Inter- and intra-annual variations of Pb/Ca ratios in clam shells (Mercenaria mercenaria): A record of anthropogenic lead pollution?
Introduction
Coastal and estuarine environments are important natural resources supporting recreational activities and commercial fishing as well as providing a host of ecological services. The pollution of these regions can have serious adverse effects and thus has been closely monitored in the past several decades. The Mussel Watch program, where soft tissues of bivalves have been used to monitor pollution in the coastal zone (e.g., Goldberg, 1975, Claisse, 1989), has been monumental in this regard. Nevertheless, pre-1970 data are scarce (Cantillo, 1998) and data are limited to certain estuaries. For example, there are currently only seven Mussel Watch sites along the entire North Carolina (USA) coast (Lauenstein et al., 2002). Although new Mussel Watch sites could be started in other estuaries, it would require several years of monitoring to determine temporal pollution trends and would not allow reconstruction of past Pb concentration levels. There are other substrates that can retrospectively extend the record back through time and into other locations, which would not require extensive monitoring, such as sediments (e.g., Chillrud et al., 2003, Cooper et al., 2004, Kim et al., 2004), tree rings (e.g., Watmough et al., 1999) and biogenic carbonates (e.g., Shen and Boyle, 1987, Pitts and Wallace, 1994, Lazareth et al., 2000); each with its own advantages and drawbacks. For instance, sediments may be bioturbated and often provide low resolution profiles (e.g., Sharma et al., 1987, Cooper et al., 2004). On the other hand, biogenic carbonates can provide high-resolution profiles and once incorporated the proxy remains more or less stable as long as diagenetic processes do not occur. However, the biology of the animal may affect the record (Vander Putten et al., 2000). Both corals and sclerosponges have been shown to accurately trace anthropogenic Pb inputs in tropical and subtropical waters (Shen and Boyle, 1987, Lazareth et al., 2000, Swart et al., 2002, Ramos et al., 2004), but long term chronologies (>50 years) based on bivalve shells have not been attempted.
Similar to sclerosponges, bivalve carbonate may be a superior recorder of Pb because bivalves accumulate higher Pb concentrations in their skeletons. Sclerosponge skeletons contain 10–35 times more Pb than corals (based on the 1970’s Pb peak; Shen and Boyle, 1987, Lazareth et al., 2000, Swart et al., 2002). Bivalve shell Pb/Ca ratios from polluted sites have been reported to be higher than 7 μmol/mol (Price and Pearce, 1997), whereas corals from polluted sites can have Pb/Ca ratios reaching only 0.23 μmol/mol (Fallon et al., 2002).
There have been many studies on trace metal concentrations in bivalve shells. However, many of these studies did not include Pb due to its low levels (Szefer et al., 2002, Nicholson and Szefer, 2003, Cravo et al., 2004). Of the studies that did measure Pb, many analyzed whole shells (Koide et al., 1982, Yap et al., 2003), thus averaging several years of shell growth and including the outer layer of the shell which may exchange with the external medium. Other studies, which did sample only the most recently formed shell material have shown that shell Pb concentrations are linearly related to tissue, particulate and dissolved Pb concentrations (Bourgoin, 1990, Pitts and Wallace, 1994). However, Bourgoin (1990) analyzed the inner nacreous shell layer and Pitts and Wallace (1994) analyzed the last formed section of the shell (the outer layer). This could effect the Pb levels they measured because Pb concentrations have been shown to vary by a factor of more than 10 between inner and outer shell layers (Fuge et al., 1993, Raith et al., 1996). Richardson et al. (2001) analyzed Pb concentrations in Modiolus modiolus shells from a polluted and non-polluted site covering 10 years of growth. They found elevated levels in shells from the polluted site, as well as a decrease of concentrations through time, which they attributed to the decline in pollution at the polluted site. However, they could not deconvolve age and time, and age has been shown to influence Pb concentrations in some mollusks (e.g., Hirao et al., 1994). Despite the large interest in using bivalve shells as records of past pollution, there has not been an attempt to create a continuous chronology back through time. Although bivalves are commonly short-lived, several shells can be strung together to form a master chronology, much longer than any one individual’s lifespan (Schöne, 2003).
The general objective of this study was to test if indeed bivalve shells can provide a long term record of anthropogenic Pb pollution. To reach this objective we first attempt to obtain pristine background Pb/Ca ratios from a fossil Pliocene Mercenaria mercenaria shell in order to have a baseline to compare the modern shells to. Secondly, the intra-annual Pb/Ca variation is assessed by sampling three shells across several annual growth increments at a high resolution. Finally, by analyzing Pb/Ca ratios of the annual growth increments in eleven M. mercenaria shells collected at different times we can construct a chronology back through time at an annual resolution. Using shells collected at different dates and of different ages also allows us to assess any effect of age on the records. As no data are available on environmental Pb concentrations at our collection sites, we compare our measured Pb/Ca profile with an expected Pb profile based on published data from biogenic carbonates and total national US Pb emissions.
Section snippets
Sample collection, preparation and analysis
Living M. mercenaria were collected from the Cape Lookout region of North Carolina, USA (Fig. 1) at about 1 m water depth in 1980 (n = 2), 1982 (n = 3), 2002 (n = 3) and 2003 (n = 3) (full data are listed in Table 1). More data on environmental conditions can be found in Peterson et al. (1985), Peterson, 1986, Peterson, 2002, and Gillikin et al. (2005a). Additionally, a Pliocene shell (∼3.2 million years old) was collected from the Duplin formation in South Carolina (1.5 km northwest of Timmonsville) in
Diagenic indicators in the fossil shell
The δ18O values of the Pliocene shell are within the values obtained from the modern shells (Fig. 4) indicating minimal recrystallization, if any (cf. Labonne and Hillaire-Marcel, 2000). Generally, during diagenesis, a number of other chemical changes occur and these changes can be used to identify chemically altered pre-recrystallized carbonates. High trace element contents of Mn, U, and Fe usually indicate some degree of diagenetic alteration, especially if they are accompanied by low Sr and
Inter- and intra-annual variations of Pb/Ca ratios
One test to asses if a proxy is primarily driven by environmental conditions is to determine its variability among individuals that grew under the same environmental conditions. Many studies have proposed that bivalve shells are better than soft tissues for monitoring pollution because the degree of trace metal variation is lower (Bourgoin, 1990, Yap et al., 2003, Cravo et al., 2004). However, we found high variability between shells (Fig. 5), as well as a high intra-annual variability (Fig. 4
Acknowledgements
We thank C.H. Peterson (University of North Carolina, Chapel Hill), who kindly provided the M. mercenaria shells collected in the early 1980’s; W.C. Gillikin and L. Daniels, who both assisted with sample collection in NC; and L. Campbell (University of South Carolina) who kindly provided the Pliocene shell. We express our gratitude to L. Monin for laboratory assistance, T. Haifeng and L. Meng, who helped with sampling the shells, and to C.E. Lazareth for providing the sclerosponge data. A.
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Present address: UR Thetis, IRD-CRH (Centre de Recherche Halieutique Méditerranéenne et Tropicale), Avenue Jean Monnet—BP 171, 34203 Sète Cedex, France.