Paleoceanographic and paleoclimatic variability in the Western Mediterranean during the last 25cal. kyr BP. New insights from contourite drifts

摘要

The Western Mediterranean Deep Water (WMDW), on its way out toward the Atlantic Ocean, has favored the formation of contourite drifts in the Alboran Sea (SW Mediterranean) since the opening of the Strait of Gibraltar. Resolving the nature of these deposits is crucial for reconstructing the WMDW variability at a millennial scale, deciphering its bottom current paleo-veloc-ity, and establishing paleoclimatic implications over the last 25 cal. kyr BP. Two sediment cores retrieved from elongated separated and plastered contourite drifts formed along its path are investigated by means of multi-sedimentological data (terrigenous grain-size, sortable silt, terrigenous and carbonate sediment fluxes, bioturbation and ichnofabric changes), geochemical data (Zr/Al and Si/(Si + Al) ratios), chronostratigraphic data (δ~(18)O, and ~(14)C data) as well as statistical analyses (grain-size end-member modelling and spectral analysis). Integration of these data confirms the contouritic nature of Alboran drift deposits. The high-resolution paleocurrent records of the WMDW inferred from the sortable silt of contourite sequences led us to define two regimes in terms of WMDW flow energy. Regime 1 (weak to moderate velocity) defined by paleo-velocities of 4 to 23 cm s~(-1) is dominant during the last 24 cal kyr BP. Regime 2 (strong velocity) is characterized by estimated paleo-velocities of about 36 cm s ' during Heinrich Stadial 2. The spectral analysis of bottom current proxies (sortable silt and Zr/Al ratio) matches four cyclic climatic signals (1900 yr, 2300 yr, 4000 ye and 6100 yr), corroborating the occurrence of millennial-scale cyclicity. These cycles are related to atmospheric climate variability, in turn linked to variations in solar activity. Our results, when combined with published data from a neighboring NW Mediterranean contourite drift, provide for a better regional understanding of the WMDW millennial-scale dynamics.