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Abstract Precise measurements of dissolved noble gases along the GP15 GEOTRACES Pacific Meridional Transect reveal the oldest northern bottom waters equilibrated with the atmosphere at a higher barometric pressure than more recent waters. Here, using a radiocarbon-calibrated multi-tracer-based diagnostic model, we reconstruct the magnitude and timing of this palaeo-barometric pressure anomaly. We hypothesize this multi-millennial trend in sea-level pressure results from local and regional processes extant in Antarctic Bottom Water formation regions. 

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⁻¹glacial 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 R_A(atmospheric³He/⁴He ratio) in the south to around 8 R_Anear the equator. North of 30–40°N, it systematically decreases northward to a low of ∼2 R_Anorth 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 water³He_XS/⁴He_XSare consistent with what is known about bottom water flow patterns and the input of low³He/⁴He sedimentary helium. 

Abstract Using a recently compiled global marine data set of dissolved helium isotopes and helium and neon concentrations, we make an estimate of the inventory of hydrothermal³He in the Southern Ocean to be 4.9 ± 0.6 × 10⁴ moles. Under the assumption that the bulk of the hydrothermally sourced³He is upwelled there, we use recent estimates of the global hydrothermal³He flux to determine ane‐folding residence time of 99 ± 18 years, depending on assumptions of water mass and upwelling boundaries. Our estimate is within the broad range of values obtained from recent Southern Ocean circulation models.