Studying the past of Mediterranean outflow based on 230th excess inventories and contourites

摘要

The Mediterranean Outflow water (MOW) comes out from the Mediterranean Sea and then contours the northern slope ofthe Cadiz Gulf. Along its way to the southern Portuguese Margin, it divides itself into three levels flowing at differentdepths, 400 m, 800 m and 1200 m, respectively. These different pathways induce a series of contourites along the Cadizslope as well as some sedimentary drifts, such as the Faro Drift. Based on the assumption that the sedimentologiccharacteristics of these contourites should give some light on the history of MOW velocity and intensity variability, two longsedimentary cores collected during the Marion Dufresnes 114/Images cruise in 1999 have been studied. The sampling sitesof these two cores, MD99-2336 and MD99-2339, located in the Cadiz Gulf at 690 and 1177 m water column depthsrespectively, are thus, actually, below the first level and in the main core of the MOW third level. Along time, variations inthese current levels, parallel to the slope, should then influence the existence and characteristics of contourites in bothsedimentary records. For this purpose, thorium-230 (230Th) as well as granulometric and micropaleontologic analysis havebeen undergone at high resolution on the 4 uppermost meters spanning MIS1 to LGM times. The referred current prints canbe detected by analysing surface and down core sediment for its 230Th content. This radioisotope is produced by theradioactive decay of uranium-234 which content in oceanic waters is known. Therefore, its production rate in the watercolumn can be estimated as a linear function of the water depth (~ 2.6 dpm/cm2.ka for 1 km water depth). As 230Th isalmost insoluble, it will sink to the oceanic floor together with the settling particles. This vertical flux to the underlyingsediment is considered, in a first order approximation, equal to its production rate in the water column. On this basis, the230Th excess in the sediment becomes a proxy for sedimentation versus erosion processes accordingly to the sign of thedifference between the total and the vertical 230Th flux, i.e. if it is, respectively, positive or negative. With this method itis then possible to extrapolate on the location of the high velocity core area and whether its intensity changed or notlooking at the inventory of excess 230Th in the contourite units. We acknowledge FEDER and OE that financed this studythrough the Portuguese Foundation for Science and Technology (PDCTM/PP/MAR/15297/1999).