Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial

Biological productivity and carbon export in the equatorial Atlantic are thought to have been dramatically higher during the last glacial period than during the Holocene. Here we reconstruct the pH and CO2 content of surface waters from the eastern equatorial Atlantic Ocean over the past ~30 k.y. using the boron isotope composition of Globigerinoides ruber (a mixed-layer–dwelling planktic foraminifera). Our new record, combined with previously published data, indicates that during the last glacial, in contrast to today, a strong west to east gradient existed in the extent of air:sea equilibrium with respect to ρCO2 (ΔρCO2), with the eastern equatorial Atlantic acting as a significant source of CO2 (+100 μatm) while the western Atlantic remained close to equilibrium (+25 μatm). This pattern suggests that a five- fold increase in the upwelling rate of deeper waters drove increased Atlantic productivity and large-scale regional cooling during the last glacial, but the higher than modern ΔρCO2 in the east indicates that export production did not keep up with enhanced upwelling of nutrients. However, the downstream decline of ΔρCO2 provides evidence that the unused nutrients from the east were eventually used for biologic carbon export, thereby effectively negating the impact of changes in upwelling on atmospheric CO2 levels. Our findings indicate that the equatorial Atlantic exerted a minimal role in contributing to lower glacial-age atmospheric CO2

Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records

International audienceWe present simulations of water-stable isotopes from the LMDZ general circulation model (the LMDZ-iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ-iso reproduces reasonably well the spatial and seasonal variations of both delta O-18 and deuterium excess. When nudged with reanalyses, LMDZ-iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid-Holocene (6 ka). A particularity of LMDZ-iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high-latitude regions, consistent with observations. Finally, we use LMDZ-iso to explore the relationship between precipitation and delta O-18 in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical delta O-18 is weak. Large regional changes in delta O-18 can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local delta O-18 records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate

Dynamical reconstruction of the global ocean state during the Last Glacial Maximum

The global ocean state for the modern age and for the Last Glacial Maximum (LGM) was dynamically reconstructed with a sophisticated data assimilation technique. A substantial amount of data including global seawater temperature, salinity (only for the modern estimate), and the isotopic composition of oxygen and carbon (only in the Atlantic for the LGM) were integrated into an ocean general circulation model with the help of the adjoint method, thereby the model was optimized to reconstruct plausible continuous fields of tracers, overturning circulation and water mass distribution. The adjoint‐based LGM state estimation of this study represents the state of the art in terms of the length of forward model runs, the number of observations assimilated, and the model domain. Compared to the modern state, the reconstructed continuous sea‐surface temperature field for the LGM shows a global‐mean cooling of 2.2 K, and the reconstructed LGM ocean has a more vigorous Atlantic meridional overturning circulation, shallower North Atlantic Deep Water (NADW) equivalent, stronger stratification, and more saline deep water

Rapid glaciation and a two-step sea-level plunge into The Last Glacial Maximum

The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth’s recent climate history1. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago4 and was followed by about 10,000 years of rapid deglaciation into the Holocene1. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to −118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago1,5, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth’s response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about −125 to −130 metres.Financial support of this research was provided by the JSPS KAKENHI (grant numbers JP26247085, JP15KK0151, JP16H06309 and JP17H01168), the Australian Research Council (grant number DP1094001), ANZIC, NERC grant NE/H014136/1 and Institut Polytechnique de Bordeaux

Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum - Supplementary material

Observation-based reconstructions of sea surface temperature from relatively stable periods in the past, such as the Last Glacial Maximum, represent an important means of constraining climate sensitivity and evaluating model simulations. The first quantitative global reconstruction of sea surface temperatures during the Last Glacial Maximum was developed by the Climate Long-Range Investigation, Mapping and Prediction (CLIMAP) project in the 1970s and 1980s. Since that time, several shortcomings of that earlier effort have become apparent. Here we present an updated synthesis of sea surface temperatures during the Last Glacial Maximum, rigorously defined as the period between 23 and 19 thousand years before present, from the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project. We integrate microfossil and geochemical reconstructions of surface temperatures and include assessments of the reliability of individual records. Our reconstruction reveals the presence of large longitudinal gradients in sea surface temperature in all of the ocean basins, in contrast to the simulations of the Last Glacial Maximum climate available at present

Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum: report of the MARGO Project

International audienceReconstructions of sea surface temperature (SST) from relatively stable periods in the past, such as the Last Glacial Maximum (LGM), represent one of the best means to constrain climate sensitivity and provide targets to evaluate coupled atmosphere-ocean general circulation models. The first quantitative global reconstruction of SST at the LGM was developed by the CLIMAP (Climate: Long-Range Investigation, Mapping and Prediction) project. Since then, there has not been any concerted effort to synthesize existing paleodata at the global scale, although several shortcomings of CLIMAP pioneering work have become apparent.We present a LGM global synthesis of SST reconstructions undertaken by the MARGO (Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface) project. The objective has been to compile and analyse available estimates of LGM SSTs based on all prevalent microfossil-based (i.e., transfer functions based on planktonic foraminifera, diatoms, dinoflagellate cysts and radiolarians abundances) and geochemical (i.e., alkenones and planktonic foraminifera Mg/Ca) paleothermometers. The MARGO project approach is to argue that no current proxy method is objectively better than another to provide an accurate picture of past SST, and that consequently the multiproxy approach yields the least biased representation of past reality. As originally suggested by CLIMAP, the strongest annual mean cooling (up to -10Ë C) occurred in the mid-latitude North Atlantic and extended into the western Mediterranean (-6Ë C). However, in contrast to CLIMAP, MARGO data indicate that the cooling was more pronounced in the eastern than in the western basin. The magnitude and position of a steep temperature gradient between 60 and 45Ë N are supported by four different proxies. In contrast with the CLIMAP reconstruction, all proxies also agree on ice-free conditions in the Nordic Seas during glacial summer. However, large discrepancies with respect to glacial temperatures recorded by different microfossil proxies remain. The best convergence between the various proxy estimates occurs within the 30Ë N to 30Ë S band. Strong inter-basins differences as well as clear west-east gradients within each basin mark the equatorial oceans, although anomalies are smaller in the Pacific and Indian Ocean than in the Atlantic. Tropical cooling is more extensive than that proposed by CLIMAP. Large cooling of the Eastern Boundary Current (EBC) systems in the Southern hemisphere is reconstructed by all proxies, making this a very robust feature of the climate and ocean circulation during the LGM. Existing coupled atmosphere-ocean general circulation models (AO-GCMs) simulations for the LGM show significant disagreement with respect to the location and magnitude of the North Atlantic cold anomaly while exhibiting stronger glacial cooling in the western than in the eastern Atlantic (http://pmip2.lsce.ipsl.fr/). This demonstrates that the robust MARGO North Atlantic East-West SST anomaly gradient is a good target with which the skill of models can be evaluated. With the advent of the multi-proxy method, we have not only been able to produce a new reconstruction of the glacial ocean surface, but also to deliver uncertainty estimates. Taken together, this yields new observational bounds on the sensitivity of Earth's climate system, with the perspective of improving existing climate models that are being used in the assessment of ongoing and future climate change

MARGO LGM planktonic oxygen isotopic data

Estimates of the change in surface seawater d18O (d18Osw) between the Last Glacial Maximum (LGM) and Late Holocene (LH) are derived from homogenous data sets with rigorous age control, namely MARGO sea surface temperature (SST) estimates and oxygen isotopic ratios (d18O) of planktonic foraminifers. Propagation of uncertainties associated with each proxy allows the identification of robust patterns of change in d18Osw. Examination of these patterns on a regional scale highlights which changes in surface currents and hydrological cycle are consistent with both planktonic isotopic data and reconstructed SST