Abstract
ABSTRACTThe concentration of radiocarbon (14C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their14C 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 intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal2014C curve and reconstructed changes in CO2obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric14C 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 atmospheric14C 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 forΔR, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data basehttp://calib.org/marine/.
Highlights
Marine Specific CalibrationIn order to convert the radiocarbon (14C) date of a sample into a calendar age we need to perform calibration
The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 14C curve and reconstructed changes in CO2 obtained from ice core data
To further complicate marine calibration, the level of depletion varies according to both location and time due to spatio-temporal differences in ocean mixing, various factors affecting the rates of air-sea exchange and wider changes to the carbon cycle, notably the atmospheric CO2 mixing ratio (Bard 1988)
Summary
In order to convert the radiocarbon (14C) date of a sample into a calendar age we need to perform calibration. LSG OGCM estimates for the MRA in a particular calendar year within polar regions can vary by over 1000 14C yr between a simulation assuming a climate scenario suitable for the present day (its PD scenario which assumes little sea ice, Butzin et al 2005, 2020 in this issue) and runs in climate scenarios appropriate for more extreme glacial periods (e.g. its GS/CS scenarios where the extent of sea ice is much greater in addition to significant changes to ocean ventilation and wind stress) This difference indicates that uncertainty in past polar climate conditions can have a significant effect on calibration in these regions. Such users should be aware that each individual LSG OGCM simulation only considers a single climate scenario. The calibrated age estimates obtained under any single fixed scenario are likely to be considerably over-precise
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