Marine Geology, 100 (1991) 125-136 125 Elsevier Science Publishers B.V., Amsterdam Possible diagenetic mobilization of barium in sapropelic sediment from the eastern Mediterranean Bertil J.H. van Os, Jack J. Middelburg and Gert J. de Lange Department (~f Geochemistry, Universi O, o/" Utrecht, P.O. Box 80.021, 3508 TA Utrecht, The Netherhmds (Received May 6, 1990; revision accepted October 24, 1990) ABSTRACT Van Os, B.J.H., Middelburg, J.J. and De Lange, G.J., 1991. Possible diagenetic mobilization of barium in sapropelic sediment from the eastern Mediterranean. In: M.B. Cita, G.J. de Lange and E. Olausson (Editors), Anoxic Basins and Sapropel Deposition in the Eastern Mediterranean: Past and Present. Mar. Geol., 100 (Spec. Sect.): 125 136. In the last few years it has frequently been suggested that Ba is a useful indicator of paleoproductivity. The formation of some sapropels in the Eastern Mediterranean is considered to be related to, or to coincide with, periods of enhanced producti- vity. A high-resolution sampling study has been undertaken in order to investigate whether the Ba distribution in sapropels reflects a primary input signal or whether it has been altered by diagenetic processes. On the basis of our results we suggest that three diagenetic stages determine the distribution of Ba. During deposition of the sapropel (stage I) Ba is mobilized as anoxic conditions prograde. After deposition of the sapropel (stage 2), a progressive oxidation front develops. This front induces the formation of Mn and Fe enrichments and barite precipitation at the oxic/ anoxic boundary. Barite precipitation is believed to he caused mainly by a rise in the porewater sulphate concentration after sulphides have been oxidized by the front. Upon burial (stage 3), suboxic conditions develop as the oxygen becomes exhausted again. In contrast to Fe- and Mn- oxyhydroxides which dissolve and reprecipitate at higher levels, barite is preserved because dissolved sulphate is not depleted. The interpretation of the Ba distribution in organic-rich sediment is not straightforward. Diagenetic reallocation of a primary Ba signal will possibly disturb the relationship between Ba and organic production. Consequently, one mus! be very cautious when invoking Ba as a paleoproductivity indicator. Introduction Organic-rich layers (sapropels) were deposited in the Eastern Mediterranean during the Pleisto- cene and Holocene, and these sapropels have been reported to contain Ba concentrations that are significantly higher than those reported in normal Mediterranean sediments (Calvert, 1983; Suther- land et al., 1984; Klinkhammer and Lambert, 1989) Enhanced preservation of organic matter due to anoxia or increased surface productivity is consid- ered to be the most important factor controlling the formation of sapropels (e.g. Cita et al., 1977; Stanley and Blanpied, 1980; Buckley et al., 1982; Rossignol-Strick et al., 1982; Calvert, 1983; De Lange and Ten Haven, 1983; Shaw and Evans, 1984; Sutherland et al., 1984; Mangini and Schlos- ser, 1986; Ten Haven et al., 1987; Buckley and Johnson, 1988; Sarmiento et al.,1988; Boyle and Lea, 1989; Rohling and Gieskes, 1989; Thunell and Williams, 1989). Therefore the Ba enrichment in sapropelic sediments (Calvert, 1983; Sutherland et a1.,1984) is believed to represent important evidence for the increased productivity hypothesis. Salinity excursions are an alternative explana- tion for the formation of sapropels (Klinkhammer and Lambert, 1989). These excursions could be the result of leaching and dissolution of evaporites. Enhanced salinity is thought to reduce the effi- ciency of the microbial community and hence decreases the degradation of organic matter, result- ing in a higher rate of accumulation of organic matter. 0025-3227/91/$03.50 ~ 1991 - - Elsevier Science Publishers B.V.