Ratios of phosphorous (P) to iron (Fe) in Precambrian banded iron formations (BIFs) have previously been used to estimate dissolved seawater phosphate concentrations in the ancient oceans. Such studies rely on an assumed composition of the primary iron minerals, the concentrations of the major ions in seawater, and empirical partitioning coefficients for phosphate sorption to Fe(III) (oxyhydr)oxides. There is limited data, however, regarding the post-depositional stability of phosphate associated with presumed primary BIF iron minerals, such as ferrihydrite under low-grade metamorphic temperature and pressure conditions. Here we experimentally formed ferrihydrite in the presence of silica, which was abundant in the Precambrian oceans, and then incubated it at 170 °C and 1.2 kbar in the presence or absence of organic carbon (C_org; either glucose or microbial biomass) as a proxy for ancient planktonic biomass. We found that the post-metamorphic mineral assemblage resulting from thermochemical Fe(III) reduction of Si-doped ferrihydrite depended on C_org reactivity: In the presence of highly reactive glucose, siderite, magnetite, and vivianite were formed, with less than 1.2 mol% of phosphate (0.5 M NaCl extractable) being mobilized. In contrast, the reaction of Si-doped ferrihydrite with less reactive microbial biomass resulted in the formation of hematite and siderite, but not vivianite, and approximately 10 mol% of phosphate was remobilized into the pore fluids. Collectively, our data suggest that the fidelity with which BIFs record ancient oceanic phosphate concentrations depends on the mineralogy and diagenetic history of individual BIFs but should be reliable within 10%.

前寒武纪带状铁地层 (BIF) 中磷 (P) 与铁 (Fe) 的比率以前曾被用于估计古代海洋中溶解的海水磷酸盐浓度。此类研究依赖于主要铁矿物的假设成分、海水中主要离子的浓度以及磷酸盐吸附到 Fe(III)（羟基）氧化物的经验分配系数。然而，关于在低品位变质温度和压力条件下与假定的原生 BIF 铁矿物（例如水铁矿）相关的磷酸盐的沉积后稳定性的数据有限。在这里，我们在前寒武纪海洋中丰富的二氧化硅存在下通过实验形成了水铁矿，然后在存在或不存在有机碳（C_组织; 葡萄糖或微生物生物量）作为古代浮游生物量的代表。我们发现 Si 掺杂的水铁矿的热化学 Fe(III) 还原导致的后变质矿物组合取决于 C _org反应性：在高反应性葡萄糖存在的情况下，形成菱铁矿、磁铁矿和蓝铁矿，其中移动的磷酸盐（可提取 0.5 M NaCl）少于 1.2 mol%。相比之下，Si 掺杂的水铁矿与反应性较低的微生物生物质的反应导致赤铁矿和菱铁矿的形成，但没有紫铁矿，并且大约 10 mol% 的磷酸盐被重新迁移到孔隙流体中。总的来说，我们的数据表明，BIFs 记录古代海洋磷酸盐浓度的保真度取决于单个 BIFs 的矿物学和成岩历史，但应该在 10% 以内可靠。