20. SEDIMENTARY SULFUR AND IRON CHEMISTRY IN RELATION TO THE FORMATION OF EASTERN MEDITERRANEAN SAPROPELS¹

Hilde F. Passier² and Gert J. de Lange²

ABSTRACT

Detailed analyses of total sulfur, pyritic sulfur, humic sulfur, NaCl-extractable sulfur, elemental sulfur, acid-volatile sulfide, and organic polysulfide in and around the most recent Sapropel S1 (maximum C_org = 2.3%) and two other sapropels recovered during Ocean Drilling Program Leg 160 (maximum C_org = 7.4% and 23.5%), show that the main sulfur species in and immediately below each sapropel is pyrite. Directly above each sapropel sulfur is rarely present in the solid phase, but occurs as porewater sulfate (SO₄^2–). Microbial SO₄^2– reduction took place in the sapropels during sapropel formation. Addition of reactive iron to sapropel layers occurred via upward diffusion of Fe²⁺ from underlying sediments and/or through water-column iron sulfide precipitation. All reactive iron that was available in the sediments was used for pyrite formation. As a result, diffusion of sulfide out of the sapropels and sulfidization of the sediments underlying each sapropel have occurred. Only in the most C_org–rich sapropel did large-scale uptake of reduced sulfur by organic molecules occur, and SO₄^2– reduction probably still continues.

¹Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998. Proc. ODP, Sci. Results, 160: College Station, TX (Ocean Drilling Program).
²Department of Geochemistry, Institute of Earth Sciences, Utrecht University, PO Box 80.021, 3508 TA Utrecht, The Netherlands. hpassier@earth.ruu.nl