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Dissertation zugänglich unter
The Nitrogen Cycle of the Eastern Mediterranean Sea from a Stable Isotope Perspective
Untersuchungen am Stickstoffkreislauf des östlichen Mittelmeeres anhand von stabilen Isotopen
Dokument 1.pdf (3.102 KB)
Stickstoffkreislauf, Mittelmeer <Ost>, Atmosphärisches Aerosol
Freie Schlagwörter (Deutsch):
Freie Schlagwörter (Englisch):
Nitrogen cycle, Eastern Mediterranean Sea, Atmospheric deposition, Stable isotopes
Emeis, Kay-Christian (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
The Eastern Mediterranean Sea (EMS) is a nutrient-poor ocean with unusually
high nitrate to phosphate ratio (up to 28: 1) and relatively depleted 15N/14N ratios (expressed as d15N) in deep-water nitrate and sediments, compared to other oceanic settings.
Up to now the principal hypothesis for the depleted d15N values observed in the region was the biological nitrogen fixation. The nutrient budgets and the surveys that were conducted in the EMS favoured the idea of nitrogen fixation since there was no direct and measurable evidence for the isotopic fingerprint of the atmospheric NOx as there was the case in other oceanic environments. Furthermore other possible procedures in the marine environment that could reasonably explain the low d15N values were perhaps underestimated because of this hypothesis. In order to elucidate the possible reasons and to provide new information on the biogeochemical nitrogen cycling in the EMS, this thesis uses stable isotopic ratios (d15Ν-ΝΟ3, d18O-ΝΟ3) to investigate sources and processes that could provide a
comprehensive data set for the N-cycling in the region. For this reason atmospheric samples (rain and dry deposition as well as aerosol samples) were collected as part of a one-year field programme and analysed for their d15Ν composition. Furthermore, samples of dissolved total reduced nitrogen (DON+NH4), inorganic nitrogen (DIN in nitrate), the suspended particulate (Νsusp) as well as the sinking particulate N (intercepted by sediment traps, SPN), were isolated from multiple depths of the water column at 17 stations, in a sampling cruise conducted on January 2007 across the EMS with R/V METEOR (M71-3 cruise). N isotope data in both dry and wet deposition samples showed constantly negative δ15N values compared to air N2, implying a strongly depleted atmospheric source calculated to be -3.1 ‰. The low d15Ν of wet deposition is in agreement with data from other environments, but the consistently depleted nature of dry deposition is unusual and supports the formation of atmospheric NO3 - compounds with dust and sea-salt particles. In the water column of the EMS, ongoing nitrate assimilation showed depleted nitrate concentrations in the mixed layer, causing an enrichment in both d15N-NO3 (average 3.1‰ ±2.0 ‰) and d18O-NO3 (average 6.0‰ ±2.4 ‰) of residual nitrate over the deep-water nitrate pool (d15N-NO3 average 2.1‰ ±0.3 ‰; d18O-NO3 average ii
4.0‰ ±1.3 ‰). Products of assimilation (PNsusp and dissolved organic nitrogen, DON) were more abundant in the mixed layer than below the thermocline (below 200 m), and δ15Nsusp of suspended matter (average 2.3‰ ±2.3 ‰) and 15(DON+NH4) (average 1.6‰ ±2.2 ‰), were isotopically more depleted in the mixed layer than in deeper water (15PN average 7.3‰ ±0.8 ‰; 15(DON+NH4 -) average 5.1‰ ±4.0 ‰). SPN intercepted by sediment traps at two depths (1600 m and 2700 m) at one station (deployed during the cruise and recovered 216 days later) had an average d15SPN of 0.9‰ ±0.8‰ in the shallow trap and 0.8‰ ± 1.0‰ in the deep trap.
Altogether the obtained data will provide additional insights and will present a
different prospect for the depleted d15Ν values reported in the nitrogen cycling of EMS. In addition they underline the importance of continuing to make measurements in order to evaluate a) the eventual fate of anthropogenic nitrogen entrained to the EMS and b) the contribution of the deep water mass to thed 15Ν of the region.