ALBERT

Description: The last deglaciation represents the most recent example of natural global warming associated with large-scale climate changes. In addition to the long-term global temperature increase, the last deglaciation onset is punctuated by a sequence of abrupt changes in the Northern Hemisphere. Such interplay between orbital- and millennial-scale variability is widely documented in paleoclimatic records but the underlying mechanisms are not fully understood. Limitations arise from the difficulty in constraining the sequence of events between external forcing, high- and low- latitude climate and environmental changes. Greenland ice cores provide sub-decadal-scale records across the last deglaciation and contain fingerprints of climate variations occurring in different regions of the Northern Hemisphere. Here, we combine new ice d-excess and 17O-excess records, tracing changes in the mid-latitudes, with ice δ18O records of polar climate. Within Heinrich Stadial 1, we demonstrate a decoupling between climatic conditions in Greenland and those of the lower latitudes. While Greenland temperature remains mostly stable from 17.5 to 14.7ka, significant change in the mid latitudes of northern Atlantic takes place at ~16.2ka, associated with warmer and wetter conditions of Greenland moisture sources. We show that this climate modification is coincident with abrupt changes in atmospheric CO2 and CH4 concentrations recorded in an Antarctic ice core. Our coherent ice core chronological framework and comparison with other paleoclimate records suggests a mechanism involving two-step freshwater fluxes in the North Atlantic associated with a southward shift of the intertropical convergence zone.

Description: The last deglaciation represents the most recent example of natural global warming associated with large-scale climate changes. In addition to the long-term global temperature increase, the last deglaciation onset is punctuated by a sequence of abrupt changes in the Northern Hemisphere. Such interplay between orbital- and millennial-scale variability is widely documented in paleoclimatic records but the underlying mechanisms are not fully understood. Limitations arise from the difficulty in constraining the sequence of events between external forcing, high- and low- latitude climate, and environmental changes. Greenland ice cores provide sub-decadal-scale records across the last deglaciation and contain fingerprints of climate variations occurring in different regions of the Northern Hemisphere. Here, we combine new ice d-excess and 17O-excess records, tracing changes in the midlatitudes, with ice δ18O records of polar climate. Within Heinrich Stadial 1, we demonstrate a decoupling between climatic conditions in Greenland and those of the lower latitudes. While Greenland temperature remains mostly stable from 17.5 to 14.7 ka, significant change in the midlatitudes of the northern Atlantic takes place at ∼16.2 ka, associated with warmer and wetter conditions of Greenland moisture sources. We show that this climate modification is coincident with abrupt changes in atmospheric CO2 and CH4 concentrations recorded in an Antarctic ice core. Our coherent ice core chronological framework and comparison with other paleoclimate records suggests a mechanism involving two-step freshwater fluxes in the North Atlantic associated with a southward shift of the Intertropical Convergence Zone.

Description: There is increasing concern with anthropogenic greenhouse gas emissions that ocean warming, in concert with summer and winter precipitation changes, will induce anoxia in multiple ocean basins. In particular the Mediterranean Sea is susceptible to severe hydrological changes. Mediterranean hydroclimate is controlled primarily by two phenomena – the latitudinal migration of the Inter-Tropical Convergence Zone and the North Atlantic climatic processes. While the former brings about the African summer monsoon rainfall the latter drives the wintertime storm tracks into the western Mediterranean. Although the hydrological changes in the eastern Mediterranean are quite well constrained, evidence of past changes in temperature and rainfall in the western Mediterranean across the past interglacials is relatively scarce. In this study, we use trace element and stable isotope composition of planktonic foraminifera from a sediment core off Corsica at the mouth of Golo river in the western Mediterranean to reconstruct variations in sea surface temperature (SST) and sea surface salinities (SSS) during the Holocene and warm periods of the past two interglacials. Our data suggest that the warm periods of the last interglacials were characterised by high river discharge and lower SSS in the northern Tyrrhenian Sea, suggesting increased winter rainfall. We find evidence that enhanced winter rainfall during periods of precession minima and high seasonality across interglacials coincide with changes in the respective eccentricity maxima suggesting a causal link. Our model simulations for representative orbital configurations such as the mid-Holocene support increased south-westerly moisture transport into the western Mediterranean originating from the North Atlantic. We suggest that these hydrologic changes in the western and the northern Mediterranean borderlands were a contributing factor to basin-wide anoxia in the past. Our findings offer new insights into the cause and impact of winter rainfall changes in the Mediterranean during past warm periods.