RATES OF REACTION BETWEEN SILICATE IRON AND DISSOLVED SULFIDE IN PERUMARGIN SEDIMENTS

Porewaters in site 680 Peru Margin sediments contain dissolved sulfide over a depth of approximately 70 m which, at a sedimentation rate of 0.005 cm yr(-1), gives a sediment exposure time to dissolved sulfide o f about 1.4 x 10(6) yr. Reactions with dissolved sulfide cause the sit e 680 sediments to show a progressive decrease in a poorly-reactive si licate iron fraction, defined as the difference between iron extracted by dithionite (FeD) at room temperature and that extracted by boiling concentrated HCl (FeH), normalised to the total iron content (FeT). S traight line plots are obtained for in [(FeH - FeD)/FeT] against time of burial, from which a first order rate constant of 0.29 x 10(-6) yr( -1) (equivalent to a half-life of 2.4 x 10(6) yr) can be derived for t he sulfidation of this silicate iron. Comparable half-lives are also f ound for the same poorly-reactive iron fraction in the nearby site 681 , and 684 sediments. This silicate Fe fraction comprises 0.8-1.0% Fe, only 30-60% of which reacts even with 1.5-3 million years exposure to dissolved sulfide. Diagenetic models based on porewater concentrations of sulfate and sulfide, and solid phase iron contents, at site 680 ar e consistent in indicating that this poorly-reactive iron fraction is only sulfidized on a million year time scale. Silicate iron not extrac ted by HCl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments.