Siderophore-promoted transfer of rare earth elements and iron from volcanic ash into glacial meltwater, river and ocean water
Highlights
► Incubation experiments were performed with river waters from Iceland that are rich in suspended volcanic ash. ► Siderophores (DFOB) strongly enhance mobilization of REE and Fe from suspended volcanic ash. ► Siderophores produce a distinctive REE fractionation during incongruent particle dissolution. ► Siderophores promote the oxidative dissolution of Ce, producing positive Ce anomalies in ambient waters. ► REE systematics and Nd isotopic composition in meltwater, river and seawater may be affected by a siderophore-bound REE flux.
Introduction
The total rare earth element (REE) inventory of natural surface waters can be subdivided into three different pools (Elderfield et al., 1990): (i) the “particulate” REE that are bound to solid particles larger than 0.45 μm or 0.2 μm (the pore sizes of the membrane filters commonly used in hydrochemical studies), (ii) the “colloidal” REE that are bound to colloids and nanoparticles of <0.45 μm or <0.2 μm size, and (iii) the “truly dissolved” REE comprising the “free” aquo ions and aqueous chemical REE complexes. The sum of the latter two pools represents the “dissolved” REE, the concentrations of which are usually determined in river and seawater studies. Although the geogenic and the anthropogenic characteristics of the REE distribution in river water have been the focus of numerous studies (e.g., Elderfield et al., 1990, Sholkovitz, 1995, Gaillardet et al., 2003, Kulaksiz and Bau, and references therein) and the importance of organic complexation is well recognized (e.g., Tang and Johannesson, 2003, Pourret et al., 2007), rather little is known of the effects of specific organic chelators on REE mobilization from lithic particulates. Similar to other major and trace elements/nutrients and their isotopes (e.g., Jones et al., 2012a, Jones et al., 2012b), mobilization processes from the particulate into the dissolved REE pool significantly impact the concentrations and distribution of dissolved REE, and hence of the isotopic composition of Nd in river water, in the riverine REE input into the oceans, and in seawater itself, where REE distribution and Nd isotopes are frequently used as (paleo)proxies. However, some details of the (bio)geochemical behaviour of the REE are still only poorly constrained, such as the “missing Nd flux” to seawater and the “Nd paradox” (i.e. the decoupling of concentration and isotopic composition of Nd in seawater), for example (e.g., Tachikawa et al., 1999; Lacan and Jeandel, 2005, Arsouze et al., 2009). Moreover, because microbes appear to play an important role in subglacial weathering (Montross et al., 2012), biogenic organic compounds may also have a substantial impact on solute concentrations and distribution in (sub)glacial meltwaters which feed sub- and proglacial lakes and rivers that both are typically rich in rock flour, i.e. silt-sized lithic particles. Widespread melting of glaciated areas during deglaciation periods in the past or as a possible result of global warming in the future suggest that the impact of biogenic chelators on the REE systematics of glacial meltwaters (and hence on seawater) could also be profound.
Explosive volcanic eruptions may result in the input of micro- to nanometer-sized volcanic ash particles into rivers, lakes and oceans where they represent an important source of nutrients (e.g., Frogner et al., 2001, Duggen et al., 2010, Olgun et al., 2011, Ayris and Delmelle, 2012; and references therein), such as iron and phosphorus. The explosive 2010 eruption of Eyjafjallajökull volcano in southern Iceland, for example, produced large amounts of glass-rich volcanic ash of predominantly andesitic composition (Gislason et al., 2011, Sigmarrson et al., 2011). After the eruption, rivers in southern Iceland (Fig. 1) close to Eyjafjallajökull, such as the Markarfljöt River, carried large amounts of this volcanic ash to the sea, whereas more distal rivers transported ash that had been produced during earlier volcanic events in southern Iceland. These river waters represent natural suspensions of volcanic ash and freshwater that are well-suited for geochemical studies.
