Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 4 2009 10.5194/bg-6-681-2009 http://www.biogeosciences.net/6/681/2009/ http://www.biogeosciences.net/6/681/2009/bg-6-681-2009.html http://www.biogeosciences.net/6/681/2009/bg-6-681-2009.pdf 681 703 2009-04-27 Estimating the storage of anthropogenic carbon in the subtropical Indian Ocean: a comparison of five different approaches M. Álvarez marta.alvarez@uib.es C. Lo Monaco T. Tanhua A. Yool A. Oschlies J. L. Bullister C. Goyet N. Metzl F. Touratier E. McDonagh H. L. Bryden IMEDEA (CSIC-UIB), Miquel Marqués 21, 07190 Esporles, Spain LOCEAN/IPSL, Université Paris 6 – place Jussieu 4, 75252 Paris, France IFM-GEOMAR, Düsternbrooker Weg 20, 24105 Kiel, Germany NOCS, Waterfront Campus European Way, Southampton, SO14 3ZH, UK NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington, USA IMAGES, Université de Perpignan, 52 avenue Paul Alduy, 66860 Perpignan, France 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 (C_ANT) 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 C_ANT inventory. Below the mixed-layer, the comparison suggests that C_ANT 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 C_ANT 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 C_ANT specific inventory is 28±2 molC m^−2. 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