Stable isotope “anomalies” in Mediterranean Pleistocene records

Abstract

Oxygen and carbon isotope records of twelve Mediterranean cores raised from west (Alboran sea) to east (off Israel), and of four Atlantic cores raised from the western side of Gibraltar and located respectively inside, below and beyond the modern Mediterranean outflow are compared to open ocean records of core 71–139C from the North Atlantic and core V19–30 from the East Pacific. Oxygen isotope stratigraphies can be established for deep Mediterranean records and the major events of the last 150 ka are easily identifiable. For instance, the five substages of stage 5 or the 18 ka event (stage 2) are identified but the 10 ka event (= Younger Dryas) is only recorded by Globigerina bulloides and benthic foraminifera, suggesting that the cooling affected only winter SST. 14C dates for the two terminations, I A and I B, of the last deglaciation which are recorded in sediment cores from areas with high sedimentation rates (Faro Ridge, Alboran Sea, Sicilian Strait), are in good agreement with the dates published for the same termination in Atlantic cores. An intermediate step, however, has been identified in some cores at 14 ka and divides termination I A into two phases. Oxygen isotope anomalies are recorded by the shallow water species Globigerinoides ruber. They correspond to (1) a higher amplitude glacial-interglacial change in δ 18 O values which exceeds that of the open ocean by 1‰ in the western basin and by 1.7‰ in the eastern basin; (2) large isotopic depletions during the deposition of sapropel S 1, S 3 and S 5. They are interpreted as the result of fresh water run-off responsible for a strong dilution in the shallower layers of the east Mediterranean waters. The isotopic effect of this dilution averages −0.75‰ for sapropel S 1 and −1.6‰ for sapropel S 5. During the last glacial maximum, the Mediterranean still acted as a concentration basin and a gradient of δ 18 O values similar to the modern one ( ≅ +0.75‰) existed from west to east in the deep waters. Apparently, deep water temperatures were lower during the last glacial but the decrease did not exceed 4°C in the eastern basin. Major anomalies are also contained in the 13C records of G. bulloides and benthic foraminifera. The 13C records from surface, intermediate and deep waters of the eastern basin and from intermediate and deep waters of the western basin are opposite to that of the open ocean. During glacial time δ 13 C values were higher in intermediate and deep waters of the whole Mediterranean, and also in surface water of the eastern basin. At the same time, the 13C gradient of surface to deep waters was low. This gradient increased during the last deglaciation while δ 13 C values of G. bulloides and benthic foraminifera decreased. This decrease, interrupted at 10 ka, may have been related to increased fresh water run-off from the bordering contintents due to increased precipitation, especially in the tropical belt. The 13C depletion is clearly recorded in the Levantine and deep water masses and can be traced westward in the Mediterranean outflow. Oxygen and carbon isotope anomalies recorded simultaneously in the east Mediterranean by the shallow water species Globigerinoides ruber during sapropel deposition indicate that increased precipitation on the bordering continents and strong fresh water run-off during the time of the last deglaciation, may have caused a density stratification and deep stagnation in the eastern basin. Uncertainties arise from the fact that, in the mixing model, deglacial Mediterranean waters are mixed with continental fresh waters, the isotopic characteristics of which are not entirely understood. Although increased surface water production of biogenic material can not be excluded as a possible cause of 13C depletion, it appears that the river discharge including the Nile, as well as Black Sea input, are also plausible sources for the fresh water layering.