Fig. 4.

Schematic of the role that reversible isotope exchange plays in Pb cycling within particle-rich veils at the (A) equator, compared to (B) outside of the particle veil and at the (C) subarctic–subtropical transition zone in the North Pacific 30 to 45°N. The background shading represents the ambient seawater-dissolved ²⁰⁶Pb/²⁰⁷Pb gradients inside and outside of particle veil regions. Brown ovals with a light-green outline represent particles transporting “low” (²⁰⁶Pb/²⁰⁷Pb = 1.155 to 1.17), anthropogenically influenced dissolved ²⁰⁶Pb/²⁰⁷Pb signals from the intermediate layer (1,000 m) to the deep central Pacific. Low dissolved ²⁰⁶Pb/²⁰⁷Pb signatures will adsorb to all particles in the intermediate layer. Particle flux is represented by the number of particles in the intermediate layer as well as by the size of the vertical flux arrows. At the equator and in the North Pacific, the vertical particle flux is large enough for reversible scavenging of low ²⁰⁶Pb/²⁰⁷Pb signatures off particles to overwhelm the influence of “high” (²⁰⁶Pb/²⁰⁷Pb = 1.19 to 1.21) dissolved ²⁰⁶Pb/²⁰⁷Pb that mixes horizontally into the region. In contrast, outside of the veils, the insufficient vertical particle flux does not reversibly scavenge enough low ²⁰⁶Pb/²⁰⁷Pb of anthropogenic origin to overwhelm the impact of horizontal mixing, leaving behind no anthropogenic imprint at depth.