Organic matter and trace element rich sapropels and black shales: a geochemical comparison

A distinct Pliocene eastern Mediterranean sapropel (i-282), recovered from three Ocean Drilling Program (ODP) Leg 160 Sites, has been investigated for its organic and inorganic composition. This sapropel is characterized by h igh organic carbon (C-org) and trace element contents, and the presence of isorenieratene derivatives. The latter suggests that the base of the photic zone was sulphidic during formation of the sapropel. Combined with evidenc e of bottom water anoxia (preservation of laminae, high redox-sensitive tra ce element contents, and the abundance and isotopic composition of pyrite) this leads to the tentative conclusion that almost the entire water column may have been anoxic. This anoxia resulted from high productivity and not f rom stagnation, because an approximation of the trace element budget during sapropel formation shows that water exchange with the western Mediterranea n is needed. Entire water column anoxia has been suggested earlier for seve ral black shales. With regard to the depositional environment and the C-org content, however, only the Cenomanian/Turonian Boundary Event (CTBE) black shales appear to be comparable to this sapropel. The proposed trace elemen t removal mechanism of scavenging and (co-)precipitation in an anoxic water column, is thought to be similar for both types of deposits. The ultimate trace element source for the sapropel, however, is seawater, whereas it is hydrothermal and fluvial input for CTBE black shales (because they have a l arger temporal and spatial distribution). Nonetheless, the C-org-rich easte rn Mediterranean Pliocene sapropel discussed here may be considered to be a younger analogue of CTBE black shales. (C) 1999 Elsevier Science B.V. All rights reserved.