Upper ocean and mesopelagic particle export at Station ALOHA in the NPSG derived from 234Th

The surface waters of the oligotrophic regions of the world's oceans are areas of significant carbon sequestration. This includes the subtropical North Pacific Gyre (NPSG) which, over the past decade, appears to be in the process of a climate induced shift in food web dynamics. In this study, we focused on the temporal variability of particle formation throughout the water column at Station ALOHA using the short-lived natural tracer ²³⁴Th from April 1999 to September 2002. Our results suggest that there are significant zones of particle formation (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) not only in the surface waters, but also at depths greater than 400 m. Large particle export events were associated with the upper 100 m followed by a clear zone of particle remineralization between 150 and 300 m. Mesopelagic scavenging was often associated with high particle export at the surface, with large deep water scavenging events occurring in Feb. and July 2001 and Feb. and June 2002. Mesopelagic particle formation and export was equivalent to as much as 20% of the magnitude of particle export measured at the surface and may be due to zooplankton mediated repackaging. Although limited, these data provide tantalizing evidence that the mesopelagic, often ignored as a zone of particle formation, may play a substantial role in carbon export. Similar observations have been recently proposed to occur in the Sargasso Sea. Hence, the oligotrophic ocean, and the mesopelagic zone in particular, may play an even larger role in long term CO₂ sequestration.