Changes in the ?13C of surface water particulate organic matter across the subtropical convergence in the SW Indian Ocean
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We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59-degrees-S and 30-degrees-S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC-deltaC-13 and [CO2aq], we concurrently measured surface water pCO2, temperature, salinity, nitrate concentration, POM concentration, chlorophyll a and the deltaC-13 of total dissolved inorganic carbon. While we found the previously reported general negative correlation between POC-deltaC-13 and [CO2aq], we also observed a prominent maximum in POC-deltaC-13 in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a, and with a minimum in pCO2. The increase in POC-deltaC-13 between 59-degrees-S and die Subtropical Convergence is consistent with the trend expected if [CO2aq] were the main factor controlling the isotopic composition of POM. In contrast, data from the region north of the Subtropical Convergence clearly illustrate that POC-deltaC-13 can also vary independently of [CO2aq], as a 5 per mil decrease in POC-deltaC-13 was found in a region characterized by nearly constant [CO2aq]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO2aq] from the carbon isotopic composition of sedimentary organic matter.