Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

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/.

24The stable carbon isotope ratio of atmospheric CO 2 (! 13 C atm ) is a key parameter to decipher 25 past carbon cycle changes. Here we present ! 13 C atm data for the last 24,000 years derived 26 from three Antarctic ice cores. We conclude that a pronounced 0.3‰ decrease in ! 13 C atm 27 during the early deglaciation can be best explained by upwelling of old, carbon-enriched 28 waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial 29 biosphere, changes in sea surface temperature, and ocean circulation governed the ! 13 C atm 30 evolution. During the Last Glacial Maximum, ! 13 C atm and CO 2 were essentially constant, 31suggesting that the carbon cycle was in dynamic equilibrium and that the net transfer of 32 carbon to the deep ocean had occurred before then. showing pronounced differences in atmospheric CO 2 rates of change in the course of the 47 last glacial/interglacial transition (3). Many processes have been involved in attempts to 48 explain these CO 2 variations, but it has become evident that none of these mechanisms 49 alone can account for the 90 ppmv increase in atmospheric CO 2 . A combination of 50 processes must have been operating (4, 5), with their exact timing being crucial. However, 51 a unique solution to the deglacial carbon cycle changes has not been yet found. 52 53

[1] Chemical weathering is an integral part of both the rock and carbon cycles and is being affected by changes in land use, particularly as a result of agricultural practices such as tilling, mineral fertilization, or liming to adjust soil pH. These human activities have already altered the terrestrial chemical cycles and land-ocean flux of major elements, although the extent remains difficult to quantify. When deployed on a grand scale, Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be used to remove CO 2 from the atmosphere. The release of cations during the dissolution of such silicate minerals would convert dissolved CO 2 to bicarbonate, increasing the alkalinity and pH of natural waters. Some products of mineral dissolution would precipitate in soils or be taken up by ecosystems, but a significant portion would be transported to the coastal zone and the open ocean, where the increase in alkalinity would partially counteract "ocean acidification" associated with the current marked increase in atmospheric CO 2 . Other elements released during this mineral dissolution, like Si, P, or K, could stimulate biological productivity, further helping to remove CO 2 from the atmosphere. On land, the terrestrial carbon pool would likely increase in response to Enhanced Weathering in areas where ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution. In the ocean, the biological carbon pumps (which export organic matter and CaCO 3 to the deep ocean) may be altered by the resulting influx of nutrients and alkalinity to the ocean. This review merges current interdisciplinary knowledge about Enhanced Weathering, the processes involved, and the applicability as well as some of the consequences and risks of applying the method.

International audienceUnderstanding the role of atmospheric CO2 during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO2 and Antarctic temperature for the past 800,000 years. Here we propose a revised relative age scale for the concentration of atmospheric CO2 and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO2 concentration and Antarctic temperature at four times when their trends change abruptly. We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO2, as has been suggested by earlier studie

10 CO 2 -consumption by chemical weathering of silicates and resulting silicate/carbonate weathering 11 ratios influences long-term climate changes. However, little is known of the spatial extension of highly 12 active weathering regions and their proportion of global CO 2 -consumption. As those regions may be of 13 significant importance for global climate change, global CO 2 -consumption is calculated here at high 14 resolution, to adequately represent them. In previous studies global CO 2 -consumption is estimated 15 using two different approaches: i) a reverse approach based on hydrochemical fluxes from large rivers 16and ii) a forward approach applying spatially explicit a function for CO 2 -consumption. The first 17 approach results in an estimate without providing a spatial resolution for highly active regions and the 18 second approach applied six lithological classes while including three sediment classes (shale, 19 sandstone and carbonate rock) based at a 1°or 2°grid resolution. It remained uncertain, if the applied 20 lithological classification schemes represent adequately CO 2 -consumption from sediments on a global 21 scale. This is due to the large variability of sediment properties, their diagenetic history and the 22 contribution from carbonates apparent in silicate dominated lithological classes. To address these 23 issues, a CO 2 -consumption model, trained at high resolution data, is applied here to a global vector 24 based lithological map with 15 lithological classes. The calibration data were obtained from areas 25 representing a wide range of weathering rates. Resulting global CO 2 -consumption by chemical 26 weathering is similar to earlier estimates (237 Mt C a -1 ) but the proportion of silicate weathering is 27 63%, and thus larger than previous estimates (49 to 60%). The application of the enhanced lithological 28 classification scheme reveals that it is important to distinguish among the various types of sedimentary 29 rocks and their diagenetic history to evaluate the spatial distribution of rock weathering. Results 30 highlight the role of hotspots (> 10 times global average weathering rates) and hyperactive areas (5 to 31 10 times global average rates). Only 9 % of the global exorheic area is responsible for about 50 % of 32 CO 2 -consumption by chemical weathering (or if hotspots and hyperactive areas are considered: 3.4% 33 of exorheic surface area corresponds to 28% of global CO 2 -consumption). The contribution of 34 endorheic areas to the global CO 2 -consumption is with 3.7 Mt C a -1 only minor. A significant impact on 35 * Manuscript Click here to view linked References 2 the global CO 2 -consumption rate can be expected if identified highly active areas are affected by 36 changes in the overall spatial patterns of the hydrological cycle due to ongoing global climate change. 37Specifically if comparing the Last Glacial Maximum with present conditions it is probable that also the 38 global carbon cycle has been affected by those changes. It is expected that results...

