Export and transfer of Southern Ocean particulate organic carbon through the lens of ramped oxidation

The euphotic and mesopelagic zones are the most dynamic depth intervals for particulate organic carbon (POC) flux in the water column. The region encompassing the Great Calcite Belt- a calcite-rich band produced by a coccolithophore bloom across the Southern Ocean - spans a wide range in primary productivity and POC transport from surface to 1000 m, offering an opportunity to explore the mechanisms that control POC transfer to depth. Bulk compositional measurements from this region show that the efficiency of POC export and transfer through the euphotic and mesopelagic zones varies with phytoplankton community composition, suggesting that different assemblages generate and transfer compositionally distinct POC to the deep ocean. To directly assess whether such compositional differences influence POC transport down the water column, we submitted two size-fractions (>51 μm and <51 μm diameter) of POC from five depth profiles in the Great Calcite Belt region to ramped-oxidation from room temperature to 700°C. Plots of released CO₂ concentration against temperature, or thermograms, provide a unique, higher-resolution perspective of organic matter composition by differentiating POC by thermal stability, which we interpret as a proxy for biochemical reactivity. The data show that a diatom-rich station exports more reactive POC out of the euphotic zone than do coccolithophore-rich stations, which may explain why diatom-rich communities typically exhibit the highest [POC] attenuation coefficients in the mesopelagic zone. We plan to statistically deconvolve thermograms into a sum of simpler component pools to explore how changes in pools with distinct thermochemical stability translate to bulk features of the POC export regime, such as Martin curve-derived attenuation coefficients and flux transfer efficiency. Stable isotope measurements of CO₂ generated at distinct temperature intervals of the ramped oxidation will help diagnose these component peaks.