The distributions of helium isotopes and tritium along the U.S. GEOTRACES North Atlantic sections (GEOTRACES GAO3)

Abstract

We present the distributions of helium isotopes (in the form of helium isotope ratio anomaly relative to the atmospheric ratio) and tritium along two sections occupied in the subtropical North Atlantic as part of the first U.S. GEOTRACES survey (GEOTRACES GA03). The general distributions of these isotopes are consistent with the continuing penetration and evolution of bomb-produced tritium and its daughter isotope 3He in the main thermocline and along the western boundary current system. We combine these two distributions to compute a tritium–3He age, which is related to the elapsed time since the water was at the ocean surface. Although it is an indicator biased by the effects of mixing and influenced by the time history and spatial distribution of bomb tritium delivery to the ocean surface, it still remains a useful measure of ventilation time-scales. Aside from the continued propagation of the tritium–3He transient into the ocean interior, there are three notable features of interest in these distributions. The first is the clear signature of upwelling in the water column near the coast of Mauritania, characterized by the upward bowing of isochrones in the thermocline and discernable 3He excess at the ocean surface. A simple 3He mass balance calculation suggests an upwelling flux of order 1.8×106m3s−1 (1.8Sv) along the Mauritanian coast. The second is a mid-depth (~1500–2000m) core of ventilated waters centered over the Mid-Atlantic Ridge, an anticyclonic circulation of waters likely originating in the Labrador Sea. The third notable feature is a volcanic 3He plume at about 3500m depth emanating from the TAG Hydrothermal Area that is detectable as much as 500km away on each side of the Mid-Atlantic Ridge. We estimate a 3He:heat ratio of ~7×10–18molJ−1 and a 3He flux from the TAG site of ~15mmoly−1. Since 3He is a conserved tracer in the absence of measureable tritium, the correlation of volcanic 3He with other hydrothermally influenced TEIs (e.g., Fe) can be used as a dilution tracer as probe of non-conservative behavior in the water column. Also, since the regional and global fluxes of volcanic 3He are known, the correlations can be used as a regional/global flux gauge for hydrothermal input of those TEIs.