Controls of dissolved carbon fractions along a meridional transect through the South Pacific Ocean

The South Pacific Ocean is the largest and deepest, yet least explored, of the Earth’s ocean basins. On a meridional transect from Chile to New Zealand crossing the most ultraoligotrophic waters of the South Pacific Gyre, we investigated the distribution of dissolved carbon fractions accessible via bulk and molecular analyses. The spatial distribution of bulk and solid-phase extractable dissolved organic carbon (DOC, SPE-DOC) concentrations as well as dissolved organic matter (DOM) molecular composition derived from ultrahigh resolution mass spectrometry of the SPE-DOC could largely be explained by mixing examined via correlations with density, temperature and apparent oxygen utilization. SPE is known for its lower affinity to very small, polar compounds or colloids often related to recently produced and usually rapidly turned over DOM, we thus argue that the SPE-DOC better reflects carbon compounds of semi-labile to refractory character. Overall, more than half of the DOM compositional variability could be attributed to water mass mixing, while subtle differences especially in the deep and central gyre surface waters were not resolved by a two-component mixing model. DOM molecular composition captured low N/C ratios, higher lability and indications of photochemical transformation in the DOM of the surface gyre waters. With this study, we show that combining the information on different methodologically defined carbon fractions on a basin scale, thereby covering short (days to years) to extremely long-lived (thousands of years) carbon pools, can improve our understanding of global carbon cycling mechanisms.