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Dissertation zugänglich unter
Nitrogen cycling in the northern Benguela Upwelling system based on the δ15N of chlorophyll pigment
Stickstoffkreislauf im nördlichen Benguela Auftriebssystem nach dem δ15N von Chlorophyll Pigment
Dokument 1.pdf (3.619 KB)
Freie Schlagwörter (Deutsch):
Stickstoffisotop , Sauerstoffminimumzone , Benguela Auftriebssystem , Chlorophyll Pigment , Stickstoffverlust
Freie Schlagwörter (Englisch):
nitrogen isotope , oxygen minimum zone , Benguela upwelling system , chlorophyll pigment , nitrogen loss
Emeis, Kay-Christian (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Coastal upwelling ecosystems account for only 7% of the Earth surface, but sustain 25 % of global biological productivity, and serve as the major regions for CO2 exchange between atmosphere and ocean. The global climate warming has invoked significant pressure on the development of coastal upwelling systems since the mid-20th century. The climate-driven fluctuations in ocean circulation imprint the thermocline through wind stress and heat fluxes, and modify its depth and strength that both govern the communication between intermediate waters and upper layer waters at coastal margins. As a result, the nutrient and oxygen concentrations in the upwelled waters vary and together influence the cycling of nitrogen, one of the key elements to primary production in coastal upwelling systems.
The Benguela Upwelling System (BUS) belongs to the Eastern Boundary Upwelling Systems (EBUSs), and has the highest primary productivity while it suffers massive nitrogen loss in association with a severe Oxygen Minimum Zone (OMZ). The upwelled nitrate is one of the most important nutrient species to local primary production, and its inventory in shelf waters is governed prominently by the upwelling intensity as a major source, and the nitrogen loss (N-loss) in OMZs and plankton assimilation as major sinks. The isotopic signature of the nitrate pool is imprinted on planktonic biomass via nitrate assimilation, a process through which chlorophyll-a (Chl-a) is produced with the assimilated nitrate. The nitrogen atoms embedded in Chl-a molecular skeleton thus link the δ15N of nitrate with the biological production by recording the isotopic fractionations that are associated with various nitrogen cycling (N-cycling) processes in shelf waters. The perennial low oxygen conditions in shelf waters of the BUS assist the preservation of Chl-a in the surface and core sediment, which makes the δ15N of Chl-a a suitable isotope marker to track the N-source, utilization, and N-sink processes by recording the δ15N signals of N-nutrients.
This thesis focuses on the spatial and temporal variations of N-cycling in association with upwelling intensity in the northern BUS. For this goal, I firstly develop the HPLC method to acquire purified chlorophyll pigment and the analytical method to measure its δ15N. Then, I analyzed the δ15N of chlorophyll pigment in the surficial sediment and in one dated sediment core of the northern BUS to investigate the spatial pattern of N-cycling and the past response of N-loss to upwelling variation in the northern BUS. The results are presented in three chapters.
In the first chapter, I find that the recrystallized K2S2O8 that is used as oxidization reagent has a low N-content but a depleted 15N background of -15‰. The 15N background of K2S2O8 would cause -1- -2‰ deviation on the δ15N of sample that contains nanomolar level N. This finding requires examining the δ15N of recrystallized K2S2O8 when it is used to oxidize reductive N-samples. The method that is established in this work is reliable for δ15Npigment measurement and has an average analytical precision better than ±0.5‰ (1σ).
In the second chapter, I focus on the spatial pattern of nitrate sources and sinks in the coastal waters of the northern BUS. The spatial δ15NChl-a distribution helps to demarcate the northern BUS into two sectors: in the northern sector, the nitrate of fresh South Atlantic Central Water (SACW) origin is removed mainly by plankton assimilation in coastal waters, while in the southern sector, the prominent OMZ invokes denitrification as the major nitrate sink. The upwelling dynamics over the shelf play a major role in governing the spatial pattern of nitrate supply, OMZ state and denitrification intensity, which are partially constrained by the local shelf topography.
The third chapter is aimed at investigating the past N-loss variation in response to the upwelling intensity in the northern BUS. The δ15NChl-a record reveals that the N-loss intensity in the northern BUS varied considerably over the past two centuries, and inversely tracked the upwelling intensity as shown by a significant inverse correlation with a -SST record (unsaturated alkenone ratio based surface seawater temperature). This result suggests that the δ15NChl-a can be applied together with -SST to link the variation of N-loss with upwelling intensity at other Eastern Boundary Upwelling Systems (EBUSs).