¹⁴C age estimation of the surface waters reservoir of the Mediterranean sea during the last 18000 years. Relations with the oceanic circulation

¹⁴C ages of surface water must be corrected for the ¹⁴C difference between the atmosphere and the sea surface. This difference, or marine ¹⁴C apparent age, is due to oceanic circulation processes that tend to advect intermediate and deep ¹⁴C-depleted water masses to the surface, to atmospheric ¹⁴C changes and to air-sea CO₂ exchange processes. In the past, circulation changes may have strongly influenced the sea surface reservoir age. The aim of this work is to evaluate ¹⁴C difference between the atmosphere and the sea surface waters for the Mediterranean sea during the last 18000 yrs. To compare the past and present sea surface reservoir age, we initially improved our knowledge of the modern reservoir age in the Mediterranean Sea by AMS ¹⁴C dates measured for mollusk shells collected alive before 1950. Results indicate a weighted average reservoir age of 390 yrs for the Mediterranean Sea basin, similar to that of the incoming Atlantic surface waters. In order to evaluate the past reservoir age during the last 18000 yrs we measured the ¹⁴C content of planktonic foraminifera contemporaneous to marine ash-layers in a high sedimentation deep sea core collected in the South Adriatic Sea and that of the paired terrestrial deposits recognized by timing and geochemistry. Results reveals that in the Mediterranean sea, reservoir ages fluctuations occurred during this time span. Reservoir ages reach about 700-800 years during the Oldest Dryas (between 13000 and 15000 yrs BP) and 530 yrs during the last sapropel deposition (between 8000 and 9000 yrs BP). During the Oldest Dryas, increased reservoir ages depends on the northward incoming of the radiocarbon depleted antarctic waters in the North Atlantic Ocean. During the sapropel increased reservoir ages is synchronous with a reversal circulation pattern at Gibraltar sill with incoming intermediate atlantic waters and mixing with mediterranean surface waters. (author)