Extractions and analyses of reactive phosphorus in the Bering Sea from IODP Holes 323-U1341A and 323-U1341B@en

To reconstruct the cycling of reactive phosphorus (P) in the Bering Sea, a P speciation record covering the last ~ 4 Ma was generated from sediments recovered during Integrated Ocean Drilling Program (IODP) Expedition 323 at Site U1341 (Bowers Ridge). A chemical extraction procedure distinguishing between different operationally defined P fractions provides new insight into reactive P input, burial and diagenetic transformations. Reactive P mass accumulation rates (MARs) are ~ 20-110 µmol/cm2/ka, which is comparable to other open ocean locations but orders of magnitude lower than most upwelling settings. We find that authigenic carbonate fluorapatite (CFA) and opal-bound P are the dominant P fractions at Site U1341. An overall increasing contribution of CFA to total P with sediment depth is consistent with a gradual \"sink switching\" from more labile P fractions (fish remains, Fe oxides, organic matter) to stable authigenic CFA. However, the positive correlation of CFA with Al content implies that a significant portion of the supposedly reactive CFA is non-reactive \"detrital contamination\" by eolian and/or riverine CFA. In contrast to CFA, opal-bound P has rarely been studied in marine sediments. We find for the first time that opal-bound P directly correlates with excess silica contents. This P fraction was apparently available to biosiliceous phytoplankton at the time of sediment deposition and is a long-term sink for reactive P in the ocean, despite the likelihood for diagenetic re-mobilisation of this P at depth (indicated by increasing ratios of excess silica to opal-bound P). Average reactive P MARs at Site U1341 increase by ~ 25% if opal-bound P is accounted for, but decrease by ~ 25% if 50% of the extracted CFA fraction (based on the lowest CFA value at Site U1341) is assumed to be detrital. Combining our results with literature data, we present a qualitative perspective of terrestrial CFA and opal-bound P deposition in the modern ocean. Riverine CFA input has mostly been reported from continental shelves and margins draining P-rich lithologies, while eolian CFA input is found across wide ocean regions underlying the Northern Hemispheric \"dust belt\". Opal-bound P burial is important in the Southern Ocean, North Pacific, and likely in upwelling areas. Shifts in detrital CFA and opal-bound P deposition across ocean basins likely occurred over time, responding to changing weathering patterns, sea level, and biogenic opal deposition.为重建白令海反应性磷（reactive phosphorus, P）的生物地球化学循环，研究基于综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）第323航次在U1341站位（鲍尔斯海岭，Bowers Ridge）获取的沉积物，构建了覆盖过去约4百万年的磷形态记录。采用可区分不同操作定义磷组分的化学萃取方法，本研究为反应性磷的输入、埋藏及成岩转化过程提供了新认知。反应性磷质量累积速率（mass accumulation rates, MARs）约为20~110 μmol/cm²/ka，与其他开阔大洋站位相当，但较多数上升流区低数个数量级。研究发现，U1341站位的主要磷组分为自生碳酸盐氟磷灰石（authigenic carbonate fluorapatite, CFA）与结合于蛋白石的磷（opal-bound P）。随着沉积物深度增加，CFA占总磷的比例整体呈上升趋势，这与反应性磷汇从更易变的磷组分（鱼残骸、氧化铁、有机质）逐渐向稳定的自生CFA转变的“汇转换”过程一致。然而，CFA与铝（Al）含量的正相关关系表明，原本被认为具有反应性的CFA中，有相当一部分为风成和/或河流成因的CFA带来的非反应性“碎屑污染”。与CFA不同，结合于蛋白石的磷在海洋沉积物中鲜有研究。本研究首次发现，结合于蛋白石的磷与过剩二氧化硅含量直接相关。该磷组分在沉积物沉积时可被硅质浮游植物利用，尽管在深部沉积物中该磷可能发生成岩再活化（由过剩二氧化硅与结合于蛋白石的磷的比值随深度增加得到指示），但其仍是海洋中反应性磷的长期汇。若将结合于蛋白石的磷计入反应性磷，U1341站位的平均反应性磷质量累积速率将提升约25%；但若假设50%的萃取得到的CFA组分（基于U1341站位的最低CFA值）为碎屑成因，则该速率将下降约25%。结合本研究结果与已有文献数据，我们对现代海洋中陆源CFA及结合于蛋白石的磷的沉积过程给出了定性认识。河流成因CFA的输入多报道于发育富磷岩性的大陆架及陆缘区域，而风成CFA的输入则广泛分布于北半球“尘埃带”覆盖的广阔大洋区域。结合于蛋白石的磷埋藏在南大洋、北太平洋及上升流区均占据重要地位。不同大洋盆地的碎屑CFA与结合于蛋白石的磷沉积模式随时间发生转变，这与风化模式、海平面及生物成因蛋白石沉积的变化相对应。