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

热带太平洋拥有世界上最大的海洋温水，是大气热量和水分的主要来源。热带太平洋温度梯度的变化导致了热带环流和全球气候的重大变化。对上新世(距今300万至500万年)热带太平洋的研究可以提供重要的洞察，以了解地球气候在过去地质历史中的表现，当时大气中的二氧化碳浓度与今天相似(比工业化前的数值高出约100ppm)。使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型模拟相吻合，但另一些则不符合。这一悬而未决的问题正是本课题研究的重点。该项目的目的是通过量化贝壳保存的变化是否导致有偏见的温度估计，重新评估基于化石贝壳的镁钙比(镁/钙)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型提供重要的验证。该项目包括对一名研究生的专业培训，对STEM领域中代表性不足群体的本科生的培训，以及向旧金山和蒙特利湾地区的本科生进行外联。热带太平洋现有的许多海表面温度记录来自浮游有孔虫的镁/钙组成，这种有孔虫很容易在海底和沉积物中溶解；溶解作用增强导致温度偏低。标准做法是使用沉积物岩心位置和水深的碳酸盐离子浓度来校正溶解的影响。然而，溶解作用可以随时间而变化，而溶解作用的变化对基于镁/钙的温度估计的影响是热带太平洋最有可能产生代理偏差的来源。该项目将测量过去5年热带太平洋公布的镁/钙温度估算所用样本中底栖有孔虫的硼/钙比(B/Ca)。B/Ca是碳酸盐离子浓度的一个替代指标，因此可以用来适当地调整镁/钙温度估计值以考虑溶解作用。新的B/Ca分析将应用于评估西太平洋暖池大洋钻探计划(ODP806)洞的潜在溶解偏差，但也将使用一套更有限的分析来评估东太平洋冷舌的ODP847。一旦考虑到溶解的影响，绝对温度和东西温差都将被量化，并与上新世暖期的气候模型模拟相比较。

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.