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

Abstract

a b s t r a c tWe 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 3 He in the main thermocline and along the western boundary current system. We combine these two distributions to compute a tritium-3 He 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-3 He 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 3 He excess at the ocean surface. A simple 3 He mass balance calculation suggests an upwelling flux of order 1.8 Â 10 6 m 3 s À 1 (1.8 Sv) along the Mauritanian coast. The second is a mid-depth ($ 1500-2000 m) 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 3 He plume at about 3500 m depth emanating from the TAG Hydrothermal Area that is detectable as much as 500 km away on each side of the Mid-Atlantic Ridge. We estimate a 3 He:heat ratio of $ 7 Â 10 -18 mol J À 1 and a 3 He flux from the TAG site of $ 15 mmol y À 1 . Since 3 He is a conserved tracer in the absence of measureable tritium, the correlation of volcanic 3 He 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 3 He are known, the correlations can be used as a regional/global flux gauge for hydrothermal input of those TEIs.

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

“…We adopt an empirical relationship between deep water ∆Ne and ∆He in an area without significant hydrothermal helium contributions, namely the North Atlantic away from Mid Atlantic Ridge. Using recent data from the U.S. North Atlantic GEOTRACES GA03 cruise (Jenkins et al, 2014a;Jenkins et al, 2014b) we find a molar ratio of 0.22 ± 0.01 between excess He and excess Ne below 1000 m depth. The fact that this value is significantly lower than the atmospheric abundance ratio of 0.288 is a reflection of the intertwined effects of thermal disequilibrium (Hamme and Severinghaus, 2007) and partial vs. complete bubble injection processes (Jenkins, 1988;Stanley et al, 2009b;Liang et al, 2013) during water mass formation.…”

TAG Plume: Revisiting the Hydrothermal Neodymium Contribution to Seawater

“…From 420 m, [Nd] is constant to 2,100 m and increases from 17.5 ± 0.5 to 19.6 pmol/kg at 2,990 m. The typical increase below 2,000 m was documented in other studies in the Atlantic Ocean and most recently, based on high resolution profiles, has been attributed to a combination of increase from particle release and lateral advection of preformed REE along with reversible scavenging (Siddall et al, 2008;Stichel et al, 2015;Lambelet et al, 2016;Zheng et al, 2016). This increasing slope in [Nd] is interrupted by a sudden decrease at 3,184 m, coinciding with the upper FIGURE 2 | Vertical distribution of dissolved Nd concentration (A), Nd isotopic composition (B) and (C) mantle derived excess He (xs 3 He, Jenkins et al, 2015a). Color coding the same as in Figure 1.…”

“…The figure is designed with ODV ® software (Schlitzer, 2016a). extent of the TAG hydrothermal plume (Jenkins et al, 2015a).…”

“…Station BATS is located at the western edge of the North Atlantic Gyre, where the Deep Western Boundary Current (DWBC) transports North Atlantic Deep Water (NADW) to the south (Figure 1). The presence of the hydrothermal plume at TAG is indicated by a significant increase in primordial He (δ 3 He, Jenkins et al, 2015a). In this study, we will use this enrichment in δ 3 He, expressed in excess He (xs 3 He), to identify the extent of the plume, referring to all samples with this enrichment as "within the plume.…”

The cobalt cycle in the Tropical Pacific Ocean

“…The range of included isopycnal surfaces was chosen so that layers reside predominantly in the mesopelagic, between 100-1500 m. Shallower isopycnal surfaces were excluded (>25.375 kg m -3 for the GP16 section and >26.125 kg m -3 for the GA03 section), in part because they are mixed on timescales < 5 years, especially in the North Atlantic, and therefore do not represent the same level of integration as the underlying surfaces Jenkins et al, 2015). As a result, aggregate dCo:PO 4 relationships are more complex and require fine-scale approaches to derive stoichiometry (Saito et al submitted).…”