Aline Govin

Abrupt change of Antarctic moisture origin at the end of Termination II

The deuterium excess of polar ice cores documents past changes in evaporation conditions and moisture origin. New data obtained from the European Project for Ice Coring in Antarctica Dome C East Antarctic ice core provide new insights on the sequence of events involved in Termination II, the transition between the penultimate glacial and interglacial periods. This termination is marked by a north–south seesaw behavior, with first a slow methane concentration rise associated with a strong Antarctic temperature warming and a slow deuterium excess rise. This first step is followed by an abrupt north Atlantic warming, an abrupt resumption of the East Asian summer monsoon, a sharp methane rise, and a CO2 overshoot, which coincide within dating uncertainties with the end of Antarctic optimum. Here, we show that this second phase is marked by a very sharp Dome C centennial deuterium excess rise, revealing abrupt reorganization of atmospheric circulation in the southern Indian Ocean sector.

Equatorial Pacific forcing of western Amazonian precipitation during Heinrich Stadial 1.

Abundant hydroclimatic evidence from western Amazonia and the adjacent Andes documents wet conditions during Heinrich Stadial 1 (HS1, 18–15 ka), a cold period in the high latitudes of the North Atlantic. This precipitation anomaly was attributed to a strengthening of the South American summer monsoon due to a change in the Atlantic interhemispheric sea surface temperature (SST) gradient. However, the physical viability of this mechanism has never been rigorously tested. We address this issue by combining a thorough compilation of tropical South American paleorecords and a set of atmosphere model sensitivity experiments. Our results show that the Atlantic SST variations alone, although leading to dry conditions in northern South America and wet conditions in northeastern Brazil, cannot produce increased precipitation over western Amazonia and the adjacent Andes during HS1. Instead, an eastern equatorial Pacific SST increase (i.e., 0.5–1.5 °C), in response to the slowdown of the Atlantic Meridional Overturning Circulation during HS1, is crucial to generate the wet conditions in these regions. The mechanism works via anomalous low sea level pressure over the eastern equatorial Pacific, which promotes a regional easterly low-level wind anomaly and moisture recycling from central Amazonia towards the Andes.

Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left

Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species-specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 and 18°C, where Mg/Ca = 1.006 ± 0.032 * e0.065 ± 0.003*Tiso (R2 = 0.82). G. bulloides is found to calcify deeper in the Southern Ocean (∼ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between ∼ −1 and 9°C, of the form Mg/Ca = 0.580 ± 0.016 * e0.084 ± 0.006*Tiso (R2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1σ propagated error in Mg/Ca-derived temperatures is 1.1°C for G. bulloides and 0.9°C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.

Astronomically forced variations in western African rainfall (21°N–20°S) during the Last Interglacial period

This study documents the long-term evolution of western African precipitation during the Last Interglacial (LIG). We compare geochemical records obtained on nine sediment cores from the western African margin to a transient simulation (130-115 ka) performed with an ocean-atmosphere general circulation model and insolation as sole forcing. Good agreement between proxy records and model outputs indicates that long-term changes in western African precipitation largely responded to insolation variations during most of the LIG. After an early LIG dry phase (related to high-latitude iceberg melting or dating uncertainties), boreal summer insolation controlled the intensification of the North African monsoon between 127 and 122 ka, perhaps facilitating human migrations out of Africa. Equatorial African rainfall slightly increased throughout the LIG in response to increasing annual insolation. East-west contrasting rainfall evolutions at 10-20°S illustrate the complex southern African response, in contrast to more direct responses of North and equatorial western Africa, to insolation forcing.

