Modes of sapropel formation in the eastern Mediterranean: some constraintsbased on pyrite properties

Pyrite formation within and directly below sapropels in the eastern Mediter ranean was governed by the relative rates of sulphide production and Fe lib eration and supply to the organic-rich layers. At times of relatively high SO42- reduction, sulphide could diffuse downward from the sapropel and form ed pyrite in underlying sediments. The sources of Fe for pyrite formation c omprised detrital Fe and diagenetically liberated Fe(ll) from sapropel-unde rlying sediments. In organic-rich sapropels, input of Fe from the water col umn via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slo w euhedral pyrite formation at low saturation levels occurred. delta(34)S v alues of pyrite are -33 parts per thousand to -50 parts per thousand. Below the sapropels delta(34)S is lower than within the sapropels, as a result o f increased sulphide re-oxidation at times of relatively high sulphide prod uction and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It rang es from 34% to 80%, varying significantly between sapropels. Increased pala eoproductivity as well. as enhanced preservation contributed to magnified a ccumulation of organic matter in sapropels. (C) 1999 Elsevier Science B.V. All rights reserved.