Co-diagenesis of iron and phosphorus in hydrothermal sediments from the southern East Pacific Rise: Implications for the evaluation of paleoseawater phosphate concentrations
Co-diagenesis of iron and phosphorus in hydrothermal sediments from the southern East Pacific Rise: Implications for the evaluation of paleoseawater phosphate concentrations
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DOI：
10.1016/j.gca.2006.01.030
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发表时间：
2006-12
影响因子：
5
通讯作者：
S. Poulton;D. Canfield
中科院分区：
文献类型：
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作者：
S. Poulton;D. Canfield
我们详细研究了东太平洋隆起南部~ 19°S热液羽流沉降沉积物中铁和磷的共成岩作用。在DSDP Leg 92期间收集的距离山脊340 - 1130公里的三个远端沉积物岩心，使用顺序化学萃取技术分析了固相铁和P的关联。所有地点的沉积物都富含水热铁（氧合）氧化物，但在成岩作用过程中，大部分原生水合铁沉淀转化为更稳定的针铁矿矿物形式，并在较小程度上转化为粘土矿物，导致被清除的磷释放到溶液中。然而，其中很大一部分磷被保留在沉积物中，通过结合到次生针铁矿中，以自生磷灰石的形式沉淀。再吸附为铁（氧）氧化物。这些沉积物的摩尔磷/铁比明显低于沿南东太平洋隆起的更北部地区的羽流颗粒，并显示出明显的下核下降至约12m的深度。在~ 35m深度内，P/Fe摩尔比相对恒定。铁和磷的形态数据表明，沉积物的成岩作用在2.5m深度内基本完成，因此摩尔磷/铁比值的深度变化趋势不能仅仅用早期成岩作用中沉积物中磷的扩散损失来解释。相反，这些沉积物很可能记录了SEPR外溶解磷浓度的变化，这可能是由于过去1000万年海洋环流变化导致的营养物质重新分配的结果。此外，在这些沉积物中观察到的相对较低的摩尔P/Fe比率并不一定仅仅是由于成岩过程中被清除的P通过扩散到上覆水柱而损失，也可能反映了来自东太平洋隆起南部富含挥发物的喷口的黄铁矿的沉积后氧化。本研究认为富氧富铁沉积物的摩尔P/Fe比值可以作为古海水磷酸盐浓度相对变化的代表，特别是当铁硫化物矿物在运输和沉积过程中不是重要成分时。
We present a detailed study of the co-diagenesis of Fe and P in hydrothermal plume fallout sediments from ∼19°S on the southern East Pacific Rise. Three distal sediment cores from 340–1130km from the ridge crest, collected during DSDP Leg 92, were analysed for solid phase Fe and P associations using sequential chemical extraction techniques. The sediments at all sites are enriched in hydrothermal Fe (oxyhydr)oxides, but during diagenesis a large proportion of the primary ferrihydrite precipitates are transformed to the more stable mineral form of goethite and to a lesser extent to clay minerals, resulting in the release to solution of scavenged P. However, a significant proportion of this P is retained within the sediment, by incorporation into secondary goethite, by precipitation as authigenic apatite, and by readsorption to Fe (oxyhydr)oxides. Molar P/Fe ratios for these sediments are significantly lower than those measured in plume particles from more northern localities along the southern East Pacific Rise, and show a distinct downcore decrease to a depth of ∼12m. Molar P/Fe ratios are then relatively constant to a depth of ∼35m. The Fe and P speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2.5m, and thus depth trends in molar P/Fe ratios can not solely be explained by losses of P from the sediment by diffusion to the overlying water column during early diagenesis. Instead, these sediments are likely recording changes in dissolved P concentrations off the SEPR, possibly as a result of redistribution of nutrients in response to changes in oceanic circulation over the last 10 million years. Furthermore, the relatively low molar P/Fe ratios observed throughout these sediments are not necessarily solely due to losses of scavenged P by diffusion to the overlying water column during diagenesis, but may also reflect post-depositional oxidation of pyrite originating from the volatile-rich vents of the southern East Pacific Rise. This study suggests that the molar P/Fe ratio of oxic Fe-rich sediments may serve as a proxy of relative changes in paleoseawater phosphate concentrations, particularly if Fe sulfide minerals are not an important component during transport and deposition.