Biological oxygen production from oxygen-to-argon ratios and oxygen isotopologues in the Atlantic Ocean

González-Posada, Alba (2012) Biological oxygen production from oxygen-to-argon ratios and oxygen isotopologues in the Atlantic Ocean. Doctoral thesis, University of East Anglia.

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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.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Users 2259 not found.
Date Deposited: 11 Mar 2014 11:38
Last Modified: 11 Mar 2014 11:38

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