Description: Numerical models are important tools for understanding the processes and feedbacks in the Earth system, including those involving changes in atmospheric CO2 (CO2,atm) concentrations. Here, we compile 55 published model studies (consisting of 778 individual simulations) that assess the impact of six forcing mechanisms on millennial-scale CO2,atm variations: changes in freshwater supply to the North Atlantic and Southern Ocean, the strength and position of the southern-hemisphere westerlies, Antarctic sea ice extent, and aeolian dust fluxes. We generally find agreement on the direction of simulated CO2,atm change across simulations, but the amplitude of change is inconsistent, primarily due to the different complexities of the model representation of Earth system processes. When freshwater is added to the North Atlantic, a reduced Atlantic Meridional Overturning Circulation (AMOC) is generally accompanied by an increase in Southern Ocean- and Pacific overturning, reduced Antarctic sea ice extent, spatially varying export production, and changes in carbon storage in the Atlantic (rising), in other ocean basins (generally decreasing) and on land (more varied). Positive or negative CO2,atm changes are simulated during AMOC minima due to a spatially and temporally varying dominance of individual terrestrial and oceanic drivers (and compensating effects between them) across the different models. In contrast, AMOC recoveries are often accompanied by rising CO2,atm levels, which are mostly driven by ocean carbon release (albeit from different regions). The magnitude of simulated CO2,atm rise broadly scales with the duration of the AMOC perturbation (i.e., the stadial length). When freshwater is added to the Southern Ocean, reduced deep-ocean ventilation drives a CO2,atm drop via reduced carbon release from the Southern Ocean. Although the impacts of shifted southern-hemisphere westerlies are inconsistent across model simulations, their intensification raises CO2,atm via enhanced Southern Ocean Ekman pumping. Increased supply of aeolian dust to the ocean, and thus iron fertilisation of marine productivity, consistently lowers modelled CO2,atm concentrations via more efficient nutrient utilisation. The magni- tude of CO2,atm change in response to dust flux variations, however, largely depends on the complexity of models' marine ecosystem and iron cycle. This especially applies to simulations forced by Antarctic sea ice changes, in which the direction of simulated CO2,atm change varies greatly across model hierarchies. Our compilation highlights that no single (forcing) mechanism can explain observed past millennial-scale CO2,atm variability, and identifies important future needs in coupled carbon cycle-climate modelling to better understand the mechanisms governing CO2,atm changes in the past.

Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of American Association for the Advancement of Science for personal use, not for redistribution. The definitive version was published in Science 349 (2015): 706-710, doi:10.1126/science.aaa9554.

Description: Changes in the formation of dense water in the Arctic Ocean and Nordic Seas (the ‘Arctic Mediterranean’, AM) likely contributed to the altered climate of the last glacial period. We examine past changes in AM circulation by reconstructing 14C ventilation ages of the deep Nordic Seas over the last 30,000 years. Our results show that the deep glacial AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest the deep glacial AM was ~2-3°C warmer than modern; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to melting sea-ice and icebergs, as well as proximal terminal ice-sheet margins.

Description: Funding was provided by a WHOI OCCI scholarship and OCCI grant 27071264 (DJRT); WHOI OCCI and NSF grants OIA-1124880 and OCE-1357121 (GG); the WHOI J. Lamar Worzel Assistant Scientist Fund, the Penzance Endowed Fund in Support of Assistant Scientists and NSF ANT-1246387 (WG); EU FP7 Marie Curie grant No. 298513 (MZ), grant DP140101393 (JY); and NERC grant NE/J008133/1 (SB).

Description: Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Radiocarbon 58 (2016): 205-211, doi:10.1017/RDC.2015.22.

Description: Reservoir age offsets are widely used to correct marine and speleothem radiocarbon age measurements for various calibration purposes. They also serve as a powerful tracer for carbon cycle dynamics. However, a clear terminology regarding reservoir age offsets is lacking, sometimes leading to miscalculations. This note seeks to provide consistent conventions for reporting reservoir 14C disequilibria useful to a broad range of environmental sciences. This contribution introduces the F14R and δ14R metrics to express the relative 14C disequilibrium between two contemporaneous reservoirs and the R metric as the associated reservoir age offset.

Description: G.S. acknowledges the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution with funding provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE-1239667). S.R.B acknowledges Dean Minghua Zhang and Provost Dennis Assanis of Stony Brook University for financial support.

Description: Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Waelbroeck, C., Lougheed, B. C., Riveiros, N. V., Missiaen, L., Pedro, J., Dokken, T., Hajdas, I., Wacker, L., Abbott, P., Dumoulin, J., Thil, F., Eynaud, F., Rossignol, L., Fersi, W., Albuquerque, A. L., Arz, H., Austin, W. E. N., Came, R., Carlson, A. E., Collins, J. A., Dennielou, B., Desprat, S., Dickson, A., Elliot, M., Farmer, C., Giraudeau, J., Gottschalk, J., Henderiks, J., Hughen, K., Jung, S., Knutz, P., Lebreiro, S., Lund, D. C., Lynch-Stieglitz, J., Malaize, B., Marchitto, T., Martinez-Mendez, G., Mollenhauer, G., Naughton, F., Nave, S., Nuernberg, D., Oppo, D., Peck, V., Peeters, F. J. C., Penaud, A., Portilho-Ramos, R. d. C., Repschlaeger, J., Roberts, J., Ruehlemann, C., Salgueiro, E., Goni, M. F. S., Schonfeld, J., Scussolini, P., Skinner, L. C., Skonieczny, C., Thornalley, D., Toucanne, S., Van Rooij, D., Vidal, L., Voelker, A. H. L., Wary, M., Weldeab, S., & Ziegler, M. Consistently dated Atlantic sediment cores over the last 40 thousand years. Scientific Data, 6, (2019): 165, doi:10.1038/s41597-019-0173-8.

Description: Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Description: The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013 Grant agreement n° 339108). New 14C dates for cores EW9209-1JPC and V29-202 were funded by NSF OCE grants to DWO. FN, ES and AV acknowledge FCT funding support through project UID/Multi/04326/2019. We thank T. Garlan and P. Guyomard for having given us access to cores from the Service Hydrographique et Océanographique de la Marine. We acknowledge N. Smialkowski for help with formatting the data into text files, and L. Mauclair, L. Leroy and G. Isguder for the picking of numerous foraminifer samples for radiocarbon dating. We are grateful to S. Obrochta, E. Cortijo, E. Michel, F. Bassinot, J.C. Duplessy, and L. Labeyrie for advice and fruitful discussions. This paper is LSCE contribution 6572.

Description: Sea surface temperature calculated from counts of the planktonic foraminifera N. pachyderma; planktonic and benthic foraminifera d18O and d13C for cores MD07-3076Q (covering the last deglaciation) and MD07-3077 (covering Termination V).