39. PLANKTONIC ¹⁸O AND U^k´₃₇ TEMPERATURE ESTIMATES FROM ORGANIC-RICH SEDIMENTS AT SITES 974 AND 975, TYRRHENIAN SEA AND BALEARIC RISE¹

ABSTRACT

Sapropels at Sites 974 and 975 as well as sediment samples from immediately below and above sapropels were analyzed for alkenone concentrations. Paleo-sea-surface temperatures (SSTs) were inferred from the U^k´₃₇ unsaturation index in Pleistocene sapropel sequences. These SST estimates vary between 13° and 25°C. The upper range of SST estimates is distinctly warmer than modern mean annual SST at both sites and closely groups around the modern summer SST. The cold end of the inferred SST range is still warmer than the mean glacial winter SSTs from the Climate Long-Range Investigation Mapping and Prediction Project (CLIMAP) planktonic foraminifer census counts (Thiede, 1978). We infer, therefore, that sapropel formation at both sites was coeval with warm climates. Using planktonic ¹⁸O and U^k´₃₇ SST estimates as input into an oxygen isotope paleotemperature equation, we can deduce seawater ¹⁸O which, in turn, we can use to infer the paleosalinity for two Pleistocene sapropels at Site 975. These paleosalinity estimates are between 32 and 34, considerably lower than the modern salinity of 37.5 at this location. If we use more negative freshwater _w values in the equation, then the salinity offsets are reduced by 2-4 units. Depleted salinities, whatever their true values might have been, in conjunction with increased SSTs, imply that sapropel formation in the western Mediterranean occurred during warm and moist climates. This agrees with the idea that sapropel formation in the eastern Mediterranean is a result of enhanced northern hemisphere insolation. Whether or not climatic forcing caused similar marine environmental responses (such as enhanced biological productivity and/or reduced deep-water circulation) in both the eastern and western Mediterranean remains to be determined.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume. 
²GEOMAR Research Center for Marine Geosciences, Wischhofstrasse 1-3, D-24148 Kiel, Federal Republic of Germany.
³Present address: Bundesanstalt fuer Geowissenschaften und Rohstoffe Postfach 510153, 30631 Hannover, Federal Republic of Germany. doose@bgr.de
⁴Geologisches Institut, ETH-Zentrum, 8092 Zürich, Switzerland.
⁵Present address: Dipartimento di Scienze della Terra, Università degli Studi di Parma, Viale delle Scienze 78, 43100 Parma, Italy.
⁶INTECHMER, BP 324, Cherbourg Cedex, France.
⁷Université Pierre et Marie Curie, Laboratoire d'Océanographie Dynamique et de Climatologie, 4 Place de Jussieu, Case 100, F-75252 Paris Cedex 05, France.
⁸Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, U.S.A. (Present address: 4429 Valerie St., Bellaire, TX 77401-5626, U.S.A.)

Date of initial receipt: 12 May 1997
Date of acceptance: 16 February 1998
Reproduced online: 12 February 2004
Ms 161SR-237