Biogeochemistry of Dissolved Manganese in the Western North Pacific Subtropical Gyre: Sources, Fluxes, and Cycling Drivers

In the oligotrophic North Pacific Subtropical Gyre (NPSG), manganese (Mn) cycling, despite its ecological importance and role as a tracer, has received less attention than macronutrient dynamics. Here, we present high-resolution dissolved Mn (dMn) data across the western NPSG during the Chinese GEOTRACES GP09 cruise. Surface dMn showed regional characteristics, with an average concentration of 1.41 ± 0.50 nmol kg⁻¹ (n = 142). Atmospheric deposition could explain approximately 35% of mixed-layer dMn, with highly variable wet deposition fluxes. The discrepancy between the observed dMn and atmospheric inputs suggests that additional sources, such as lateral transport or island inputs, are significant. In the euphotic zone, a dMn flux budget revealed that horizontal transport (189 ± 160 μmol m⁻² year⁻¹) exceeded atmospheric deposition by more than an order of magnitude, while oxidative scavenging (140 ± 90 μmol m⁻² year⁻¹) dominated removal processes. This balance yields a dMn residence time of 1.1 ± 0.9 years in the euphotic zone. Intermediate water masses (σ₀ = 26.6–27.4 kg m⁻³) displayed a robust inverse dMn–salinity relationship (r = 0.89, p < 0.01), with dMn concentrations ranging from 0.25 to 0.36 nmol kg⁻¹, reflecting isopycnal mixing. Deep water (>1,500 m) dMn distributions showed basin-scale depletion (<0.2 nmol kg⁻¹) interrupted by two distinct enrichment processes: (a) Hydrothermal inputs from the Mariana Trough vent field generated extreme dMn enrichment (up to 3.05 nmol kg⁻¹), with the signal transported conservatively over hundreds of kilometers. (b) Abyssal boundary interactions at the Luzon Strait and Yap–Mariana Junction generated localized peaks (0.92–1.55 nmol kg⁻¹) through resuspension by strong deep currents.