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Volume 9, issue 7
Biogeosciences, 9, 2603–2624, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Low oxygen in marine environments from the Cretaceous to the...

Biogeosciences, 9, 2603–2624, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 16 Jul 2012

Research article | 16 Jul 2012

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

P. Kraal1,*, C. P. Slomp1, D. C. Reed1, G.-J. Reichart1,2, and S. W. Poulton3 P. Kraal et al.
  • 1Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, Budapestlaan 4, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
  • 2Marine Biogeosciences, Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
  • 3School of Civil Engineering and Geosciences, Newcastle University, Drummond Building, Newcastle upon Tyne NE1 7RU, UK
  • *now at: Southern Cross GeoScience, Southern Cross University, P.O. Box 157, Lismore, 2480 New South Wales, Australia

Abstract. 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 (Corg/Porg) and to total reactive P (Corg/Preactive) in surface sediments indicate that the overall burial efficiency of P relative to Corg 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 O2: ~14 μmol L−1). 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 Corg/P ratio leads to an overestimation of the burial efficiency of reactive P relative to Corg 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.

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