Bivalve shell microstructures - exploring a novel marine temperature proxy

在过去几个世纪到几千年间，以年度和更好的时间分辨率定量重建海洋温度和盐度是古气候学中的一个紧迫问题。这些数据对于更好地理解海洋在全球气候变化中的作用和限制预测性的数值气候模型至关重要。最有前景的档案之一，可以提供海洋几乎任何地方的此类数据，最近引起了相当大的关注：双壳类软体动物的贝壳，特别是长寿(&gt；30岁)物种。尽管在过去十年中双壳类硬化症取得了重大研究进展，但从贝壳中定量重建水温仍然是一项具有挑战性的任务。例如，双壳类中最常用和被广泛接受的替代物，即贝壳氧同位素值，可以同时告知温度的变化和水的氧同位素特征(与盐度相关)。为了从贝壳氧同位素数据中重建精确的温度，盐度(或水的氧同位素特征)必须是已知的，这一信息对于古代环境来说是很少获得的，因为没有合适的替代品。最近，我们证明了短寿命(约6岁)的龟壳的微结构可以作为水温的替代品(Milano等人)。2017年)。随着温度的升高，单个生物矿物单元变得更大、更细长。我们的研究得到了Gilbert等人的证实。(2017)，适用于不同种的天南星属和羽扇属。他们的结果表明，这些同样生长迅速、寿命相对较短的双壳类贝类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符合热力学预测。(Ii)新的温度代理可以应用于保存完好的化石，即使原始的化学特征在成岩过程中丢失了。(3)一旦训练图像处理软件识别生物矿物单元，就可以在短时间内进行大量测量。有了独立的温度指标，海洋盐度可以根据贝壳氧同位素值计算出来。计划中的项目的主要目标是开发可以用来根据长寿命双壳类贝壳微结构的形态特征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片层和珍珠状结构的短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，有必要从温带环境中对这种简单易用但功能强大的长寿命双壳类(具有不同微结构)的新方法进行测试和校准。这项计划中的研究结果将极大地推动双壳类菌体年代学领域的发展。

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.