Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyondLimited influence of basalt weathering inputs on the seawater neodymium isotope composition of the northern Iceland Basin☆
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).
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
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2022, Geochimica et Cosmochimica ActaCitation 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).
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2021, Chemical GeologyCitation 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 ReviewsCitation 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).
Ice-sheet driven weathering input and water mass mixing in the Nordic Seas during the last 25,000 years
2019, Earth and Planetary Science Letters
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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.