Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions

Melanie Grenier,Thomas W. Trull,Catherine Jeandel,Nolwenn Lemaitre,Pieter van Beek,Marc Souhaut,Philippe Nonnotte,Ester Garcia-Solsona,Francois Lacan,Vincent Bouvier

doi:10.3389/fmars.2018.00426

Abstract

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as eNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October-November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative eNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic eNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic eNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica.

Highlights

The Southern Ocean is recognized as the major high-nutrient, low-chlorophyll (HNLC) oceanic region, where phytoplankton growth is limited by a low availability of the micronutrient iron (Fe; Martin, 1990; de Baar et al, 1995; Boyd et al, 2007)
For the sake of simplicity, we have shown the concentration vertical profiles of only some rare earth element (REE): the lightest (La), the most particle-reactive REE in seawater (Ce), the two ones used for the isotope dilution
The results described above highlight that the distribution of dissolved REEs and εNd in the Kerguelen Plateau area are influenced by the following factors: 1) Recent lithogenic supplies, deduced from concentration increases and from weakening of Ce anomaly and of Ndn/Ybn slope, which impact most of the waters sampled between the surface and 500 m depth; 2) Two kinds of fractionation processes observed below 500 m depth: (i) constant low La anomalies and positive Eu anomalies, and (ii) high jerky La anomalies and no Eu anomalies

Summary

Introduction

The Southern Ocean is recognized as the major high-nutrient, low-chlorophyll (HNLC) oceanic region, where phytoplankton growth is limited by a low availability of the micronutrient iron (Fe; Martin, 1990; de Baar et al, 1995; Boyd et al, 2007). A recurrent annual phytoplankton bloom is notably observed in the Indian sector of the Southern Ocean, in the vicinity of the Kerguelen Plateau (Blain et al, 2001, 2007). This bloom—one of the largest of the Southern Ocean—extends more than 1,000 km downstream of the Kerguelen Islands, on either side of the Polar front (PF; Figure 1). The KEOPS1 cruise was conducted in the Central Kerguelen Plateau area in austral summer 2005, during the decline of the bloom (Blain et al, 2007). The complementary KEOPS2 cruise was mostly conducted in the Northern Kerguelen Plateau area in spring 2011, during the onset of the bloom (Carlotti et al, 2015; Figure 1)

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Discussion

Conclusion