Submarine groundwater discharge: A significant source of dissolved trace metals to the North Western Mediterranean Sea
Graphical abstract
Introduction
The availability of bioactive trace metals (e.g. Fe, Mn, Co, Ni, Cu, Zn and Cd) plays an important role in supporting primary productivity in the oceans (Bruland et al., 1991, Morel and Price, 2003, Sunda, 2012). Understanding the biogeochemical cycling of these micronutrients requires a detailed knowledge of their diverse sources and sinks. The main continental sources of trace metals to the ocean are riverine discharge (e.g. Bewers and Yeats, 1989, Jeandel and Oelkers, 2015, Martin and Whitfield, 1983, Oelkers et al., 2011), atmospheric deposition (e.g. Bowie et al., 2002, Duce et al., 1991, Jickells, 1995, Mackey et al., 2012, Mahowald et al., 2005), benthic fluxes from sediments (e.g. Elrod et al., 2004, Jeandel et al., 2011) and submarine groundwater discharge (SGD), although the latter has received attention only recently (e.g. Beck et al., 2007, Beck et al., 2009, Charette and Buesseler, 2004, Windom et al., 2006). SGD includes both fresh meteoric groundwater and recirculated seawater through permeable sediments of the coastal aquifer. Indeed, the mixing interface between fresh and salty water is a chemical reaction zone called the subterranean estuary, where groundwater can become enriched or depleted in chemical compounds before discharging into the sea (Moore, 1999). The chemical composition of SGD is influenced by several factors, such as the geological matrix and the geochemical conditions of the coastal aquifers (Charette et al., 2005, Mcallister et al., 2015, Santos et al., 2012), the potential impact of human activities (Beck et al., 2009, De Sieyes et al., 2008, Trezzi et al., 2016) and the type of discharge (e.g. karstic or detrital) (Tovar-Sánchez et al., 2014b).
Globally SGD has been estimated to be 3 to 4 times higher than riverine discharge into the oceans (Kwon et al., 2014). In the Mediterranean Sea, a semi-enclosed oligotrophic basin, SGD is comparable or even larger (up to 16 times) than the riverine water flow (Rodellas et al., 2015a). In this basin, the role of SGD in biogeochemical cycles is also prominent since the estimated SGD-driven macronutrient fluxes (DIN, DIP and DSi) are comparable or even higher than the riverine and atmospheric fluxes (Rodellas et al., 2015a). The relevance of SGD in delivering dissolved trace metals (dTM) to the Mediterranean Sea has been highlighted in some local areas, characterized by the absence of riverine discharge (Rodellas et al., 2014, Tovar-Sánchez et al., 2014b, Trezzi et al., 2016).
The aim of this study is to quantify SGD-driven dTM fluxes at regional scale and evaluate their significance. We determine these fluxes in the North Western (NW) Mediterranean Sea and compare them to other external sources (i.e. riverine discharge and atmospheric deposition). This area was considered an appropriate study site for conducting such a comparison for three main reasons: 1) the existence of several studies reporting local estimations of SGD flows that can be used to obtain a reliable regional estimate of SGD-driven dTM fluxes (i.e., Baudron et al., 2015, Garcia-Solsona et al., 2010a, Garcia-Solsona et al., 2010b, Mejías et al., 2012, Ollivier et al., 2008, Rodellas et al., 2015b, Rodellas et al., 2014, Rodellas et al., 2012, Stieglitz et al., 2013, Tovar-Sánchez et al., 2014b, Trezzi et al., 2016); 2) the large riverine discharge in this area compared with most zones of the Mediterranean basin (Ludwig et al., 2009); 3) the influence of atmospheric deposition originating from Europe and the Saharan region (Guerzoni et al., 1999, Guieu et al., 1997).
Section snippets
The NW Mediterranean Sea
The Mediterranean Sea is a semi-enclosed basin connected to the Atlantic Ocean through the Strait of Gibraltar, characterized by a net export of nutrients to the Atlantic Ocean, that leads to oligotrophic conditions in the whole basin (Béthoux et al., 1998). Climate conditions of the Mediterranean Sea generally consist of dry summers and rainy autumns and winters, with larger mean annual precipitation in the north and western parts of the basin.
The study area comprises the NW Mediterranean zone
Metal characterization of SGD
SGD end-members spanned a wide range of salinities: karstic SGD had salinities ranging between 0.3 and 9.7, whereas detrital SGD samples were in the range of 0.2–36 (Table 1). The higher variability in salinity for detrital SGD with respect to karstic SGD is related to the intrusion of recirculated seawater through permeable beach sediments (Moore, 1999).
The variability of trace metal concentrations in SGD systems depends on the geological composition of the coastal aquifers, the possible
Conclusions
This study is a first effort to estimate regional inputs of dTM (Cd, Co, Cu, Fe, Ni, Pb and Zn) through SGD to the oligotrophic Mediterranean Sea. SGD-driven dTM fluxes, calculated for the NW basin and compared to riverine discharge and atmospheric deposition, reveal that SGD is a significant source of dTM to the coastal NW Mediterranean Sea. The relevance of SGD may be even higher in coastal areas of the Southern and Eastern Mediterranean Sea or in many islands, which are characterized by
Acknowledgments
This project has been partially funded by the European Union Seventh Framework Programme through the MetTrans Initial Training Network, Marie Curie Actions, which also provide financial support through a PhD fellowship to G.T. (EU FP7-People-2011-ITN-290336). Support provided by the Generalitat de Catalunya to MERS (2014 SGR – 1356) is also acknowledged. V.R. acknowledges financial support from the European Union's FP7 (Marie Curie Actions PCOFUND-GA-2013-609102), through the PRESTIGE programme
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