210Po and 210Pb Distributions Along the GEOTRACES Pacific Meridional Transect (GP15): Tracers of Scavenging and Particulate Organic Carbon (POC) Export

Distributions of the natural radionuclide ²¹⁰Po and its grandparent ²¹⁰Pb along the GP15 Pacific Meridional Transect provide information on scavenging rates of reactive chemical species throughout the water column and fluxes of particulate organic carbon (POC) from the primary production zone (PPZ). ²¹⁰Pb is in excess of its grandparent ²²⁶Ra in the upper 400–700 m due to the atmospheric flux of ²¹⁰Pb. Mid-water ²¹⁰Pb/²²⁶Ra activity ratios are close to radioactive equilibrium (1.0) north of ∼20°N, indicating slow scavenging, but deficiencies at stations near and south of the equator suggest more rapid scavenging associated with a “particle veil” located at the equator and hydrothermal processes at the East Pacific Rise. Scavenging of ²¹⁰Pb and ²¹⁰Po is evident in the bottom 500–1,000 m at most stations due to enhanced removal in the nepheloid layer. Deficits in the PPZ of ²¹⁰Po (relative to ²¹⁰Pb) and ²¹⁰Pb (relative to ²²⁶Ra decay and the ²¹⁰Pb atmospheric flux), together with POC concentrations and particulate ²¹⁰Po and ²¹⁰Pb activities, are used to calculate export fluxes of POC from the PPZ. ²¹⁰Po-derived POC fluxes on large (>51 μm) particles range from 15.5 ± 1.3 mmol C/m²/d to 1.5 ± 0.2 mmol C/m²/d and are highest in the Subarctic North Pacific; ²¹⁰Pb-derived fluxes range from 6.7 ± 1.8 mmol C/m²/d to 0.2 ± 0.1 mmol C/m²/d. Both ²¹⁰Po- and ²¹⁰Pb-derived POC fluxes are greater than those calculated using the ²³⁴Th proxy, possibly due to different integration times of the radionuclides, considering their different radioactive mean-lives and scavenging mean residence times.