Pore water and solid phase geochemistry of sediment core US5B

Studies of authigenic phosphorus (P) minerals in marine sediments typically focus on authigenic carbonate fluorapatite, which is considered to be the major sink for P in marine sediments and can easily be semi-quantitatively extracted with the SEDEX sequential extraction method. The role of other potentially important authigenic P phases, such as the reduced iron (Fe) phosphate mineral vivianite (Fe(II)3(PO4)*8H2O) has so far largely been ignored in marine systems. This is, in part, likely due to the fact that the SEDEX method does not distinguish between vivianite and P associated with Fe-oxides. Here, we show that vivianite can be quantified in marine sediments by combining the SEDEX method with microscopic and spectroscopic techniques such as micro X-ray fluorescence (µXRF) elemental mapping of resin-embedded sediments, as well as scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) and powder X-ray diffraction (XRD). We further demonstrate that resin embedding of vertically intact sediment sub-cores enables the use of synchrotron-based microanalysis (X-ray absorption near-edge structure (XANES) spectroscopy) to differentiate between different P burial phases in aquatic sediments. Our results reveal that vivianite represents a major burial sink for P below a shallow sulfate/methane transition zone in Bothnian Sea sediments, accounting for 40-50% of total P burial. We further show that anaerobic oxidation of methane (AOM) drives a sink-switching from Fe-oxide bound P to vivianite by driving the release of both phosphate (AOM with sulfate and Fe-oxides) and ferrous Fe (AOM with Fe-oxides) to the pore water allowing supersaturation with respect to vivianite to be reached. The vivianite in the sediment contains significant amounts of manganese (~4-8 wt.%), similar to vivianite obtained from freshwater sediments. Our results indicate that methane dynamics play a key role in providing conditions that allow for vivianite authigenesis in coastal surface sediments. We suggest that vivianite may act as an important burial sink for P in brackish coastal environments worldwide.海洋沉积物自生磷（P）矿物的相关研究通常聚焦于自生碳酸氟磷灰石，该矿物被认为是海洋沉积物中磷的主要储汇，且可通过SEDEX连续提取法实现简易半定量提取。然而，其他潜在重要的自生磷物相——如还原态磷酸铁矿物蓝铁矿（vivianite，化学式为Fe(II)₃(PO₄)₂·8H₂O）——在海洋系统中的作用迄今尚未得到足够重视。这一现象的部分原因在于，SEDEX法无法区分蓝铁矿与铁氧化物结合态磷。本研究证实，通过将SEDEX法与显微及光谱技术相结合，包括树脂包埋沉积物的微X射线荧光（µXRF）元素成像、扫描电镜-能谱仪（SEM-EDS）以及粉末X射线衍射（XRD），可实现海洋沉积物中蓝铁矿的定量分析。本研究进一步证明，对直立完整的沉积物亚岩心进行树脂包埋，可开展同步辐射微区分析（X射线吸收近边结构（XANES）光谱），以此区分水生沉积物中不同的磷埋藏物相。研究结果显示，在波的尼亚湾沉积物的浅层硫酸盐-甲烷转换带下方，蓝铁矿是磷的主要埋藏储汇，其贡献可达总磷埋藏量的40%~50%。本研究还证实，甲烷厌氧氧化（anaerobic oxidation of methane, AOM）过程可驱动磷的储汇转换：从铁氧化物结合态磷转向蓝铁矿。该过程通过将磷酸盐（由硫酸盐型AOM与铁氧化物反应生成）与亚铁离子（由铁氧化物型AOM反应生成）释放至孔隙水，使孔隙水达到蓝铁矿形成所需的过饱和状态。沉积物中的蓝铁矿含有显著含量的锰（约4%~8%质量百分比），这与淡水沉积物中提取的蓝铁矿特征一致。本研究结果表明，甲烷动力学过程为滨海表层沉积物中蓝铁矿的自生形成提供了关键环境条件。本研究提出，全球范围内的半咸水滨海环境中，蓝铁矿可能是磷的重要埋藏储汇。