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

认识到温室气体变率在地球上最大的气候变化中发挥了主要作用，现在的挑战是发现伴随大气候变化的大气温室气体变化的过程是什么。更新世冰期/间冰期旋回可以说是地球上与大气二氧化碳分压和甲烷变化相关的最大和最剧烈的气候变化。但是经过三十年的研究，科学界对温室气体浓度变化的机制并没有达成共识。最近的发现揭示了地球碳系统的秘密，促使该假说获得了该奖项。该假说认为，在冰期-间冰期旋回期间，二氧化碳进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。这是因为在冰期温度下降时，来自热液喷口的二氧化碳和富含二氧化碳的液体比海水密度大。密度较大的二氧化碳积聚在岩石和沉积物中，直到海洋在冰川终点变暖。当冰期结束时温度升高，二氧化碳变得浮力，并被释放到海洋中。热液系统释放的碳带有独特的地球化学指纹，这些指纹会印在海洋沉积物上。该项目将使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的过渡时期被印在沉积物上。初步证据支持这一假设。这项调查的结果不仅可以让我们了解地质过程如何影响地球过去的气候，还可以让我们了解地质碳储层对未来海洋温度上升有多敏感。该奖项还将通过为研究生、本科生和高中毕业生提供研究机会，支持南加州大学持续参与STEM培训。来自海洋沉积物的硼同位素和14C新记录表明，在末次冰期终止期间，随着大气pCO2的上升，赤道东太平洋向大气中损耗14C的CO2通量显著增加。过量碳的来源以及消耗14c的二氧化碳通量增加的原因目前尚不清楚。然而，人们提出了两种不同的假说来解释冰川史。一种假设要求一个正式孤立的深海水团通过南大洋通风，并通过中间水域将通风的碳输送到热带地区。另一种假说认为，在冰川消退期间，由于海洋变暖，地质碳从热液储层释放到热带海洋中。这项研究试图确定这两种假设中哪一种是正确的。每种假设的机制都会在赤道太平洋上升流系统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，如果赤道太平洋的碳通量增加是由于局部地质源的碳释放造成的，那么最耗尽的Delta14C值和最低的[CO3=]将发生在碳释放点附近。这两种机制将通过测量底栖有孔虫的14C年龄来区分，14C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的结果，用于表征赤道潜流系统的中间水性质和源水信号的混合。南加州大学的研究人员将进行这些代理测量，并使用中等复杂性的地球系统模型（cGENIE）来评估结果，该模型能够模拟如果碳从南大洋和/或热液喷口附近的局部来源释放，太平洋中深度的同位素和[CO3=]梯度将如何变化。

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.