Sulphur contents and partition of iron in ODP Leg 112 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, gives a sediment exposure time to dissolved sulfide of about 1.4 Myr. Reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly-reactive silicate 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). Straight line plots are obtained for ln[(FeH - FeD)/FeT] against time of burial, from which a first order rate constant of 0.29 1/Myr (equivalent to a half-life of 2.4 Myr) can be derived for the sulfidation of this silicate iron. Comparable half-lives are also found 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 are consistent in indicating that this poorly-reactive iron fraction is only sulfidized on a million year time scale. Silicate iron not extracted by HCl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments.秘鲁大陆边缘680站位沉积物的孔隙水中含有溶解态硫化物，其分布深度约为70米；在0.005厘米/年的沉积速率条件下，该站位沉积物的溶解态硫化物暴露时长约为1.4百万年（Myr）。与溶解态硫化物的反应会导致680站位沉积物中的弱反应性硅酸盐铁组分逐渐减少，该组分被定义为室温下连二亚硫酸盐（dithionite）提取的铁（FeD）与沸腾浓盐酸（HCl）提取的铁（FeH）的差值，并以总铁含量（FeT）进行归一化后的数值。针对埋藏时间绘制ln[(FeH - FeD)/FeT]的散点图可得到线性关系，由此可推导出该硅酸盐铁硫化作用的一级速率常数为0.29每百万年（1/Myr，对应半衰期为2.4百万年（Myr））。邻近的681和684站位沉积物中的同一弱反应性铁组分也测得相近的半衰期。该硅酸盐铁组分的铁含量为0.8%~1.0%，即便经历150万至300万年的溶解态硫化物暴露，其中也仅有30%~60%发生反应。基于680站位孔隙水硫酸盐、硫化物浓度及固相铁含量构建的成岩作用模型，均表明该弱反应性铁组分仅能在百万年级别的时间尺度上发生硫化作用。在海洋沉积物常见的时间尺度下，未被浓盐酸提取的硅酸盐铁可被视为与溶解态硫化物不发生反应。