双壳类生物壳体的clumped同位素在渤海湾河口海岸地区水温重建中的应用

The paleotemperature of the seawater, which could reflect the climate and hydrological change in the geological past, takes an important role in the study of global climate change. The shell of marine bivalve is an ideal high-resolution archive of the past ocean and it can record the annual, seasonal and even monthly change of the aquatic environment. However, the Sr/Ca and Mg/Ca thermometry of bivalve shell are affected by kinetic effects, and the application of oxygen isotope thermometry is limited especially in the coastal and estuarine regions for the difficulty in constraining the influence of the isotopic composition of ancient seawater and its variation. The clumped isotope technique is a newly developed homogeneous thermometry. It is independent from the isotopic composition of the ambient fluid from which the carbonate is precipitated. In this study, we will establish the methodology for clumped isotope analysis and data correction using dual-inlet gas-source isotope ratio mass spectrometry, measure the Δ47 of modern oyster shells grown at monitored conditions in Bohai Bay, and calibrate the Δ47 proxy against the calcification temperature. Then we will measure the Δ47 of the ligamental increments that indicate seasonality of buried Holocene oyster shells. We attempt to use this thermometry for a seasonally resolved reconstruction of the sea surface temperature, and the oxygen isotopic composition of the seawater when combined with the oxygen isotope thermometry. This study will shed new light on the reconstruction of temperature and climate short-scale oscillation and seasonality in the geological past.

海水古温度可以反映过去的气候和水文变化，是全球气候变化的重要研究内容。海洋双壳类壳体是一种理想的高分辨率古海洋信息载体，记录了年际，季节甚至月分辨率的水体环境变化。传统的壳体Sr/Ca，Mg/Ca温度计存在动力学因素的影响，而氧同位素温度计则受到古海水氧同位素组成和变化的影响，在示踪海水古温度时受到限制，在海岸带和河口等地区尤为突出。碳酸盐的clumped同位素是一种新的单相温度计，与环境流体的同位素组成无关。本申请拟采用双路进样气体源同位素比质谱法建立可靠的碳酸盐clumped同位素分析和校正方法；测量渤海湾地区有生长环境记录的现生牡蛎壳体的Δ47，建立Δ47与水温变化的函数关系；测量渤海湾地区全新世埋藏牡蛎壳体具有季节指示意义的微生长层的Δ47，尝试开展季节分辨率的表层古水温重建，结合氧同位素温度计计算同期海水的氧同位素组成，为重建古海水温度及古气候的短尺度振荡和季节变化提供新的思路。

双壳类生物的碳酸盐壳体对海洋物理化学条件较为敏感，是较理想的高分辨率海洋古气候信息载体，其时间分辨率可达年际，季度甚至月。双壳类壳体的微量元素（如Sr/Ca，Mg/Ca）、氧同位素对温度较为敏感，但适合其大量生长的河口及海岸地区易受淡水、大气降水、地下水等因素影响，水文条件变化较大，限制了氧同位素温度计的应用。碳酸盐的clumped同位素是一种新型单相温度计，有望解决双壳类微量元素和氧同位素温度计使用受限的问题。课题组分析了采自渤海湾的7个太平洋牡蛎壳体韧带部位年生长纹层的碳氧同位素和牡蛎全壳的clumped同位素。在使用高温磷酸化连续流质谱法分析壳体碳氧同位素时，发现硝酸盐污染物会干扰碳酸盐样品的测量。课题组提出了NO2假说阐述了硝酸盐干扰碳酸盐测量的机制，并提出低温磷酸化方法解决了硝酸盐污染的问题。课题组分析了7个壳体的碳氧同位素剖面，所得夏季至冬季δ18O值变化范围为-4~2 ‰，冬夏两季相差约6 ‰，大于温度的贡献（~4 ‰）；根据渤海湾地区的盐度-海水δ18Ow关系，海水盐度季节变化的贡献约为1 ‰；牡蛎壳体的δ18O可能存在生命效应，而对太平洋牡蛎冬季休克温度的估计也可能存在较大误差。碳酸盐clumped同位素可能有助于解决这一问题。通过对全壳样品的clumped同位素分析发现，17个均一化的样品所得Δ47值的范围在0.6-0.7 ‰。课题组所在实验室的clumped同位素分析精度难以满足水温重建需求，其原因可能是由于课题组使用的MAT 253气体源同位素比质谱仪在进行clumped同位素测量之外还承担了大量常规样品同位素的测量任务，导致仪器很难处于测量clumped同位素的最佳状态。课题组将在实验室于2019年完成安装的专门用于clumped同位素测量的253 Plus同位素质谱仪上继续开展工作。