Phosphate d18O pore-water profiles of sediment core GeoB11807-2 and GeoB11804-4

Phosphorus cycling in the ocean is influenced by biological and geochemical processes that are reflected in the oxygen isotope signature of dissolved inorganic phosphate (Pi). Extending the Pi oxygen isotope record from the water column into the seabed is difficult due to low Pi concentrations and small amounts of marine porewaters available for analysis. We obtained porewater profiles of Pi oxygen isotopes using a refined protocol based on the original micro-extraction designed by Colman (2002). This refined and customized method allows the conversion of ultra-low quantities (0.5 - 1 µmol) of porewater Pi to silver phosphate (Ag3PO4) for routine analysis by mass spectrometry. A combination of magnesium hydroxide co-precipitation with ion exchange resin treatment steps is used to remove dissolved organic matter, anions, and cations from the sample before precipitating Ag3PO4. Samples as low as 200 µg were analyzed in a continuous flow isotope ratio mass spectrometer setup. Tests with external and laboratory internal standards validated the preservation of the original phosphate oxygen isotope signature (d18OP) during micro extraction. Porewater data on d18OP has been obtained from two sediment cores of the Moroccan margin. The d18OP values are in a range of +19.49 to +27.30 per mill. We apply a simple isotope mass balance model to disentangle processes contributing to benthic P cycling and find evidence for Pi regeneration outbalancing microbial demand in the upper sediment layers. This highlights the great potential of using d18OP to study microbial processes in the subseafloor and at the sediment water interface.

海洋磷循环受生物与地球化学过程共同调控，这些过程的特征可通过溶解态无机磷（dissolved inorganic phosphate, Pi）的氧同位素信号体现。由于孔隙水中Pi浓度极低且可用于分析的海洋孔隙水样品量极少，将水柱中的Pi氧同位素记录拓展至海底沉积物中颇具挑战。本研究基于Colman（2002）提出的原始微萃取法，通过优化实验流程获取了孔隙水Pi的氧同位素剖面。该经优化与定制化改良的方法，可将超微量（0.5~1 μmol）的孔隙水Pi转化为磷酸银（silver phosphate, Ag3PO4），以供质谱法开展常规分析。在沉淀磷酸银之前，研究团队通过联用氢氧化镁共沉淀与离子交换树脂处理步骤，去除样品中的溶解性有机质、阴离子与阳离子。低至200 μg的样品可通过连续流同位素比值质谱仪平台完成分析测试。借助外部标准物质与实验室内部标准物质开展的验证实验，证实了微萃取过程中原始磷酸氧同位素特征（d18OP）得以完整保留。研究人员从摩洛哥陆缘的两个沉积岩芯中获取了d18OP的孔隙水数据，其数值范围为+19.49‰至+27.30‰。本研究采用简易同位素质量平衡模型，解析了驱动底栖磷循环的各类过程，并发现沉积物上层中Pi再生速率超过了微生物的磷需求。该结果凸显了利用d18OP研究次海底及沉积物-水界面微生物过程的巨大潜力。