Elsevier

Geochimica et Cosmochimica Acta

Volume 169, 15 November 2015, Pages 82-98
Geochimica et Cosmochimica Acta

Ni cycling in mangrove sediments from New Caledonia

https://doi.org/10.1016/j.gca.2015.07.024Get rights and content

Abstract

Covering more than 70% of tropical and subtropical coastlines, mangrove intertidal forests are well known to accumulate potentially toxic trace metals in their sediments, and thus are generally considered to play a protective role in marine and lagoon ecosystems. However, the chemical forms of these trace metals in mangrove sediments are still not well known, even though their molecular-level speciation controls their long-term behavior. Here we report the vertical and lateral changes in the chemical forms of nickel, which accumulates massively in mangrove sediments downstream from lateritized ultramafic deposits from New Caledonia, where one of nature’s largest accumulations of nickel occurs. To accomplish this we used Ni K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data in combination with microscale chemical analyses using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDXS). After Principal Component and Target Transform analyses (PCA-TT), the EXAFS data of the mangrove sediments were reliably least-squares fitted by linear combination of 3-components chosen from a large model compound spectral database including synthetic and natural Ni-bearing sulfides, clay minerals, oxyhydroxides, and organic complexes. Our results show that in the inland salt flat Ni is hosted in minerals inherited from the eroded lateritic materials, i.e. Ni-poor serpentine (44–58%), Ni-rich talc (20–31%), and Ni-goethite (18–24%). In contrast, in the hydromorphic sediments beneath the vegetated Avicennia and Rhizophora stands, a large fraction of Ni is partly redistributed into a neoformed smectite pool (20–69% of Ni-montmorillonite), and Ni speciation significantly changes with depth in the sediment. Indeed, Ni-rich talc (25–56%) and Ni-goethite (15–23%) disappear below ∼15 cm depth in the sediment and are replaced by Ni-sorbed pyrite (23–52%) in redox-active intermediate depth layers and by pyrite (34–55%) in the deepest sediment layers. Ni-incorporation in pyrite is especially observed beneath an inland Avicennia stand where anoxic conditions are dominant. In contrast, beneath a Rhizophora stand closer to the ocean, where the redox cycle is intensified due to the tide cycle, partial re-oxidation of Ni-bearing pyrites favors nickel mobility, as confirmed by Ni-mass balance estimates and by higher Ni concentration in the pore waters. These findings have important environmental implications for better evaluating the protective role of mangroves against trace metal dispersion into marine ecosystems. They may also help in predicting the response of mangrove ecosystems to increasing anthropogenic pressure on coastal areas.

Introduction

Mangrove intertidal forests cover more than 70% of tropical and subtropical coastlines and play a major role in the ecological balance of these areas (Valiela et al., 2001, Duke et al., 2007, Nagelkerken et al., 2008). They receive inputs from natural or anthropogenically impacted watersheds and may act as filters between land and sea for various metallic contaminants (Harbison, 1986, Lacerda et al., 1993, Tam and Wong, 1995, Tam and Wong, 2000). As a result, and due to their potential toxicity to living organisms (Di Toro et al., 1992), the distribution of trace metals in mangrove ecosystems has received significant attention during the last decade (Otero et al., 2009, Lewis et al., 2011, Bayen, 2012, Nath et al., 2013). This is especially true in New Caledonia where mangroves act as a buffer zone between massive lateritic deposits enriched in transition metals such as Fe, Cr, Ni, Co, and Mn (Becquer et al., 2001, Quantin et al., 2001, Dublet et al., 2012), and a lagoon of over 20,000 km2, recently registered as an UNESCO World Heritage site.

