The 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/.

Significance: The ocean’s role in regulating atmospheric carbon dioxide on glacial–interglacial timescales remains an unresolved issue in paleoclimatology. Many apparently independent changes in ocean physics, chemistry, and biology need to be invoked to explain the full signal. Recent understanding of the deep ocean circulation and stratification is used to demonstrate that the major changes invoked in ocean physics are dynamically linked. In particular, the expansion of permanent sea ice in the Southern Hemisphere results in a volume increase of Antarctic-origin abyssal waters and a reduction in mixing between abyssal waters of Arctic and Antarctic origin.

A Drop in the Ocean: The concentration of atmospheric CO ₂ rose by ∼80 parts per million (ppm), from ∼190 to 270 ppm, during the last deglaciation. It is widely believed that the primary source of that CO ₂ was the deep Southern Ocean. Burke and Robinson (p. 557 , published online 15 December) present a 25,000-year-long record of the radiocarbon content of deep-sea corals collected from the Southern Ocean, which shows evidence of the ¹⁴ C-depletion that must have accompanied CO ₂ sequestration. ¹⁴ C depletion and ocean stratification ended between 15,000 and 14,000 years ago, in a manner consistent with the transfer of large amounts of CO ₂ from the deep Southern Ocean to the atmosphere. The observed ¹⁴ C drop can explain the atmospheric CO ₂ rise between 17,500 and 14,500 years ago, adding support to the existing model of deglacial CO ₂ dynamics.

The last deglaciation, which marked the transition between the last glacial and present interglacial periods, was punctuated by a series of rapid (centennial and decadal) climate changes. Numerical climate models are useful for investigating mechanisms that underpin the climate change events, especially now that some of the complex models can be run for multiple millennia. We have set up a Paleoclimate Modelling Intercomparison Project (PMIP) working group to coordinate efforts to run transient simulations of the last deglaciation, and to facilitate the dissemination of expertise between modellers and those engaged with reconstructing the climate of the last 21 000 years. Here, we present the design of a coordinated Core experiment over the period 21–9 thousand years before present (ka) with time-varying orbital forcing, greenhouse gases, ice sheets and other geographical changes. A choice of two ice sheet reconstructions is given, and we make recommendations for prescribing ice meltwater (or not) in the Core experiment. Additional focussed simulations will also be coordinated on an ad hoc basis by the working group, for example to investigate more thoroughly the effect of ice meltwater on climate system evolution, and to examine the uncertainty in other forcings. Some of these focussed simulations will target shorter durations around specific events in order to understand them in more detail and allow for the more computationally expensive models to take part.