Isotopic fractionation of Cu in plants Charlotte Weinstein a , Frédéric Moynier a, ⁎, Kun Wang a , Randal Paniello a , Julien Foriel a , Jeff Catalano a , Sylvain Pichat b a Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St Louis, One Brookings Drive, St Louis, MO 63130, USA b Laboratoire de Sciences de la Terre, Ecole nomale superieure de Lyon, 46 allée d'Italie, 69007 Lyon, France abstract article info Article history: Received 3 January 2011 Received in revised form 11 May 2011 Accepted 16 May 2011 Available online 25 May 2011 Editor: J.D. Blum Keywords: Cu Isotopes Plants Soils Copper cycle Knowledge of the copper cycle in the plant–soil–water system is needed in order to better constrain proper plant micronutrient nutrition, control pollution, and determine sustainable soil management practices. Here, we report the Cu isotopic compositions of different components (seeds, germinated seeds, leaves, and stems) of the dicot, lentil (Lens culinaris), and of two monocots, Virginia wild rye (Elymus virginicus) and hairy-leaved sedge (Carex hirsutella). According to our data, the isotopic compositions of these plants are systematically enriched in the lighter isotope of Cu ( 63 Cu) in comparison to the soil in which they grow. Furthermore, different components within the plants themselves are isotopically fractionated. The shoots (stems, leaves and seeds) are systematically lighter than the germinated seeds of the plants and the Cu isotopic compositions of individual leaves correlate with their heights on the plant. These results are similar to what has been observed for Zn isotopes, which are assumed to be transported through plants by means of diffusion and kinetic fractionation across cell membranes. Because of this similarity, we suggest that the same transport mechanisms are also responsible for the observed isotopic fractionation of Cu. As a side-note, the Cu isotopic variations measured in plants are similar in magnitude to the differences previously measured in various soils, and therefore should not be neglected while interpreting the isotopic composition of soils. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Copper is a vital micronutrient in organisms, particularly plants, most notably because Cu participates in protein synthesis, membrane activities, and photosynthesis (Koch et al., 1997; Frausto da Silva and Williams, 2001). However, Cu may also be toxic at high concentra- tions (Shioi et al., 1978; Maksymiec, 1997; Manceau et al., 2008). Cu is often applied to soil and crops as fertilizer to ameliorate micronutrient deficiencies caused by prolonged, intensive agriculture (Karamanos et al., 2005). It is also often used as a fungicide, especially in vini- culture (Epstein and Bassein, 1998). Given this widespread use and the expansion of urban areas, pollution by Cu is an important issue that could affect plant growth and increase the concentration of heavy metals within the food chain. The theory behind mass-dependent stable isotopic fractionation was developed in two seminal papers (Bigeleisen and Mayer, 1947; Urey, 1947). The theory specifies that the fractionation between two isotopes is proportional to their relative difference in mass. Until recently, it was very difficult to measure the small natural variations in the stable isotopes of transition metals, and so the technique was applied almost exclusively to light elements (e.g. H, C, N, O). With the advent of high precision multi collection-inductively coupled plasma- mass spectrometry (MC-ICP-MS), precise measurement of “non- traditional” stable isotopes of alkali earth elements and transition metals became possible (for reviews, see Johnson, et al., 2004). Analyses of the ‘new’ stable isotope systems, notably those of Mg (Young and Galy, 2004), Cr (Ellis et al., 2002; Moynier et al., 2011a), Fe (Dauphas and Rouxel, 2006; Johnson et al., 2008), Ni (Moynier et al., 2007; Cameron et al., 2009), Cu (Maréchal et al., 1999; Albarede, 2004), and Zn (Albarede, 2004; Cloquet et al., 2008; Moynier et al., 2009a,b,2011b) are starting to provide novel insights into geochem- ical cycles and biological processes. A major preliminary conclusion of these early studies is that biological processes play an important role as one of the few types of processes that can fractionate transition metal stable isotopes (Albarede, 2004; Johnson et al., 2008). Recently, use of the Cu isotopic ratio has been shown to be a good method to trace long-term processes in soil that are difficult to quantify by other means (e.g. analysis of element transport and variations in redox conditions) (Bigalke et al., 2010a, b). However, the influence of plant-induced isotopic fractionation on soil composition has not yet been quantified, significantly limiting the practicality of this technique (Bigalke et al., 2010a,b). The main known processes that can induce Cu isotopic fractionation in soils are 1) adsorption onto mineral surfaces and organic matter (Pokrovsky et al., 2008; Navarette et al. 2011), 2) inorganic and organic complexation to some Chemical Geology 286 (2011) 266–271 ⁎ Corresponding author. E-mail address: moynier@levee.wustl.edu (F. Moynier). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.05.010 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo