Vertical profiles of nitrous oxide, methane and nutrients in coastal water of eastern South Pacific

Between February 8th and March 4th 2007 an oceanographic cruise (Galathea 3 expedition) on board of Research Vessel Vædderen (Denmark) was performed Legs 13 and 14 of that expedition were devoted to the study of the oxygen minimum zone off northern Chile and Peru The cruise track covered the area between Valparaiso-Chile (32°S) and northern Peru (7.5°S) from total amount of 18 stations visited, we present 21 station with relatively high vertical resolution of Nitrous oxide and other variables. The main objective of the cruise was to study of low oxygen conditions over microbial communities and metabolic pathways related to the N and C cycles. In this region, an emblematic Oxygen Minimum Zone (OMZ), mainly defined by the presence of Subsurface Equatorial Waters (ESSW) between 50 and 400 m depth. These waters determine an important role of the eastern South Pacific in global climate related to the production of green house gases such as CO2, CH4 and N2O. During the cruise researcher was focused on several mechanisms of N and C transformation in the OMZ such as Fixation of molecular nitrogen (N2) and nitrous oxide (N2O) in surface waters and the core of the oxygen minimum zone and production and consumption of N2O by denitrificion and even nitrification. Continuously sampled variables are temperature, salinity and disolved oxygen, obtained unsing a CTD-O probe (SeaBird). Discretely sampled variables are water samples, collected using Niskin bottles attached to rosette sampler, in order to obtain discrete measurements of dissolver oxygen (O2) and nutrients (NO3, NO2, SiO4 and PO43). Discrete samples of DO (in triplicate) were analyzed using the AULOX measurement system, an automatic Winkler method. Samples for NO2, NO3 and PO4 (15 mL in triplicate) were filtered (using a 0.45 μm GF/F glass filter) and stored (frozen) until analysis, using standard colorimetric techniques (Grasshoff, 1983). Measurements of NH4 (40 mL, in triplicate) were carried out using the fluorometric method proposed by Holmes et al. (1999). N2O and CH4 samples were taken in triplicate in 20 mL vials and carefully sealed to avoid air bubbles. They were then preserved with 50 μL of saturated HgCl2 and stored in darkness until analysis. N2O and CH4 was analyzed by creating a 5 mL headspace of ultrapure Helium and then equilibrated to 40◦C. Mesurements were carried out through gas chromatography using an electron capture detector (Varian 3380) and Flame Ionization detector (Shimadzu 17A), respectively. The calibration curves used 0.1, 0.5, and 1 ppm of N2O standards and 1, 2.5, 5 and 10 ppm of CH4 standards, both curves used pure He as 0 value. The analytical error for N2O measurements was ~3% and ~5% for CH4. CH4 was manually analyzed through a capillary column GS-Q (J&W, 0.53 mm×30 m) and 30°C oven temperature The uncertainty of the measurements was calculated from the standard deviation of the triplicate measurements by depth. Samples with a variation coefficient above 10% were not considered.