Thermohaline Circulation and Deep Ocean Carbonate Chemistry across the Mid-Pleistocene Transition

“中更新世过渡”（MPT）是近代地质史上全球气候系统最大的一次重组。它指的是大约发生在100万年前的间隔，当时冰河时代周期的长度从4万年变为10万年，冰河时代变得更加极端。在MPT期间，大气二氧化碳（CO2）向较低值显著转移，这将增强气候冷却。由于丢失的二氧化碳可能已经储存在海洋中，因此人们推断，二氧化碳的减少是对深海环流（也被称为“温盐环流”）变化的反应。这种气候变率模式的转变在系统能量输入（即天文强迫）没有显著变化的情况下发生，因此意味着气候系统的非线性响应和/或海洋-大气-冰冻圈内部的变化是最终的原因。这项研究将有助于量化这一关键气候时期深海环流、碳循环和全球气候变化之间的相互作用。由于mpt前期反映了一种以冰川大气二氧化碳升高和冰盖变小为特征的气候状态，因此提高对这一转变的理解将有助于科学家和决策者更好地预测和准备未来可预测的高大气二氧化碳水平。具体而言，本研究探索了过去深海环流和碳酸盐化学的地质档案，以帮助阐明MPT上海洋环流和二氧化碳的历史。本工作采用了新的地球化学指标（Nd同位素、B同位素和B/Ca比值）来(1)重建和量化深海温盐环流和CO2变化；(2)应用这些指标来确定这一主要气候转变的性质和时间。利用来自北大西洋、热带大西洋和南大西洋不同水深的沉积物岩心，研究小组可以在MPT之前、期间和之后追踪温盐环流的横向和垂直变化。Nd同位素数据将补充已发表的底栖碳稳定同位素数据，后者也反映了热盐环流，但也受到生物（生产力）、海气平衡和全球碳收支变化的影响。该项目还将通过估算硼同位素的海水ph值和碳酸盐饱和度以及底栖有孔虫的B/Ca比来研究深海二氧化碳储存。对这些代用物的配对分析提供了解决深海环流促进冰期同期大气pCO2下降的假设的机会，这反过来又促进了高纬度冰盖的生长，并为100-kyr旋回的发展建立了边界条件。资金支持研究生和本科生的研究，并通过拉蒙特-多尔蒂地球观测站年度开放日向公众宣传。

The "Mid-Pleistocene Transition" (MPT) was the biggest reorganization of the global climate system in the recent geological past. It refers to the interval, occurring roughly a million years ago, when the lengths of ice age cycles shifted from ~40,000 to 100,000 years and ice ages became more extreme. During the MPT, atmospheric carbon dioxide (CO2) shifted significantly towards lower values, which would enhance climate cooling. Since the missing CO2 would have been stored in the ocean, it has been inferred that the decrease is a response to changes in the deep-ocean's circulation (also known as "thermohaline circulation"). This shift in the mode of climate variability occurred without significant change in the system's energy inputs (i.e., astronomical forcing), thus implying that non-linear responses of the climate system and/or internal changes within the ocean-atmosphere-cryosphere were ultimately responsible. This research will help quantify the interaction between deep-ocean circulation, the carbon cycle, and global climate change during this critical climatic period. Because the pre-MPT reflects a climate state characterized by elevated glacial atmospheric CO2 and smaller ice sheets, improving understanding of this transition will help scientists and policy makers to better predict and prepare for predicted high atmospheric CO2 levels in the future.Specifically, this research explores geological archives of past deep-ocean circulation and carbonate chemistry to help elucidate the history of ocean circulation and CO2 over the MPT. The work applies novel geochemical proxies (Nd isotopes, B isotopes and B/Ca ratios) to (1) reconstruct and quantify thermohaline circulation and CO2 changes in the deep-ocean and (2) apply these indicators to establish the nature and timing of this major climatic transition. Using sediment cores from different water depths in the northern, tropical, and south Atlantic will allow the research team to track lateral and vertical changes in the thermohaline circulation before, during and after the MPT. The Nd isotope data will complement the published benthic carbon stable isotope data, which also reflect thermohaline circulation but are also affected by biology (productivity), air-sea equilibration and changes in the global carbon budget. The project will also investigate deep-ocean CO2 storage by estimating seawater-pH and carbonate saturation from boron isotopes and B/Ca ratios in benthic foraminifers. Paired analysis of these proxies provides the opportunity to address the hypothesis that deep-ocean circulation facilitated the coeval drawdown of atmospheric pCO2 during glacial periods, which in turn enhanced high-latitude ice sheet growth, and established the boundary conditions for the development of 100-kyr cycles. Funding supports graduate and undergraduate research, and public outreach via the Lamont-Doherty Earth Observatory Annual Open House.