Particulate organic carbon export fluxes estimates by 234Th238U disequilibrium in the oxygen minimum zone off the Peruvian coast

The Peruvian upwelling zone is one of the most productive marine ecosystems in the world with a spectacular, pronounced oxygen minimum zone (OMZ). Globally OMZs are increasing in size and intensity with far-reaching consequences for the marine biological carbon pump and carbon export; thus, these zones need to be carefully monitored to be able to understand future climate change impacts. The current study was carried out in 2013 and 2017 to quantify the vertical flux of organic matter exported out of the productive surface layer by measuring ²³⁴Th²³⁸U disequilibria in the water column. Samples were collected in January 2013 and May 2017 along an identical transect located at 12°S off the Peruvian coast near Lima, Peru. Th-234 fluxes ranged from 0 to 2088 ± 95 dpm m⁻² d⁻¹ in 2013 and 698 ± 63 to 3648 ± 113 dpm m⁻² d⁻¹ in 2017. The corresponding POC fluxes varied between 0 and 164.2 ± 7.9 mg C m⁻² d⁻¹ in 2013 and 22.7 ± 2.7 to 133.1 ± 15.2 mg C m⁻² d⁻¹ in 2017, with POC fluxes gradually decreasing with distance from the coast. Despite higher POC fluxes, the export efficiencies were found to be extremely low due to high particle remineralization rates observed within the euphotic zone.