Solid phase phosphorus geochemistry in surface sediments of sediment cores GeoB3718-4, GeoB3702-2 and GeoB3707-4

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High Corg/Porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory. Release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle. Dissolved phosphate and iron in the pore water are closely coupled, showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment-water interface. The abundant presence of Fe(III)-bound phosphorus in the sediments document the co-precipitation of both constituents as P-containing iron (oxyhydr)oxides. However, highly dissolved phosphate concentrations in pore waters cannot be explained, neither by simple mass balance calculations nor by the application of an established computer model. Under the assumption of steady state conditions, phosphate release rates are too high as to be balanced with a solid phase reservoir. This discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail. We assume that the known, active, fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron (oxyhydro)oxides from the oxic surface sediments. Accordingly, a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone.

本研究针对纳米比亚沿海上升流区外三处站位的现代大陆边缘沉积物，开展底栖磷循环（benthic phosphorus cycling）研究。沉积物分析结果显示，有机磷与生物成因磷是沉积物中主要的含磷物相。沉积物表层极高的有机碳/有机磷（Corg/Porg）比值表明，相较于有机碳，磷的优先再生要么已在悬浮颗粒物负荷阶段发生，要么此处沉积的有机质本身已具备较强难降解性。底栖微生物介导的有机质降解过程所释放的磷酸盐，并非观测到的内源性底栖磷循环的主导过程。孔隙水中溶解态磷酸盐与铁元素呈现显著耦合关系：在沉积物含氧表层之下浓度较高，而在沉积物-水界面处浓度较低。沉积物中大量存在的三价铁结合态磷（Fe(III)-bound phosphorus）证实，磷与含铁（羟基）氧化物（iron (oxyhydr)oxides）以含磷铁氧化物形式发生了共沉淀。然而，孔隙水中高浓度的溶解态磷酸盐既无法通过简单的质量平衡计算（mass balance calculations）解释，也无法通过已成熟的计算机模型予以阐明。在稳态条件假设下，磷酸盐释放速率过高，无法通过固相储库（solid phase reservoir）进行平衡。这一差异表明，在次氧条件（suboxic conditions）占主导的沉积物深度处，固相磷存在明显缺失。我们推测，已知的由穴居大型底栖生物（burrowing macrobenthic organisms）介导的活跃、快速且间歇性的沉积物混合过程，可反复为铁还原的微生物催化过程提供来自含氧表层沉积物的自生铁（羟基）氧化物（authigenic iron (oxyhydro)oxides）。据此，在磷酸盐与铁元素被埋藏至铁还原带（iron reduction zone）之下前，二者会经历多次内循环过程。