17. THE FORMATION OF PLIOCENE MEDITERRANEAN SAPROPELS: CONSTRAINTS FROM HIGH-RESOLUTION MAJOR AND MINOR ELEMENT STUDIES1R. Wehausen2 and H.-J. Brumsack2 |
ABSTRACTTwo Pliocene Mediterranean sediment cores from Sites 964 (Pisano Plateau, Ionian Basin) and 967 (lower northern slope of Eratosthenes Seamount, south of Cyprus), which comprise a total of 428 samples, have been analyzed for major and minor elements (Si, Ti, Al, Fe, Mg, Ca, K, Ba, Cr, Ni, V, Zn, and Zr) by X-ray fluorescence analysis at a depth resolution of 3–4 cm. Core 160-964C-9H comprises five sapropels with up to 9.7% total organic carbon (TOC), one 2.5-m-thick turbidite, and a 50-cm segment of a second turbidite, whereas Core 160-967B-9H contains eight sapropels with TOC content of up to 6.7%. Cyclic variations in carbonate content and detrital-matter composition are evident at both sites. At Site 964 (Pisano Plateau) terrigenous detrital matter chemistry reflects periods of stronger Saharan dust input alternating with fluvial input from the northern borderlands (higher K/Al and Mg/Al ratios). Cyclicity is not that well expressed at this location because turbidites or other short-term resedimentation processes occur. Site 967 (Eratosthenes Seamount) sediments are strongly influenced by discharge from the Nile River during humid periods (low K/Al and Mg/Al ratios). At Site 967 sapropels are occurring only during periods of enhanced Nile discharge and the corresponding lower Si/Al, Ti/ Al, Mg/Al, K/Al, and Zr/Al ratios. Accumulation rates of carbonate and terrigenous detrital matter were 30% lower during such episodes because of the reduction in eolian input, carbonate production, and/or carbonate dissolution. Ba enrichments, which are not only seen in the sapropels, but also very periodically in every K/Al minimum, document the triggering effect of enhanced bioproductivity on sapropel formation. As calculated from excess Ba concentrations, productivity increased by up to a factor of five during time intervals of intensified Nile runoff. Fluviatile discharge seems to be responsible for an enhanced input of nutrients and suspended matter containing low Mg/Al and K/Al ratios. Based on organic-carbon concentrations, episodes of sapropel formation did last at least from 1000 to 4000 yr at Site 964 and from 2000 to 6000 yr at Site 967. Periods of enhanced bioproductivity, as defined by Ba enrichments, did last from 8000 to 12,000 yr at Site 967. Episodes of high river discharge and corresponding nutrient input, as well as conditions of sapropel formation, seem to have lasted longer in the eastern part of the Eastern Mediterranean (Site 967) as compared to the Pisano Plateau (Site 964). |
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