Nitrous oxide and hydroxylamine in the eastern tropical Atlantic and Pacific Oceans

This thesis compiles different studies that help to identify where, how and by whom N2O is produced in the ocean. A mixed layer budget of N2O in the upwelling region off Mauritania is calculated to quantify the N2O transport from the ocean interior to the atmosphere. N2O production mechanisms are investigated in a comparison study of N2O and δ15N-Nitrate measurements in the tropical Atlantic and Pacific Oceans. Evidence for N2O production from archaeal nitrification is presented from field studies as well as from culture experiments, and a method for improved measurements of hydroxylamine in seawater is presented that can help to identify the production mechanisms of N2O during nitrification

Nitrite removal improves hydroxylamine analysis in aqueous solution by conversion with iron(III)

Dissolved hydroxylamine (NH2OH) is a hort-lived compound produced in the oceanic environment during nitrification and dissimilatory eduction of nitrate to ammonium (DNRA). The ferric ammonium sulfate (FAS) conversion method is the only method available so far to determine dissolved NH2OH in nanomolar concentrations in seawater. We show that side reactions of dissolved nitrite (NO2-) can result in a significant bias in the NH2OH concentration measurements when applying the FAS conversion method. We propose to scavenge dissolved NO2- by addition of sulfanilamide to suppress effectively the undesired side reactions by NO2-. This modification of the FAS conversion method will allow a NH2OH determination even in oceanic regions with high NO2- concentrations. A reliable detection of NH2OH in seawater samples can give us a clue about the occurrence of active nitrification or DNRA in the ocean and,therefore, will provide further insights about the oceanic nitrogen cycle

Lachgas und Hydroxylamin im tropischen Ostatlantik und -pazifik

This thesis compiles different studies that help to identify where, how and by whom N2O is produced in the ocean. A mixed layer budget of N2O in the upwelling region off Mauritania is calculated to quantify the N2O transport from the ocean interior to the atmosphere. N2O production mechanisms are investigated in a comparison study of N2O and δ15N-Nitrate measurements in the tropical Atlantic and Pacific Oceans. Evidence for N2O production from archaeal nitrification is presented from field studies as well as from culture experiments, and a method for improved measurements of hydroxylamine in seawater is presented that can help to identify the production mechanisms of N2O during nitrification.In dieser Dissertation sind mehrere Arbeiten zusammengefasst, die dazu beitragen, zu identifizieren, wo, von wem und auf welche Weise N2O im Ozean produziert wird. Im Auftriebsgebiet vor Mauretanien wurde eine Deckschichtbilanz von N2O aufgestellt um den Transport von N2O aus der Tiefe in die Deckschicht zu quantifizieren. Die Produktionswege von N2O wurden in einer Vergleichsstudie von N2O und δ15N-Nitrat-Messungen im tropischen Atlantik und Pazifik untersucht. N2O Produktion durch nitrifizierende Archaeen konnte in Feldstudien sowie in Kulturexperimenten gezeigt werden, und eine verbesserte Methode zur Bestimmung von Hydroxylamin in Seewasser, die dazu beitragen kann, die N2O-Produktionsmechanismen während der Nitrifizierung zu untersuchen, wurde entwickelt

High-resolution measurements of atmospheric molecular hydrogen and its isotopic composition at the West African coast of Mauritania

Oceans are a net source of molecular hydrogen (H-2) to the atmosphere, where nitrogen (N-2) fixation is assumed to be the main biological production pathway followed by photochemical production from organic material. The sources can be distinguished using isotope measurements because of clearly differing isotopic signatures of the produced hydrogen. Here we present the first ship-borne measurements of atmospheric molecular H-2 mixing ratio and isotopic composition at the West African coast of Mauritania (16-25 degrees W, 17-24 degrees N). This area is one of the biologically most active regions of the world's oceans with seasonal upwelling events and characterized by strongly differing hydrographical/ biological properties and phytoplankton community structures. The aim of this study was to identify areas of H-2 production and distinguish H-2 sources by isotopic signatures of atmospheric H-2. For this more than 100 air samples were taken during two cruises in February 2007 and 2008. During both cruises a transect from the Cape Verde Islands towards the Mauritanian Coast was sampled to cover differing oceanic regions such as upwelling and oligotrophic regimes. In 2007, additionally, four days were sampled at high resolution of one sample per hour to investigate a possible diurnal cycle of atmospheric H-2. Our results indicate the influence of local sources and suggest the Banc d'Arguin as a pool for precursors for photochemical H-2 production, whereas oceanic N-2 fixation could not be identified as a source for atmospheric H-2 during these two cruises. The variability in diurnal cycles is probably influenced by released precursors for photochemical H-2 production and also affected by a varying origin of air masses. This means for future investigations that only measuring the mixing ratio of H-2 is insufficient to explain the variability of an atmospheric diurnal cycle and support is needed, e.g. by isotopic measurements. Nevertheless, measurements of atmospheric H-2 mixing ratios, which are easy to conduct online during ship cruises, could be a useful indicator of production areas of biological precursors such as volatile organic compounds (VOCs) for further investigations

