Collaborative Research: Resolving the LGM ventilation age conundrum: New radiocarbon records from high sedimentation rate sites in the deep western Pacific

一个多世纪以来，科学家们一直试图理解为什么冰盖从极地扩展到中纬度地区，使地球进入冰河时代。在20世纪上半叶，科学家们关注的是由于地球绕太阳轨道的变化而引起的太阳辐射的变化。到20世纪中期，地球科学界开发了利用化石重建气候变量（如温度和冰量）的新方法。这一重要进展使人们认识到，在过去的250万年里，地球的气候有一种节律性，就像地球的轨道周期一样。但是，轨道引起的气候变化的原始理论存在一个问题。到20世纪后期，人们已经清楚，两个半球的冰是同时前进和后退的。这与轨道理论是不相容的，除非有其他的东西会增加太阳的影响，同时影响整个地球。这正是科学家们从冰芯中发现的。科学家们利用南极冰层中的空气样本表明，在上一个冰河时代，大气中的二氧化碳含量比现在低30%。随着这一重大科学进步，人们认识到轨道变化会以某种方式影响二氧化碳水平的变化。但如何?三十多年来，轨道周期与地球碳循环之间的联系一直是古气候研究和辩论的主要焦点。这个项目通过测试一个流行的假设来解决科学争论，即“海洋在冰河时期储存大气中的二氧化碳吗？”在这项工作中，科学小组将测试在最后一个冰河时期，深海的循环是否放缓到足以让二氧化碳在深海中积聚。放射性碳技术将用于确定深水在太平洋深处停留了多长时间才返回水面。如果海洋环流在上一个冰河时期显著减缓，这将有力地支持海洋调节大气二氧化碳的观点。目前可用的放射性碳数据无法回答这个问题，主要是因为深海沉积物积累太慢，无法进行精确的年龄测定。新的研究将通过从沉积物积聚非常迅速的地区恢复深海记录来解决这一挑战，这将使研究小组能够精确地确定太平洋深水在上一个冰河时期的年龄。如果研究结果表明，当二氧化碳浓度较低时，深海水域并没有明显变老，这将促使科学家们寻找其他替代解释。该项目将为本科生和研究生提供培训，特别是来自传统上被排斥群体的学生。学生也将有机会参加研究游船收集样本。目前可获得的来自深太平洋低沉积速率点（10 cm/kyr）的放射性碳数据表明，深水通风年龄比今天早800-1000年，而高沉积速率点（20 cm/kyr）表明没有实质性差异。这种对比是由于：1)低积累率位置的生物扰动驱动的偏差，或2)缺乏3公里以下水深的高积累率记录。提出的研究将通过从北太平洋西部高沉积速率地点获取新的高分辨率稳定同位素和放射性碳数据来解决这一问题。填补太平洋深处的数据空白对于评估这样一种假设至关重要，即更缓慢的海洋环流允许呼吸碳在深海太平洋积累，从而导致末次盛冰期（LGM）期间大气二氧化碳水平降低。该项目的目标是从西太平洋棉兰老岛的边缘获得一套岩心，以重建水深在1000到3800米之间的通风年龄。棉兰老岛的边缘非常适合这项任务，因为该地区的沉积速率非常高，从30厘米/平方公里到75厘米/平方公里不等。通过从高累积率的位置创建新的通风年龄重建，将有可能确定地测试在LGM期间太平洋深处的通风率是否较低，正如海洋环流假说所预测的那样。这项研究将解决古气候科学中的一个重大挑战：在冰期/间冰期时间尺度上，是什么控制着大气中的二氧化碳？有人提出，在冰期极大期，深海通过生物和物理过程的结合封存CO2，在降低大气CO2方面发挥了主要作用。测试海洋是否吸收了二氧化碳将是解决冰期/间冰期二氧化碳问题的重要一步。拟议的研究对这一努力至关重要，因为之前评估冰期通风年龄是否更大的尝试，由于缺乏来自太平洋深处（3公里）的高分辨率数据集而受到阻碍，而太平洋深处是代谢衍生碳储存的最大潜在库。在目前可用于验证流行假设的数据中，大多数来自可能受到生物扰动污染的极低分辨率（低沉积速率岩心）。此外，没有来自太平洋2800米以下沉积物积累速率大于20 cm/kyr的地点的通风年龄估计。该项目将克服现有数据库的局限性，从北太平洋西部的深度处获得高度分辨率的记录，这些记录将确认在LGM期间通风年龄是否大大延长。该项目将支持对学生的培训，重点是从传统上被排斥的群体中招收学生。此外，来自三所院校的学生将参与研究巡航，并在实验室合作。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。

For more than a century, scientists have sought to understand why ice sheets expanded from the polar regions into the middle latitudes, plunging the Earth into an Ice Age. In the first part of the 20th century, scientists focused on changes in solar radiation due to changes in the Earth’s orbit around the Sun. By the mid-20th century, the Earth Science community developed new methods to reconstruct climate variables, like temperature and ice volume using fossils. That important advance led to the recognition that there is a rhythmicity to Earth’s climate during the last 2.5 million years that is like that of the Earth’s orbital cycles. But there was a problem with the original theory of orbital-induced climate variability. By the late 20th century, it had become clear that ice had advanced and retreated in both hemispheres simultaneously. This is incompatible with the orbital theory unless there was something else that would increase the sun’s impact to affect the entire planet simultaneously. That was exactly what scientists discovered from ice cores. Using samples of air trapped in Antarctic ice, scientists showed atmospheric CO2 levels were 30% lower during the last ice age. With that major scientific advance came the realization that somehow orbital variations influence changes in carbon dioxide levels. But how? The connection between orbital cycles and the Earth’s carbon cycle has been a primary focus of paleoclimate research and debate for more than three decades. This project addresses the scientific debate by testing one of the prevailing hypotheses, that is “Do the ocean’s store atmospheric CO2 during ice ages?” In this work, the scientific team will test whether the circulation of the deep ocean slowed down enough during the last ice age to allow CO2 to build up in the deep ocean. Radiocarbon techniques will be used to date how long deep waters resided in the deep Pacific before returning the surface. If the ocean circulation slowed significantly during the last ice age, it would lend strong support to the notion that the oceans regulate