ESH: Mg/Ca Thermometry for the Marine Mussels Mytilus californianus and M. trossulus: Refinement of an Elemental Thermometer Based on Laboratory Culture Experiments

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9809374LohmannThis project will refine the Mg/Ca paleothermometer (Klein et al., 1996a) for the marine mussels Mytilus trossulus and M. californianus through a series of laboratory culturing experiments. The importance of developing alternative proxies for paleotemperature is emphasized by the inadequacy of utilizing only d18O for temperature determinations where decreases in seawater salinities bias estimates towards warmer values. For example, in coastal regions where an abundance of historical and fossil materials are available in archeological middens and raised beach sequences, the effects of salinity biasing preclude examination of these materials. Paleoclimate information from shallow marine coastal sites, which accurately record variations in air temperatures, will be extremely valuable because they represent the interface between the marine and continental settings. Although a preliminary field-based study of M. trossulus has demonstrated a consistent relationship between temperature and the Mg/Ca of the outer prismatic calcite layer of the shell, the precision of this paleothermometer can be further refined with a series of well-controlled laboratory experiments. Furthermore, we will extend the use of the Mg/Ca thermometer to M. californianus, a species that is easily identified in ancient fossilized specimens and extends across a greater latitudinal range than M. trossulus. Such calibration will enable reconstruction of paleoclimate conditions that existed on the west coast of North America during the late Pleistocene and Holocene, a goal of future studies.Specifically, this project will culture two species of Mytilus, (M. trossulus and M. californianus) in randomly intermixed assemblages at 8, 16 and 23 C in two successive experiments: the first at 35ppt salinity and; the second at 20ppt salinity. Growth conditions (temperature and salinity) and water compositions (d13C, d18O and Mg/Ca, Sr/Ca and B/Ca) will be monitored throughout the duration of the experiments. Following 6 months of shell growth for each experiment, shells will be sectioned and the outer prismatic calcite layer will be microsampled for elemental and isotopic analysis. Analysis of variance techniques (Model I and mixed-model nested ANOVAs) will be applied to determine the sources of compositional variation (i.e., within individuals, among individuals within a species, among species at each temperature, and among temperatures). If no differences are observed between M. trossulus and M. californianus, a single Mg/Ca paleothermometer will be calculated.Preliminary work (Klein et al., 1996b and 1997) suggests that combined Sr/Ca, d18O and d13C measures provide a means determining salinity and the d18O of the seawater and freshwater endmembers. Moreover, work of Kitano et al. (1978) and Fudrst et al. (1976) suggest that boron concentrations may also serve as independent proxies of salinity. Therefore, during the course of this study, we will also examine variations in shell Sr/Ca, B/Ca and d13C to evaluate the utility of these ratios as measures of paleosalinity.Given the cosmopolitan distribution of Mytilus, validation of Mg/Ca content of skeletal calcite as a new proxy of temperature will allow the reconstruction of paleoclimate conditions across a broad latitudinal range in the shallow marine environment, a setting that represents the interface between marine and continental regimes.