Chemical cycles in Pliocene sapropel-bearing and sapropel-barren eastern Mediterranean sediments

Abstract

High-resolution (3–4 cm) investigations of major and minor elements were carried out on sediment cores from ODP Leg 160 Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 967 and 966 (northern slope and top of Eratosthenes Seamount, south of Cyprus). At Site 967, element/Al ratios display pronounced cyclic records that are related to astronomically-forced climatic variations. Sediments from the top of Eratosthenes Seamount (Site 966) show a similar pattern to those from Site 967, but they are more disturbed by bioturbation and also strongly influenced by long-term variations in sediment dispersal. Sediments from Site 969 display similar trends to those from Site 964, i.e. they are characterized by higher Mg/Al, slightly higher K/Al and lower Ti/Al ratios during sapropel formation. Higher Ti/Al ratios at Site 969 compared with sediments from Site 964 may be explained by a westward transport of material from a source in the east, where Ti/Al ratios are highest (Sites 966 and 967). Elevated Mg/Al ratios at Site 969 during sapropel intervals are due to river-derived material, transported through the Kithira Strait between Peloponnesus and Crete from the northwestern Aegean Sea, which originates from ultramafic rocks. A change in intermediate water currents (current reversal?) during sapropel formation may explain the enhanced supply of Mg-rich and the relatively diminished supply of Ti-rich material. We propose that, during deposition of the reddish intervals at Site 969, a change in circulation did not occur, because a drastic decrease in Ti/Al ratios is not evident. In these sediments Ba peaks, indicative of former sapropels, are not present. Ba enrichments in the same sapropel-barren intervals at Site 967 (northern slope of Eratosthenes Seamount) suggest that, during such intervals, the ‘burn-down’ process was more effective than during the sapropel-bearing intervals. Mn contents in all investigated Pliocene sediments correlate with the carbonate contents. Higher Mn contents of the sapropel-barren sediments suggest that, during time intervals of sapropel formation, either Mn accumulation at the margins of the permanently oxic upper water column, or Mn export into the western Mediterranean, might have occured. The latter could only be explained by sub- or anoxic conditions in large parts of the water column.