Elsevier

Chemical Geology

Volume 511, 20 April 2019, Pages 358-370
Chemical Geology

Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond
Limited influence of basalt weathering inputs on the seawater neodymium isotope composition of the northern Iceland Basin

https://doi.org/10.1016/j.chemgeo.2018.10.019Get rights and content

Highlights

  • Icelandic input of radiogenic Nd essentially limited to coastal waters

  • Offshore bottom water Nd isotope signatures consistent with conservative mixing of intermediate and deep water masses

  • Decreased bottom water Nd concentrations likely reflect removal by particle scavenging

Abstract

Radiogenic neodymium (Nd) isotopes have been widely used as a proxy for tracing present and past water masses and ocean circulation, yet relatively few data exist for seawater from the important deep water formation area around Iceland. We have analyzed the dissolved seawater Nd isotope compositions (expressed as ƐNd) of 71 seawater samples, as well as Nd concentrations [Nd] of 38 seawater samples, collected at full water column profiles from 18 stations in the shelf area off the southern coast of Iceland. The goal of this work was to determine to what extent weathering inputs from Icelandic basalts, which are characterized by a distinctly radiogenic ƐNd signature within the North Atlantic, contribute to the Nd isotope and concentration signatures of water masses in the northern Iceland Basin.

Radiogenic ƐNd values of up to −3.5 and elevated concentrations of up to 21 pmol/kg compared to nearby open ocean sites were found in surface waters at shallow sites closest to shore and to river mouths of Iceland. This documents partial dissolution of highly radiogenic basaltic particles, which are transported northwards by the coastal currents. A comparable signal is not observed, however, in offshore surface waters likely as a result of the advection of surface currents mainly directed onshore, thus isolating these sites from Icelandic weathering contributions. The dominance of Subpolar Mode Waters and Intermediate Water unaffected by Icelandic contributions in the offshore study area is supported by unradiogenic ƐNd signatures between −15 and −12.

In agreement with hydrographic data, highly radiogenic bottom waters at one site on the Iceland-Faroe Ridge (ƐNd = −7.5) reveal the presence of almost pure Iceland Scotland Overflow Water (ISOW) near its formation site further to the east. In bottom waters of all deeper offshore sites, the combination of depleted Nd concentrations and similar ƐNd values (averaging at ≃−11.75 for the R/V Poseidon data and ≃−11 for the R/V Thalassa data) confirms the rapid entrainment of Atlantic mid-depth and deep waters into the overflow waters, which is accompanied by near bottom Nd removal via particle scavenging. Overall, our findings demonstrate that at present, apart from the radiogenic isotope signature of ISOW itself, the direct contribution of radiogenic Nd originating from weathering of Iceland basalts to the water column of the Iceland Basin is limited. This supports the reliable application of ƐNd values to trace changes in the mixing of open North Atlantic water masses (including ISOW).

This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Introduction

Radiogenic neodymium isotopes have been shown to be a valuable proxy for quantifying water mass mixing due to the “quasi conservative” behavior of Nd in seawater (cf. Piepgras et al., 1979; Frank, 2002; Goldstein and Hemming, 2003). Overall, Nd isotope signatures of major open ocean water masses broadly depict the signature of the surrounding continents and are influenced by subsequent mixing and particle cycling along the advection pathways (van de Flierdt et al., 2016; Tachikawa et al., 2017). Radiogenic Nd isotope ratios are generally expressed in the epsilon (ƐNd) notation, which represents the deviation of the 143Nd/144Nd of a sample from that of CHUR (Chondritic Uniform Reservoir) in parts per 10,000 (Jacobsen and Wasserburg, 1980).εNd=N143dN144dsampleN143dN144dCHURN143dN144dCHUR×10000

The Rare Earth Element Nd is primarily introduced into the ocean through rivers, which carry the dissolved and particulate products of continental weathering. Atmospheric dust (Tachikawa et al., 1999; Rickli et al., 2010), marine sediment pore waters (e.g. Abbott et al., 2015), and submarine groundwater discharge (SGD; e.g. Johannesson et al., 2011, Johannesson et al., 2017; Kim and Kim, 2011) are additional contributors to the Nd budget of the global ocean, whereby the global importance of SGD is not yet clear (Molina-Kescher et al., 2018). Importantly, it has been shown that the exchange between seawater and shelf sediments along continental margins can substantially modify the dissolved seawater Nd isotope composition without significantly altering the seawater Nd concentration, which has been termed “boundary exchange” (Lacan and Jeandel, 2005a; Arsouze et al., 2007; Rempfer et al., 2011). While it has been shown that this process exerts a strong control on the global Nd budget, the exact mechanisms controlling Nd cycling during boundary exchange remain poorly understood.

