TAG Plume: Revisiting the Hydrothermal Neodymium Contribution to Seawater

Abstract

We present results on the distribution of ε Nd and [Nd] from the TAG hydrothermal vent field and adjacent locations collected during the GEOTRACES GA03 cruise in October 2011. Our results show that Nd isotopes directly below and above the plume do not significantly deviate from average NADW (ε Nd = −12.3 ± 0.2). Within the plume, however, isotope values are shifted slightly toward more radiogenic values up to ε Nd = −11.4. Interestingly at the same time a significant decrease in [Nd] along with rare earth element (REE) fractionation is observed, indicating enhanced scavenging within the plume despite the change in Nd isotopes. Elemental concentrations of Nd are reduced by 19.6-18.5 pmol/kg, coinciding with the maximum increase of mantle derived helium (xs 3 He) from 0.203 to 0.675 fmol/kg, resulting in an average 1.8 pmol/kg decrease in [Nd] relative to an expected linear increase with depth. The inventory loss of Nd within the plume sums up to 614 nmoles/m 2 , or 6%, if a continuous increase of [Nd] with depth is assumed. Compared to BATS and the western adjacent station USGT11-14, the local inventory loss is even higher at 10%. The tight relationship of xs 3 He increase and [Nd] decrease allows us to estimate scavenging rates at TAG suggesting 40 mol/year are removed within the TAG plume. A global estimate using power output along ocean ridges yields an annual Nd removal of 3.44 × 10 6 mol/year, which is about 71% of riverine and dust flux combined or 6-8% of the estimated global flux of Nd into the ocean. The change in Nd isotopic composition of up to 0.7 more radiogenic ε Nd values suggests an exchange process between hydrothermally derived particles and seawater in which during the removal process an estimated 1.1 mol/year of hydrothermal Nd is contributed to the seawater at the TAG site. This estimate is only 0.1% of the global Nd signal added to the ocean by boundary exchange processes at ocean margins, limiting the ability of changing the Nd isotopic composition on a global scale in contrast to the more significant estimated sink of elemental Nd in hydrothermal plumes from this study.

Replacement Times of a Spectrum of Elements in the North Atlantic Based on Thorium Supply

“…For most REEs the replacement time appears to increase with integration depth relatively abruptly in the deep eastern Atlantic basin below 4‐km depth, compared with the western basin (Figure ). In investigating this pattern, it is worth noting that the Mid‐Atlantic Ridge hydrothermal vents are not a source of REEs to the ocean and are actually a sink from scavenging onto hydrothermal particles (Stichel et al, ; Zheng et al, ). The apparent increase in replacement time in the deep eastern basin therefore may be explained by the presence of preformed REEs transported by Antarctic Bottom Water (AABW).…”

Influence of Elevated Nd Fluxes on the Northern Nd Isotope End Member of the Atlantic During the Early Holocene

“…Because the Nd sources and their distributions changed substantially in the model since the implementation by Rempfer et al (2011), we retuned the Nd module by minimizing the mean absolute error (MAE) of the dissolved Nd concentration and ε Nd (weighted by the provided measurement uncertainty) between the Bern3D model and the updated NEOSYMPA database of observational data (Tachikawa et al, 2017). In this updated version we included the most recent studies (Amakawa et al, 2019; Rahlf et al, 2020; Stichel et al, 2018; Zieringer et al, 2019). Note that seawater data are biased toward the Atlantic (~50% of all data) as well as toward shallower water depths (median water depth = 600 m).…”

Neodymium in the oceans: a global database, a regional comparison and implications for palaeoceanographic research

“…Measures et al . [ 110 ] proposed a hydrothermal origin for the high Al concentrations in the deep eastern basin, but no such hydrothermal source can be invoked to explain high concentrations of Nd in the area [ 40 , 129 , 130 ].…”