234th and 210po as tracers for particle export in the surface ocean
- Autores: Elisabet Verdeny Colominas
- Directores de la Tesis: Pere Masqué Barri (dir. tes.), Jordi García i Orellana (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2009
- Idioma: español
- Tribunal Calificador de la Tesis: Ken O. Buesseler (presid.), Sergio Rossi (secret.), Michiel Rutgers van Der Loeff (voc.)
- Materias:
- Texto completo no disponible (Saber más ...)
- Resumen
The atmospheric concentration of carbon dioxide (CO2) has increased since the onset of the industrial revolution about two centuries ago. The rising concentration of this greenhouse gas, as well as other such as water vapour, chlorouorocarbons, methane and nitrous oxide, causes the global surface temperature of our planet to increase and plays a crucial role in controlling the Earth's climate.
Despite the increase in CO2 emissions the rise in atmospheric CO2 is only about one half of what we would expect indicating that the terrestrial biosphere and the oceans are actively storing the missing CO2 and counteracting the increase in atmospheric CO2. In fact, it is believed that a considerable proportion of the CO2 is absorbed by the ocean (about 30%). One of the processes that remove carbon from the ocean surface waters is the biological pump, an uptake mechanism based on the conversion of CO2 into biomass via the process of photosynthesis that results in a temporary or permanent storage of carbon.
Four general approaches for measuring new production and its equivalent export production are usually employed. The first method is based on direct measurements of nitrate utilization; this technique involves bottle incubation experiments using compounds labelled with 15N isotope as a tracer to estimate new production. The second method is based on mass balance, e.g. using the seasonal changes in oxygen or nutrient stocks to examine regional tendencies in geochemical balances from which net fluxes into or out of the surface could be inferred. These geochemical balance methods integrate over time as well as for all forms of carbon transport: particle settling, migration of active zooplankton and physical mixing of POC and DOC. A third method consists of the use of upper-ocean sediment traps for direct measurements of the flux of particles, which have long been used to measure export production.
The final method, and the subject of this thesis, involves the use of naturally occurring particle-reactive radionuclides as tracers for sinking particles. The radiotracer pair 238U-234Th is well known and has long been used to estimate particle fluxes and to determine the strength of the biological pump in the upper ocean (for a review see Cochran and Masque, 2003). Besides, the 210Pb-210Po pair has long been proposed as a potential tool for tracing particle transport processes and removal rates although not until recently experimental studies have provided further support for the use of 210Po as a tracer for organic matter in the ocean. As a result, the 210Pb-210Po disequilibrium has only been applied in a very few instances to date to estimate particle fluxes in the surface ocean in a similar manner to that of 238U-234Th (for a review see Verdeny et al., 2009).
One of the primary objectives of this thesis is to present an integrated study of the two pairs of radioactive tracers, 238U-234Th and 210Pb-210Po, and assess their use as tracers for particle export in the surface ocean. Both approaches provide an independent indirect means of measuring export production and, taken together, may provide comparative and/or complementary estimates.
The other primary objective of this thesis is the practical application of 234Th and 210Po as tracers for particle export in the upper ocean. During the course of this thesis the coupled use of both tracers could be applied at a variety of regions of the world's ocean that display very diverse oceanic and biogeochemical regimes. Results from the application of these tracers in the lee of Hawaii and the Canary Current are presented in this thesis. Results from other regions, such as the Mediterranean Sea, the Southern Ocean and the Gulf of California, are still in progress and final results will be soon available.
References Cochran, J.K. and Masque, P., 2003. Reviews in Mineralogy and Geochemistry 52, 461-492.
Verdeny, E. et al., 2009. Deep-Sea Research Part II, 56, 1502-1518.