Reconstruction of centennial-scale fluxes of chemical elements in the Australian coastal environment using seagrass archives
Graphical abstract
Introduction
The natural fluxes of chemical elements to coastal ecosystems (e.g. by volcanic eruptions, aeolian dust deposition, delivery of suspended solids and chemical species in solution by rivers, etc.) are now supplemented by anthropogenic activities (e.g. mining, fossil fuel burning, industrial activities, coastal development and agriculture). Some metals play a key role in biological processes in living organisms (Zn, Fe, Co, Mn, Cu and Ni), but others such as Al, As, Pb, Cr and Cd are toxic non-essential elements (e.g. Shanker et al., 2005). Some metals, such as Pb, are considered harmful pollutants in trace amounts in aquatic ecosystems because of their toxicity and persistence in the environment (Schüürmann and Markert, 1998).
While anthropogenic contamination began with the domestication of fire, the intensity of metal emissions accelerated during and after the industrial revolution due to, for instance, increased mining, metallurgy and coal combustion, with anthropogenic sources now dominating the global cycling of metals (Nriagu, 1979, Nriagu, 1996). In Europe and Asia, contamination from mining and metallurgical activities has a long history dating back, at least, a few millennia ago (e.g. Davis et al., 2000, Lee et al., 2008), but this may not be the case for Australia where aboriginal hunter-gatherers were abruptly replaced by an industrialized society only recently (since the 19th century; Coghlan, 2011). Australia has widespread metal and coal reserves, which have been extensively mined since European settlement (first settlement in 1788; Coghlan, 2011) to become one of the largest suppliers of metals worldwide (Martin et al., 1993, Mudd, 2007). Following European settlement, mining, smelting, coal combustion and agriculture (including land clearance and use of fertilizers) have led to large emissions of Cr, Pb, Ni, Zn and Cd to the environment (Marx et al., 2010, Morelli et al., 2012). Therefore, it is expected that European settlement and industrialization of Australia led to an increase in the inputs of metals and other chemical elements to coastal ecosystems. Testing this hypothesis requires appropriate records.
There is broad agreement that natural archives are useful in reconstructing fluxes and trends of chemical elements at local, regional, and even global scales (Biester et al., 2007). Previous studies have focused on estimating the natural and anthropogenic contribution of past metal inputs based on the analysis of natural environmental archives such as cores from bogs (e.g. Shotyk et al., 1996, Martínez et al., 1999), lake sediments (e.g. Renberg et al., 1994, Smol, 2009), glacial ice (e.g. Boutron et al., 1994, Schuster et al., 2002), and river and marine sediments (e.g. Valette-Silver, 1993). Although most of the studies reconstructing metal fluxes in continental and coastal areas are concentrated in the northern hemisphere (e.g. Martin and Richardson, 1991), there are several investigations focusing on metal fluxes in the southern hemisphere (e.g. Wolff et al., 1999), including Australia (e.g. Cox and Preda, 2005, Marx et al., 2010, Morelli et al., 2012). To our best knowledge, however, all previous reconstructions of metal fluxes in Australia focused on the east and southern regions, while the vast region of Western Australia remains unexplored.
There is no clear evidence supporting a long-scale dispersion of Northern Hemisphere pollution across the intertropical convergence zone to the Southern Hemisphere (Duce et al., 1975), nor via oceanic circulation pathways (Staudt et al., 2001, Wagener et al., 2008). As a result, metal contamination in the Southern Hemisphere may have started by AD ~ 1500 in South America (e.g. Miller et al., 2002). Within Australia, local and regional development (mining, metal and coal production, and agriculture) may constitute the major sources of chemical elements to the ocean, while run-off and atmospheric soil dust transport may constitute the major dispersion pathways (e.g. Guieu et al., 1991).
