26. STABLE ISOTOPE AND ALKENONE TEMPERATURE RECORDS OF SAPROPELS FROM SITES 964 AND 967: CONSTRAINING THE PHYSICAL ENVIRONMENT OF SAPROPEL FORMATION IN THE EASTERN MEDITERRANEAN SEA1Kay-Christian Emeis,2 Hans-Martin Schulz,2 Ulrich Struck,2 Tatsuhiko Sakamoto,3 Heidi Doose,4 Helmut Erlenkeuser,5 Michael Howell,6 Dirk Kroon,7 and Martine Paterne8 |
ABSTRACTSapropels formed in response to changes in the climatic background and in water mass circulation of the Mediterranean Sea. To examine the magnitude of change in surface waters, which are a prominent source of both deep and intermediate waters today, we measured the alkenone unsaturation index of sedimentary lipids (a sea-surface temperature proxy; at Ocean Drilling Program (ODP) Sites 967 and 964, as well as in PALEOFLUX Cores KC01/01B) and the stable oxygen isotope composition of planktonic foraminifers (at Site 967) in closely spaced samples across sapropels. With these data we evaluate the temperature and salinity history of surface waters in Ionian and Levantine Basins of the Eastern Mediterranean at times of sapropel deposition over the last 3 m.y. Average sea-surface temperatures (SST) are high (23°C) and show little fluctuation in the time interval from 2 to 3 Ma at ODP Site 964 in the Ionian Basin. During this period, we discern no change in the SST going into and out of the sapropel or temperature decreases at the onset of sapropel deposition. Between 0.9 and 1.3 Ma, the average temperatures were between 16°C and 21°C and varied by as much as 6°C within individual sapropel layers. Sapropel SST are markedly higher than those immediately below the organic-rich layers. From 650 ka to the last sapropel (S1) of Holocene age, the average SST across sapropels follow the global climatic background and range from as low as 15°C (during glacial isotope Stage 6) to 21°C (during interglacial isotope Stages 5 and 9), which corresponds to a glacial/interglacial temperature change of 6°C. The ranges of individual SST values are much higher and exceed a 10°C difference between glacial and interglacial samples. Superimposed on the global glacial/interglacial temperature pattern was a warming trend of at least 2°C at the onset of each sapropel event. Temperature gradients between coeval sapropels from the two basins are less than 2°C for the last 400 ka. Parallel to the warming trends in the sapropels, the delta18O of planktonic foraminifer calcite decreases by values between 0.7‰ and 3.4‰, of which the temperature change explains only a portion between 0‰ and 1.1‰. The remainder must be caused by salinity and global ice-volume changes of up to 3.4‰ in the surface waters. Published estimates of global ice-volume variations over periods corresponding to individual sapropel intervals account for a maximum of 0.32‰ delta18O in the change from below to within the sapropels. The corrected isotope data suggest that the surface-water salinity was lower by as much as 7.7 in Sapropel S4 and usually more than 1 for all other sapropels in the late Pleistocene to Holocene. The data validate the hypothesis that the thermohaline circulation was reduced and that the intermediate- and deep-water formation in the Eastern Mediterranean was weakened or impeded. |
|