PYRITE CONTENTS, MICROTEXTURES, AND SULFUR ISOTOPES IN RELATION TO FORMATION OF THE YOUNGEST EASTERN MEDITERRANEAN SAPROPEL

Pyrite within and below sapropels in the eastern Mediterranean is a re sult of microbial SO42- reduction within the sapropel, and the subsequ ent reaction of sulfide (HS-) with detrital Fe and Fe2+ diffusing upwa rd from underlying sediments. Below the youngest Mediterranean saprope l, S1, pyrite (as much as 281 mu mol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 mu mol pyritic S/g) has been detected. Framboidal mic rotextures indicate pyrite formation at the site of HS- production wit hin the sapropel. Euhedral pyrite, below the sapropel, forms when sulf ate reduction in the sapropel outcompetes iron liberation and supply, and HS- diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (-37.3 parts per thousand to -38.2 parts per thousand) as well as below the sapropel (-45.6 parts per th ousand and -49.6 parts per thousand), indicating that HS- has formed i n a system with abundant SO42- and in the presence of oxidants.