ENSO Variability and Mean State during the Late Holocene and Last Glacial Maximum Based on Mg/Ca of Individual Foraminifera

在包括美国在内的全球大部分地区，厄尔尼诺-南方涛动(ENSO)是每年气候变化的主要原因。区域性影响可能是灾难性的，包括干旱、野火、洪水、泥石流和渔业崩溃。此外，ENSO对全球平均气温有重大影响，而一次强烈的厄尔尼诺事件促成了2015-2016年间创纪录的温暖。尽管ENSO对现代气候变异性具有明显的重要性，但用于进行气候预测的计算机模型并没有就ENSO的未来行为提供明确的共识。各种计算机模型可以通过评估它们模拟过去ENSO不同行为的时间段的能力而得到改进，从而为社会带来广泛的好处。然而，这种方法需要从海洋沉积物岩心等地质档案中进行更多、更好的古ENSO观测。这个项目的目的是找到一种新的方法来重建过去地质时期的ENSO变化.它依赖于这样一个事实，即微观贝类(有孔虫)的化学物质反映了贝壳形成的温度。通过测量大量单个贝壳的化学成分，研究人员将能够重建温度受ENSO现象支配的地区的海洋表面温度的变异性。该项目还将为一名女博士生和两名本科生提供广泛的学习和培训经验。更具体地说，这里提出的工作将开发一种新的海表面温度变化指标，即单个浮游有孔虫的“湿化学”镁/钙比，用于探测过去ENSO的变化。初步测量和蒙特卡罗模拟表明，保存完好的沉积物中的单个有孔虫是钙化温度的可靠记录。因此，ENSO方差的过去变化应该会在精心选择的核心位置上留下个人镁/钙分布的印记。然而，在热带太平洋，CaCO3的海底溶解和低沉积速率对这一方法提出了双重挑战，因此重要的是利用晚全新世沉积物来实地验证这一方法。这项工作将使用两种浮游有孔虫中的单标本镁/钙来检验两个主要假设：(1)观察到的部分溶解样品中浮游有孔虫镁/钙的降低是由每个贝壳中的镁损失百分比引起的，而不是由摩尔量的损失引起的，也不是由在较温暖的水域形成的较高镁个体的分解和损失引起的；(2)无论溶解人工产物的确切性质如何，关于温度变异性(包括ENSO)的单标本镁/钙分布中都保留了来自于溶跃层以上的热带太平洋柱心顶部沉积物的镁/钙分布信息。假设1将使用热带西太平洋Ontong爪哇高原深度样带的4个核心顶部进行测试，而假设2将使用另外10个覆盖热带太平洋一系列年和年际温度变化体系的核心顶部进行测试。

Over large parts of the globe including the United States, El Niño-Southern Oscillation (ENSO) is a leading cause of climate variations from one year to the next. Regional impacts can be catastrophic, including drought, wildfires, flooding, mudslides, and fisheries collapse. Furthermore, ENSO has a significant influence on global mean air temperature, and a strong El Niño event contributed to the record warmth experienced during 2015-2016. Despite the clear importance of ENSO to modern climate variability, the computer models used to make climate projections offer no clear consensus on ENSO's future behavior. The various computer models can be improved, with broad benefit to society, by evaluating their ability to simulate past time periods when ENSO behaved differently. However, that approach requires more and better paleo-ENSO observations from geologic archives like ocean sediment cores. This project aims to ground-truth a new method to reconstruct ENSO variability during the geologic past. It relies on the fact that the chemistry of microscopic seashells (foraminifera) reflects the temperature at which the shell formed. By measuring the chemistry of numerous individual shells, researchers will be able to reconstruct the variability of sea surface temperature in regions where temperature is dominated by the ENSO phenomenon. This project will also provide extensive learning and training experiences for a female PhD student and two undergraduates.More specifically, the work proposed here will develop a new proxy for sea surface temperature variability, "wet chemistry" Mg/Ca ratios in individual planktic foraminifera, for probing ENSO variance during the past. Preliminary measurements and Monte Carlo simulations show that individual foraminifera from well-preserved sediments are reliable recorders of calcification temperature. Past changes in ENSO variance should therefore leave imprints on the distributions of individuals' Mg/Ca at carefully chosen core sites. In the tropical Pacific however, seafloor dissolution of CaCO3 and low sedimentation rates present dual challenges to this method, so it will be important to ground-truth the approach using late Holocene sediments. This work will use single-specimen Mg/Ca in two species of planktic foraminifera to test two main hypotheses: (1) The observed lowering of planktic foraminiferal Mg/Ca in partially dissolved samples is caused by a percentage Mg loss from each shell, and not by a molar amount lost, nor by breakup and loss of higher-Mg individuals formed in warmer waters; (2) Regardless of the exact nature of the dissolution artifact, information about temperature variability (including ENSO) is retained in single-specimen Mg/Ca distributions from tropical Pacific core top sediments above the lysocline. Hypothesis 1 will be tested using four core tops from a depth transect on Ontong Java Plateau in the western tropical Pacific, and Hypothesis 2 will be tested using 10 additional core tops spanning a range of annual and interannual temperature variability regimes across the tropical Pacific.