Mangrove sediments are known to have a large capacity to accumulate metallic elements (Tam and Wong, 2000, Qiu et al., 2011, Marchand et al., 2012), which is partly attributed to their large content of organic matter that can act as a trace metal complexing agent (Nissenbaum and Swaine, 1976). In addition, mangrove sediments are subjected to sulfate-reduction processes (Howarth and Merkel, 1984, King, 1988), which lead to precipitation of sulfide minerals that are able to trap chalcophile elements as well as Co and Ni in sulfidic coastal or marine sediments (Huerta-Diaz and Morse, 1992, Morse and Luther, 1999, Burton et al., 2006, Burton et al., 2008). In mangrove sediments, trace metal trapping processes are, however, expected to depend on biogeochemical gradients in salinity and in redox and organic matter contents that are driven by the distance to the seafront, the magnitude of the tide, and the types of mangrove tree species (Otero et al., 2009, Marchand et al., 2011, Marchand et al., 2012). In addition, oxic and anoxic diagenetic reactions can operate in coastal sediments as a function of time, due to tide cycles (Otero et al., 2009), seawater flooding (Johnston et al., 2010, Wong et al., 2010, Wong et al., 2013), as well as freshwater inputs (Burton et al., 2008). Complex redox cycling in mangrove sediments may thus significantly impact the speciation of metallic elements across the intertidal zone (Clark et al., 1998, Tam and Wong, 2000, Marchand et al., 2006a, Marchand et al., 2012, Otero et al., 2009). Such biogeochemical cycles were studied in the mangrove sediments of the Island of Pai Matos (Cananeia — SP, Brazil), where Fe speciation, determined from sequential chemical extactions, changes significantly as a function of the redox boundaries delineating intertidal and depth zonations (Otero et al., 2009). Similarly, Noël et al. (2014) determined iron speciation in mangrove sediments from New Caledonia using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and confirmed that the major variations in the distribution of Fe-oxyhydroxides, pyrite and iron-bearing clays, rely on the redox gradients generated by both the topographic position across the intertidal zone and the depth within the sediments. Compared to that of iron, the speciation of nickel in mangrove sediments has been less studied. Chemical extraction studies have however reported the potential role of sulfides, oxyhydroxides and carbonate minerals as well as of organic matter in nickel trapping (Clark et al., 1998, Marchand et al., 2012).

The objective of the present study was to improve our understanding of the biogeochemistry of nickel in mangrove sediments as a function of chemical gradients and of vegetal cover changes across the intertidal zone. For this purpose, we chose mangrove sediments that exhibit exceptional nickel content downstream from lateritized ultramafic ore deposits from New Caledonia. Complemented by X-ray Diffraction (XRD) and Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDXS) analyses, EXAFS spectroscopy was used for this purpose because it has been shown to be suitable for quantifying the variety of Ni-species in lateritic materials that are the source of nickel in the mangrove sediments in New Caledonia (Dublet et al., 2012, Dublet et al., 2014). The spatial distribution of nickel species was quantified in three cores drilled beneath typical mangrove tree covers (Rhizophora spp., Avicennia marina, and salt flat) in order to evaluate the long-term influence of the redox gradients on the speciation and mobility of Ni in sediments of these fragile ecosystems.

Section snippets

Sampling site and sampling procedures

The site we chose for study is typical of the topographic, vegetal and redox gradients of New Caledonian mangroves. It is located in Vavouto Bay (20°59′S; 164°49′E) at the mouth of the Talea-Coco River that drains the ultramafic Koniambo regolith (Noël et al., 2014). It stands on the west coast of the main New Caledonia island (20°S–23°S), which is characterized by a semi-arid climate (∼750 mm of rain/year; Maitrepierre, 2004). As in many mangrove ecosystems, the intertidal zone in New Caledonia

Mineralogy of the mangrove sediments

Located upstream of the mangrove forest studied, the Koniambo regolith consists of deeply weathered ultramafic rocks and thus hosts high concentrations of Fe, and to a lesser extent, Ni, Co, Cr, and Mn (Becquer et al., 2001, Quantin et al., 2001, Fandeur et al., 2009, Dublet et al., 2012, Dublet et al., 2015). It is composed of an upper laterite unit consisting of goethite with minor amounts of hematite and Mn-oxides, and of a deeper saprolite unit consisting of serpentine and talc, with minor

Evolution of the Ni-speciation along the major redox gradients

The present study shows that nickel in the salt flat sediments as well as in the upper horizon beneath the Avicennia and Rhizophora stands is hosted primarily by phyllosilicate minerals and goethite, which have been shown by Dublet et al. (2012) to be major Ni species in the Koniambo regolith, located upstream of the mangrove forest. This result confirms that a major Ni input to the mangrove sediments is related to erosion of the lateritized ultramafic outcrops. Beyond these

Acknowledgments

This study was performed in the framework of a UPMC Paris 6/CNRS (IMPMC)/IRD – Koniambo Nickel SAS (KNS) joint project. Field sampling campaigns were greatly facilitated by Sylvain Capo (KNS-GLENCORE), Andy Wright (KNS), Gregory Marakovic (KNS), and Jacques Loquet (Koniambo Environmental Committee). The authors thank Imene Esteve (IMPMC) for her help during Scanning Electron Microscope observations and analyses and Ludovic Delbes (IMPMC) for carrying out the X-ray diffraction experiments under

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