Iron and phosphorus speciations in DSDP Leg 92 sediments

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 340-1130 km 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 ~12 m. Molar P/Fe ratios are then relatively constant to a depth of ~35 m. The Fe and P speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2.5 m, 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.

本研究针对东太平洋海隆南段（southern East Pacific Rise）约19°S区域的热液羽流沉降沉积物中的铁（Fe）与磷（P）成岩共生过程展开了详尽的系统分析。本研究采用连续化学萃取技术，对深海钻探计划（Deep Sea Drilling Project, DSDP）第92航次采集的、距脊顶340~1130 km的3根远端沉积岩芯，开展了固相铁与磷的赋存状态分析。所有站位的沉积物均富集热液成因铁（氢）氧化物，但在成岩作用过程中，大部分原生水铁矿沉淀物会转变为性质更稳定的针铁矿，少量则转化为黏土矿物，由此释放出此前被吸附的磷。然而，相当比例的释放磷仍留存于沉积物中：其一为结合至次生针铁矿晶格，其二是以自生磷灰石形式沉淀，其三为重新吸附于铁（氢）氧化物表面。上述沉积物的摩尔P/Fe比值显著低于东太平洋海隆南段北侧区域热液羽流颗粒物的对应比值，且随岩芯深度增加呈现明显下降趋势，直至约12 m深度；至约35 m深度后，该比值趋于相对稳定。铁、磷赋存形态数据表明，沉积物的成岩改造作用在约2.5 m深度已基本完成，因此摩尔P/Fe比值的深度变化趋势无法仅通过早期成岩阶段磷通过扩散作用流失至上覆水柱来解释。取而代之的是，上述沉积物很可能记录了东太平洋海隆南段（southern East Pacific Rise, SEPR）外围溶解态磷浓度的变化，这一变化可能与过去1000万年间海洋环流变迁引发的营养盐再分布有关。此外，上述沉积物整体偏低的摩尔P/Fe比值，未必仅由成岩阶段吸附态磷通过扩散流失至上覆水柱所致，还可能反映了源自东太平洋海隆南段富挥发性喷口的黄铁矿在沉积后的氧化过程。本研究表明，富氧富铁沉积物的摩尔P/Fe比值可作为古海水磷酸盐浓度相对变化的代用指标，尤其当铁硫化物矿物在物质搬运与沉积过程中并非关键组分时。