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一次重组。它指的是大约发生在100万年前的时间间隔，当时冰河时代的周期长度从约4万年转变到10万年，冰河时代变得更加极端。在MPT期间，大气二氧化碳(CO2)显著向较低的值移动，这将加强气候降温。由于丢失的二氧化碳将被储存在海洋中，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。气候变化模式的这种转变在系统的能量输入没有发生重大变化(即天文强迫)的情况下发生，因此意味着气候系统的非线性反应和/或海洋-大气-冰冻圈内部的变化是最终的责任。这项研究将有助于量化深海环流、碳循环和全球气候变化在这一关键气候时期的相互作用。由于前MPT反映了以冰川大气二氧化碳升高和冰盖变小为特征的气候状态，提高对这种转变的理解将有助于科学家和政策制定者更好地预测和准备未来预测的高大气二氧化碳水平。具体地说，这项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。这项工作应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)重建和量化深海的温盐循环和二氧化碳变化，以及(2)应用这些指标来确定这一重大气候转变的性质和时间。利用北大西洋、热带大西洋和南大西洋不同水深的沉积物岩心，研究小组将能够跟踪MPT之前、期间和之后温盐环流的横向和垂直变化。ND同位素数据将补充已公布的海底碳稳定同位素数据，后者也反映了温盐循环，但也受到生物(生产力)、海-气平衡和全球碳收支变化的影响。该项目还将通过根据底栖有孔虫中的硼同位素和B/Ca比率估计海水pH值和碳酸盐饱和度来研究深海二氧化碳的储存。对这些指标的配对分析使我们有机会提出这样一种假设，即深海环流促进了冰川时期大气二氧化碳的同年下降，进而促进了高纬度冰盖的增长，并为100-KYR循环的发展奠定了边界条件。资金支持研究生和本科生的研究，以及通过拉蒙特-多尔蒂地球天文台年度开放参观活动进行的公众宣传。

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.