Studies of the oxygen and carbon cycles in the surface ocean

The thesis work consists of two projects that study carbon and oxygen cycles in the surface ocean. In the first project, net community O₂ production (NCP) and gross primary O₂ production (GPP) were estimated using [O₂]/[Ar] ratios and the triple isotope composition of dissolved O₂ in samples collected during 4 consecutive January cruises from 2008-2011 in the West Antarctica Peninsula (WAP) region of the Southern Ocean. Our study indicates large spatial and interannual variability of NCP and GPP, and suggests that light and iron are the most important factors that regulate NCP and GPP in the study region. While light availability is controlled by the surface irradiance and the mixed layer depth (MLD), iron could be largely regulated by deep winter mixing that supplies iron from the deep water. The relationships between NCP (NCP/GPP) and a number of physical properties implies that NCP is positively correlated with the strength of deep winter mixing. The deep mixing could be weakened when increasing heat content in the ocean and increasing air temperature inhibit sea ice growth and winter water formation. Overall, our study indicates that ocean productivity tends to decrease in the study region in response to changes in physical conditions associated with rapid warming in this region. In the second project, an automatic system using isotope dilution as its core method has been developed to obtain high-frequency measurements of dissolved inorganic carbon concentration ([DIC]) in the surface ocean. This system accurately mixes seawater samples and a NaH13CO₃ solution. The mixed solution is acidified and sent through a gas permeable membrane contactor. CO₂ derived from DIC in the mixture is extracted and sent to a cavity ringdown spectrometer to for 13C/12C analysis. [DIC] of the seawater can be calculated from the measured 13C/12C, the known mixing ratio and the [NaH13CO₃]. The method has been tested in the laboratory and on a cruise. At 15 samples/hour, the precision and the accuracy of the method are <0.1 %. Results of these tests indicate that the method can provide accurate shipboard [DIC] measurements at high resolution in most oceanic regions.