Phosphorus geochemistry of eastern equatorial Pacific Ocean sediments

We determined phosphorus (P) concentrations in Leg 138 sediment samples from Sites 844, 846, and 851, using a sequential extraction technique to identify the P associated with five sedimentary components. Total concentrations of P (sum of the five components) ranged from 4 to 35 µmol P/g sediment, with mean values relatively similar between the three sites (11, 14, and 12 for Sites 844,846, and 851, respectively). Authigenic/biogenic P was the most important component in terms of percentage of total P (about 75%), with iron-bound P (13%), adsorbed P (2%-9%), and organic P (4%) of secondary importance; detrital P was a minor P sink (1%) in these sediments. Profiles of adsorbed P and iron-bound P show decreasing concentrations with age, indicating that these components have been affected by diagenesis and reorganization of P. A peak in iron-bound P may reflect higher fluxes of hydrothermally derived Fe to eastern equatorial Pacific Ocean sediments from 11 to 8 Ma. Lower detrital P values for western Site 851 reflect a greater distance of this site from a terrigenous source area, compared to that of Sites 844 and 846.
Phosphorus mass accumulation rates (P-MARs; units of µmol P/cm**2/k.y.) were calculated using total P concentrations (not including the minor and oceanically unreactive detrital P component) and sedimentation rates and dry-bulk densities averaged over time intervals of 0.5 m.y. P-MARs generally decrease from 17 Ma to the present. Eastern transect Sites 844 and 846 display a decrease in P-MARs from about 30 to 10 in the interval from 17 to 8 Ma, while western transect Site 851 is highly variable during this interval. P-MARs increase to about 45 and stay relatively high from 8 to 6 Ma, then decrease toward the present to some of the lowest values of the record (about 10). The general trend of high P-MARs at about 6 Ma and decreasing values toward the present is correlated with other geochemical and sedimentary trends through this interval and may reflect (1) a change in net sediment and P burial, (2) a reorganization of fluxes with no change of net burial, or (3) a combination of the two.

本研究采用连续萃取技术（sequential extraction technique），对大洋钻探第138航次（Leg 138）844、846、851站位的沉积物样品中的磷（phosphorus, P）浓度进行了测定，以识别与五类沉积组分（sedimentary components）结合的磷形态。总磷浓度（五类组分之和）介于4~35 μmol P/g沉积物之间，三个站位的平均总磷浓度较为接近，分别为11、14和12 μmol P/g沉积物（对应站位844、846、851）。自生/生物成因磷（authigenic/biogenic P）是占总磷比例最高的组分（约75%），铁结合态磷（iron-bound P，13%）、吸附态磷（adsorbed P，2%~9%）与有机磷（organic P，4%）次之；碎屑磷（detrital P）在该类沉积物中仅为次要的磷储库，占比仅1%。吸附态磷与铁结合态磷的浓度剖面随沉积年代增加而降低，表明这两类组分受到成岩作用（diagenesis）与磷元素再分配的影响。11 Ma至8 Ma期间，铁结合态磷出现峰值，这可能反映了东赤道太平洋沉积物中热液成因（hydrothermally derived）铁的输入通量升高。西侧站位851的碎屑磷浓度更低，这反映该站位距离陆源物源区（terrigenous source area）较站位844与846更远。
磷质量累积速率（phosphorus mass accumulation rates, P-MARs；单位为μmol P/cm²/k.y.）的计算基于总磷浓度（不含占比极低且在海洋中不活跃的碎屑磷组分）、沉积速率以及以0.5百万年为时间间隔求取的平均干容重（dry-bulk densities）。整体而言，P-MARs从17 Ma至今呈持续降低的趋势。东侧断面的站位844与846的P-MARs在17 Ma至8 Ma期间从约30降至10，而西侧断面的站位851在此时间段内的P-MARs波动剧烈。在8 Ma至6 Ma期间，P-MARs升至约45并维持在较高水平，随后逐渐降低至本记录中的部分最低值（约10），直至现代。约6 Ma时P-MARs处于高位、随后向现代逐渐降低的整体趋势，与该时段内其他地球化学与沉积学趋势具有相关性，其成因可能为：1）沉积物与磷的净埋藏（net burial）过程发生改变；2）物质通量（fluxes）发生再分配但净埋藏量未发生变化；或3）上述两种情况共同作用。