Investigating Sources of Excess Carbon and DELTA14C Anomalies During the Last Glacial Termination

调查末次冰期终止期间过量碳和 DELTA14C 异常的来源

• 批准号: 1558990
• 负责人: Lowell Stott
• 金额: $ 43.14万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-15 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558990&HistoricalAwards=false
• 关键词: Investigating; Sources; Excess; Carbon; DELTA14C

Recognizing that greenhouse gas variability played a primary role in Earth's largest climate changes, the challenge now is to discover what processes are responsible for variations in atmospheric greenhouse gases that accompany large climate changes. The Pleistocene glacial/interglacial cycles are arguably some Earth's largest and most dramatic climatic changes associated with variations in atmospheric pCO2 and methane. But after three decades of research there is no scientific consensus on the mechanisms responsible for the variations in greenhouse gas concentrations. Recent discoveries have revealed secretes about the Earth's carbon system that prompted the hypothesis being investigated with this award. The hypothesis argues that the flux of CO2 into and out of rocks and sediments that blanket marine hydrothermal systems varied with temperature during glacial-interglacial cycles. This is because CO2 and CO2-rich fluids from hydrothermal vents become denser than seawater as temperatures cool during glaciations. The denser CO2 accumulates in rocks and sediments until the oceans warm at the glacial terminations. When temperatures rise during a glacial termination, CO2 becomes buoyant and is released into the Ocean. Carbon released from hydrothermal systems carries distinct geochemical fingerprints that are imprinted on marine sediments. This project will use geochemical measurements to investigate whether these geochemical signals were imprinted on sediments at the last glacial-to-interglacial transition. Preliminary evidence supports this hypothesis. The results of this investigation could inform not only our understanding of how geologic processes influenced the Earth's past climate, it could also inform our understanding of how sensitive geologic reservoirs of carbon are to rising ocean temperature in the future. This award will also support ongoing participation in STEM training at the University of Southern California through research opportunities for graduate and undergraduate students and high school seniors.New Boron isotope and 14C records from marine sediments indicate there was a significant increase in the flux of 14C-depleted CO2 from the eastern equatorial Pacific to the atmosphere during the last glacial termination as atmospheric pCO2 was rising. The source of excess carbon and the reason for the increased flux of 14C-depleted CO2 is not currently known. However, two different hypotheses have been put forth to explain the deglacial history. One hypotheses calls upon ventilation of a formally isolated abyssal water mass through the Southern Ocean and transport of the ventilated carbon to the tropics via intermediate waters. An alternative hypothesis calls upon release of geologic carbon into the tropical Ocean from hydrothermal reservoirs in response to ocean warming during the deglaciation. This research seeks to determine which of these two hypotheses is correct. Each of the hypothesized mechanisms would have left a different geochemical fingerprint on the intermediate waters that fed the equatorial Pacific upwelling system. The so-called "abyssal ventilation hypothesis" predicts that the source waters that contributed carbon to the equatorial undercurrent would have had the lowest Delta14C signature and the lowest [CO3=]. Alternatively, if the increased carbon flux in the equatorial Pacific resulted from release of carbon from localized geologic sources, the most depleted Delta14C values and the lowest [CO3=] would have occurred near the sites of carbon release. These two mechanisms will be distinguishable with measurements of 14C ages of benthic foraminifera, a proxy for changes in Delta14C and in B/Ca, a proxy for changes in [CO3=]. Stable isotope data (benthic foraminiferal delta13C and delta18O) will augment the 14C and B/Ca results for characterizing intermediate water properties and mixing of source water signals in the equatorial undercurrent system. Researchers at USC will make these proxy measurements and evaluate the results using an Earth System model of intermediate complexity (cGENIE) that is able to simulate how the isotope and [CO3=] gradients would change at intermediate depths in the Pacific if carbon was released from the Southern Ocean and/or from localized sources near hydrothermal vents.

认识到温室气体变异在地球最大的气候变化中发挥了主要作用，现在的挑战是发现哪些过程导致了伴随大规模气候变化的大气温室气体的变化。 更新世冰川/间冰期循环可以说是地球上最大、最剧烈的气候变化，与大气二氧化碳分压和甲烷的变化有关。但经过三十年的研究，科学界对于温室气体浓度变化的机制尚未达成共识。最近的发现揭示了地球碳系统的秘密，促使该奖项对这一假设进行了研究。该假说认为，在冰期-间冰期循环期间，二氧化碳流入和流出覆盖海洋热液系统的岩石和沉积物的通量随着温度的变化而变化。这是因为随着冰川作用期间温度降低，来自热液喷口的二氧化碳和富含二氧化碳的流体变得比海水密度更大。 密度较大的二氧化碳在岩石和沉积物中积累，直到冰川末端的海洋变暖。当冰川终止期间温度升高时，二氧化碳会变得有浮力并释放到海洋中。 热液系统释放的碳带有独特的地球化学指纹，印在海洋沉积物上。该项目将利用地球化学测量来调查这些地球化学信号是否在最后一次冰期到间冰期过渡期间印在沉积物上。初步证据支持这一假设。这项调查的结果不仅可以帮助我们了解地质过程如何影响地球过去的气候，还可以帮助我们了解碳地质储库对未来海洋温度上升的敏感程度。该奖项还将通过为研究生、本科生和高中生提供研究机会，支持南加州大学持续参与 STEM 培训。海洋沉积物中的新硼同位素和 14C 记录表明 14C- 通量显着增加。在末次冰期结束期间，随着大气中二氧化碳分压（pCO2）的上升，二氧化碳从赤道东太平洋耗尽到大气中。目前尚不清楚过量碳的来源以及贫 14C CO2 通量增加的原因。然而，人们提出了两种不同的假设来解释冰消历史。一种假设要求通过南大洋对正式隔离的深海水团进行通风，并通过中间水域将通风的碳运输到热带地区。另一种假说呼吁将地质碳从热液库释放到热带海洋中，以应对冰消过程中的海洋变暖。本研究旨在确定这两个假设中哪一个是正确的。每一种假设的机制都会在为赤道太平洋上升流系统提供水源的中间水域留下不同的地球化学指纹。所谓的“深海通风假说”预测，为赤道潜流贡献碳的源水将具有最低的 Delta14C 特征和最低的 [CO3=]。或者，如果赤道太平洋碳通量的增加是由于当地地质来源的碳释放造成的，那么最耗尽的 Delta14C 值和最低的 [CO3=] 将出现在碳释放地点附近。这两种机制可以通过底栖有孔虫 14C 年龄的测量来区分，14C 年龄是 Delta14C 变化的代表，B/Ca 是 [CO3=] 变化的代表。稳定同位素数据（底栖有孔虫 delta13C 和 delta18O）将增强 14C 和 B/Ca 结果，用于表征赤道暗流系统中的中间水性质和源水信号混合。南加州大学的研究人员将使用中等复杂度的地球系统模型（cGENIE）进行这些代理测量并评估结果，该模型能够模拟如果碳从太平洋中释放出来，同位素和[CO3=]梯度将如何在太平洋的中间深度发生变化。南大洋和/或来自热液喷口附近的局部来源。