Changes in the δ¹³C of surface water particulate organic matter across the subtropical convergence in the SW Indian Ocean

We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59°S and 30°S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC‐δ 13 C and [CO 2aq ], we concurrently measured surface water pCO 2 , temperature, salinity, nitrate concentration, POM concentration, chlorophyll a and the δ 13 C of total dissolved inorganic carbon. While we found the previously reported general negative correlation between POC‐δ 13 C and [CO 2aq ], we also observed a prominent maximum in POC‐δ 13 C in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a , and with a minimum in pCO 2 . The increase in POC‐δ 13 C between 59°S and the Subtropical Convergence is consistent with the trend expected if [CO 2aq ] 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‐δ 13 C can also vary independently of [CO 2aq ] as a 5 per mil decrease in POC‐δ 13 C was found in a region characterized by nearly constant [CO 2aq ]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO 2aq ] from the carbon isotopic composition of sedimentary organic matter.