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Niño-Southern涛动（ENSO）是每年气候变化的主要原因。区域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对全球平均气温有显著影响，强El Niño事件促成了2015-2016年创纪录的温暖。尽管ENSO对现代气候变率具有明显的重要性，但用于进行气候预测的计算机模型并未对ENSO的未来行为提供明确的共识。通过评估各种计算机模型模拟ENSO表现不同的过去时间段的能力，可以改进这些模型，为社会带来广泛的利益。然而，这种方法需要从海洋沉积物岩心等地质档案中获得更多更好的古enso观测。本项目旨在实地验证一种重建过去地质时期ENSO变率的新方法。它依赖于这样一个事实，即微观贝壳（有孔虫）的化学性质反映了贝壳形成时的温度。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面温度变化。该项目还将为一名女博士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洋表面温度变化的代理，即单个浮游有孔虫的“湿化学”Mg/Ca比值，用于探测过去ENSO的变化。初步测量和蒙特卡罗模拟表明，来自保存完好的沉积物的个体有孔虫是钙化温度的可靠记录者。因此，过去ENSO方差的变化应该会在精心选择的核心位点上留下个体Mg/Ca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该方法提出了双重挑战，因此使用晚全新世沉积物对该方法进行地面验证将是重要的。本研究将使用两种浮游有孔虫的单样品Mg/Ca来检验两个主要假设：(1)在部分溶解的样品中观察到的浮游有孔虫Mg/Ca的降低是由每个壳中Mg的百分比损失引起的，而不是由摩尔量损失引起的，也不是由在温暖的水域中形成的高Mg个体的分解和损失引起的；(2)无论溶蚀人工产物的确切性质如何，溶斜层上方热带太平洋岩心顶部沉积物的单样品Mg/Ca分布都保留了温度变化（包括ENSO）的信息。假设1将使用来自热带太平洋西部Ontong爪哇高原的四个深度样带的核心顶部进行测试，假设2将使用跨越热带太平洋年度和年际温度变化制度范围的另外10个核心顶部进行测试。

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.