Chronology and sources of trace elements accumulation in the Rhône pro-delta sediments (Northwestern Mediterranean) during the last 400 years
Introduction
As stated by the international Geotraces program (www.geotraces.org), many trace elements (TEs) are critical for marine life, and therefore influence the functioning of ocean ecosystems and the global carbon cycle. Indeed, they are micronutrients essential to life (e.g., Fe, Zn, Cu), tracers of modern processes in the ocean (e.g., Al, Mn), and are significantly perturbed by human activities (e.g., Pb). In the Western Mediterranean, the biogeochemical cycling of TEs is largely governed by atmospheric inputs (Migon et al., 2002). However, the influence of rivers on TE distributions is discernible in the sediments of the adjacent continental shelf (Ferrand et al., 1999, Roussiez et al., 2005, Roussiez et al., 2006, Radakovitch et al., 2008). According to the Mermex Group review (The Mermex Group, 2011), present and past inputs of riverine particulate TEs to the Mediterranean, especially anthropogenic TEs, need to be more precisely estimated.
Estuary and delta sediments are suitable environments to reconstruct historical accumulation of elements and anthropogenic chemicals from continental and marine sources during last centuries or even millennia (e.g., Valette-Silver, 1993, Santschi et al., 2001, Callender, 2003). In particular, this is the case for the Rhône prodeltaic prism, which receives particulate inputs from the Rhône River (RR), and has archived its inputs for centuries (Fanget et al., 2013). Studying this area provides a unique opportunity to document the chronology and the sources of the TE inputs from the largest riverine sediment source to the Mediterranean Sea.
Here, we document the temporal accumulation of TEs (Ag, Cd, Co, Cr, Cu, Ni, Pb and its stable isotopes, and Zn) in sediment recovered in the Roustan lobe within the Rhône prodelta area, located on the continental shelf of the North side of the Western Mediterranean (Fig. 1). Based on a multiproxy study, including major elements and radiometric analyses, we have been able to identify the major changes in TE accumulation in the Rhône prodelta sediments during the last four centuries. These changes are discussed in terms of hydrological regime, channelization of the RR mouth, climate change, and TE contamination history.
Section snippets
Study area
The RR and its tributaries drain various catchments including sedimentary rocks, granitic, and metamorphic rocks from the Alps, Massif Central, and Vosges. In addition, RR carries alluvium from one of the most industrialized and urbanized European catchments (The Mermex Group, 2011). The RR sediment load has varied significantly during the past few centuries Since the XIXth Century, it has decreased by a factor close to ten, mainly because of stabilization of mountain slopes due to decline of
Core collection
This study is based on a multi-proxy analysis of a 7.71 m-long piston core (KS57, Fig. 1) collected on the Roustan prodeltaic lobe (43°17.10′N; 4°50.97′E) at 79 m water depth during the Rhosos cruise in September 2008 aboard R/V Le Suroît. The core was sliced every 2 cm from 2 cm below the sediment-water interface (SWI) to 100 cm, then every 4 cm down to the bottom of the core, which was 767 cm below the SWI. Subsamples were frozen (−18 °C), freeze-dried and stored under cold (4 °C) and dark conditions
Characterization of sediment core
The dating of core has been established based on 210Pb, 137Cs vertical profiles and microfossil assemblages (ostracods, benthic foraminifera, and coccoliths) in combination with sedimentological proxies (Fanget et al., 2013). Herein, we present our geochemical results based on this well-constrained chronology. This 7.67 m core covers a period of just over 400 years. The first 3.75 m below the SWI has accumulated since the middle of the XIXth Century, corresponding roughly to the beginning of the
Summary and conclusions: The Rhône prodelta anthropization in the context of Mediterranean Sea
Trace element concentrations in surface coastal sediments are largely documented in the Western Mediterranean and the Eastern Mediterranean (e.g., Scoullos and Oldfield, 1986, Abdel-Moati and El-Sammak, 1997, Abi-Ghanem et al., 2009, Abi-Ghanem et al., 2010, Schintu et al., 2009, Schintu et al., 2015, Mzoughi and Chouba, 2011, Roussiez et al., 2011). Concentrations range between geological backgrounds, with the example of Sardinia coasts (Schintu et al., 2009), and very high concentrations. In
Acknowledgements
We thank the scientific team as well as the captain and crew of the cruise RHOSOS-2008 cruise on board R/V Le Suroît. This study was carried out as a part of the WP3 Mermex/Mistrals and is a contribution to the international LOICZ projects. This work was partly supported by the European project Hermione (FP7-ENV-2008-1-226354) and ANR (France) EXTREMA (contract number ANR-06-VULN-005). We are grateful to O. Radakovich for his comments on the manuscript.
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Present address: Aarhus University, Department of Geoscience, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark.