Triple oxygen isotope and oxygen/argon ratio measurements to enhance coastal and open ocean production/respiration comparisons

Topic: 10. Oxygen isotope measurements to enhance production/respiration comparisons (WP 2.1, SO 1) Aquatic photosynthesis (P) and respiration (R) are drivers of the biogeochemical budgets of many climate-relevant gases (for example, CO2, DMS, many halocarbons), and it is therefore crucial to be able to accurately determine P and R in both the coastal and open ocean. However, despite many years of effort, the balance between P and R continues to be a matter of debate, both concerning its sign and its magnitude. For example, results from bottle incubations suggest that the oligotropic Atlantic gyre regions are net carbon sources, yet geochemical evidence indicates that they are in metabolic balance. The best strategy to tackle this conundrum is to concurrently compare and contrast several techniques which estimate P or R over a range of time and space scales alongside measures of the abiotic and biotic environment. This project will enhance the range of estimates of P measured at PML (TILSTONE et al 2003; ROBINSON et al. 2002) using a relatively new geochemical approach based on measurements of oxygen/argon (O2/Ar) ratios and the triple oxygen isotopic composition of dissolved O2 (CRAIG and HAYWARD, 1987; KAISER et al., 2005; LUZ et al., 1999; LUZ and BARKAN, 2000). This new approach allows the derivation of net and gross productivity integrated over the depth of the mixed layer and the time-scale of O2 gas exchange (10-20 days). To this end, they complement instantaneous rate measurements by bottle incubations, such as 14C and O2 and estimates from bio-optical models using remotely sensed ocean colour. This project will undertake these comparisons of P within the framework of two time series studies maintained by PML / where a full description of environmental factors such as temperature, light and nutrients will be made alongside measures of the composition of the plankton community. Samples will be collected from both the open ocean oligotrophic gyres (during an AMT cruise) and during a seasonal study at a coastal site in the English Channel (L4/ WCO). This will be the first application of the triple oxygen isotope technique at such a productive site, and the first intercomparison anywhere of so many methods to measure P. Oxygen isotope measurements made on the AMT cruise will be compared with those made on two previous AMT cruises by the supervisors, to provide the first interannual comparison of spatially integrated P and R in the Atlantic gyres. The student will: * Derive gross and net production on a north-south transect through the Atlantic Basin from measurements of dissolved oxygen isotopes * Derive gross and net production during an annual cycle at the L4 site of the Western Channel Observatory from measurements of dissolved oxygen isotopes * Make all oxygen isotope measurements concurrently with measurements derived from 14C incubations, FRRF (fast repetition rate fluorometry) and bio-optical models * Compare and contrast the range of estimates of P and R at the coastal and open ocean sites * Use statistical methods to discern the environmental and ecological controls on both the magnitude of P and R and any variability in the difference between the magnitude of P derived from the different methods The project will provide training for the student in the most recent methodological and instrument advances in oxygen stable isotopes, phytoplankton photo-physiology and analytical chemistry. The student will be actively encouraged to network through attendance at conferences, workshops and meetings at departmental, institutional, regional, national and international levels. The student will bring UEA's expertise in membrane inlet mass spectrometry and oxygen isotope analysis to PML's primary production and optics groups in order to contribute to the comparison of primary production methodologies outlined in Theme 2 and Theme 10 of Oceans 2025. This project will therefore substantially enhance the delivery