SUBPROJECT - A8-E
Transport and fluxes across the bottom boundary layer
The main objective of the subproject is to estimate transport and fluxes of solutes between the bottom boundary layer, the stratified interior ocean and the ocean mixed layer on the continental slope and shelf regions of the Peruvian and Mauritanian oxygen minimum zones (OMZ). Data from physical-biogeochemical process studies carried out on cruises to the Peruvian (Jan-Feb. 2013) and to the Mauritania (May-June 2014) continental margin were analyzed.
A benthic-pelagic nutrient transport budget that also allowed determining of nitrogen (N)- cycling rates and associated N-loss was calculated from the observations for the Peruvian upper continental slope and for the shelf. Three major conclusions were inferred from the study: (1) Unexpectedly, the results showed that diapycnal nutrient fluxes, driven by turbulent mixing caused by the breaking of non-linear internal waves, was the dominant physical mechanism driving N-cycling; (2) the relative contribution of benthic nutrient fluxes to nutrient cycling was between 30% and 50%; (3) rates of N-loss on the shelf were significantly larger than those at the continental slope, most likely due to the presence of sulfidic bottom waters on the shelf.
The water column distribution of the sediment-derived four radium isotopes (224Ra (T1/2 = 3.7 d), 223Ra (T1/2 = 11.4 d), 228Ra (T1/2 = 5.7 y), 226Ra (T1/2 = 1600 y) were used as tracers for mixing processes on different time scales. During the cruises radium was sampled in the water column and, for the first time, in the benthic boundary layer. Based on the 224Ra/223Ra distributions two different mixing environments were observed: (i) Rapid near bottom mixing on short time scales (few days); (ii) different mixing intensities between the boundary layer and the water column above. 224Ra/223Ra-derived benthic nutrient fluxes at selected locations off Peru are within the same range as flux estimates based on microstructure and/or benthic chamber measurements.
In collaborative studies with B1, B6 and A7 it was found that non-linear internal waves off Peru strongly influence the distribution of epibenthic megafauna at the continental slope and that the internal wave generation cites play a dominant role in regional variability of present and past (last 10,000 years) sedimentation rates. Additionally, the physical and biogeochemical implications of eddy generation at the continental slope were investigated in cooperation with B9 and B4.
In the northeastern Atlantic upwelling region, physical processes contributing to the oxygen balance were investigated. A budget of benthic oxygen uptake and pelagic oxygen consumption indicated that the benthic oxygen uptake plays a negligible role in maintaining the deep OMZ at about 400m depth. Diapycnal oxygen fluxes are of similar magnitude as benthic oxygen uptake in water depth less than 100m on the shelves, suggesting a direct oxygen supply from the surface ocean. Within the shallow OMZ region (100m-200m) however, benthic oxygen uptake must be balanced by advective oxygen transport within the eastern boundary current system.
Collaborative studies in the Mauritanian upwelling with SFB 754 subprojects included estimates of the diapycnal oxygen flux and its role in ventilating the deep OMZ (A3 and A4), a study on the atmospheric N-input in the central tropical Atlantic utilizing diapycnal nutrient fluxes and N-isotopes of different functional groups of epipelagic zooplankton (B8 and A4), and the blocking of near-bottom ventilation on the Mauritanian shelf by elevated poleward currents (B6).
Banyte, D., T. Tanhua, M. Visbeck, D.W.R. Wallace, J. Karstensen, G. Krahmann, A. Schneider, L. Stramma and M. Dengler (2012), Diapycnal diffusivity at the upper boundary of the tropical North Atlantic oxygen minimum zone. J. Geophys. Res., 117, C09016, doi: 10.1029/ 2011JC007762
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
Dale, A.W., S. Sommer, U. Lomnitz, I. Montes, T. Treude, J. Gier, C. Hensen, M. Dengler, K. Stolpovsky, L.D. Bryant and K. Wallmann (2015) Organic carbon production, mineralization and preservation on the Peruvian margin. Biogeosciences, 12, 1537-1559, doi: 10.5194/bg-12-1537-2015
Fischer, T., D. Banyte, P. Brandt, M. Dengler, G. Krahmann, T. Tanhua and M. Visbeck (2013) Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone. Biogeosciences, 10, pp. 5079-5093, doi: 10.5194/bg-10-5079-2013
Hummels, R., M. Dengler, P. Brandt and M. Schlundt (2014) Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue. Clim. Dynam., 43, 3179–3199, doi: 10.1007/s00382-014-2339-6
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
Kock, A., J. Schafstall, M. Dengler, P. Brandt and H.W. Bange (2012), Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean, Biogeosciences, 9, 957-964, doi: 10.5194/bg-9-957-2012
Mosch, T., S. Sommer, M. Dengler, A. Noffke, L. Bohlen, O. Pfannkuch, V. Liebetrau, and K. Wallmann (2012), Factors influencing the distribution of epibenthic megafauna across the Peruvian oxygen minimum zone. Deep-Sea Res. Pt. I, 68, 123-135, doi: 10.1016/ j.dsr.2012.04.014
Scholten, J.C., I. Osvath and M. Khanh Pham, (2013) 226Ra measurements through gamma spectrometric counting of radon progenies: How significant is the loss of radon? Mar. Chem., 156, 146-152, doi: 10.1016/j.marchem.2013.03.001
Schubert, M., J. Scholten, A. Schmidt, J.F. Comanducci, M. Khanh Pham, U. Mallast and K. Knoeller (2014) Submarine Groundwater Discharge at a Single Spot Location: Evaluation of Different Detection Approaches. Water, 6(3), 584-601, doi: 10.3390/w6030584