Nd isotopes preserve distinct signatures along the water mass pathways because the average Nd residence time in seawater of 400 to 1000 years is short enough to prevent global ocean mixing but long enough for local source signatures to be advected out of and between individual ocean basins (Tachikawa et al., 2003; Siddall et al., 2008; Rempfer et al., 2011). Nd concentrations in the deep ocean are on the order of 10–50 pmol/kg and profiles generally follow a nutrient-type pattern, increasing with depth and with time advected along the global conveyor belt in deep waters (Elderfield et al., 1988; Siddall et al., 2008). Very high Nd concentrations of up to 150 pmol/kg have been measured in coastal surface waters of the North Atlantic, reflecting local inputs; otherwise, Nd concentrations in deep and intermediate waters are generally lowest in the North Atlantic and highest in the North Pacific (Lacan et al., 2012; van de Flierdt et al., 2016; Tachikawa et al., 2017). In contrast, ƐNd signatures vary with water mass distribution, particularly when advection is strong and dominant over other vertical Nd cycling processes and diapycnal mixing, thus enabling the distinctive Nd isotopic composition of individual water masses to be conserved. Prompted by the ongoing international GEOTRACES program (Henderson et al., 2007), the available database has expanded greatly over recent years (Mawji et al., 2015; van de Flierdt et al., 2016) and new data for the North Atlantic have been generated (Stichel et al., 2015; Lambelet et al., 2016; Dubois-Dauphin et al., 2017).

Iceland is a uniquely radiogenic Nd source in the Northern North Atlantic due to its location on the Mid Atlantic Ridge. Ninety percent of Iceland consists of basaltic rocks (e.g. Gíslason et al., 1996) characterized by ƐNd signatures between +7 and +8.5 (Shorttle et al., 2013). Some of the most important deep water formation areas of the North Atlantic Ocean, which ultimately contribute to the formation of North Atlantic Deep Water (NADW), are located near Iceland. It is thus crucial to constrain if and how basaltic inputs from Iceland contribute to the Nd isotope composition of the surrounding seawater.

The Iceland Basin is located within the Subpolar Gyre of the North Atlantic, which is a key area of the Atlantic overturning circulation (e.g. van Aken and de Boer, 1995). Interactions between warm and saline Atlantic waters and colder Arctic waters result in the formation of two major deep water masses in this area which substantially contribute to the formation of NADW: Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) which originate in the Iceland and Greenland Seas to the north of Iceland (Lee and Ellett, 1967; Swift et al., 1980; Swift, 1984; Fig. 1). Both overflow waters eventually mix with deep waters from other sources in the western North Atlantic basin (most importantly Labrador Sea Water “LSW”) to form dense and saline NADW (van Aken and de Boer, 1995). Temporal and spatial changes in these circulation patterns, overflow strength, and water mass distribution are linked to heat and moisture transport in the Northern Hemisphere and are thus impacted by - and influence - regional and global climate (e.g. Rhein et al., 2011).

The hydrography of waters around Iceland is complex due to sea floor topography and admixture of a number of different water masses (van Aken and de Boer, 1995; Malmberg, 2004; Pollard et al., 2004; Logemann et al., 2013; Fig. 1). The Iceland Basin is located to the south of Iceland and is bordered by the Denmark Strait in the west and the Iceland-Faroe Ridge in the east (Fig. 1). Warm and saline subtropical Atlantic Water originating from the Gulf Stream is advected into the Nordic Seas where it mixes with cold and fresh Arctic Water flowing southward. Cooling and freezing processes further transform and subduct these water masses, which are eventually carried back into the Atlantic as overflow waters both across the Iceland-Faroe Ridge into the Iceland Basin, and across the Denmark Straight into the Irminger Basin. The properties and proportions of major water masses in the Iceland Basin vary on inter-annual time scales (van Aken and de Boer, 1995).