Correction of interstitial water changes in calibration methods applied to XRF core‐scanning major elements in long sediment cores: Case study from the South China Sea

Fast and non-destructive X-ray fluorescence (XRF) core scanning provides high-resolution element data that are widely used in paleoclimate studies. However, various matrix and specimen effects prevent the use of semiquantitative raw XRF core-scanning intensities for robust paleoenvironmental interpretations. We present here a case study of a 50.8 m-long piston Core MD12-3432 retrieved from the northern South China Sea. The absorption effect of interstitial water is identified as the major source of deviations between XRF core-scanning intensities and measured element concentrations. The existing two calibration methods, i.e., normalized median-scaled calibration (NMS) and multivariate log-ratio calibration (MLC), are tested with this sequence after the application of water absorption correction. The results indicate that an improvement is still required to appropriately correct the influence of downcore changes in interstitial water content in the long sediment core. Consequently, we implement a new polynomial water content correction in NMS and MLC methods, referred as NPS and P_MLC calibrations. Results calibrated by these two improved methods indicate that the influence of downcore water content changes is now appropriately corrected. We therefore recommend either of the two methods to be applied for robust paleoenvir-onmental interpretations of major elements measured by XRF-scanning in long sediment sequences with significant downcore interstitial water content changes.

The evolution of deep‐ocean flow speeds and δ13C under large changes in the Atlantic overturning circulation: Toward a more direct model‐data comparison

To investigate the dynamics of the Atlantic meridional overturning circulation (AMOC) on timescales longer than the observational records, model-data comparisons of past AMOC variability are imperative. However, this remains challenging because of dissimilarities between different proxy-based AMOC tracers and the difficulty of comparing these to model output. We present an iLOVECLIM simulation with tuned AMOC evolution and focus on AMOC tracers that are directly comparable to reconstructions: flow speeds and δ13C. We deduce their driving factors and show that they yield different but complementary information about AMOC changes. Simulated flow speed changes are only linked to AMOC changes in regions bathed by North Atlantic Deep Water; however, in those regions they do provide details on vertical migration and thickness changes of the water masses. Simulated δ13C changes in the North Atlantic Deep Water region are again related to AMOC changes. Yet in regions bathed by Antarctic Bottom Water or Antarctic Intermediate Water, the δ13C evolution is driven by Southern Hemisphere source water δ13C changes, while in the Nordic Seas and the two major overflow regions it is driven by Northern Hemisphere source water δ13C changes. This shows that AMOC changes are not necessarily recorded by δ13C and stresses the need for combining both tracers in paleoclimate studies. A preliminary model-data comparison for Last Interglacial flow speeds and δ13C changes in the Deep Western Boundary Current shows that this integrated approach is far from straightforward and currently inconclusive on the Last Interglacial AMOC evolution. Nonetheless, the approach yields potential for more direct and in-depth model-data comparisons of past AMOC changes.

What Ends an Interglacial? Feedbacks Between Tropical Rainfall, Atlantic Climate and Ice Sheets During the Last Interglacial

How long the present interglacial will last remains under debate. This project aims to determine the climatic mechanisms and sequence of events terminating an interglacial period. By comparing new paleoclimate reconstructions and climate model experiments, we investigate the impact of South American rainfall changes on tropical Atlantic sea-surface salinity and Atlantic thermohaline circulation at the end of the Last Interglacial (LIG). Model and proxy data show gradually intensifying South American monsoonal precipitation and enhanced Amazon discharge through the LIG, in response to increasing austral summer insolation. However, an increased meridional temperature gradient at the end of the LIG caused a strengthening of the North Brazil Current retroflection which deflected eastward the Amazon freshwater plume. Such changes in South American river discharge contributed to decrease tropical and North Atlantic surface salinities, resulting in a shift in regions of North Atlantic deep water convection and small reduction in deep water formation.

Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials demonstrated on a sediment core transect

J. A. Collins, A. Govin, S. Mulitza, D. Heslop, M. Zabel, J. Hartmann, U. Röhl, and G. Wefer MARUM – Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia Institute for Biogeochemistry and Marine Chemistry, University of Hamburg, Bundesstraße 55, 20146 Hamburg, Germany