Methane emissions from the upwelling area off Mauritania (NW Africa)

Coastal upwelling regions have been identified as sites of enhanced CH4 emissions to the atmosphere. The coastal upwelling area off Mauritania (NW Africa) is one of the most biologically productive regions of the world's ocean but its CH4 emissions have not been quantified so far. More than 1000 measurements of atmospheric and dissolved CH4 in the surface layer in the upwelling area off Mauritania were performed as part of the German SOPRAN (Surface Ocean Processes in the Anthropocene) study during two cruises in March/April 2005 (P320/1) and February 2007 (P348). During P348 enhanced CH4 saturations of up to 200% were found close to the coast and were associated with upwelling of South Atlantic Central Water. An area-weighted, seasonally adjusted estimate yielded overall annual CH4 emissions in the range from 1.6 to 2.9 Gg CH4. Thus the upwelling area off Mauritania represents a regional hot spot of CH4 emissions but seems to be of minor importance for the global oceanic CH4 emissions

A new method for continuous measurements of oceanic and atmospheric N2O, CO and CO2: performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR)

A new system for continuous, highly-resolved oceanic and atmospheric measurements of N2O, CO and CO2 is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR) both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N2O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N2O and CO respectively, as well as detection limits of < 40 ppt and precision better than 0.3 ppb Hz−1/2. Results from a direct comparison of the method presented here and well-established discrete methods for oceanic N2O and CO2 measurements showed very good consistency. The favorable agreement between underway atmospheric N2O, CO and CO2 measurements and monthly means at Ascension Island (7.96°S 14.4°W) further suggests a reliable operation of the underway setup in the field. The potential of the system as an improved platform for measurements of trace gases was explored by using continuous N2O and CO2 data to characterize the development of the seasonal equatorial upwelling in the Atlantic Ocean during two RV/ Maria S. Merian cruises. A similar record of high-resolution CO measurements was simultaneously obtained offering for the first time the possibility of a comprehensive view on the distribution and emissions of these climate relevant gases on the area. The relatively simple underway N2O/CO/CO2 setup is suitable for long-term deployment on board of research and commercial vessels although potential sources of drift such as cavity temperature and further technical improvements towards automation still need to be addressed

Deflating the shale gas potential of South Africa’s Main Karoo basin

The Main Karoo basin has been identified as a potential source of shale gas (i.e. natural gas that can be extracted via the process of hydraulic stimulation or ‘fracking’). Current resource estimates of 0.4–11x109 m3 (13–390 Tcf) are speculatively based on carbonaceous shale thickness, area, depth, thermal maturity and, most of all, the total organic carbon content of specifically the Ecca Group’s Whitehill Formation with a thickness of more than 30 m. These estimates were made without any measurements on the actual available gas content of the shale. Such measurements were recently conducted on samples from two boreholes and are reported here. These measurements indicate that there is little to no desorbed and residual gas, despite high total organic carbon values. In addition, vitrinite reflectance and illite crystallinity of unweathered shale material reveal the Ecca Group to be metamorphosed and overmature. Organic carbon in the shale is largely unbound to hydrogen, and little hydrocarbon generation potential remains. These findings led to the conclusion that the lowest of the existing resource estimates, namely 0.4x109 m3 (13 Tcf), may be the most realistic. However, such low estimates still represent a large resource with developmental potential for the South African petroleum industry. To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin. It is acknowledged that the drill cores we investigated fall outside of currently identified sweet spots and these areas should be targets for further scientific drilling projects