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

9809374lohmann本项目将通过一系列实验室培养实验，改进用于海洋贻贝（Mytilus trossulus）和加利福尼亚贻贝（m.c ananus）的Mg/Ca古温度计（Klein et al., 1996a）。仅利用d18O进行温度测定的不足之处强调了开发替代古温标的重要性，因为海水盐度的降低使估计偏向于较暖的值。例如，在沿海地区，考古丘和凸起的海滩序列中有丰富的历史和化石材料，盐度偏差的影响阻碍了对这些材料的检查。来自浅海沿岸遗址的古气候信息准确地记录了气温的变化，因为它们代表了海洋和大陆环境之间的界面，因此将非常有价值。虽然对M. trossulus的初步野外研究已经证明了温度与壳的外棱柱方解石层的Mg/Ca之间的一致关系，但这种古温度计的精度可以通过一系列控制良好的实验室实验进一步提高。此外，我们将把Mg/Ca温度计的使用范围扩大到m.c ananus，这是一个很容易在古代化石标本中识别出来的物种，并且比m.c rossulus在更大的纬度范围内扩展。这样的校准将使北美西海岸在晚更新世和全新世期间存在的古气候条件得以重建，这是未来研究的目标。具体地说，这个项目将在两个连续的实验中，在8、16和23℃的条件下随机混合培养两种Mytilus (M. trossulus和M. californianus)：第一个实验在35ppt的盐度下；第二次盐度为20ppt。在整个实验期间，将监测生长条件（温度和盐度）和水成分（d13C、d18O和Mg/Ca、Sr/Ca和B/Ca）。每个实验经过6个月的贝壳生长后，将对贝壳进行切片，并对外层棱柱状方解石层进行微采样，以进行元素和同位素分析。将应用方差分析技术（模型1和混合模型嵌套方差分析）来确定组成变异的来源（即个体内、物种内个体之间、每种温度下的物种之间以及不同温度下的物种之间）。如果在M. trossulus和M. californianus之间没有观察到差异，则将计算单个Mg/Ca古温度计。初步工作（Klein et al.， 1996b和1997）表明，Sr/Ca、d18O和d13C联合测量提供了一种确定海水和淡水末端成员盐度和d18O的方法。此外，Kitano et al.（1978）和Fudrst et al.（1976）的研究表明，硼浓度也可以作为盐度的独立代表。因此，在本研究过程中，我们还将研究贝壳Sr/Ca、B/Ca和d13C的变化，以评估这些比率作为古盐度测量的效用。鉴于贻贝的世界性分布，验证骨骼方解石的Mg/Ca含量作为温度的新代理，将允许在浅海环境中重建广泛的纬度范围内的古气候条件，这一设置代表了海洋和大陆制度之间的界面。

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.