Multi-decadal variability of anthropogenic and natural carbon in central Labrador Sea

Lorenza Raimondi, ETH Zürich, D-USYS, Zürich, Switzerland, Toste S Tanhua, Helmholtz Centre for Ocean Research Kiel, GEOMAR, Kiel, Germany, Kumiko Azetsu-Scott, Bedford Inst Oceanography, Dartmouth, NS, Canada, Igor Yashayaev, Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, NS, Canada and Douglas Wallace, Dalhousie University, Department of Oceanography, Halifax, NS, Canada
Abstract:
The North Atlantic has been identified as one of the most intense sink regions for anthropogenic carbon (Cant) in the global ocean. In particular, the Labrador Sea represents the region with the highest column inventory of Cant due to deep water formation (of Labrador Sea Water; LSW) that transports dissolved gases from the surface to the ocean interior on a short timescale.

The formation of LSW displays strong inter-annual to multi-decadal variability and this can affect the column inventory and storage rate of Cant in the region. Climate change projections suggest that increased freshwater supply may already be reducing the depth of convection. These conditions make the Labrador Sea a key location to monitor changes in both hydrographic and carbonate chemistry processes.

A long-term monitoring program by the Bedford Institute of Oceanography has involved annual occupation since 1992 of a repeated hydrography section (AR7W) that crosses the Labrador Sea. This one of a kind dataset offers the possibility to observe annual to multi-decadal changes in column inventory and storage rate of both Cant and total dissolved inorganic carbon (DIC).

For this study we estimated the Cant concentrations using a modified version of the Transit Time Distribution (TTD) method. We tested, and modified, the assumption of a constant transient tracers’ saturation using CFC-12 and SF6 observations. Further we tested the validity of a constant air-sea disequilibrium of CO2 assumption using measurements of DIC and total alkalinity (TA) to calculate pCO2. With this study we present the first multi-decadal dataset of both Cant and DIC in central Labrador Sea, and compare the variability of these two parameters to tease apart the anthropogenic and natural changes of inorganic carbon in this region. Furthermore we will highlight differences in estimation of Cant using the traditional and the modified TTD, and identify possible limitations of the latter.

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