As a result, the main water masses found in the Iceland Basin include Subpolar Mode Water (SPMW), a modification of Atlantic Water convectively formed in the Subpolar Gyre through mixing of subtropical and polar waters. It has a temperature of ≃8 °C and a salinity of ≃35.2, and is found below the surface mixed layer extending as deep as 1000 m (McCartney and Talley, 1982; van Aken and de Boer, 1995). Intermediate Water (IW) is another modification of Atlantic Water found below SPMW and is primarily identified by a distinct oxygen minimum and temperatures of ≃6 °C (van Aken and de Boer, 1995; Beaird et al., 2016). Icelandic Slope Water (ISW) is a water mass found at deep levels on the Icelandic slope and is comprised of a mixture of SPMW and ISOW, with high salinity and oxygen values (van Aken and de Boer, 1995). Labrador Sea Water (LSW) spreads from its source in the Labrador Basin across the Mid-Atlantic Ridge into the Iceland Basin and is one of the important deep water masses contributing to the formation of NADW, with temperatures of ≃3–4 °C, a somewhat variable salinity near ≃34.9, and oxygen content >275 μmol/kg (van Aken and de Boer, 1995; Malmberg, 2004). Iceland Scotland Overflow Water (ISOW) is another important deep water mass contributing to NADW which forms by mixing of Atlantic waters and LSW and flows westwards across the Iceland-Faroe Ridge into the Iceland Basin. This water mass is characterized by low temperatures of ≃1.75–3 °C, a salinity of ≃35, and high oxygen concentrations around 300 μmol/kg (van Aken and de Boer, 1995; Malmberg, 2004; Beaird et al., 2016).

Several studies on the Nd isotope composition of water masses have been performed in this area of the North Atlantic which defined distinctly radiogenic ƐNd signatures of ISOW (−7.7 ± 0.6 by Piepgras and Wasserburg, 1987; −8.2 ± 0.6 by Lacan and Jeandel, 2004b) and DSOW (−8.6 ± 0.5 by Piepgras and Wasserburg, 1987; −8.4 ± 1.2 by Lacan and Jeandel, 2004a). Other major water masses have been isotopically characterized in the northern North Atlantic including parts of the southern Iceland Basin. In the northeastern Iceland Basin near the Iceland-Faroe Ridge, SPMW has an ƐNd signature of −13 ± 0.6, and in the southwestern Iceland Basin ƐNd signatures of SPMW and LSW are −14.8 ± 0.2 and −14.1 ± 0.4, respectively (Lacan and Jeandel, 2004c; Lacan and Jeandel, 2005b; Dubois-Dauphin et al., 2017). Overall, however, very few data exist for locations close to Iceland, in particular in shallow waters.

Here we determine to what extent weathering inputs from Icelandic basalts, which are characterized by a distinctly radiogenic ƐNd signature within the North Atlantic, modify the Nd isotope and concentration signatures of water masses in the northern Iceland Basin. We have analyzed 71 samples collected at 18 stations off the southern coast of Iceland for seawater Nd isotopic compositions, and 38 of those samples for seawater Nd concentrations. The goal was to further elucidate the Nd isotope signatures of the water masses and their mixing, and therefore the influence of processes such as boundary exchange (e.g. Lacan and Jeandel, 2005a, Lacan and Jeandel, 2005b; Wilson et al., 2012) and basalt dissolution (e.g. Pearce et al., 2013; Fröllje et al., 2016) on the Nd isotope composition of Iceland Basin seawater.

Section snippets

Materials and methods

The entire purification and measurement techniques applied by the GEOMAR and GEOPS laboratories followed approved GEOTRACES protocols and both laboratories successfully participated in the international GEOTRACES intercalibration study (van de Flierdt et al., 2012). In total, 9 nearshore stations at <500 m depth (883, 899, 890, 913, and one “surface” sample from cruise P457, and 6, 9, 15, and 16 from cruise ICE-CTD), and 9 offshore stations (889, 903, and 905 from cruise P457, and 5, 10, 11,

Results

Combined temperature, salinity, and oxygen data from both cruises served to characterize the hydrography of the study area (Fig. 2, Fig. 3; temperature distribution shown in Fig. S1). Subpolar Mode Water was identified at all stations by its high temperature and salinity immediately below the surface mixed layer and extended down to depths between 300 m (at station 913) and 750 m (at station 905). At offshore stations, oxygen minima marked the presence of Intermediate Water (IW) below SPMW.

