Biogeosciences (Jul 2012)

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

  • P. Kraal,
  • C. P. Slomp,
  • D. C. Reed,
  • G.-J. Reichart,
  • S. W. Poulton

DOI
https://doi.org/10.5194/bg-9-2603-2012
Journal volume & issue
Vol. 9, no. 7
pp. 2603 – 2624

Abstract

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In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (C<sub>org</sub>/P<sub>org</sub>) and to total reactive P (C<sub>org</sub>/P<sub>reactive</sub>) in surface sediments indicate that the overall burial efficiency of P relative to C<sub>org</sub> decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ~25 cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O<sub>2</sub>: ~14 μmol L<sup>−1</sup>). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the C<sub>org</sub>/P ratio leads to an overestimation of the burial efficiency of reactive P relative to C<sub>org</sub> along the depth transect. Moreover, the unreactive Ca-P accounts for ~85% of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.