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

“中更新世过渡”（MPT）是最近地质历史中全球气候系统最大的重组。它指的是大约100万年前的一段时间，当时冰河时代的周期长度从大约40，000年变为100，000年，冰河时代变得更加极端。在MPT期间，大气中的二氧化碳（CO2）显著地向低值转移，这将增强气候变冷。由于消失的二氧化碳本来会储存在海洋中，因此可以推断，二氧化碳的减少是对深海环流（也称为"温盐环流"）变化的反应。气候变率模式的这种转变发生时，系统的能量输入没有发生重大变化（即，天文强迫），从而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在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。由于前MPT反映了一种气候状态，其特征是冰川大气CO2升高和冰盖变小，因此提高对这一转变的理解将有助于科学家和政策制定者更好地预测未来大气CO2高水平并为此做好准备。这项研究探索了过去深部的地质档案，海洋环流和碳酸盐化学，以帮助阐明海洋环流和CO2的MPT的历史。这项工作采用了新的地球化学代用指标（Nd同位素、B同位素和B/Ca比值），以（1）重建和量化深海的温盐环流和CO2变化，（2）应用这些指标确定这一重大气候转变的性质和时间。使用来自北方，热带和南大西洋不同水深的沉积物岩心将使研究小组能够在MPT之前，期间和之后跟踪温盐环流的横向和垂直变化。钕同位素数据将补充已公布的海底碳稳定同位素数据，后者也反映了温盐环流，但也受到生物（生产力）、海气平衡和全球碳收支变化的影响。该项目还将研究深海二氧化碳储存，方法是根据硼同位素和底栖有孔虫中的B/Ca比率来估计海水pH值和碳酸盐饱和度。这些代理的配对分析提供了一个机会，以解决这一假设，即深海环流促进了同时代的大气pCO2下降在冰期，这反过来又增强了高纬度冰盖的增长，并建立了边界条件的发展100-kyr周期。资金支持研究生和本科生的研究，并通过拉蒙特-多尔蒂地球天文台年度开放日进行公众宣传。

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.