Biological oxygen production from oxygen-to-argon ratios and oxygen isotopologues in the Atlantic Ocean
Marine primary producers play an important role in controlling the atmospheric carbon dioxide through photosynthesis and export of organic matter into the deep ocean. In this thesis I used two biogeochemical techniques based on O2/Ar ratios and O2 isotopologues to measure net community production (N) and gross oxygen production (G) in two different areas relevant for the marine C cycle: the South Atlantic subtropical gyre and the North Atlantic during a spring bloom. The former despite small rates of production it is relevant for the global marine C cycle due to the large area it covers; in addition the study of this gyre may contribute to clarify the ongoing debate on whether subtropical gyres are net autotrophic or heterotrophic. The North Atlantic is a main component of the global C production because of the large the amount of C that is fixed during the spring bloom. N was estimated from continuous measurements of O2/Ar ratios with a Membrane Inlet Mass Spectrometer and G from discrete samples both in the USW of RSS James Cook (South Atlantic) and R/V Knorr (North Atlantic). Discrete samples were processed in a newly built gas extraction line and analyze in an Isotope Ratio Mass Spectrometer at UEA. The gas extraction line was built for the separation of O2 and Ar from N2 without isotopic fractionation. This thesis shows that the South Atlantic Gyre produced O2 at a rate of 10±3 mmol m-2 d-1 in March/April 2009, thus was net autotrophic. Interestingly G (177±96 mmol m-2 d-1) was much higher than previous estimates. This difference could be due to other processes that might fractionate the O2 without fixing any carbon such as the Mehler reaction. Measurements of N made in the North Atlantic during a spring bloom in May 2008 showed the highly variable and heterogeneous nature of the area. Within the bloom N had an average value of 67 mmol m-2 d-1 with N increasing from 60 to 170 mmol m-2 d-1, followed by a short period of net heterotrophy and finishing at 5 mmol m-2 d-1. These changes were associated with changes in the species composition, with diatoms dominating first followed by the increase of nanophytoplankton. These techniques show O2/Ar and O2 isotopologues to be useful and accurate proxies for production with the potential for complete automation of O2/Ar measurements.
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