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温暖海水，是大气热量和水分的主要来源。横跨热带太平洋的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300万至500万年前）热带太平洋的研究可以提供重要的见解，了解地球气候在地质学过去的某个时期是如何表现的，当时大气中的二氧化碳浓度与今天相似（比工业化前高出约100 ppm）。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模拟结果一致，但与其他结果不一致。这个悬而未决的问题是本研究项目的重点。该项目的目的是重新评估上新世的温度估计是根据镁钙比（镁/钙）的化石贝壳量化是否在保存的变化导致的温度估计偏差。这项工作的结果将提供重要的验证气候模式用于模拟温暖的气候状态。该项目涉及研究生的专业培训，STEM领域代表性不足群体的本科生的培训，以及对旧金山弗朗西斯科和蒙特雷湾地区本科生的推广。 热带太平洋现有的许多海洋表面温度记录都来自于浮游有孔虫的Mg/Ca组成，这种成分容易在海底和沉积物中溶解;溶解作用的增强会导致冷温度偏差。标准做法是利用沉积物岩心所在位置和水深处的碳酸根离子浓度来校正溶解的影响。然而，溶解可以随着时间的推移而变化，溶解变化对镁/钙基温度估计的影响是热带太平洋代理偏差最有可能的来源。本项目将测量过去500万年来用于估算热带太平洋镁钙温度的同一样本中底栖有孔虫的硼钙（B/Ca）比。B/Ca是碳酸根离子浓度的代表，因此可用于适当调整Mg/Ca-温度估计值以考虑溶解。新的B/Ca分析将用于评估西太平洋暖池大洋钻探项目（ODP）806孔的潜在溶解偏差，但也将使用更有限的分析集，评估东太平洋冷舌ODP 847。一旦考虑到溶解的影响，绝对温度和东西温差都将被量化，并与上新世温暖期的气候模型模拟进行比较。

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.