Biogeochemical cycling of iron and phosphate under low oxygen conditions

This subproject investigates the feedbacks between bottom water oxygenation and the cycling of iron (Fe) and phosphorus (P) in sediments underneath oxygen minimum zones (OMZ). Under low oxygen conditions the supply of P and Fe from sediments is typically enhanced compared to normal marine conditions. Our ongoing work revealed that dissolved Fe and P fluxes into the overlying bottom from sediments of the Peruvian OMZ are higher than at any other continental margin. Bottom water oxygen concentrations have to fall below a critical threshold value in the order of 20 μM to trigger enhanced benthic Fe and P fluxes. Under fully anoxic conditions periodically occurring on the Peruvian shelf, P release is further amplified while the release of Fe is then limited by the precipitation of sulphide minerals. Fluxes are partitioned into one fraction transported vertically into the surface ocean and another fraction advected horizontally out of the OMZ. The fraction reaching the surface ocean may support primary production and amplify the ongoing expansion of OMZs in a positive feedback loop. However, a significant P-fraction of particles in the euphotic zone is not bound in biomolecules but fixed in inorganic phases, hence; a certain fraction of P does not contribute to biomass production even when it reaches the surface ocean. The horizontally exported fraction is partly transported into the open ocean or, mainly with respect to Fe, precipitated at the outer rim of the OMZ where it accumulates in surface sediments. The accumulation of authigenic Fe in sediments at the margins of the Peruvian OMZ may cause pulses of dissolved Fe upon OMZ expansion when bottom water oxygen values fall below the critical threshold value.

In the final phase of SFB 754, the project will focus on remaining open questions which need to be addressed to balance the modern Fe and P turnover in the Peruvian OMZ and to predict the response of the benthic Fe and P system to future OMZ expansion: (1) How much of the dissolved Fe load released at the seabed precipitates in bottom waters and accumulates at the outer rim of the OMZ before it can reach the surface ocean? (2) How much of the dissolved benthic P load is fixed in particulate inorganic phases and does not contribute to the biological pump? (3) What are the oxygen threshold values for Fe and P release from sediments and how do these depend on sedimentary Fe and P speciation and binding forms? Planned work will comprise pore water and solid phase analyses of sediments as well as water column particles (CTD casts, pumps, mesocosm experiments), including sequential extractions, REM-EDX, XRF/XRD, and synchrotron radiation-based (Xanes) investigations on Fe and P speciation. These activities are complementary to other sub-projects and will be carried out in close collaboration specifically with TP B1, B2, B6, B7, and B10.

Bohlen, L., A. Dale and K. Wallmann, (2012). Simple transfer functions for calculating benthic fixed nitrogen losses and C:N:P regeneration ratios in global biogeochemical models Glob. Biogeochem. Cy., 26, GB3029, doi: 10.1029/2011GB004198

Bohlen, L., A.W. Dale, S. Sommer, T. Mosch, C. Hensen, A. Noffke, F. Scholz and K. Wallmann (2011) Benthic nitrogen cycling traversing the Peruvian oxygen minimum zone. Geochim. Cosmochim. Acta, 75, 6094-6111, doi: 10.1016/j.gca.2011.08.010Haese, R., K.

Wallmann, A. Dahmke, U. Kretzmann, P.J. Müller and H.D. Schulz (1997) Iron species determination to investigate early diagenetic reactivity in marine sediments. Geochim. Cosmochim. Acta, 61, 63-72, doi: 10.1016/S0016-7037(96)00312-2

Hensen, C., H. Landenberger, M. Zabel and H.D. Schulz (1998) Quantification of diffusive benthic fluxes of nitrate, phosphate and silicate in the southern Atlantic Ocean. Glob. Biogeochem. Cy., 12, 193-210, doi: 10.1029/97GB02731

Noffke, A., C. Hensen, S. Sommer, F. Scholz, L. Bohlen, T. Mosch, M. Graco and K. Wallmann, (2012) Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone. Limnol. Oceanogr., 57, 851-867, doi: 10.4319/lo.2012.57.3.0851

Scholz, F., J. McManus, A.C. Mix, C. Hensen and R.R. Schneider (2014) The impact of ocean deoxygenation on iron release from continental margin sediments. Nat. Geosci., 7, 433-437, doi: 10.1038/ngeo2162

Scholz, F., S. Severmann, J. McManus and C. Hensen (2014) Beyond the Black Sea paradigm: The sedimentary fingerprint of an open-marine iron shuttle. Geochim. Cosmochim. Acta, 127, 368-380, doi: 10.1016/j.gca.2013.11.041

Scholz, F., C. Hensen, A. Noffke, A. Rohde, V. Liebetrau and K. Wallmann (2011) Early diagenesis of redox-sensitive trace metals in the Peru upwelling area – response to ENSO- related oxygen fluctuations in the water column. Geochim. Cosmochim. Acta, 75, 7257- 7276, doi: 10.1016/j.gca.2011.08.007

Wallmann, K. (2010) Phosphorus imbalance in the global ocean? Glob. Biogeochem. Cy., 24, GB4030, doi: 10.1029/2009GB003643

Wallmann, K. (2003) Feedbacks between oceanic redox states and marine productivity: A model perspective focused on benthic phosphorus cycling. Glob. Biogeochem. Cy., 17, 1084, doi: 10.1029GB001968


Dr. Christian Hensen                       Prof. Klaus Wallmann
Tel.: +49 431 600 2567                   Tel.: +49 431 600 2287
chensen(a)                     kwallmann(a)