Assessing the effect of dissolution on key Pliocene Mg/Ca-based tropical temperature records

热带太平洋拥有世界海洋中最大的温水，是大气中热量和水分的主要来源。热带太平洋温度梯度的变化驱动着热带环流和全球气候的重大变化。对上新世（距今300万年至500万年）热带太平洋的研究，可以提供重要的见解，了解在过去的地质时期，大气二氧化碳浓度与今天相似（比工业化前高100 ppm）时，地球气候是如何变化的。利用地球化学技术重建的上新世热带太平洋温度与一些上新世气候模式的模拟结果一致，但与其他模式不一致。这个尚未解决的问题是本研究项目的重点。该项目的目的是通过量化贝壳保存变化是否导致温度估计偏差，重新评估基于化石贝壳镁钙比（Mg/Ca）的上新世温度估计。这项工作的结果将为用于模拟温暖气候状态的气候模式提供重要的验证。该项目包括对一名研究生进行专业培训，对来自STEM领域代表性不足群体的本科生进行培训，并向旧金山和蒙特利湾地区的本科生提供服务。热带太平洋现有的许多海表温度记录是根据浮游有孔虫的镁/钙组成得出的，浮游有孔虫很容易在海底和沉积物中溶解；增强的溶解导致冷偏温。使用沉积物岩心位置和水深处的碳酸盐离子浓度来校正溶解的影响是标准做法。然而，溶解度可以随时间变化，而溶解度变化对基于Mg/ ca的温度估计的影响最有可能是热带太平洋代用偏差的来源。该项目将测量底栖有孔虫的硼钙比（B/Ca），这些样品在过去5万年中被用于产生已发表的热带太平洋Mg/Ca温度估计。B/Ca是碳酸盐离子浓度的代表，因此可以用来适当地调整Mg/Ca温度估计值以考虑溶解。新的B/Ca分析将应用于评估西太平洋暖池中海洋钻探项目（ODP） 806孔的潜在溶解偏差，但也将使用一组更有限的分析来评估东太平洋冷舌中的ODP 847。一旦考虑溶蚀的影响，绝对温度和东西温差将被量化，并与上新世暖期的气候模式模拟结果进行比较。

The tropical Pacific harbors the largest mass of very warm water of the world's ocean and is a primary source of heat and moisture to the atmosphere. Changes in the temperature gradients across the tropical Pacific drive major changes in tropical circulation and global climate. Studies of the tropical Pacific during the Pliocene Epoch (three to five million years before present) can provide important insight into how the Earth's climate behaved at a time in the geologic past when atmospheric carbon dioxide concentrations were similar to today (~100 ppm higher than preindustrial values). Reconstructions of Pliocene tropical Pacific temperatures using geochemical techniques agree with some Pliocene climate model simulations, but not others. This unresolved issue is the focus of this research project. The aim of this project is to re-evaluate the Pliocene temperature estimates that are based on the magnesium to calcium ratio (Mg/Ca) of fossil shells by quantifying whether changes in the preservation of the shells resulted in biased temperature estimates. The results of this work willl provide important validation of climate models used to simulate warm climate states. This project involves the professional training of a graduate student, training of undergraduate students from underrepresented groups in STEM fields, and outreach to undergraduates in the San Francisco and Monterey Bay Areas. Many of the existing sea surface temperature records from the tropical Pacific are derived from the Mg/Ca composition of planktonic foraminifera, which is susceptible to dissolution on the seafloor and in the sediments; enhanced dissolution results in a cold temperature bias. It is standard practice to correct for the effect of dissolution using carbonate ion concentrations at the location and water depth of the sediment core. However, dissolution can change with time and the effects of changing dissolution on Mg/Ca-based temperature estimates is the most likely source of proxy bias in the tropical Pacific. This project will measure the boron to calcium (B/Ca) ratio of benthic foraminifera in the same samples that were used to produce published Mg/Ca-temperature estimates in the tropical Pacific over the last 5 Myrs. B/Ca is a proxy for carbonate ion concentration, and thus can be used to properly adjust the Mg/Ca-temperature estimates to account for dissolution. The new B/Ca analyses will be applied to evaluating potential dissolution biases at Ocean Drilling Project (ODP) Hole 806 in the West Pacific warm pool, but will also, using a more limited set of analyses, evaluate ODP 847 in the East Pacific cold tongue. Both the absolute temperatures and the West-East temperature difference will be quantified once the effect of dissolution is taken into account, and compared to climate model simulations of the Pliocene warm period.