A North Pacific Meridional Section (U.S. GEOTRACES GP15) of Helium Isotopes and Noble Gases II: Shallow Distributions

Abstract

The distribution of helium isotopes in the upper kilometer of the water column along the GP15 section in the central Pacific reflects the large-scale patterns of upwelling hydrothermal ³He in the tropics and sub-polar gyre, tracing two important pathways whereby bottom water exits from the deep Pacific. Heavy noble gas saturation anomalies, particularly in the upper two hundred meters of the water column, are more strongly increased by seasonal radiative heating, while lighter noble gas saturation anomalies are increased more by air injection processes. A similar, seasonally persistent radiative heating feature was observed in the Equatorial Undercurrent, and appears to be replicated in climate system model simulations. The origin of this feature, however, remains a mystery. A heuristic component model explains the noble gas saturation anomaly distributions, separating the influences of air injection, barometric pressure and radiative heating/cooling. Results show cohesive spatial patterns consistent with where water masses originate, their circulation, and gas exchange dynamics in relation to their formation regions. Using this model, we diagnose the distribution of “non-atmospheric” ⁴He in shallow waters, which parallels the helium isotope anomaly and silica distributions.