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Carbon Pumps in the Benguela Current Upwelling System

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CARBON PUMPS IN THE BENGUELA CURRENT UPWELLING SYSTEM Dissertation zur Erlangung des Doktorgrades an der Fakultät für Mathematik, Informatik und Naturwissenschaften Fachbereich Geowissenschaften der Universität Hamburg vorgelegt von Anita Flohr Hamburg, 2014 Als Dissertation angenommen vom Fachbereich Geowissenschaften der Universität Hamburg auf Grund der Gutachten von Prof. Dr. Kay-Christian Emeis und Dr. Tim Rixen Tag der Disputation: 13.01.2015 The study has been realised from April 2009 to September 2014 at the Leibniz Center for Tropical Marine Ecology in Bremen in the frame of the GENUS programme (Geochemistry and Ecology of the Namibian Upwelling System). This project has been funded by the German Federal Ministry of Education and Research (03F0497D-ZMT) and is an endorsed project of the Integrated Marine Biogeochemistry and Ecosystem Research (IMBER). CONTENT Summary ..................................................................................................................................... 1 Zusammenfassung ...................................................................................................................... 4 1. Scientific background and objectives .................................................................................. 7 1.1. The marine carbon cycle ......................................................................................... 7 1.1.1. Significance in global carbon cycling ................................................................. 7 1.1.2. Chemical basis - The carbonate system of seawater ........................................... 9 1.2. Drivers, strength and efficiency of marine CO2 sequestration ............................. 12 1.3. Relevance of eastern boundary upwelling systems in marine carbon cycling ...... 14 1.3.1. The Benguela Current upwelling system .......................................................... 15 1.4. Objectives and outline........................................................................................... 18 2. Outline of publications ...................................................................................................... 22 3. Chapter I ............................................................................................................................ 25 4. Chapter II .......................................................................................................................... 51 5. Chapter III ......................................................................................................................... 69 6. Chapter IV ......................................................................................................................... 81 7. Synoptic discussion and conclusion .................................................................................. 97 7.1. Aspects of carbon pumping in the Benguela Current upwelling system .............. 97 7.2. Impacts of climate change .................................................................................. 104 8. Perspectives ..................................................................................................................... 106 Figure captions ....................................................................................................................... 108 Table captions ......................................................................................................................... 117 References .............................................................................................................................. 119 List of co-author publications ................................................................................................. 135 Curriculum Vitae .................................................................................................................... 137 Danksagung ............................................................................................................................ 138 Erklärung ................................................................................................................................ 