[1] It is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments. Sr/Ca ratios have been successfully used as a temperature proxy in corals and sclerosponges. Previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on Sr/Ca ratios. As many studies have used bivalve growth rates to determine temperature, we tested if Sr/Ca ratios could predict temperature through its relationship with growth rate. Shells from the two species of clams from the same family (veneroidea) studied here, Saxidomus giganteus and Mercenaria mercenaria, show vastly different seasonal Sr/Ca profiles. A strong relationship between average annual Sr/Ca ratios and annual growth rate was found in S. giganteus shells from both Washington (R 2 = 0.87) and Alaska (R 2 = 0.64), USA, but not in M. mercenaria shells from North Carolina, USA. Furthermore, the Sr/Ca-growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in S. giganteus (R 2 = 0.73). Although there were significant positive correlations between Sr/Ca ratios and temperature in S. giganteus shells, the correlations were weak (0.09 < R 2 < 0.27), and thus Sr/Ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves. It is clear from this study that Sr/Ca ratios are not under thermodynamic control in either clam species, since thermodynamics predict a negative correlation between Sr/Ca ratios and temperature in aragonite. This points toward dominance of biological processes in the regulation of Sr 2+ . This is also reflected by the largely differing Sr/Ca partition coefficients (D Sr ) in these shells (D Sr % 0.25), when compared to inorganic, coral, and sclerosponge studies (D Sr % 1), all of which show a negative dependence of Sr/Ca on temperature. We suggest that caution be taken when using Sr/Ca in any biogenic aragonite as a temperature proxy when the D Sr greatly deviates from one, as this indicates the dominance of biological controls on Sr/Ca ratios.
, et al.. Barium uptake into the shells of the common mussel (Mytilus edulis) and the potential for estuarine paleo-chemistry reconstruction. Geochimica et Cosmochimica Acta, Elsevier, 2006, 70 (2), pp.395-407. hal-00452801 Barium uptake into the shells of the common mussel (Mytilus edulis) and the potential for estuarine paleo-chemistry reconstruction
Bivalve shells can potentially record the carbon isotopic signature of dissolved inorganic carbon (d 13 C DIC ) in estuarine waters, thereby providing information about past estuarine biogeochemical cycles. However, the fluid from which these animals calcify is a 'pool' of metabolic CO 2 and external dissolved inorganic carbon (DIC). The incorporation of respired 13 Cdepleted carbon into the skeletons of aquatic invertebrates is well documented, and may affect the d 13 C record of the skeleton. Typically, less than 10% of the carbon in the skeleton is metabolic in origin, although higher amounts have been reported. If this small offset is more or less constant, large biogeochemical gradients in estuaries may be recorded in the d 13 C value of bivalve shells. In this study, it is assessed if the d 13 C values of Mytilus edulis shells can be used as a proxy of d 13 C DIC as well as providing an indication of salinity. First, the d 13 C values of respired CO 2 (d 13 C R ) were considered using the d 13 C values of soft tissues as a proxy for d 13 C R . Along the strong biogeochemical gradient of the Scheldt estuary (The Netherlands-Belgium), d 13 C R was linearly related to d 13 C DIC (r 2 = 0.87), which in turn was linearly related to salinity (r 2 = 0.94). The mussels were highly selective, assimilating most of their carbon from phytoplankton out of the total particulate organic carbon (POC) pool. However, on a seasonal basis, tissue d 13 C varied differently than d 13 C DIC and d 13 C POC , most likely due to lipid content of the tissue. All shells contained less than 10% metabolic carbon, but ranged from near zero to 10%, thus excluding the use of d 13 C in these shells as a robust d 13 C DIC or salinity proxy. As an example, an error in salinity of about 5 would have been made at one site. Nevertheless, large changes in d 13 C DIC (>2&) can be determined using M. edulis shell d 13 C.
Although Sr/Ca ratios in abiogenic calcite are strongly controlled by precipitation rates, such a kinetic effect has never been demonstrated in calcitic bivalve shells. Therefore, we report Sr/Ca ratios together with daily growth rates in the calcitic shells of four individuals of the bivalve Pecten maximus (age class I). Ratios of Sr/Ca were found to be variable among individuals that grew at the same location, illustrating that vital effects dominate over environmental controls. Although daily growth rate was correlated with shell Sr/Ca ratios, it explained only half of the Sr/Ca variations. However, daily shell surface area increment, an estimation of the total quantity of carbonate precipitated for a given time, explained 74% of the Sr/Ca variability in the shells of P. maximus. This proves, for the first time in a calcitic bivalve, that shell Sr/Ca partitioning is mainly controlled by kinetic effects. The Sr/Ca ratio should therefore be tested as a potential proxy of calcification rate in modern or fossil calcitic biocarbonates.
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