Carbon dioxide (CO2) emissions (from fossil fuel burning and land use change) increased strongly in recent years, from 5.5 PgC yr−1 (Pg = 1015g) in 1970 to 8.4 PgC yr−1 in 2000 (Raupach et al., 2007) and 9.9 PgC yr−1 in 2006 (Canadell et al., 2007). As a consequence, the accumulation of anthropogenic CO2 in the atmosphere reached the benchmark of 100 ppm in 2007 (Le Quéré et al., 2009), which raises the issue of global warming and the associated climate change, the well-known consequences of increasing atmospheric CO2 (IPCC, 2007). An evidence for this human-induced perturbation, aside from atmospheric CO2 measurements, is the modification of the isotopic ratio of CO2 known as the 13C Suess effect. Indeed, since plants preferentially use the lightest carbon isotope (12C) during photosynthesis, the burning of fossil fuel releases mainly 12CO2 in the atmosphere (Gruber et al., 1999), resulting in a decrease in the 13C/12C ratio of atmospheric CO2 (δ13CCO2). The 13C Suess effect was evaluated to –0.02‰ yr−1 since 1988 based on atmospheric measurements (Quay et al., 1992) but modelling studies have recently shown that the magnitude of this phenomenon depends on the time frame over which it is evaluated (Tagliabue and Bopp, 2008). As the ocean takes up between 15 and 20% of current CO2 emissions (Takahashi et al., 2002, 2009), the 13C Suess effect is nevertheless transmitted to surface waters; several studies thus showed a decrease in the 13C/12C ratio of dissolved inorganic carbon (δ13CDIC, e.g. Kroopnick, 1985; Broecker and Maier-Reimer, 1992; Gruber et al., 1999; Sonnerup et al., 2000; McNeil et al., 2001; Quay et al., 2003; Tagliabue and Bopp, 2008). This signal, the oceanic 13C Suess effect, reflects the accumulation of anthropogenic CO2 in the ocean, and therefore provides an additional constraint for estimating the global carbon budget. Consequently observing and understanding the seasonal to decadal variability of both atmospheric CO2 and 13C budget could help in separating the carbon sources and sinks at planetary scale between land and ocean systems (e.g. Ciais et al., 1995). On the ocean side, despite regular observations conducted since 1980s at few time-series stations and during WOCE/JGOFS era (1990s), the seasonal variability of surface δ13CDIC is not yet very well described at global ocean scale. In the high northern latitudes of the North Atlantic ocean the winter/summer difference of δ13CDIC may be as large as ∼1‰ (Gruber et al., 1999) that potentially masked the 13C Suess effect observed in the surface water (McNeil et al., 2001). It has been also suggested from ocean carbon models, that upwelling of deep waters brings an older δ13CDIC signal from the atmosphere in the surface layers and thus dilutes the ocean 13C Suess effect (Tagliabue and Bopp, 2008). In these models, processes such as biological activity, thermodynamics and ocean circulation that control the ocean δ13CDIC distribution (and ocean CO2) in the surface and deep waters, need to be compared against seasonal δ13CDIC fields in order to best represent the δ13CDIC distribution that could be used for both geological (testing the proxies) and predictive simulations including the coupling between climate and carbon changes. Understanding better the temporal evaluation of surface δ13CDIC distribution and the driving processes would also reduced uncertainties when using δ13CDIC observations to evaluate the anthropogenic CO2 uptake in the ocean (Quay et al., 1992, 2003; Tans et al., 1993; Gruber et al., 1999; Sonnerup et al., 2000, 2007; McNeil et al., 2001; McNeil and Tilbrook, 2009).
Since about two decades, δ13CDIC was regularly sampled at the time-series stations Hawaii Ocean Time Series (HOTS, 23°N–158°W) and Bermuda Atlantic Time Series (BATS, 31°40′N–64°10′W). These time-series provided a long-term monitoring of δ13CDIC that allowed to investigate the seasonal, interannual and decadal variability of δ13CDIC and to identify the driving processes. At station BATS, authors estimated a surface δ13CDIC decrease of –0.025‰ yr−1± 0.002‰ (Gruber et al., 1998, 1999; Quay et al., 2003) between 1984 and 1993 and validated oceanic models which evaluate the anthropogenic carbon uptake in the ocean (e.g. Box Diffusion Model of Bascatow et al., 1996). δ13CDIC measurements were also collected in other oceanic regions during GEOSECS expedition (Geochemical Ocean SECtion Study) at the end of 1970s, Transient Tracers in the Ocean cruises (TTO) in the 1980s, international World Ocean Circulation Experiment program (WOCE) in the 1990s, as well as during several national initiatives. The synthesis of these observations have shown a large spatial variability in both δ13CDIC distribution and associated oceanic 13C Suess effect (e.g. Gruber et al., 1999; Sonnerup et al., 2000; Quay et al., 2003). Most of these studies focused on two major processes to explain the observed δ13CDIC variability in surface waters, the air–sea gas exchange and the biological activity (organic matter production/respiration and calcium carbonate formation/dissolution); however, recent modelling work also highlighted the importance of ocean circulation and mixing, especially in the high latitudes where δ13CDIC data are sparse (Tagliabue and Bopp, 2008).
In the Southern Ocean, the seasonality of δ13CDIC as well as the oceanic 13C Suess effect remains poorly quantified due to the lack of good spatial and temporal coverage of δ13CDIC measurements (Broecker and Maier-Reimer, 1992; Lynch-Stieglitz et al., 1995; Gruber et al., 1999; McNeil et al., 2001; Tagliabue and Bopp, 2008). Studies of the distribution of δ13CDIC and the processes controlling its evolution on seasonal to decadal scales are needed, in this region in particular, to provide insights into the ocean carbon cycle and its evolution in recent years (Le Quéré et al., 2009; Lenton et al., 2009; Metzl, 2009), and to help reducing the uncertainty in anthropogenic carbon estimates in the Southern Ocean (Lo Monaco et al., 2005; Vazquez-Rodriguez et al., 2009).
Since 1998, surface and water column δ13CDIC observations were collected one or twice a year in the Southwest Indian Ocean in the frame of the OISO program (Océan Indien Service d’Observation). Our study aims at providing and describing the distribution of δ13CDIC in an undocumented area, for both austral summer and winter. Such analysis complements the times series of δ13CDIC mainly conducted in the subtropics since two decades. Here, we evaluate for the first time the mean distribution and the summer/winter variations of δ13CDIC in this region (20°S–60°S) by analysing surface measurements obtained over the period 1998–2005. Following the method of δ13C* developed by Gruber et al. (1999), we also try to identify processes controlling the δ13CDIC distribution in the surface ocean of this region. It is our hope that these results will help to evaluate the oceanic uptake of anthropogenic carbon in this region and to constrain and validate atmospheric inversions and ocean carbon models.