The concentration of radiocarbon (14C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their 14C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally-agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0 – 55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realisations of our Northern Hemispheric atmospheric IntCal20 14C curve and reconstructed changes in CO2 obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric 14C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric 14C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for 𝛥𝑅, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/.This data set includes:- the data plotted in the related manuscript, including Marine20, and IntCal20, the most recent version of the radiocarbon age calibration curves- the 500 northern hemispheric atmospheric Δ14C realisations of IntCal20 used as input for the calculation of Marine20- a netCDF file from the LSG OCGCM with spatially resolved marine reservoir ages.

The netCDF file included here contains results of marine reservoir age (MRA) simulations applying the LSG ocean general circulation model forced with various climatic background conditions and with atmospheric radiocarbon changes according to the IntCal20 reconstruction for the years 55,000-0 cal BP (one data point every 50 years), oceanic depth range, from which the MRA has been calculated, is 0-50 m. Land values are flagged with -100. The different results denote upper (MRA_MAX) and lower (MRA_MIN) bounds as well as the average (MRA_AVG) and the median (MRA_MED) of the ensemble of nine simulations ((3 versions of atmosheric Delta14C from IntCal20) x (3 climate forcing scenarios)). The range of results decreases for 10,700-0 cal BP, since this period covers only 3 ensemble members based on one climate forcing. This dataset complements PANGAEA dataset doi:10.1594/PANGAEA.902301, which was used in the construction of IntCal20 and employed preliminary atmospheric radiocarbon forcing based on the Hulu Cave speleothem record.