Rapid Environmental Fluctuations Recorded over the Last Glacial/Interglacial Cycle in the Sediments from Borehole PRGL1-4 (Gulf of Lions; Western Mediterranean) using Radiogenic Isotopes (Sr & Nd)

Abstract:

The study of sediments deposited and preserved in oceanic basins unable us to examine how terrigenous sediment supply varied through time in relation to paleoenvironmental and climatic changes on land. The geochemical and isotopic compositions of marine sediment are used to unravel its provenance, and provide information about its formation. Providing that the paleoclimatic frame is known, such data give an insight into the rock-source location and allow us to decipher between genetic processes of mechanical erosion and chemical alteration. Borehole PRGL1-4 (European project PROMESS), located in the Gulf of Lions (W Mediterranean) at 300 mwd, was investigated geochemically at high-temporal resolution over the last glacial/interglacial cycle (i.e. 130 ka) to study sediment-source variations during rapid climate changes. Besides, sediments originating from the Rhône’s and the Pyreneo-Languedocian’s catchment areas have been analyzed to measure the isotopic composition of five source end-members that are the Alps, the Higher Rhône valley, the Lower Rhône valley, the Languedoc and the Pyrenees. Epsilon Nd and ⁸⁷Sr/⁸⁶Sr were measured on 60 samples encompassing 4 marine isotopic stages with an emphasis on Heinrich events. The epsilon Nd values of PRGL1-4 lean towards the Lower Rhône valley unradiogenic end-member during cold stadial intervals, and towards the Alpine radiogenic end-member during warm interstadials. The presence of an ice cap over the crystalline Alpine watershed during cold phases could prevent the sediments originating from this region from reaching the Gulf of Lions. The same pattern is observed during the time of Heinrich events. An influence of the sea level variations on the sedimentation at the borehole site during the Heinrich events is unlikely since they are only 10 to 15 m in amplitude. Furthermore, a major isotopic shift in epsilon Nd mean values is displayed around 40 ka that coincides with the connection of the Durance to the Rhône River. We therefore propose that, at this time scale, the major control on sediment’s provenance and composition in the area is related to climatic conditions on land.