1107 / 2024-09-20 12:23:03

Zonal coupling of surface export flux and subsurface remineralization in the Northwest Pacific Ocean

234Th/238U disequilibrium,POC Flux,Remineralization

The spatial variability of the basin scale export flux in the western Pacific Ocean is largely unknown. This study examined the particle fluxes across a long distance (1500 km) meridional transect using ²³⁴Th/²³⁸U disequilibrium, covering three distinctive nutrient regimes: the oligotrophic subtropical gyre (SG), the mixing front of Kuroshio extension (KE) and the eutrophic sub-Arctic water (SA). In the upper 100 m, ²³⁴Th was deficient relative to ²³⁸U, and the degree of this deficit increased with latitude. Interestingly, ²³⁴Th excess induced by particle remineralization was seen right below 100 m and synchronously changed with ²³⁴Th deficit in the surface. Based on 1D steady state model, the export flux was estimated as 1.10±0.20 - 10.0±0.42 mmol C m^-2d^-1(averaged 3.87±0.28 mmol C m^-2d^-1) at the depth of 100 m, with the lowest value of 1.67±0.13 mmol C m^-2d^-1 in the gyre region but the highest of 6.30±0.35 mmol C m^-2d^-1 in SA. Such north to south basin scale variability of the export flux became insignificant at the depth of 160 m, with 1.25±0.17 mmol C m^-2d^-1 in the SG, 2.56±0.17 mmol C m^-2d^-1 in the KE, and 3.20±0.35 mmol C m^-2d^-1 in the SA. This suggested that export fluxes at the depth of 100 m could be enhanced in the eutrophic region at the north due to the higher primary production. However, immediate shallow remineralization at the subsurface was also significant possibly mediated by the higher grazing rate, and thus offset the flux enhancement in the upper layer.