The use of marine sediment cores to reconstruct historical trends in contamination is widespread. However, the complex nature of sedimentary processes (e.g. erosion and bioturbation) and the low resolution of most marine records in coastal areas often preclude detailed studies over Holocene timescales. Chemical elements tend to concentrate in coastal vegetated ecosystems such as mangroves, salt marshes and seagrasses (e.g. Weis and Weis, 2004) due to the bioaccumulation capacity of living organisms and the larger sedimentation rates in the habitats they form, which makes them potentially useful to attempt reconstruction of historical trends in contamination (Serrano et al., 2011, Serrano et al., 2013). Seagrasses of the genus Posidonia, which are widespread in the Mediterranean and Australian coasts, have been found to form reliable sedimentary archives of the past history of the marine and terrestrial environment over the late Holocene (López-Sáez et al., 2009, Mateo et al., 2010, Serrano et al., 2011, Serrano et al., 2013). Posidonia meadows support significant carbon accumulation rates (Lavery et al., 2013, Serrano et al., 2014), because they accrete mats composed of a mixture of roots and rhizomes binding sediments that, apart from their importance as biogeochemical sinks (Fourqurean et al., 2012), can be used as archives of long-term environmental data (Serrano et al., 2012). Heavy metal concentrations in Posidonia tissues and seagrass mats provide convenient proxies for variations in metal fluxes to the sea ranging from decades (ca. 30 years; e.g. Pergent-Martini and Pergent, 2000) to millennia (ca. 5000 years; Serrano et al., 2011, Serrano et al., 2013). The peculiarities of seagrass meadows may have contributed to the enhancement of chemical elements in the mat: i) enhancement of in-situ settlement and burial of seston particles and fine-grained sediments (Madsen et al., 2001, Gacia and Duarte, 2001, De Boer, 2007); ii) higher stabilization of the sediment compared to bare sediments (e.g. Gacia and Duarte, 2001); iii) high bioaccumulation capacity (Pergent-Martini, 1998); and iv) high amounts of organic matter preserved in the mat (Fourqurean et al., 2012). Redox-sensitive processes occurring in the oxic-anoxic interface in estuarine sediments, enhanced by the oxidizing conditions predominant at the plants' rhizosphere, can affect metal behavior after burial (Du Laing et al., 2009). However, previous studies have shown that redox potential in seagrass mats are stable (highly anoxic) below 5 cm depth (e.g. Mateo et al. 2006), which could reduce the impact of diagenetic processes in the contamination signal recorded in the sediment. Indeed, some seagrass meadows are always submerged (i.e. not subjected to fluctuating water levels) and thus reducing metal mobility.
Here we reconstruct element fluxes over the last 3000 years in Oyster Harbor (Albany, SW Australia) using the sedimentary archive beneath the Posidonia australis meadows. We also compare the P. australis record of metal contamination with other records of past deposition of chemical elements in the Southern Hemisphere and Europe and demonstrate that contamination in Australia is most probably related to recent historic land use transformations and industrial development associated with European colonization.
Section snippets
Study site
Oyster Harbor is a large (16 km2), marine-dominated estuary located on the south coast of Western Australia, near Albany (Appendix A). The harbor is permanently connected to the open ocean through a 200 m-wide channel and is primarily saline but receives freshwater and land-derived organic-rich siliceous sediments from the King and Kalgan rivers. The two rivers carry materials from a rural catchment (3041 km2) into Oyster Harbor during winter, when high rainfall can cause flooding (D'Adamo, 1991).
Results
The mat beneath P. australis meadows at Oyster Harbor is mainly composed of siliciclastic (~ 64%) and biogenic carbonate (~ 26%) sediments, and to a lesser extent of organic matter (~ 10%; Fig. 2 and Table 1). Dry bulk density values ranged from 0.29 to 0.65 g cm− 3, while the organic matter and carbonate contents ranged from 6 to 18% and 14 to 34%, respectively. The P. australis mat deposit is relatively homogenous in density, organic matter and carbonate contents, with only slight variations with
Discussion
The analysis of the P. australis core reported here provided a record of the concentrations and fluxes of elements throughout the last ~ 3000 years in Oyster Harbor. The concentration records and the PCA results suggest that contamination in Oyster Harbor began ~ 100 cal years BP (AD 1900), likely related to local and regional development (mining, metal and coal production, and agriculture). The abrupt increase in Al, Fe, Mn, Cr, Zn, Cd, Pb and Co concentrations since ~ 50 cal years BP signals a recent
Conclusions
The study of a P. australis sedimentary archive has provided a record of changes in element concentrations over the last 3000 cal years BP in the Australian marine environment. Contamination in Oyster Harbor began ~ 100 cal years BP (AD 1900) and exponentially increased until present, likely related to European colonization of Australia and the subsequent impact of local and regional development (mining, metal and coal production, and agriculture). Although sediments beneath seagrass meadows of the
Acknowledgements
This work was supported by the ECU Faculty Research Grant Scheme, the ECU Early Career Research Grant Scheme, and the CSIRO Flagship Marine & Coastal Carbon Biogeochemical Cluster with funding from the CSIRO Flagship Collaboration Fund. PM was supported by a Gledden Visiting Fellowship and AAO by Obra Social “la Caixa”. The authors are grateful to N. Marbà, G. Bastyan and D. Kyrwood for their help in field and/or laboratory tasks, as well as to L. López-Merino and four anonymous reviewers for
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