The deep distributions of helium isotopes, radiocarbon, and noble gases along the US GEOTRACES East Pacific Zonal Transect (GP16)

We report the deep distributions of noble gases, helium isotopes, and radiocarbon measured during the U.S. GEOTRACES GP16 East Pacific Zonal Transect between 152 and 77 W at 12-15 degrees S in the South Pacific. The dominant feature is an intense tongue of hydrothermal effluent that extends > 4000 km westward from the East Pacific Rise (EPR) at similar to 2500 m depth. The patterns reveal significant downstream variations in water mass structure, advection, and mixing that belie the simple perception of a continuous plume extending westward from the EPR. For example, one feature observed at 120 degrees W, 14 degrees S has tracer signatures that are consistent with a water mass originating from an area as much as 2000 km south of this section, suggesting a quasi-permanent northward flow on the western flank of the EPR. Helium isotope variations in the plume show a uniquely high He-3/He-4 source in the tongue compared with typical mid-ocean ridge basalts (MORE), consistent with the anomalously high ratios observed in MORE glasses from the EPR segment just south of this transect. The water column data also reveal that the background He-3/He-4 east of the EPR is significantly lower than values characteristic of MORE, suggesting an additional, more geographically distributed radiogenic He-4 flux of order 10(7) mol/y into the deep Pacific. In the western end of the section, incoming bottom waters have relatively less hydrothermal hydrothermal helium, more radiocarbon, and more oxygen, as well as negative saturation anomalies for the heavy noble gases (Ar, Kr, and Xe). During the basin-scale upwelling of this water, diapycnal mixing serves to erase these negative anomalies. The relative magnitudes of the increases for the heavy noble gases (Ar, Kr, and Xe) are quantitatively consistent with this process. This leads us to estimate the relatively smaller effects on He and Ne saturations, which range from near zero to 0.2% and 0.3% respectively. With this information, we are able to refine our estimates of the magnitude of He-3 and He-4 excesses and the absolute He-3/He-4 ratio of non-atmospheric helium introduced into deep Pacific waters.