Lithogenic inputs to nearshore surface waters (stations <500 m water depth)

The large variability of Nd isotope compositions and Nd concentrations measured in surface waters off the southern coast of Iceland clearly demonstrates that nearshore waters are influenced by local Nd inputs in addition to conservative water mass mixing (Fig. 5, Fig. 6).

Several of the shallowest nearshore stations (883, 899, and 15) display isotope compositions significantly more radiogenic (up to ƐNd = −3.5) than both the deep waters in the rest of the study area and any so far published

Conclusions

We present the first systematic study of the Nd isotope exchange between the radiogenic rocks and sediments of Iceland and adjacent seawater. Close to shore and to the mouths of rivers, ƐNd values of near surface waters are overall significantly more radiogenic, and Nd concentrations are higher than in surface waters offshore (>500 m water depth). This documents exchange with riverine and/or aeolian basaltic particles from Iceland. These seawater signatures are also modified by water mass

Acknowledgements

We acknowledge the help of the captains and crews of R/V Poseidon and R/V Thalassa. Analyses of ICE-CTD seawater samples received funding from the CNRS (Centre National de la Recherche Scientifique, France) Project LEFE - ICE-CTD, as well as three ANR (Agence National de la Recherche, France) projects: the NEWTON project (Grant ANR-06-BLAN-0146), the HAMOC project (Grant ANR-13-BS06-0003), and the L-IPSL project (Grant ANR-10-LABX-0018). R/V Poseidon Cruise P457 was carried out in the frame of

References (70)

  • K.H. Johannesson et al.

    Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

    Geochim. Cosmochim. Acta

    (2011)
  • K.H. Johannesson et al.

    Rare earth element behavior during groundwater-seawater mixing along the Kona Coast of Hawaii

    Geochim. Cosmochim. Acta

    (2017)
  • I. Kim et al.

    Large fluxes of rare earth elements through submarine groundwater discharge (SGD) from a volcanic island, Jeju, Korea

    Mar. Chem.

    (2011)
  • F. Lacan et al.

    Denmark Strait water circulation traced by heterogeneity in neodymium isotopic compositions

    Deep-Sea Res.

    (2004)
  • F. Lacan et al.

    Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent–ocean interface

    Earth Planet. Sci. Lett.

    (2005)
  • F. Lacan et al.

    Neodymium isotopic composition of the oceans: a compilation of seawater data

    Chem. Geol.

    (2012)
  • M. Lambelet et al.

    Neodymium isotopic composition and concentration in the western North Atlantic Ocean: results from the GEOTRACES GA02 section

    Geochim. Cosmochim. Acta

    (2016)
  • G. Laukert et al.

    Transport and transformation of riverine neodymium isotope and rare earth element signatures in high latitude estuaries: a case study from the Laptev Sea

    Earth Planet. Sci. Lett.

    (2017)
  • A. Lee et al.

    On the water masses of the Northwest Atlantic Ocean

    Deep-Sea Res.

    (1967)
  • G. Merschel et al.

    Contrasting impact of organic and inorganic nanoparticles and colloids on the behavior of particle-reactive elements in tropical estuaries: an experimental study

    Geochim. Cosmochim. Acta

    (2017)
  • C.R. Pearce et al.

    The effect of particulate dissolution on the neodymium (Nd) isotope and rare earth element (REE) composition of seawater

    Earth Planet. Sci. Lett.

    (2013)
  • D.J. Piepgras et al.

    Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations

    Geochim. Cosmochim. Acta

    (1987)
  • D.J. Piepgras et al.

    The isotopic composition of Nd in different ocean masses

    Earth Planet. Sci. Lett.

    (1979)
  • J. Rempfer et al.

    Modelling Nd-isotopes with a coarse resolution ocean circulation model: sensitivities to model parameters and source/sink distributions

    Geochim. Cosmochim. Acta

    (2011)
  • M. Rhein et al.

    Deep water formation, the subpolar gyre, and the meridional overturning circulation in the subpolar North Atlantic

    Deep-Sea Res. II Top. Stud. Oceanogr.

    (2011)
  • J. Rickli et al.

    Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: implications for sources and inputs of trace metals to the ocean

    Geochim. Cosmochim. Acta

    (2010)
  • D.R. Shoosmith et al.

    Discrete eddies in the northern North Atlantic as observed by looping RAFOS floats

    Deep-Sea Res. II

    (2005)
  • O. Shorttle et al.

    Geochemical provincialism in the Iceland plume

    Geochim. Cosmochim. Acta

    (2013)
  • M. Siddall et al.

