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

Abstract

AbstractDistributions 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.