Although the deposition of volcanic ash into seawater, for example, can promote marine bioproductivity (Frogner et al., 2001, Duggen et al., 2010, Hamme et al., 2010, Langmann et al., 2010, Olgun et al., 2011), the bioavailability of iron from ash particles that dissolve under aerobic conditions in surface waters is limited due to the rapid formation of Fe(III) oxyhydroxides (e.g., Ayris and Delmelle, 2012). To mitigate iron deficiency, terrestrial and marine organisms and plants may produce siderophores which are biogenic chelators that effectively mobilize ferric iron and increase iron bioavailability even in oxic environments (e.g., Kraemer, 2004, Baker and Croot, 2010; and references therein). However, siderophores do not only form strong chemical complexes with iron, but also with a range of other polyvalent metal ions (e.g., Harrington et al., 2011; and references therein) including the REE, and therefore affect the mobilization and scavenging behaviour of these elements (Yoshida et al., 2004a, Yoshida et al., 2004b, Brantley et al., 2001, Kraemer et al., 2002, Tanaka et al., 2010, Christenson and Schijf, 2011).
Hence, we performed incubation experiments (20 °C, 72 h, pH range: 7.04–7.68) to investigate the impact of the well-characterized biogenic siderophore desferrioxamin-B (DFOB) on the mobilization of REE from volcanic ash particles (particulate REE) into river water (dissolved REE) from southern Iceland. Our results suggest that in the presence of siderophores release of iron and REE from ash particles is significantly enhanced and that the mobilized REE fraction is characterized by specific chondrite-normalized (subscript CN, chondrite from Anders and Grevesse (1989)) distribution patterns with light REE (LREE) depletion, convex shape between La and Sm, and positive Ce anomaly.
Section snippets
Samples
Water samples used in the experiments had been taken in acid-cleaned PE bottles from five glacial meltwater-dominated rivers in southern Iceland (Fig. 1) in September of 2010, i.e. about five months after the eruption of Eyjafjallajökull volcano had covered parts of southern Iceland with fine grained volcanic ash of mostly intermediate chemical composition (Gislason et al., 2011, Sigmarrson et al., 2011). From west to east we studied samples from the following rivers: Markarfljöt, Jökulsá í
Particulate REE
Although the filter residues (i.e. the >0.2 μm-sized particulate REE pool) from the river waters we studied are almost exclusively composed of volcanic ash particles, the general REE distribution of the particulates (Fig. 4) from rivers close to Eyjafjallajökull (Markarfljöt, Jökulsá í Solheima) is rather similar to that of the more distal rivers (Gigjukvísl, Hornafjardarfljöt, Jökulsá í Loni) which where rather unaffected by ash from this volcano (Online Supplementary Table S2). Only the
Discussion
The increase of iron and REE concentrations in river waters upon incubation with DFOB indicates that the siderophore mobilized iron and REE from the ash particles and caused their partitioning from the particulate into the dissolved pool. Normalizing the dissolved REE concentrations of a sample to those of its respective particulate REE pool (Fig. 6a,b) reveals the severe fractionation within the REE group during this siderophore-promoted mobilization. The strong difference between the REE
Conclusions
The results of our experiments demonstrate that strong biogenic chelators, such as siderophores, not only enhance the (bio)availability of iron, but also have the potential to significantly affect the concentrations and distribution of the REE in natural waters. They promote the oxidation of Ce and the dissolution of lithic particles, and induce incongruent release of REE. This may result in a siderophore-bound dissolved REE flux into glacial meltwater, river water and seawater, that shows a
Acknowledgements
We appreciate the help of D. Meissner and J. Mawick in the Geochemistry Lab at Jacobs University Bremen. The manuscript benefited from comments of EPSL reviewer M. Jones, University of Iceland, of an anonymous reviewer, and of EPSL editor G. Henderson. This study was performed within the framework of DFG grant BA 2289/5-1 to M.B.
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