Surface sediment and porewater chemistry from slope to deep sea in northwest Black Sea (PHOXY cruise, June 2013)

Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water column redox transect (six stations, 78-2107 m water depth) in the Black Sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin. Organic P is an important P pool under all redox regimes, accounting for up to 60% of P burial. We find a general down-core increase in the relative importance of organic P, especially on the shelf where P bound to iron (Fe) and manganese (Mn) (oxyhydr)oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth. Our chemical and spectroscopic data indicate that the carbonate-rich sediments (Unit I, ~3000 years, ~0-30 cm depth) of the sulfidic deep Black Sea contain three major P pools: calcium phosphate (apatite), organic P and P that is strongly associated with CaCO3 and possibly clay surfaces. Apatite concentrations increase from 5% to 25% of total P in the uppermost centimeters of the deep basin sediments, highlighting the importance of apatite formation for long-term P burial. Iron(II)-associated P (ludlamite) was detected with X-ray absorption spectroscopy but was shown to be a minor P pool (~5%), indicating that lateral Fe-P transport from the shelf ("shuttling") likely occurs but does not impact the P burial budget of the deep Black Sea. The CaCO3-P pool was relatively constant throughout the Unit I sediment interval and accounted for up to 55% of total P. Our results highlight that carbonate-bound P can be an important sink for P in CaCO3-rich sediments of anoxic, sulfidic basins and should also be considered as a potential P sink (and P source in case of CaCO3 dissolution) when reconstructing past ocean P dynamics from geological records.缺氧硫化环境下关键营养盐磷（P）的沉积埋藏过程尚未得到充分阐明。本研究借助化学分析与微观尺度光谱学方法，对黑海从含氧水体陆架至缺氧硫化深海盆地的水柱氧化还原断面（6个站位，水深78~2107 m）的沉积磷埋藏特征进行了系统表征。有机磷在所有氧化还原环境中均为重要的磷库，其占磷埋藏总量的比例最高可达60%。研究发现有机磷的相对占比随沉积深度整体呈升高趋势，这一特征在陆架区域尤为显著：该区域中与铁（Fe）、锰（Mn）（羟基）氧化物结合的磷在表层沉积物中含量较高，但随沉积深度增加浓度快速降低。我们的化学与光谱学数据显示，黑海硫化深海盆地的富碳酸盐沉积物（第一单元Unit I，沉积年代约3000年，深度区间0~30 cm）包含三大主要磷库：磷酸钙（磷灰石apatite）、有机磷，以及与碳酸钙（CaCO3）乃至黏土表面紧密结合的磷。在深海盆地沉积物的最上层厘米级区间内，磷灰石（apatite）占总磷的比例从5%升高至25%，这凸显了磷灰石形成对于长期磷埋藏的重要性。通过X射线吸收光谱法（X-ray absorption spectroscopy）检测到了与亚铁（Fe(II)）结合的磷（卢德拉姆石ludlamite），但其仅为次要磷库（占比约5%），这表明尽管来自陆架的铁磷横向搬运“穿梭作用”可能存在，但并未对黑海深海的磷埋藏收支产生显著影响。碳酸钙结合磷库（CaCO3-P）在第一单元沉积物的整个层段内相对稳定，最高可占总磷的55%。本研究结果表明，在缺氧硫化盆地的富碳酸盐沉积物中，碳酸盐结合磷可作为重要的磷汇；在通过地质记录重建过去海洋磷循环动态时，也应将其视为潜在的磷汇（若碳酸钙发生溶解则可作为磷源）纳入考量范围。