Abstract
Abstract. The subtropical Indian Ocean along 32° S was for the first time simultaneously sampled in 2002 for inorganic carbon and transient tracers. The vertical distribution and inventory of anthropogenic carbon (CANT) from five different methods: four data-base methods (ΔC*, TrOCA, TTD and IPSL) and a simulation from the OCCAM model are compared and discussed along with the observed CFC-12 and CCl4 distributions. In the surface layer, where carbon-based methods are uncertain, TTD and OCCAM yield the same result (7±0.2 molC m−2), helping to specify the surface CANT inventory. Below the mixed-layer, the comparison suggests that CANT penetrates deeper and more uniformly into the Antarctic Intermediate Water layer limit than estimated from the much utilized ΔC* method. Significant CFC-12 and CCl4 values are detected in bottom waters, associated with Antarctic Bottom Water. In this layer, except for ΔC* and OCCAM, the other methods detect significant CANT values. Consequently, the lowest inventory is calculated using the ΔC* method (24±2 molC m−2) or OCCAM (24.4±2.8 molC m−2) while TrOCA, TTD, and IPSL lead to higher inventories (28.1±2.2, 28.9±2.3 and 30.8±2.5 molC m−2 respectively). Overall and despite the uncertainties each method is evaluated using its relationship with tracers and the knowledge about water masses in the subtropical Indian Ocean. Along 32° S our best estimate for the mean CANT specific inventory is 28±2 molC m−2. Comparison exercises for data-based CANT methods along with time-series or repeat sections analysis should help to identify strengths and caveats in the CANT methods and to better constrain model simulations.
Highlights
The fate of the anthropogenic CO2 (CANT) emissions to the atmosphere is one of the critical concerns in our attempts to better understand and possibly predict global change and its impact on society (IPCC, 2007)
CANT estimates and transient-tracers in five water mass layers defined along the 32◦ S Indian Ocean section
Carbonbased methods are unable to properly correct for biological activity and unable to estimate CANT (Fig. 8, Table 1), while Transit Time Distribution (TTD) is based on tracer distributions independent of biological activity, but affected by seasonality; and OCCAM assumes CO2 air-sea equilibration, which is not always true
Summary
The fate of the anthropogenic CO2 (CANT) emissions to the atmosphere is one of the critical concerns in our attempts to better understand and possibly predict global change and its impact on society (IPCC, 2007). Different approaches to estimate the global oceanic CANT uptake have arrived at essentially the same number, about 2 Pg C yr−1 (Wetzel et al, 2005). Despite this general agreement, questions about the reliability of these estimates remain. The Joint SOLAS-IMBER implementation plan has identified the research priorities for ocean carbon research, among them the separation of natural from anthropogenic carbon, the oceanic storage and transport of CANT and the effect of decreasing pH, ocean acidification, on the marine biogeochemical cycles, ecosystems and their interactions Large impacts of ocean acidification are expected to occur in high latitudes, i.e. the Southern (Bopp et al, 2001; Orr et al, 2005) and the Published by Copernicus Publications on behalf of the European Geosciences Union
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