacier readvances on the Fildes Peninsula and King George Island (Isla 25 de Mayo), South Shetland Islands, NW Antarctic Peninsula

“…Calibration of radiocarbon ages from marine samples was undertaken in Oxcal v. 4.4 using the Marine20 calibration curve (Bronk Ramsey, 2009;Heaton et al, 2020Heaton et al, , 2022 Terrestrial samples, Kiteschsee Lake samples and previously published data from Yanou Lake (Roberts et al, 2017), were (re)calibrated using the Southern Hemisphere SHCal20 calibration curve (Hogg et al, 2020) in Oxcal v. 4.4 (Bronk Ramsey, 2009. Post-bomb (>1950 CE) ages were corrected according to 13 C/ 12 C isotopic ratios from measured pMC with the 'present day' pMC value defined as 107.5% (2010 CE) and calibrated using the SHCal13 SH Zone 1-2 Bomb curve in CALIBomb (Reimer and Reimer, 2004;Hua et al, 2013).…”