Separating biogeochemical cycling of neodymium from water mass mixing in the Eastern North Atlantic

Stichel, Torben, Hartman, Alison E., Duggan, Brian, Goldstein, Steven L., Scher, Howie and Pahnke, Katharina (2015) Separating biogeochemical cycling of neodymium from water mass mixing in the Eastern North Atlantic. Earth and Planetary Science Letters, 412, 245-260. (doi:10.1016/j.epsl.2014.12.008).

Abstract

The radiogenic neodymium (Nd) isotope ratio 143Nd/144Nd (expressed in ?Nd?Nd) is being used as a tracer in paleo and modern ocean circulation. However, the mechanisms controlling input, distribution, and internal cycling are far from understood. For example, globally, Nd concentration ([Nd]) commonly follows patterns of nutrient tracers, generally increasing with depth below the thermocline, while ?Nd?Nd, tends to reflect the water masses, which has often been referred to as the ‘Nd-paradox’. Here we present dissolved Nd isotopes and concentrations at unprecedented vertical and spatial resolution from the eastern part of the US GEOTRACES North Atlantic Zonal Transect (Gulf of Cadiz – Mauritanian Shelf – Cape Verde Islands).

The [Nd] of all samples ranges from 12.3 to 36.7 pmol/kg, with lowest [Nd] usually found within the layer of highest chlorophyll-a levels (chl-max), suggesting removal through scavenging. The Nd isotope compositions range between ?Nd=?13.4?Nd=?13.4 and ?9.9, with lower values at the surface within the extension of the Saharan dust plume and a benthic nepheloid layer (BNL). Less negative values are found in oligotrophic surface waters, Mediterranean Outflow Water (MOW), and near the Cape Verde Islands. Overall, water mass mixing derived from ?Nd?Nd is best visible at the Strait of Gibraltar, where MOW enters the Atlantic Ocean. Most of the sub-thermocline ?Nd?Nd varies within a small range with poor water mass distinction due to the dominance of North Atlantic Deep Water. High surface [Nd] associated with more negative ?Nd?Nd is interpreted to be the result of dust deposition and dissolution. Local [Nd] maxima with no apparent change in ?Nd?Nd compared to ambient seawater, observed within a zone of minimum oxygen concentration (OMZ) at ?500 m depth off Mauritania, suggest minor input of lithogenic Nd but a rather high contribution through desorption of previously scavenged Nd. That is, Saharan dust in this area has only a minor influence on the isotope composition of water below the uppermost surface layer. The low ?Nd?Nd values and elevated [Nd] within the BNL on the other hand indicate release from detrital material. Our new high-resolution dataset gives valuable insights into the Nd distribution near continental margins, influenced by high atmospheric inputs and changing oxygen conditions. We show how vertical and lateral processes overlap to produce the observed Nd distribution, bringing us forward in understanding the Nd-paradox.

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