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

University Of Maryland Center For Environmental Sciences, Cambridge MD

Investigators

Abstract

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.