Assessing the Extent and Causes of Fluctuations in the Seawater Sr/Ca Ratio to Improve Coral Paleothermometry Calibrations

过去气候变率的地质档案使科学家能够将最近的气候变化置于更广泛的时间背景下，并在比单独使用仪器气候数据更长的时间尺度上研究气候变率背后的机制。巨大的珊瑚就是这样一个地质档案。珊瑚骨骼中的锶-钙元素比（Sr/Ca）受珊瑚生长的水温的影响。 这种关系，加上一些珊瑚相对较快的生长速度和较长的寿命，使科学家能够从珊瑚的地球化学中重建长达几个世纪的热带海洋温度记录。 基于Sr/Ca的珊瑚温度重建的一个主要假设是海水的Sr/Ca比在海洋中的任何地方都是相同的，并且至少在珊瑚记录所跨越的时间段内是恒定的。然而，有些过程可能会导致海水中Sr/Ca比率的变化，特别是在珊瑚生长的浅水沿海地区。本研究调查的幅度随时间和空间的海水Sr/Ca波动在不同的珊瑚礁设置使用的分析方法，将开发用于快速，廉价的海水Sr/Ca测定。 该方法的目的是使海水锶/钙测量是一个常规的一部分，珊瑚古气候样品采集。 预期的研究结果将使科学界能够就未来珊瑚锶/钙基古气候研究的选址标准和海水取样提出宝贵建议，并最终提高这一重要科学工具的准确性和可靠性。在马里兰州大学切萨皮克生物实验室（CBL）外联协调员的帮助下，一名中学教师将被邀请参加实地考察。这两个教育专家将与项目的主要研究人员合作，维护一个博客，创建一个视频杂志，并开发/传播一个与新课程标准兼容的课程计划，并在课堂上使用，以及在CBL游客中心的教育活动。 通过与教师、学生、游客中心志愿者和赞助人分享科学，项目参与者将吸引当地社区的公民以及每年近2700名游客。科学方法将是在三个自然地理环境中对海缘礁的海水Sr/Ca比率和其他相关变量进行时空密集研究：一个大陆规模的碳酸盐台地（佛罗里达礁群）、一个小硅质岩岛（美属维尔京群岛圣约翰）和一个小碳酸盐岛（英属维尔京群岛阿加达）。 海水样品中Sr/Ca比率的分析将基于对文石分析的现有ICP-AES方法进行修改，以在复杂的海水基质中始终达到0.1%或更高的准确度和精密度。其他技术，包括ICP-MS或离子色谱法，将用于验证测量的Sr/Ca比率的准确性，并分析补充地球化学数据。海水样本将在冬季和夏季从佛罗里达群岛的海峡切割、泻湖、补丁礁、礁顶、前礁和开阔海洋地点收集。在间隔期间，将在这些地点用连续取样泵收集连续记录。 同时测量温度与热敏电阻，和随后的Sr/Ca分析的珊瑚nubbins放置在附近的设备将使验证和量化的水化学和温度对珊瑚文石Sr/Ca比的影响。这一取样计划将有助于评估盐度、涌升、河流和地下水输入以及生产力变化的影响。 不同的自然地理环境对沿海海水Sr/Ca的影响将通过在佛罗里达群岛和英属维尔京群岛收集的类似数据之间的比较进行评估。将时间序列和主成分分析相结合，确定各种参数在空间和时间上对整个数据集的差异影响。

Geologic archives of past climate variability enable scientists to put recent climate changes in broader temporal context and to investigate the mechanisms behind climate variability on longer timescales than is possible using instrumental climate data alone. Massive corals are one such geologic archive. Strontium-to-calcium elemental ratios (Sr/Ca) in coral skeletons are influenced by the temperature of the water in which the coral grew. This relationship, combined with the relatively fast growth rates and long life-span of some corals, enables scientists to reconstruct centuries-long records of tropical ocean temperatures from coral geochemistry. A major assumption underlying coral Sr/Ca-based temperature reconstructions is that the Sr/Ca ratio of seawater is the same everywhere in the ocean and is constant over at least the time period spanned by the coral record. However, there are processes that may cause variations in the Sr/Ca ratio of seawater, especially in shallow coastal areas where corals grow. This research investigates the magnitude of seawater Sr/Ca fluctuations over time and space in different coral reef settings using an analytical method that will be developed for rapid, inexpensive seawater Sr/Ca determinations. The method is designed to enable seawater Sr/Ca measurements to be a routine part of coral paleoclimate sample collection. The anticipated findings will enable valuable recommendations to the scientific community regarding site selection criteria and seawater sampling for future coral Sr/Ca-based paleoclimate research and should ultimately lead to greater accuracy and reliability of this important scientific tool.The project will include substantial efforts to communicate the research to lay audiences. With help from the University of Maryland's Chesapeake Biological Laboratory (CBL) outreach coordinator, a secondary school teacher will be invited to participate in a field expedition. Both of these educational specialists will work with the project Principle Investigators to maintain a blog, create a video journal, and develop/disseminate a lesson plan compatible with new curriculum standards and used in the classroom as well as in educational activities of the CBL Visitor Center. By sharing the science with teachers, their students, and Visitor Center volunteer staff and patrons, the project participants will engage citizens in the local community as well as the nearly 2700 tourists that enjoy the Visitor Center annually. The scientific approach will be to conduct a spatially and temporally intensive study of seawater Sr/Ca ratios and other associated variables on fringing reefs in three physiographic settings: a continental-scale carbonate platform (Florida Keys), a small siliciclastic island (St. John, US Virgin Islands) and a small carbonate island (Anegada, British Virgin Islands). Analysis of Sr/Ca ratios in the seawater samples will be based on the existing ICP-AES method for aragonite analysis modified to consistently attain accuracy and precision of 0.1% or better in a complex seawater matrix. Other techniques, including ICP-MS or ion chromatography will be used to verify the accuracy of measured Sr/Ca ratios and to analyze complementary geochemical data. Seawater samples will be collected in the winter and summer from channel cuts, lagoon, patch reef, reef crest, fore reef, and open ocean sites in the Florida Keys. Continuous records will be collected with osmosampler pumps in these locations during the intervening periods. Concurrent measurements of temperature with thermistors, and subsequent Sr/Ca analysis of coral nubbins placed near the equipment will enable verification and quantification of the impact of both water chemistry and temperature on coral aragonite Sr/Ca ratios. This sampling scheme will allow assessment of the influences of salinity, upwelling, river and groundwater inputs, and variations in productivity. The impacts of different physiographic environments on coastal seawater Sr/Ca will be assessed by comparisons between similar data collected in the FL Keys and British Virgin Islands. Differential effect of various parameters on the entire dataset in space and time will be determined by a combination of time series and principal components analysis.