Bivalve shell microstructures - exploring a novel marine temperature proxy

在过去的几个世纪到几千年间，以年和较好的时间分辨率定量重建海洋温度和盐度是古气候学中一个紧迫的问题。这些数据对于更好地了解海洋在全球气候变化中的作用和限制预测数值气候模式至关重要。最近，一个最有希望的档案引起了相当大的关注：双壳类软体动物的壳，特别是长寿（大约30岁）的物种。该档案几乎可以从海洋的任何部分提供此类数据。尽管近十年来双壳类动物的年代学研究取得了重大进展，但从壳中定量重建水温仍然是一项具有挑战性的任务。例如，在双壳类动物中最常用和被广泛接受的代用物，即壳氧同位素值，可以同时反映温度变化和水的氧同位素特征（与盐度相关）。为了从壳氧同位素数据中重建精确的温度，必须知道盐度（或水的氧同位素特征），由于没有合适的代用物存在，古代环境很少有这样的信息。最近，我们已经证明，短寿命（约6年）贝壳的微观结构可以作为水温的代表（Milano et al. 2017）。随着温度的升高，个体生物矿物单位变得更大，更长。我们的研究得到了Gilbert等人（2017）对不同种类的Atrina和Pinna的证实。他们的结果表明，这些同样快速生长，寿命相对较短的双壳类动物的珍珠层厚度也与温度呈正相关。与现有的古温度计相比，这种新工具有许多优点：(i)生物矿物尺寸的增加遵循热力学预测。（ii）即使原始的化学特征因成岩作用而丢失，新的温度代用物也可以应用于保存完好的化石。（iii）一旦训练图像处理软件识别生物矿物单位，就可以在短时间内进行大量测量。有了一个独立的温度代理，海洋盐度可以从壳氧同位素值计算出来。计划项目的主要目标是开发可用于从长寿双壳类贝壳微观结构的形态特性定量重建海洋温度的传递函数。到目前为止，该方法仅在少数具有交叉板层和珍珠织物的短寿命双壳类物种中进行了评估。为了在古气候学中得到更广泛的应用，有必要对这种简单易用但功能强大的新方法进行测试和校准，以研究温带环境中具有不同微观结构的长寿双壳类动物。计划中的研究结果将显著推进双壳类动物的年代学研究。

Quantitative reconstruction of ocean temperature and salinity at annual and better temporal resolution during the last centuries to millennia is a pressing issue in paleoclimatology. Such data are essential to better understand the role of the ocean in global climate change and to constrain predictive numerical climate models. One of the most promising archives that can provide such data from nearly any part of the ocean has recently attracted considerable attention: shells of bivalve mollusks, specifically long-lived (> 30 yrs-old) species.Despite major research advances in bivalve sclerochronology during the last decade, the quantitative reconstruction of water temperature from shells remains a challenging task. For example, the most frequently used and widely accepted proxy in bivalves, i.e., the shell oxygen isotope value, simultaneously informs about changes of temperature and the oxygen isotope signature of the water (which is correlated to salinity). To reconstruct precise temperatures from shell oxygen isotope data, salinity (or the oxygen isotope signature of the water) must be known, an information which is rarely available for ancient environments as no suitable proxy exists.Recently, we have demonstrated that the microstructure of shells of the short-lived (ca. 6 yrs-old) Cerastoderma edule serves as a proxy for water temperature (Milano et al. 2017). As temperature rises, individual biomineral units become larger and more elongated. Our study was corroborated by Gilbert et al. (2017) for different species of Atrina and Pinna. Their results demonstrated that the nacre tablet thickness in these likewise fast-growing, relatively short-lived bivalves is also positively correlated to temperature. There are a number of advantages of this new tool over existing paleothermometers: (i) The increase in biomineral size follows thermodynamic predictions. (ii) The new temperature proxy can be applied to well-preserved fossils even if the original chemical signature is diagenetically lost. (iii) Once the image processing software is trained to recognize the biomineral units, a large number of measurements can be done in short time. With an independent temperature proxy, ocean salinity can be computed from shell oxygen isotope values.The main objective of the planned project is to develop transfer functions that can be used to quantitatively reconstruct ocean temperature from morphological properties of shell microstructures of long-lived bivalves. So far, the method has only been evaluated in a few short-lived bivalve species with crossed-lamellar and nacreous fabrics. For the broader application in paleoclimatology, it is necessary to test and calibrate this simple and easy-to-use but powerful novel method for long-lived bivalves (with different microstructures) from extratropical settings. Results of the planned study will significantly advance the field of bivalve sclerochronology.