Benthic pelagic N-cycling under shifting redox conditions and hydro-dynamical forcing

In contrast to the common understanding of continental margin sediments representing a major sink for fixed nitrogen, the Peruvian shelf- and upper slope were identified as major recycling sites for dissolved inorganic nitrogen during previous SFB 754 phases. A second major finding was that the benthic nitrogen turnover can be shifted out of steady state depending on the redox conditions of the bottom water. As a consequence, particular focus during the 3rd phase is on the in situ and ex situ experimental investigation of the effects of bottom water fluctuations and depletion of O2, NO3-, and NO2- on nitrogen loss and recycling pathways. It will be specifically addressed to what extent benthic N-fluxes are coupled and controlled by filamentous sulphide oxidizing bacteria Beggiatoa and Thioploca, which are ubiquitous on the Peruvian shelf and upper slope and how their vast internal storage of NO3- /NO2- could contribute to non-steady state conditions in the benthic N-cycle and how this affects the benthic-pelagic nutrient exchange. This requires the determination of the turnover rate of their internal NO3-/NO2- pools as well their storage capacity of sulphur in response to changing bottom water levels of electron acceptors. Furthermore, the determination of the timing of sulphidic events after stepwise depletion of O2, NO3-, and NO2- in the bottom water will be an essential part of the proposed study. There will be close collaboration with B7 that will decipher using foraminifera cultivation experiments their quantitative contribution to the benthic nitrogen turnover and N-loss as well as to B5 and B10 with regard to phosphorus and iron turnover. Results of the experiments will be used to develop the non-steady state model by B1 to provide a process-based interpretation of the data on different time scales.

The second major goal is to quantify the seasonal and intra-seasonal variability of natural benthic nutrient release and pelagic nutrient cycling by budgeting physical fluxes of solutes. While previous studies within the SFB 754 have concentrated on nutrient cycling processes during austral summer, natural benthic as well as pelagic nutrient fluxes in combination with hydrodynamic forcing will be measured in austral fall/winter, enhancing our knowledge of natural variability of nutrient cycling processes. During austral fall/winter, productivity is reduced compared to the summer season while winds are enhanced and the ocean’s surface bouyancy flux is negative. This variability in hydrodynamic forcing and associated variability of the eastern boundary circulation as well as water column stratification require variability of physical nutrient transport processes. It is thus expected that the pathways of nutrient transport and thereby the overall benthic-pelagic coupling and nutrient budget is subject to seasonal variability. The results of the data analysis will advance understanding of the solute cycling in oxygen minimum zones and will be used for improving biogeochemical circulation models within the SFB 754 synthesis phase (B1) and elsewhere. Physical transport process analysis will be closely linked to submesoscale processes and organic matter export fluxes investigated in B9, pelagic transfer of iron (B10) and the benthic tracer release study (A3).

Brandt, P., H. Bange, D. Banyte, M. Dengler, S-H. Didwischus, T. Fischer, R. Greatbatch, J. Hahn, T. Kanzow, J. Karstensen, A. Körtzinger, G. Krahmann, S. Schmidtko, L. Stramma, T. Tanhua and M. Visbeck (2015) On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic. Biogeosciences, 12, 489-512, doi: 10.5194/bg-12-489-2015

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.2011.08.010

Dale, A.W., S. Sommer, E. Ryanbenko, A. Noffke, L. Bohlen and K. Wallmann (2014) Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic). Geochim. Cosmochim. Acta, 134, 234-256, doi: 10.1016/ j.gca.2014.02.026

Dale, A.W., S. Sommer, L. Bohlen, T. Treude, V.J. Bertics, H.W. Bange, O. Pfannkuche, T. Schorp, M. Mattsdotter and K. Wallmann (2011) Rates and regulation of nitrogen cycling in seasonally hypoxic sediments during winter (BoknisEck, SW Baltic Sea): Sensitivity to environmental variables. Estuar. Coast. Shelf S., 95: 14-28, doi: 10.1016/j.ecss.2011.05.016

Dengler, M., F. Schott, C. Eden, P. Brandt, J. Fischer and R. Zantopp (2004) Break-up of the Atlantic deep western boundary current at 8°S. Nature, 1018-1020, doi: 10.1038/ nature03134

Flögel, S., W.-C. Dullo, O. Pfannkuche, K. Kiriakoulakis and A. Rueggeberg (2014) Geochemical and physical constraints for the ocurrence of living cold-water corals. Deep-Sea Res. Pt. II, 99, 19-26, doi: 10.1016/j.dsr2.2013.06.006

Glock, N., J. Schönfeld, A. Eisenhauer, C. Hensen, J. Mallon and S. Sommer (2013) The role of benthic foraminifera in the benthic nitrogen cycle of the Peruvian oxygen minimum zone. Biogeosciences, 10, 4767-4783, doi: 10.5194/bg-10-4767-2013

Hummels, R., M. Dengler and B. Bourlés (2013) Seasonal and regional variability of upper ocean diapycnal heat flux in the Atlantic Cold Tongue. Prog. Oceanogr., 111, 52-74, doi: 10.1016/j.pocean.2012.11.001

Schafstall, J., M. Dengler, P. Brandt and H. Bange (2010), Tidal induced mixing and diapycnal nutrient fluxes in the Mauritanian upwelling region. J. Geophys. Res., 115, C10014, doi: 10.1029/2009JC005940

Sommer, S., M. Türk, S. Kriwanek and O. Pfannkuche (2008) Gas exchange system for extended in situ benthic chamber flux measurements under controlled oxygen conditions: First application – Sea bed methane emission measurements at Captain Arutyunov mud volcano. Limnol. Oceanogr. Methods, 6, 23-33, doi: 10.4319/lom.2008.6.23


Dr. Marcus Dengler                             Dr. Stefan Sommer                        
Tel.: +49 431 600 4107                       Tel.: +49 431 600 2119                     
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