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 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 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/.

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.

Changes in the upwelling and degassing of carbon from the Southern Ocean form one of the leading hypotheses for the cause of glacial-interglacial changes in atmospheric carbon dioxide. We present a 25,000-year-long Southern Ocean radiocarbon record reconstructed from deep-sea corals, which shows radiocarbon-depleted waters during the glacial period and through the early deglaciation. This depletion and associated deep stratification disappeared by ~14.6 ka (thousand years ago), consistent with the transfer of carbon from the deep ocean to the surface ocean and atmosphere via a Southern Ocean ventilation event. Given this evidence for carbon exchange in the Southern Ocean, we show that existing deep-ocean radiocarbon records from the glacial period are sufficiently depleted to explain the ~190 per mil drop in atmospheric radiocarbon between ~17 and 14.5 ka.

Antarctic ice-core data reveal that the atmosphere experienced abrupt centennial increases in CO 2 concentration during the last deglaciation (~18-11 thousand years, ka).Establishing the role of ocean circulation in these changes requires high-resolution, accurately-dated marine records. Here we report radiocarbon data from uranium-thorium dated deep-sea corals in the Equatorial Atlantic and Drake Passage over the last 25 ka. Two major deglacial radiocarbon increases occurred in phase with centennial atmospheric CO 2 rises at 14.8 ka and 11.7 ka. We interpret these radiocarbon-enriched signals to represent two short-lived (<500 years) 'overshoot' events with Atlantic meridional overturning stronger than modern. These results provide compelling evidence for a close coupling of ocean circulation and centennial climate events during the last deglaciation.

Abstract. 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.

The cause of atmospheric CO 2 change during the recent ice ages remains a first order question in climate science. Most mechanisms have invoked carbon exchange with the deep ocean, due to its large size and relatively rapid exchange time with the atmosphere 1 . The Southern Ocean is thought to play a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region 2 . However reconstructing changes in deep Southern Ocean carbon storage is challenging, so few direct tests of this hypothesis exist. Here we present new deep-sea coral boron isotope data that track the pH -and thus CO 2 chemistry -of the deep Southern Ocean over the last 40,000 years. At sites closest to the Antarctic continental margin, and most influenced by the deep Southern waters that form the ocean's lower overturning cell, we find a close relationship between ocean pH and atmospheric CO 2 : during intervals of low CO 2 ocean pH is low, reflecting enhanced ocean carbon storage; during intervals of rising CO 2 ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial to centennial-scale) pH decreases during abrupt CO 2 rise, reflecting the rapid transfer of carbon from the deep to the upper

The record of volcanic forcing of climate over the past 2500 years is based primarily on sulfate concentrations in ice cores. Of particular interest are large volcanic eruptions with plumes that reached high altitudes in the stratosphere, as these afford sulfate aerosols the longest residence time in the atmosphere, and thus have the greatest impact on radiative forcing. Sulfur isotopes measured in ice cores can be used to identify these large eruptions because stratospheric sulfur is exposed to UV radiation, which imparts a time-evolving mass independent fractionation (MIF) that is preserved in the ice. However sample size requirements of traditional measurement techniques mean that the MIF signal may be obscured, leading to an inconclusive result. Here we present a new method of measuring sulfur isotopes in ice cores by multi-collector inductively

Supplementary Information End-member decompositionFollowing the approach of Gaillardet et al. (1999), we use a series of linear equations to solve for the proportions of sodium in the river that are attributed to evaporite, carbonate, and silicate weathering. Table 1 in the main text specifies the end-member molar ratios and their associated uncertainty. In order to propagate this uncertainty in end-member values through the calculation, we solve the linear equations for each river 10,000 times using a random sampling of weathering end-member values assuming a Gaussian distribution. For all of the calculations in the main text, the median and standard deviation of these Monte Carlo simulations were used as the solution and uncertainty for each river. For all rivers, using the median of the Monte Carlo simulations gives proportions attributed to each end-member that sum to 1 (± 0.05).Although many of the rivers have distributions of Monte Carlo solutions that are roughly Gaussian for all three end-members (e.g. Supplementary Figure 1), there are a total of 12 of the 31 rivers whose solution distributions for either the evaporite or silicate (or both) end-members are not Gaussian and they have a long tail (e.g. Supplementary Figure 2). This is likely related to the fact that the end-member values for evaporites and silicates have overlapping ranges. In contrast the carbonate end-member values are more distinct resulting in Monte Carlo solution distributions for the carbonate end-member that are typically roughly Gaussian. These 12 rivers are: Brahmaputra, Fraser, Ganges, Kaoping, Kolyma, Lena, Maipo, Mekong, Orinoco, Red, Yangtze, and Yukon rivers.We have tested the influence that these poorly constrained rivers with long tailed distributions have on the results from this study. If we use the modes instead of the medians for those rivers that do not have near-Gaussian Monte Carlo distributions, the total fluxes reported in the main text change by <0.1 Tmol/y, well within the 0.2 Tmol/y uncertainty. However, it does make a slight difference for the proportions of sulfate attributed to each of the weathering end-members (and excess sulfate) for those 12 rivers. This difference influences where these rivers plot on Figure 6 (main text), and is illustrated in Supplementary Figure 3. Although these 12 rivers plot slightly differently in this figure, the major conclusions remain valid, namely that the rivers plot to the left of the 1:1 line and are farther from the line with increasing excess sulfate values, indicating that the excess sulfate likely has a low δ 34 S value. References Gaillardet, J., Dupré, B., Louvat, P., Allegre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chemical Geology 159, 3-30.