19. PALEOPRODUCTIVITY AND CLIMATE VARIATIONS DURING SAPROPEL DEPOSITION IN THE EASTERN MEDITERRANEAN SEA1Liselotte Diester-Haass,2 Christian Robert,3 Herve Chamley4 |
ABSTRACTAt four sites from the Eastern Mediterranean, three to five sapropel layers and surrounding oxygenated sediments from the late and early Quaternary and Pliocene were investigated by a multidisciplinary approach which included analyses of coarse fraction, carbon and sulfur contents, grain size, and clay-minerals. Clay-mineral associations are comparable at all sites and in all time periods, in all sapropels, and oxygenated sediments. There is no diagenetic alteration of clay minerals in the sapropels. The relative importance of the different clay species varies with location of the sites and with time. African and European provenance can be distinguished. In the sapropels clay-mineral associations show changes that suggest increased precipitation in Southern Europe (increase in chlorite, illite, and mixed-layer clays) and intensified atmospheric circulation over Northern Africa and the Middle East (increased amounts of palygorskite and sepiolite). In all sapropels silt-sized particles increase at the expense of clay-sized particles in relation to higher energy of circulation. Sand-sized terrigenous particles have maximum accumulation rates just prior to sapropel deposition. Export productivity during deposition of oxygenated sediments has been calculated from benthic foraminifer numbers. It is highest just prior to sapropel deposition than above the sapropels. Paleoproductivity decreases from west to east and from the Pliocene to the Quaternary. Values from the late Quaternary (30-60 gC/m2·yr) are comparable to values obtained from oceanographic measurements (Bethoux, 1989). On top of Eratosthenes Seamount values are much higher (130-360 gC/m2·yr). The degree of carbonate dissolution reflects three types of sapropels: (1) nonbioturbated sapropels with excellent carbonate preservation; (2) increase in carbonate dissolution towards the top of the sapropel because of "burning down" phenomena; (3) bioturbated Pliocene sapropels with carbonate dissolution as strong as in surrounding oxygenated sediments. |
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