    Towards explaining the Nd paradox using reversible scavenging in an open general circulation model

    Earth Planet. Sci. Lett.

    (2008)
  • T. Stichel et al.

    The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean

    Earth Planet. Sci. Lett.

    (2012)
  • T. Stichel et al.

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

    Earth Planet. Sci. Lett.

    (2015)
  • J.H. Swift

    The circulation of the Denmark Strait and Iceland-Scotland overflow waters in the North Atlantic

    Deep-Sea Res.

    (1984)
  • J.H. Swift et al.

    The contribution of the Denmark strait overflow to the deep North Atlantic

    Deep Sea Res. Part A

    (1980)
  • K. Tachikawa et al.

    A new approach to the Nd residence time in the ocean: the role of atmospheric inputs

    Earth Planet. Sci. Lett.

    (1999)
  • K. Tachikawa et al.

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

    Chem. Geol.

    (2017)
  • Cited by (9)

    • Dissolved neodymium isotopes in the Mediterranean Sea

      2022, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      The seawater εNd signature originates from the continental Nd supply through weathering of surrounding source rocks of different ages (Goldstein and Hemming, 2003) and mainly reflects lateral water mass advection and mixing. However, the use of εNd as a water mass tracer is challenged by non-conservative modifications that can impact its “quasi-conservative” behaviour, which include input of riverine particles and waters, aeolian-derived material, benthic fluxes of Nd, submarine groundwater discharge and exchange with the sediments at continental margins (e.g. Frank, 2002; Goldstein and Hemming, 2003; Lacan and Jeandel, 2005; Johannesson and Burdige, 2007; Abbott et al., 2015a, 2015b; Morrison et al., 2019). This has been observed in several regions of the global ocean, especially close to the continental margins, where seawater εNd does not co-vary with other conservative hydrographic parameters, such as salinity and temperature (e.g. Grenier et al., 2013; Stichel et al., 2015).

    • A decade of progress in understanding cycles of trace elements and their isotopes in the oceans

      2021, Chemical Geology
      Citation Excerpt :

      The studies also examine the influence of local processes which could compromise the use of regional εNd as a circulation tracer. For example Morrison et al. (2019) investigate the possibility of addition of uniquely-radiogenic εNd from weathering of basalts on Iceland, but find that such an addition of [Nd] only affects the εNd of local coastal waters, rather than compromising the εNd signature of other major water masses in the Iceland Basin. In contrast, Zieringer et al. (2019) show that, similar to previous work by Stichel et al. (2015) and Rickli et al. (2010), surface waters of the tropical eastern Atlantic are significantly impacted by seasonally- and spatially-variable deposition of Saharan dust, manifesting in both [Nd] and εNd, potentially weakening the use of εNd as a water mass tracer in this region.

    • Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes

      2020, Quaternary Science Reviews
      Citation Excerpt :

      In the Atlantic, the non-conservative behavior is generally weaker than in the Pacific but clearly identifiable. More evident examples include (Fig. 2E and F): a strong negative non-conservative εNd (∼−5) in the Baffin-Labrador Basin where sediments are partially sourced from the Canadian Shield (Filippova et al., 2017; Lacan and Jeandel, 2005b); a strong positive non-conservative εNd (∼+5) around Iceland, reflecting basaltic sources (Morrison et al., 2018); a pattern of consistent negative non-conservative εNd (∼−3) in the deep western North Atlantic that corresponds to the thick and persistent Benthic Nepheloid Layer in this region (Gardner et al., 2018; Pöppelmeier et al., 2019), where suspended sediments are likely transported from the Labrador Basin via the Deep Western Boundary Current (DWBC) (Grousset and Biscaye, 2005). Other regional non-conservative examples include (Fig. 2F): a positive non-conservative εNd in the Caribbean due to local volcanic sediments (Osborne et al., 2014); a negative non-conservative source signature for εNd in the Angola Basin consistent with local detrital sediments (Rahlf et al., 2019); a positive non-conservative source signature for εNd in the Pacific and Atlantic sectors of the Southern Ocean where local Antarctic sourced detrital sediments have radiogenic εNd (Carter et al., 2012; Lambelet et al., 2018), and a negative non-conservative signature for εNd in the Bay of Bengal due to unradiogenic detritus from the Himalayans (Yu et al., 2017).

    View all citing articles on Scopus

    This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

    View full text