Collaborative Research: Tracing Pacific Ocean circulation and ventilation during the warm Pliocene Epoch

人类造成的二氧化碳排放，以及它们困在地球气候系统中的热量，很大一部分正在被全球海洋吸收。人们对这种吸收对气候和海洋环流的长期影响知之甚少。对21世纪气候变化的预测表明，全球变暖将导致海洋分层加剧，全球海洋环流减弱，这将对区域和全球气候产生重大影响。然而，最近的研究表明，在地球历史早期的温暖期，海洋环流更强，而不是更缓慢。这种潜在的差异为更好地描述过去比现在更温暖的气候时期的海洋状况提供了依据。为了做到这一点，这项研究将把重点放在过去地质时期大气二氧化碳浓度与今天相似、气温比现在高几度的时间间隔上。最近符合这一标准的地质间隔是上新世（533万至258万年前）。该项目的主要目标是生成古海洋学记录，以验证上新世时期北太平洋存在强烈翻转环流的模型。该奖项将支持五名早期职业科学家的职业和培训，并培训和指导高中生和本科生。这一提议的目的是：1)验证最近的假设，即在温暖的上新世期间，北太平洋存在深水形成和活跃的太平洋经向翻转环流；2)追踪海洋通风的区域分布；3)完善使用大量沉积物和有孔虫中的氧化还原、温度和生产力指标来重建上新世北太平洋的通风、养分有效性和水混合。一个多代理方法将研究两个关键地质区间的中至深海环流模式：上新世中期暖期（北半球冰川强化）（~2.5至3.3 Ma）和上新世早期（~4.9至5.1 Ma）。该项目将在四个地点（IODP 882、883、887和1208）产生新的记录，包括：i)大量沉积物的氧化还原和生产力代用物，ii)底栖有孔虫的氧化还原、生产力和水质量混合代用物，以及iii)浮游有孔虫的生产力和温度代用物，这些地点覆盖了北太平洋的一系列深度和位置。同时，本研究将分析一系列具有活跃生物地球化学和d13C循环的耦合气候模式模拟。模式示踪剂、环流、同位素和通风年龄的结果将有助于解释新生成的数据。这种以气候模式分析为基础的多代理方法将有助于确保代理推断的动力学的可行性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A large portion of human-induced carbon dioxide emissions, and the heat that they trap within Earth’s climate system, are being absorbed by the global ocean. The long-term impacts of this absorption on climate and ocean circulation are poorly understood. Forecasts of 21st-century climate change suggest that global warming will lead to a more stratified ocean and weaker global ocean circulation, which would have major ramifications for regional and global climate. However, recent studies suggest that ocean circulation was stronger, not more sluggish, during warm intervals earlier in Earth’s history. This potential discrepancy warrants a better characterization of ocean conditions during warmer-than-present climates of the past. To do so, the study will focus on intervals in the geologic past during which atmospheric carbon dioxide concentrations were similar to today, and temperatures were several degrees warmer. The most recent geologic interval that meets this criterion is the Pliocene Epoch (5.33 to 2.58 million years ago). The primary goal of this project is to generate paleoceanographic records to test models suggesting that there was a strong overturning circulation in the North Pacific Ocean during the Pliocene Epoch. This award will support the careers and training of five early-career scientists, and train and mentor high school and undergraduate students.The goals of this proposal are to: 1) test the recent hypothesis that there was North Pacific deep-water formation and an active Pacific Meridional Overturning Circulation present at times during the warm Pliocene; 2) trace the regional distribution of ocean ventilation; and 3) refine the use of redox, temperature, and productivity proxies in bulk sediments and foraminifera for reconstructing Pliocene North Pacific Ocean ventilation, nutrient availability, and water mixing. A multi-proxy approach will examine intermediate to deep ocean circulation patterns across two key geologic intervals: the mid-Pliocene warm period through the intensification of Northern Hemisphere glaciation (~2.5 to 3.3 Ma) and the early Pliocene (~4.9 to 5.1 Ma). This project will generate new records of i) redox and productivity proxies from bulk sediments, ii) redox, productivity, and water mass mixing proxies from benthic foraminifera, and iii) productivity and temperature proxies from planktic foraminifera, at four sites (IODP 882, 883, 887, and 1208) that cover a range of depths and locations across the North Pacific Ocean. In parallel, the study will analyze a series of coupled climate model simulations with active biogeochemical and d13C cycling. The model tracer, circulation, isotope, and ventilation age results will aid in the interpretation of the newly generated data. This multi-proxy approach, underpinned by climate model analysis, will help ensure the feasibility of the proxy-inferred dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.