Ocean Temperatures Through Early Cenozoic Climate Maxima Across a Latitudinal Transect from the North to the South Pacific - A Multi-Proxy In Situ Approach

地球过去气候的记录为我们未来的气候提供了重要线索。例如，始新世早期(5600万至4900万年前)的特点是大气温室气体水平高于现代大气温室气体水平，全球平均气温远高于现代水平。这一时期可能是21世纪或22世纪的一个很好的类比。气候模型预测，到那时，温室气体排放将产生比地球至少3500万年来经历过的更温暖的条件。过去所有温暖气候时期的一个共同特征是高纬度地区的异常温暖。然而，目前的气候模型无法再现这些温暖的极地温度。古气候记录和模型模拟之间的这种不一致给气候研究带来了重大挑战。是古气候数据有偏见，是气候模型不准确，还是两者兼而有之？由于缺乏可靠的古气候数据，气候模型的测试目前受到限制。例如，过去海洋表面温度的许多记录都是基于浮游有孔虫化石贝壳的同位素和化学成分。这些微生物生活在整个表层海洋中，它们的外壳保存在海底沉积物中。然而，这些化石化学记录可能会被数百万年的沉积作用所退化。高分辨率显微成像现在可以识别化石有孔虫贝壳中保存较好的区域。最近的分析发展使测量这些微小区域的化学成分成为可能。拟议的研究将使用这些新方法重新评估始新世早期太平洋从赤道到极地的海面温度梯度。这些数据将为数据与模型的不匹配提供新的线索，并有助于提高气候模型的准确性。这项研究将支持一名研究生。该项目的成果将在向非科学家介绍古气候研究的科学外展中突出显示。我们准确模拟地球历史上温暖气候的能力是对我们对地球大气系统理解的最重要考验之一。特别令人感兴趣的是始新世早期至中期(距今5600万至4900万年前)，这是温室气体水平最后一次超过~600ppm二氧化碳(现代为412ppm，每年增加超过2ppm)。这一时期的一个令人费解的特征是，高纬度地区异常温暖，与相对凉爽的热带温度和微弱的纬度温度梯度有关。然而，即使是目前的气候模型也无法模拟这样的气候制度。这种数据模型的失配可能部分是由古气候档案的成岩改造造成的。在沉积物中存在数百万年后，成岩作用可能会改变有孔虫贝壳的原始同位素和化学成分，有孔虫贝壳是沙粒大小的海洋微化石，到目前为止，它们是地球过去气候的最重要记录。在过去的几年里，人们发现这些有孔虫贝壳中的微小(只有几十微米)区域比剩余的材料保存得更好。最近发展和改进的原位(“就地”)分析方法与高分辨率成像相结合，现在可以识别和分析有孔虫贝壳内保存较好的这些区域。通过使用这些新的和新兴的原位技术，我们的目标是通过南太平洋纵向断面上的早期始新世气候极大值来重新评估经向温度梯度。因此，我们将重点关注气候模型界最感兴趣的三个时间段：(1)早始新世气候最佳时期(~53-51 Ma)；(2)古新世-始新世最高温度时期(~56 Ma)；(3)古新世-始新世最高温度时期，以评估气候变暖前的背景条件。最先进的成像方法将与二次离子质谱仪(SIMS)相结合用于氧同位素分析，电子探针显微分析(EPMA)用于确定镁/钙(mg/Ca)比率，以及激光消融电感耦合等离子体质谱(LIBICMS)用于通过有孔虫洞壁进行多元素深度剖析(成岩作用的化学指纹)，以确定有孔虫壳体中保存更好的区域。现场数据将与传统的氧同位素测量相结合，以评估先前发布的古记录的潜在偏差。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Records of Earth’s past climate hold important clues to our future climate. For example, the early Eocene (56 to 49 million years ago) was characterized by higher-than-modern atmospheric greenhouse gas levels and much warmer mean global temperatures. This period may be a good analog for the 21st or 22nd century. By that time, climate models predict that greenhouse gas emissions will produce warmer conditions than the Earth has experienced for at least 35 million years. One common feature of all past periods of warm climate is the exceptional warmth of high latitude regions. However, current climate models fail to reproduce these warm polar temperatures. This disagreement between paleoclimate records and model simulations poses a major challenge in climate research. Are the paleoclimate data biased, are the climate models inaccurate, or both? Testing of climate models is currently limited by the scarcity of robust paleoclimate data. For instance, many records of past sea surface temperature are based on the isotopic and chemical composition of fossil shells of planktic foraminifera. These microorganisms live throughout the surface ocean and their shells are preserved in seafloor sediments. However, these fossil chemical records can be degraded by sedimentary processes acting over millions of years. High-resolution microscopic imaging now allows for the identification of better-preserved areas within fossil foraminifera shells. And recent analytical developments make it possible to measure the chemistry of these tiny areas. The proposed study will use these new methods to re-assess equator-to-pole sea surface temperature gradients in the Pacific Ocean during the early Eocene warm period. These data will shed new light on the data-model mismatches and help improve the climate models’ accuracy. The study will support a graduate student. The project results will be highlighted in a science outreach display to introduce paleoclimate studies to non-scientists.Our ability to accurately simulate warm climates in Earth history provides one of the most important tests of our understanding of the Earth’s atmospheric system. Of particular interest is the early to middle Eocene 56 to 49 million years ago) which is the last time that greenhouse gas levels exceeded ~600 ppm CO2 (modern 412 ppm, increasing more than 2 ppm/year). A puzzling feature of this time is the exceptional warmth of high-latitude regions associated with relatively cool tropical temperatures and a weak latitudinal temperature gradient. However, even current climate models fail to simulate such a climate regime. This data-model mismatch may be partially caused by diagenetic alteration of paleoclimate archives. After millions of years in the sediment, diageneses may alter the original isotopic and chemical composition of foraminifer shells, sand-grain-sized marine microfossils that are – by far – the most important recorders of the Earth’s past climate. Within the past years, it was found that minute (just tens of microns) domains within these foraminifer shells are better preserved than the remaining material. Recent developments and improvements of in situ (‘in place’) analytical approaches in combination with high-resolution imaging now allows for the identification and analysis of these better-preserved domains within foraminifer shells. By using these new and emerging in situ technologies, we aim to re-asses meridional temperature gradients through early Eocene climate maxima across a South Pacific longitudinal transect. Thereby, we will focus on three time intervals that are of highest interest for climate modeling community: (1) The Early Eocene Climate Optimum (~53 – 51 Ma); (2) The Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), (3); The period just before the PETM to assess to pre-warming background conditions. State-of-the-art imaging approaches to identify better preserved domains within foraminifer shells will be used in combination with Secondary Ion Mass Spectrometry (SIMS) for oxygen isotope analysis, Electron Probe Microanalysis (EPMA) for the determination of Magnesium/Calcium (Mg/Ca) ratios, and Laser-Ablation ICP-MS for multielement depth profiling through foraminifer chamber walls (chemical ‘fingerprinting’ of diagenesis). The in situ data will be paired with conventional oxygen isotope measurements to assess the potential bias of previously published paleorecords.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.