A North Pacific meridional section (U.S. GEOTRACES GP15) of helium isotopes and noble gases I: deep water distributions

Abstract

The noble gas signature of incoming Pacific Bottom Water (PBW), when compared to North Atlantic Deep Water, indicates the addition of 450 ± 70 GT a−1 glacial melt water to form AABW and subsequently PBW. The downstream evolution of this signature between the southern (20°S to equator) and northern (25°–45°N) bottom waters indicates a decrease in sea level pressure around Antarctica over the past two millennia. Vertical profiles of noble gases in the deep Pacific show exponential relationships with depth with scale heights identical to temperature and salinity. Unlike the other noble gases, helium isotopes show evidence of mid‐depth injection of non‐atmospheric helium. Using observed deviations from exponential behavior, we quantify its magnitude and isotope ratio. There is a clear latitude trend in the isotope ratio of this added helium that decreases from a high exceeding 9 RA (atmospheric 3He/4He ratio) in the south to around 8 RA near the equator. North of 30–40°N, it systematically decreases northward to a low of ∼2 RA north of 50°N. This decline results from a combination of northward decline in seafloor spreading, release of radiogenic helium from increased sediment thickness, and the possible emission of radiogenic helium through cold seeps along the Alaskan and North American margins. Finally, we derive an improved method of computing the excess helium isotope concentrations and that the distributions of bottom water 3HeXS/4HeXS are consistent with what is known about bottom water flow patterns and the input of low 3He/4He sedimentary helium.