The temperature on Earth varied largely in the Pleistocene from cold glacials to interglacials of different warmths. To contribute to an understanding of the underlying causes of these changes we compile various environmental records (and model-based interpretations of some of them) in order to calculate the direct effect of various processes on Earth's radiative budget and, thus, on global annual mean surface temperature over the last 800,000 years. The importance of orbital variations, of the greenhouse gases CO 2 , CH 4 and N 2 O, of the albedo of land ice sheets, annual mean snow cover, sea ice area and vegetation, and of the radiative perturbation of mineral dust in the atmosphere are investigated. Altogether we can explain with these processes a global cooling of 3.9 ± 0.8 K in the equilibrium temperature for the Last Glacial Maximum (LGM) directly from the radiative budget using only the Planck feedback that parametrises the direct effect on the radiative balance, but neglecting other feedbacks such as water vapour, cloud cover, and lapse rate. The unaccounted feedbacks and related uncertainties would, if taken at present day feedback strengths, decrease the global temperature at the LGM by −8.0 ± 1.6 K. Increased Antarctic temperatures during the Marine Isotope Stages 5.5, 7.5, 9.3 and 11.3 are in our conceptual approach difficult to explain. If compared with other studies, such as PMIP2, this gives supporting evidence that the feedbacks themselves are not constant, but depend in their strength on the mean climate state. The best estimate and uncertainty for our reconstructed radiative forcing and LGM cooling support a present day equilibrium climate sensitivity (excluding the ice sheet and vegetation components) between 1.4 and 5.2 K, with a most likely value near 2.4 K, somewhat smaller than other methods but consistent with the consensus range of 2 − 4.5 K derived from other lines of evidence. Climate sensitivities above 6 K are difficult to reconcile with Last Glacial Maximum reconstructions.

Geoengineering is a proposed action to manipulate Earth's climate in order to counteract global warming from anthropogenic greenhouse gas emissions. We investigate the potential of a specific geoengineering technique, carbon sequestration by artificially enhanced silicate weathering via the dissolution of olivine. This approach would not only operate against rising temperatures but would also oppose ocean acidification, because it influences the global climate via the carbon cycle. If important details of the marine chemistry are taken into consideration, a new mass ratio of CO 2 sequestration per olivine dissolution of about 1 is achieved, 20% smaller than previously assumed. We calculate that this approach has the potential to sequestrate up to 1 Pg of C per year directly, if olivine is distributed as fine powder over land areas of the humid tropics, but this rate is limited by the saturation concentration of silicic acid. In our calculations for the Amazon and Congo river catchments, a maximum annual dissolution of 1.8 and 0.4 Pg of olivine seems possible, corresponding to the sequestration of 0.5 and 0.1 Pg of C per year, but these upper limit sequestration rates come at the environmental cost of pH values in the rivers rising to 8.2. Open water dissolution of fine-grained olivine and an enhancement of the biological pump by the rising riverine input of silicic acid might increase our estimate of the carbon sequestration, but additional research is needed here. We finally calculate with a carbon cycle model the consequences of sequestration rates of 1-5 Pg of C per year for the 21st century by this technique.alkalinity enhancement | river alkalization | diatoms | biological production | climate engineering