Collaborative Research: The role of the Southern Ocean in Late Miocene climate change

对全球气温上升的担忧使得预测未来的气候变化变得非常重要。然而，要预测未来的气候，需要更好地了解气候系统在变暖的世界中是如何运作的。更好地了解未来暖化的一种方法是观察过去的暖化时期。该项目将探索最近的一个长期温暖时期，中新世晚期，大约500万到850万年前。中新世的特点是全球气温变暖，尤其是在极地地区。然而，目前的模式不能正确地预测中新世的温度，而且经常低估变暖。预测和理解这个变暖的世界的能力受到中新世晚期观测资料的限制。南大洋，也就是南极洲周围的海域，尤其如此。该区域在控制全球温度变化方面发挥着重要作用，因为它通过物理过程（如海洋混合）和生物生产力（如光合作用）与大气中的二氧化碳相互作用。目前，没有关于南大洋太平洋部分中新世晚期海洋表面温度或生物生产力的记录。该项目将生成该地区中新世晚期海洋条件的记录，包括海洋表面温度和生物生产力。这些数据将记录中新世晚期南大洋的温度。这些数据还将为了解变暖世界的气候如何响应大气二氧化碳的变化提供关键见解。最后，该研究将评估南大洋的过程可能对中新世末期全球变冷做出了哪些贡献。这项工作将支持研究生和本科生进行海洋学和科学教育。作为该项目的一部分，科学教育硕士学生和海洋学博士学生将共同努力，在公平的教学方法和课程设计方面发展专业知识。中新世（23.3 - 5.05 Ma）对于理解是至关重要的，因为它是一个持续变暖的时期，导致了北半球冰川作用的早期阶段。最近的中新世模式比较研究（MioMIP1）表明：中新世晚期的大部分变暖归因于大气二氧化碳（pCO2）强迫；气候对二氧化碳分压强迫的敏感性取决于状态；对日照轨道变化的区域敏感性了解甚少，且依赖于模式；虽然经向海面温度（SST）梯度减小通常是温暖气候的一个强大特征，对水文循环有深远的影响，但它并没有很好的约束。最大的数据模型偏差之一是在南部高纬度地区，那里的模型严重低估了观测到的变暖。然而，这种不匹配是由几个南方地点的观测结果决定的，这些地点都不是来自太平洋南大洋（SO）中部的广大地区。该项目将从最近钻探的一个地点（IODP Expedition 383, site U1541）中太平洋地区首次获得中新世晚期至上新世早期（8.3 - 4.0 Ma）的古海洋记录。这些记录涵盖了晚中新世冷却和晚中新世碳同位素转移，将有助于确定南太平洋经向海温梯度，评估长期气候对pCO2强迫和轨道强迫的敏感性，评估海温与古生产力变率之间的联系，以及深水环流的演变。该项目的目标是：(a)建立一个基于英国37年的高分辨率海温记录，以限制经向海温梯度的冷南端成员，并评估随着平均状态的变化，pCO2强迫的减少和轨道变率的潜在变化的响应；(b)使用古生产力代用物来验证中新世晚期海洋生产力的增强有助于去除大气中二氧化碳，从而进入上新世的假设。虽然这不是该项目的重点，但该研究还将利用U1541的底栖有孔虫d13C来限制深海d13C的纬向和经向梯度，以了解全球深水环流的潜在变化。来自U1541遗址（一个以前未被研究过的地区）的中新世晚期新数据将为气候系统在温暖气候下的运作提供新的见解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Concern over rising global temperatures makes it important to predict future climate change. However, to predict future climate requires a better understanding of how the climate system works in a warmer world. One way to better understand future warmth is to look at a warm period in the past. The project will explore one of the most recent periods of long-term warmth, the late Miocene, approximately 5 to 8.5 million years ago. The Miocene was characterized by warmer temperatures globally, and particularly in the polar regions. However, currently models are not able to correctly hindcast Miocene temperatures and often underestimate the warming. The ability to predict and understand this warmer world is limited by the availability of observations from the late Miocene. This is particularly true of the Southern Ocean, the area of ocean around Antarctica. This region plays an important role in controlling global temperature changes because it interacts with atmospheric carbon dioxide through physical processes (e.g., ocean mixing) and biological productivity (e.g., photosynthesis). Currently, there are no records of surface ocean temperature or biological productivity from the late Miocene for the Pacific sector of the Southern Ocean. This project will generate records of late Miocene oceanographic conditions in this region, including ocean surface temperature and biological productivity. These data will document how warm the Southern Ocean was during the late Miocene. The data will also provide key insights into how climate in a warmer world responds to changes in atmospheric carbon dioxide. Finally, the study will assess how processes in the Southern Ocean may have contributed to the cooling of the globe at the end of the Miocene. This work will support graduate and undergraduate students in both oceanography and science education. As part of the project, science education MS students and oceanography PhD students will work together to develop expertise in equitable teaching methods and curricular design.The Miocene (23.3 - 5.05 Ma) is critical to understand as it was a period of sustained warmth leading up to the early phase of Northern Hemisphere glaciation. A recent Miocene model comparison study (MioMIP1) indicates that: during the late Miocene the majority of warmth is attributed to atmospheric carbon dioxide (pCO2) forcing; climate sensitivity to pCO2 forcing is state dependent; regional sensitivity to orbital variations in insolation is poorly understood and model-dependent; while a reduced meridional sea surface temperature (SST) gradient is generally a robust feature of warm climates with profound impacts on the hydrologic cycle, it is not well-constrained. One of the largest data-model misfits is in the southern high latitudes where models severely underestimate observed warming. However, this misfit is determined by observations from only a few southern sites, none of which are from the vast region of the central Pacific Southern Ocean (SO). This project will produce the first paleoceanographic records from the late Miocene to early Pliocene (8.3 – 4.0 Ma) in the Central Pacific sector of the SO from a recently drilled site (IODP Expedition 383, Site U1541). These records, covering the Late Miocene Cooling and Late Miocene Carbon Isotopic Shift, will enable the determination of South Pacific meridional SST gradients, an evaluation of long-term climate sensitivity to pCO2 forcing and orbital forcing, an assessment of the links between SST and paleoproductivity variability, and of the evolution of deep-water circulation. The project objectives are to: (a) develop a high-resolution Uk’37-based SST record to constrain the cold southern end member of the meridional SST gradient and to evaluate responses to decreasing pCO2 forcing and potential changes in orbital variability as the mean state changed, and (b) use paleoproductivity proxies to test the hypothesis that enhanced marine productivity during the late Miocene contributed to the removal of atmospheric CO2 leading into the Pliocene. Although not the focus of this project, the study will also use benthic foraminiferal d13C at U1541 to constrain zonal and meridional gradients in deep ocean d13C to inform potential changes in global deep-water circulation. The new late Miocene data from Site U1541, a previously unstudied region, will provide new insight into the workings of the climate system in a warmer climate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.