The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data

Abstract

International audienceNeodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years).At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths

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

“…Note as well, as shown in Figure , that if one only considers the upper 1 km, the dissolved Nd replacement time is about 25 years, which is likely shorter than the ventilation time scale of ~40 years (Jenkins, Lott, et al, ). This suggests Nd is unlikely to trace water mass properties in the upper 1 km of the ocean, in agreement with other studies (Tachikawa et al, ). Additionally, a recent Eastern Equatorial Pacific study found significant changes in dissolved Nd isotopic composition through the upper 2.5 km of the water column from station reoccupations separated by only 3 years (2009 and 2012), suggesting a very rapid turnover time (Grasse et al, ).…”

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…The form of the [Nd] profiles in Figure 1, especially in the upper 1,000 m, does appear to share many of the characteristics of POM remineralization in the oceans (e.g., Suess, 1980;Martin et al, 1987) and of AOU (discussed by Stichel et al, 2015). Unfortunately, global compilations, such as presented by Tachikawa et al (2017), produce poor correlations between δ 13 C and [Nd] (r 2 < 0.001; not shown), and the similarity between deep (>1,000 m) [Nd] and P * (van de Flierdt et al, 2016) becomes difficult to justify under the hypothesis of POM scavenging of Nd.…”

Enhanced Mid‐depth Southward Transport in the Northeast Atlantic at the Last Glacial Maximum Despite a Weaker AMOC

“…In contrast, due to low nutrient utilization at the surface of the Southern Ocean, AABW displays a high nutrient content and low values. Neodymium (Nd) isotopic ratios ( Nd/ Nd, Nd) have also been used to infer changes in the Atlantic ocean circulation as the NADW end‐member has a much lower Nd ( ) than AABW ( ) (Tachikawa et al, ). Lower PO (Marchitto, ), and higher carbonate ions content (Yu et al, ), as well as higher benthic (Curry & Oppo, ) and Nd (Howe et al, ) in the upper 2,000 m of the North Atlantic at the LGM have thus been interpreted as indicating a shallower NADW.…”