Arevalo-Martinez, Damian L. (2012) Underway CO2 and N2O measurements in surface waters of the equatorial Atlantic during the R/V Merian cruise MSM18/2. (Master thesis), Christian-Albrechts-Universität, Kiel, Germany, 72 pp.

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Abstract

A new underway system combining infrared detection and the OA-ICOS technique was coupled to a continuous equilibrator in order to perform accurate and highly-resolved measurements of atmospheric and oceanic CO2 and N2O in the equatorial Atlantic. Measured atmospheric xCO2 was within ± 2 ppm from mean values reported for the latitudinal band occupied by the cruise track (mean: 390 ± 2 ppm), although the variability was higher. Atmospheric xN2O was somewhat low (mean: 322 ± 0.7 ppb) when compared to long-term flask measurements from the closest NOAA’s sampling location. Seawater measurements agreed with discrete samples and the slight differences were mostly due to uncertainty on the sample collection while steep gradients were crossed. Fluctuations in CN2O were well correlated with SST, whereas fCO2 was influenced by SST, SSS and possibly, biological drawdown. Calculated fCO2 and CN2O were highly variable during the cruise (331-347 μatm and 5.7-9.8 nmol L−1, respectively). Supersaturation conditions for both gases were predominant whereas near equilibrium/slight undersaturation was only attained north of 2°N due to a thermal front established on the area of convergence between the NECC and the SEC. Such effect was more notorious for CO2 than for N2O due to the absence of production/consumption processes for the latter at the ocean’s surface. No latitudinal gradients were found, and the differences between east and west side of the study area represent a temporal pattern superimposed on regional changes that follow the seasonal development of upwelling and the northward shift on the position of the ITCZ. High values of oceanic fCO2 and pN2O with respect to the overlying atmosphere led to positive sea-air differences as large as 70 μatm CO2 and 159 natm N2O, representing well the characteristics of the equatorial Atlantic which is regarded as a net source to the atmosphere. Analysis of similar cross-equatorial sections at different times revealed that the strength of this source increased in association with the onset of equatorial upwelling on early June, when lowest SST coincide with the highest (positive) ΔfCO2 and ΔpN2O close to 10°W. Further implementation of such underway system for simultaneous determinations of CO2 and N2O at the ocean’s surface will certainly improve our ability to characterize fine scale processes that bear importance on the analysis of future ocean responses to climate change.

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