Stable carbon isotope analysis of seawater samples: a new approach to assess CO2 effects on the marine carbon cycle

Abstract

Stable isotope ratio analyses offer a unique opportunity to obtain information about ecosystem dynamics, patterns and processes. The anthropogenic contribution to the global atmospheric CO2 rise through fossil fuel combustion, deforestation and other related human activities has changed the stable carbon isotope composition (δ13C) of the atmosphere over the past 200 years. Changes in the carbon isotopic patterns of terrestrial biosphere, lithosphere and oceans are also expected. The global ocean has been acting as a net sink for CO2 emissions and although it moderates the climate, it is currently in a critical state of health. While the physico-chemical consequences (ocean acidification) of the increasing CO2 uptake by the ocean are fairly well known, the perturbation to marine ecosystems and the related effects on biota still entail large uncertainties. This thesis investigates the feasibility of using measurements of δ13C of seawater samples to increase our understanding of the biogeochemical responses of marine ecosystems to human CO2 perturbation.

The isotopic composition of all the individual inorganic and organic carbon species from three long term mesocosm experiments (Sweden 2013, Gran Canaria 2014, Norway 2015) was determined. To have accurate and precise isotopic measurements, mass spectrometry instrument calibrations and method validation procedures were performed. Universal and inter-laboratory accuracy of the analysis was assessed by running standard materials provided by the International Atomic Energy Agency (IAEA, Vienna) and by the Scottish Universities Environmental Research Centre (SUERC) stable isotope laboratory, respectively. Precision and internal consistency was assessed from isotopic measurements of seawater reference materials from A.G. Dickson and D. Hansell for dissolved inorganic and organic carbon, respectively. A novel accurate, precise and rapid method, coupling a Shimadzu 5000A total organic carbon (TOC) analyser to an isotope ratio mass spectrometer (Thermo Scientific Delta V Advantage IRMS), was successfully developed in order to determine the δ13C of dissolved organic carbon in seawater samples which, due to analytical challenges, is currently not a widespread technique.

The combination of isotopic and non-isotopic measurements from mesocosm experiments provided a holistic view of the biogeochemical mechanisms that affect carbon dynamics under different CO2 conditions (up to 2000 ppm). A clear CO2 response was detected in the isotopic datasets, but increased CO2 levels had only a subtle effect on the concentrations of the dissolved and particulate organic carbon pools. Distinctive δ13C signatures of the particulate carbon pool both in the water column and the sediments were detectable for the different CO2 treatments and they were strongly correlated with the δ13C signatures of the inorganic carbon but not with the δ13C of the dissolved organic pools. Phytoplankton fractionation was positively affected by high CO2 either because of the higher CO2 availability or because of a shift in phytoplankton community composition, however, phytoplankton bloom intensity and evolution was independent of CO2 concentrations and higher CO2 levels had no significant effect on inorganic nutrient uptake or carbon production/consumption.

Overall this study proved the stable carbon isotope approach to be an effective tool for the assessment of the major biogeochemical interactions among individual compartments within the marine system opening the door to new interpretations for past, present and future changes of the global carbon cycle.

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