Quaternary sediment geochemistry as a proxy for toxic element source: A case study of arsenic in the Pecora Valley (southern Tuscany, Italy) P. Costagliola a, ⁎, M.M. Benvenuti a , M.G. Benvenuti a , F. Di Benedetto a , P. Lattanzi b a Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira, 4-50121 Firenze, Italy b Dipartimento di Scienze della Terra, Università di Cagliari, via Trentino 51-09127 Cagliari, Italy abstract article info Article history: Received 3 August 2009 Received in revised form 6 November 2009 Accepted 11 November 2009 Editor: J.D. Blum Keywords: Arsenic Quaternary sediments Geogenic anomaly Southern Tuscany The valley of the Pecora river (southern Tuscany, Italy) hosts a very large (∼ 25 km 2 ) arsenic anomaly, with rock values up to ∼ 1000 mg/kg. Its origin (natural or anthropogenic) is still debated. A systematic study of a well developed Neogene–Quaternary sequence in the middle Pecora valley (MPV) reveals that each formation of the sequence has a distinctive, definite range of arsenic contents. Specifically, all but one Upper Messinian sands and gravels (M2 formation) show 4–39 mg/kg As (mean 20, SD 12); the overlying Quaternary alluvial deposits (P1) have the highest contents (142–1157 mg/kg, mean 647, SD 288); and the topmost calcareous tufa (P2) show intermediate contents (83–243 mg/kg, mean 159, SD 50, except for a single, silica-rich sample having 420 mg/kg). This distribution cannot be explained by any anthropogenic practice, and points to a geogenic origin of the anomaly. We suggest that the arsenic anomaly of MPV is basically the result of erosion and fluvial transport processes of country rocks hosting vein mineral deposits. This hypothesis is fully consistent with a recent reconstruction of the paleogeographic evolution of the Pecora river catchment. Moreover, a recent study pointed out a similar anomaly in the neighboring Val di Cornia; thus the MPV anomaly could be just a part of a larger system. An implication of this study is that mobilization of arsenic in shallow systems may be related to gravitational transport of clastic sediments, causing a memory effect that may be preserved for relatively long times. This process of mobilization may involve very large rock volumes, resulting in regional scale anomaly. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The problem of quantifying human interferences with global element cycles has been recently stressed by many authors (e.g., Reimann and de Caritat, 2000, 2005; and references therein). In highly inhabited regions it may be difficult to find truly pristine ecosystems that can provide reference environmental parameters (Zglobicki and Rodzik, 2007). This problem is particularly relevant in mineralized areas, where waters, soils and rocks can display high natural levels of toxic elements, while human activity usually contributes to extend these geochemical anomalies. The arsenic (As) anomaly of Pecora River Valley (PRV) in southern Tuscany, Italy (Figs. 1 and 2), is a typical example of a situation where natural (“geogenic”) and anthropogenic components are difficult to discriminate. In the present contribution, we suggest that a clue on the origin of the anomaly may come from the study of the Quaternary sedimentary succession cropping out in the PRV. The geochemical study of Quaternary sediments is a commonly applied tool for reconstruction of recent climate variation and landscape evolution (e.g., Toyoda and Shinozuka, 2004; McGowan et al., 2008; and references therein), but there are relatively few studies where the composition of Quaternary (continental) sediments was used to infer the features of pristine ecosystems in terms of heavy metal contents (e.g., Martin, 2004). The Quaternary sedimentary succession of PRV is located in a belt comprised between the mining district of Colline Metallifere and the seacoast (Fig. 1). These sediments were deposited in a continental environment after the emplacement of mineral deposits, but prior to any industrial activity of this area. Hence, they could keep the record of any natural transport of As, and may serve as a reference background to discriminate “geogenic” from anthropogenic contributions. 2. Geological and environmental setting The PRV belongs to the geologic district of Southern Tuscany, which is a part of the Northern Apennines tectonic chain, consisting of several stacked units (from structural bottom to top: Palaeozoic basement, Ligurian Unit, Tuscan Unit; e.g., Bonini and Sani, 2002). Since Late Miocene, the area was the seat of emplacement of magmatic rocks (Innocenti et al., 1992; Serri et al., 1993, 2001; Fig.1). Magmatic activity triggered an intense hydrothermal Chemical Geology 270 (2010) 80–89 ⁎ Corresponding author. Tel.: +39 055 27 57 476; fax: +39 055 28 45 71. E-mail address: pilario.costagliola@unifi.it (P. Costagliola). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.11.007 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo