Published: 2015-04-01

Carbon isotopes in the Baltsem model

Baltic Nest researchers have now expanded the BALTSEM model to include carbon isotopes. The isotopic compositions of inorganic and organic carbon species in the Baltic Sea are mainly controlled by phytoplankton production and respiration and the related air-sea CO2 fluxes. This method could also have important applications for estimates of greenhouse gas emissions in other areas.

This is presented in a recent paper from Baltic Nest Institute, Stockholm University, with the title "Modelling the 13C and 12C isotopes of inorganic and organic carbon in the Baltic Sea" by the authors Erik Gustafsson, Carl-Magnus Mörth, Christoph Humborg and Bo G. Gustafsson.

In a recently expanded version of the BALTSEM model, stable carbon isotopes (12C and 13C) are explicitly simulated. Thus, all state variables that contain carbon (i.e., dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and detrital carbon of marine and terrestrial origins, carbon content of auto- and heterotrophs), are now additionally accounted for in terms of 13C/12C ratios.

Information on carbon sources

In general, isotope signatures of dissolved and particulate constituents in water can be used for characterization of sources and transformation processes. Different processes in the carbon cycling such as primary production and outgassing of CO2 leave characteristic fingerprints on carbon reservoirs in the water column and sediments.

Thus, by including carbon isotopes in a physical-biogeochemical model, it is possible to simulate not only the concentrations of e.g. DIC and DOC, but also to constrain the rates of various transformation processes in the land-water-air cycling of carbon.

Phytoplankton and river loads important

In this study we demonstrate that in the Baltic Sea, the isotopic compositions of inorganic and organic carbon species are mainly controlled by phyto-plankton production and respiration and the related air-sea CO2 fluxes. The isotopic composition is to a lesser extent influenced by river loads of DIC and DOC and by mineralization of terrestrial organic carbon within the system.

Over the last century, the changes in isotopic composition have been largely dominated by two processes: One is the preferential release of 12C to the atmosphere in association with fossil fuel burning, and the other is the eutrophication of the Baltic Sea related to increased nutrient loads under the second half of the 20th century.

Important application for other areas

While this study is entirely focused on physical and biogeochemical processes in the Baltic Sea, the method is generally applicable to other coastal and shelf seas. It is of a particular interest to develop a similar model setup for the East Siberian Arctic Shelf (ESAS). Huge quantities of carbon are stored in the Arctic permafrost; global warming and permafrost thawing are anticipated to result in increased riverine transports of organic carbon to the ESAS.

Mineralization of this carbon would then enhance the CO2 evasion and thus contribute to greenhouse gas emissions and potentially a warming-thawing-outgassing-warming feedback cycle. By including stable carbon isotopes in a model for the ESAS, it would for example be possible to use isotope signatures to determine the influence of mineralizing terrestrial organic carbon on CO2 emissions from the shelf.

Link to the article in Journal of Marine Systems

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Web editor: Marmar Nekoro

Updated: 2015-04-01
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