Sedimentary phosphorus and iron cycling in and below the oxygen minimum zone of the northern Arabian Sea

[摘要] In this study, we investigate phosphorus (P) and iron (Fe) cycling insediments along a depth transect from within to well below the oxygenminimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-waterand solid-phase analyses show that authigenic formation of calcium phosphateminerals (Ca-P) is largely restricted to where the OMZ intersects theseafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P(C_org/P_org) and to total reactive P (C_org/P_reactive) insurface sediments indicate that the overall burial efficiency of P relativeto C_org decreases under the low bottom water oxygen concentrations(BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sedimentsalong the depth transect suggest that corresponding changes in Fe burial arelimited. Sedimentary pyrite contents are low throughout the ~25 cmsediment cores at most stations, as commonly observed in the Arabian SeaOMZ. However, pyrite is an important sink for reactive Fe at one station inthe OMZ. A reactive transport model (RTM) was applied to quantitativelyinvestigate P and Fe diagenesis at an intermediate station at the lowerboundary of the OMZ (bottom water O₂: ~14 μmol L⁻¹).The RTM results contrast with earlier findings in showing that Fe redoxcycling can control authigenic apatite formation and P burial in Arabian Seasediment. In addition, results suggest that a large fraction of thesedimentary Ca-P is not authigenic, but is instead deposited from the watercolumn and buried. Dust is likely a major source of this Ca-P. Inclusion ofthe unreactive Ca-P pool in the C_org/P ratio leads to an overestimationof the burial efficiency of reactive P relative to C_org along the depthtransect. Moreover, the unreactive Ca-P accounts for ~85% oftotal Ca-P burial. In general, our results reveal large differences in P andFe chemistry between stations in the OMZ, indicating dynamic sedimentaryconditions under these oxygen-depleted waters.

[学科分类] 地球化学与岩石