Enhanced gravitational and advective particulate carbon export at a frontal region in the southern California Current Ecosystem

Open-ocean mesoscale fronts are common in the California Current Ecosystem (CCE). These fronts are often sites of increased phytoplankton and zooplankton biomass, upwelling, subduction, and eddy kinetic energy and hence may be sites of enhanced particulate organic carbon flux. On a process cruise of the CCE LTER program, we used diverse sampling strategies (broad-region SeaSoar mesoscale mapping, rapid transects across the frontal region for measurement of biogeochemical standing stocks, and Lagrangian process studies) to investigate the biogeochemical impact of a stable front that formed between two mesoscale eddies. Using a combination of ²³⁴Th and sediment trap sampling, we found that carbon export mediated by sinking particles was increased in the frontal region (up to 450 mg C m^-2 d^-1) relative to normal (non-frontal) conditions in the CCE (typically 100-200 mg C m^-2 d^-1). However, ²³⁴Th distributions were distinctly different during our initial and final sampling of the front (separated in time by 2-3 weeks) and suggested downwelling during the first transect through the front and upwelling during the second. This shift from a subductive to an upwelling front was confirmed using a data assimilative physical model (ROMS) incorporating the cruise CTD data, and was likely tied to a change in curvature of the front. Vertical velocities diagnosed from the physical model, combined with POC profiles measured on frontal transects suggested that subduction-mediated passive POC vertical flux rates were equal to or greater than gravitational particle flux during the first sampling period. Combined, these results show that the magnitude of the biological pump in frontal regions (mediated by a combination of physics and biology) may be as much as an order of magnitude greater than background POC fluxes and hence is important in closing a previously measured imbalance between new and export production in the CCE.