atmospheric CO2. The radiocarbon data currently available cannot answer this question, primarily because deep sea sediments accumulate too slowly to allow precise age dating. The new research will address this challenge by recovering deep sea records from a region where sediments accumulate very rapidly, which will allow the team to precisely date how old the deep waters in the Pacific were during the last ice age. If the findings show that the deep ocean waters were not significantly older when CO2 concentrations were low, it will prompt scientists to look to other alternative explanations. This project will provide training for undergraduate and graduate students especially students from traditionally excluded groups. Students will also have an opportunity to participate in the research cruise to collect the samples. Currently available radiocarbon data from low sedimentation rate sites (10 cm/kyr) in the deep Pacific imply deep water ventilation ages were ~800-1000 years older than today, whereas high sedimentation rate sites (20 cm/kyr) indicate there was no substantial difference. The contrast is due to either: 1) bioturbation-driven biases in low accumulation rate locations, or 2) the lack of high accumulation rate records from water depths below 3 km. The research proposed will address this problem by acquiring new high-resolution stable isotope and radiocarbon data from high deposition rate sites in the western North Pacific. Filling the deep Pacific data gap is essential to assess the hypothesis that a more sluggish ocean circulation allowed respired carbon to accumulate in the abyssal Pacific, leading to lower atmospheric CO2 levels during the Last Glacial Maximum (LGM). The goal of the project is to obtain a suite of cores from the margin of Mindanao in the western Pacific to reconstruct the ventilation ages at water depths between 1000 and 3800m. The margin of Mindanao is ideally suited for this task because sedimentation rates in the region are very high, ranging from 30 cm/kyr to 75 cm/kyr. By creating new ventilation age reconstructions from the high accumulation rate sites, it will be possible to definitively test whether ventilation rates in the deep Pacific were lower during the LGM, as predicted from the ocean circulation hypothesis. The research will address one of the grand challenges in paleoclimate science: What controls atmospheric CO2 on glacial/interglacial timescales? It has been proposed that the deep ocean played a primary role in lowering atmospheric CO2 during glacial maxima by sequestering CO2 through a combination of biological and physical processes. Testing whether the ocean sequestered CO2 would be a major step forward in solving the glacial/interglacial CO2 problem. The proposed research is critical to this effort because previous attempts to evaluate whether ventilation ages were greater during glaciations has been hampered by a lack of high-resolution data sets from the deep ( 3 km) Pacific, which is the largest potential reservoir for the storage of metabolically- derived carbon. Of the data currently available to test the prevailing hypothesis, most are from very low resolution (low deposition rate cores) that have likely been contaminated by bioturbation. Furthermore, there are no ventilation age estimates from sites below 2800 m in the Pacific that have sediment accumulation rates greater than 20 cm/kyr. This project will overcome the limitations of the current database by obtaining highly resolved records from sites in the western North Pacific at depths that will confirm whether or not ventilation ages were substantially older during the LGM. The project will support the training of students with an emphasis on recruiting students from traditionally excluded groups. In addition, students from all three institutions will participate in the research cruise and collaborate in the lab.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.