141 ABBREVIATIONS AT see TA BUS Benguela Current upwelling system C carbon CaCO3 calcium carbonate cm centimetre CT see DIC CO2 carbon dioxide d day 13 δ CCT stable isotope ratio of dissolved inorganic carbon DIC, CT dissolved inorganic carbon 15 - δ NNO3 stable isotope ratio of dissolved nitrate DOC dissolved organic carbon EBUS eastern boundary upwelling system ESACW Eastern South Atlantic Central Water fCO2 fugacity of CO2 g gram GENUS Geochemistry and Ecology of the Namibian Upwelling System H2S hydrogen sulphide km² square kilometre l litre m² square meter - - + N fixed nitrogen (NO3 , NO2 , NH4 ) N2 di-nitrogen gas N* deviation of fixed nitrogen with respect to phosphate; based on the Redfield ratio of N/P = 16/1 NatMIRC National Marine Information and Research Centre in Swakopmund NBUS northern Benguela upwelling system + NH4 ammonium - NO3 nitrate - NO2 nitrite O2 oxygen OM organic matter ΩA saturation state of aragonite in seawater ΩC saturation state of calcite in seawater P* deviation of phosphate with respect to N; based on the Redfield ratio of N/P = 16/1 PIC, CCaCO3 particulate inorganic carbon 3- P, PO4 phosphate 0 PO4 preformed phosphate pCO2 partial pressure of CO2 POC, Corg particulate organic carbon SACW South Atlantic Central Water Si(OH)4 silicate SBUS southern Benguela upwelling system TA, AT total alkalinity yr year % percent °C degree Celsius FACTORS 15 P peta 10 T terra 1012 G giga 109 M mega 106 k kilo 103 m milli 10-3 µ micro 10-6 n nano 10-9 SUMMARY SUMMARY The biological carbon (C) pump influences the flux of carbon dioxide (CO2) between the ocean and the atmosphere by carbon assimilation during the photosynthesis of particulate organic carbon (POC) and the precipitation of calcium carbonate particles (PIC). Regions of intense biological carbon pumping are eastern boundary upwelling systems, among which the Benguela upwelling system (BUS) is the most productive but the least studied one. This work is based on data obtained during several research cruises between 2009 and 2011 in the framework of the GENUS project (Geochemistry and Ecology of the Namibian Upwelling System). GENUS is a multi-disciplinary programme that aims to improve our understanding of the complex interaction between biological, biogeochemical and physical processes within the BUS and their response to climate change. The main objective of this thesis is to investigate the functioning of the biological carbon pump in the northern BUS (NBUS), to assess whether the BUS is a net source or sink for carbon and to give basis for evaluating potential responses of the BUS to global change. Dissolved inorganic nutrient and carbonate chemistry data was raised and used to study the spatio-temporal variability of the remineralisation ratios of C, nitrogen (N) and phosphate (P) (C/N/P) with emphasis on the variability of N deficiency, which is a major factor that limits the production of POC. Records of annual particle fluxes along with hydrographical data were obtained by a mooring on the shelf off Walvis Bay. In conjunction with meteorological records, these data were used to investigate the influence of upwelling on the particle fluxes with a focus on the factors that control the export of POC and PIC on the shelf. Furthermore, carbonate chemistry data allow to determine the variability of the carbonate saturation state (Ω) which influences calcification, growth and survival in many marine calcifying organisms. Along with patterns of air-sea CO2 fluxes, these results were used to evaluate the C source-sink function of the BUS. The results show that the release of C, N and P in the course of organic matter decomposition follows the Redfield stoichiometry of C/N/P of 106/16/1 in the oxygenated 1 SUMMARY water column. However, N loss and P input, which occur in sub- and anoxic bottom waters overlying the mud belt of the Namibian shelf, lower the N/P to <16 in upwelling intermediate water. Together, these processes result in an N deficiency (-N*), equivalent to a relative P excess (+P*), which surfaces and extends to the adjacent hemipelagic ocean. Around 300 km offshore, P* ranged between P* ~0.007µmol kg-1 in 2008 and P* ~0.3 µmol kg-1 in 2010/2011. The results suggest that the magnitude of exported P* is not only controlled by the degree of local O2 deficiency on the shelf, but as well by the amount of remotely supplied N* excess (N/P >16, +N*). This +N* is carried along with South Atlantic Central Water (SACW) to the NBUS shelf and varied between N* ~12 µmol kg-1 in 2008 and N* ~3-4 µmol kg-1 in 2009/2011 off Kunene. The findings point to a mechanism which replenishes the N* deficiency on the NBUS shelf and thereby counteracts the N limitation that hampers the N-driven CO2 drawdown of the biological carbon pump in the NBUS. The combined results of hydrographical, meteorological and particle flux data show that even comparatively weak upwelling of high SACW fractions fuels higher POC fluxes (>91 g POC m-2 yr-1) than intense upwelling of Eastern South Atlantic Central Water (ESACW). This reflects the higher nutrient inventory of SACW compared to ESACW, among other factors. The results further indicate that the POC export on the shelf is dominated by diatom productivity. The decomposition of the basically diatomaceous organic matter causes very low Ω (ΩAmin = 0.7, ΩCmin = 1.2) and high silicate concentrations in ascending sub-thermocline waters.
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