RUI: COLLABORATIVE RESEARCH: Surface reservoir ages and ventilation in the Bering Sea during the last deglaciation from tephras and radiocarbon

对过去海洋的放射性碳估算是有价值的，因为它们有可能追踪水团的分布和运输，这对于理解海洋环流在气候变化中的作用都很重要。通过对底栖生物和浮游有孔虫的配对测量进行放射性碳重建的一个众所周知的问题是，大多数研究假设当地地表水的表观放射性碳年龄与当代大气（“地表水库年龄”）之间的偏移一直是恒定的。模拟结果表明，在过去的百年至千年时间尺度上，地表水库年龄可能发生了数百年的变化。地表储层年龄未被解释的变化是有问题的，因为它们可以在海洋内部的放射性碳重建中引入与海洋环流相关的变化一样大的信号。在这个合作项目中，研究小组将把白令海东南部海洋沉积物岩心中的火山灰层（tephras）与附近萨纳克岛（Sanak Island）独立确定年代的tephras进行比对。这将允许从这个位置重建地表储层年龄，并测试过去数百年储层年龄变化的假设。该团队还将能够重新评估基于有孔虫放射性碳的研究结论，其中作者假设恒定的地表水库年龄，包括北太平洋中间水通风在最后一次冰川消退期间与北大西洋反相的假设。这项研究的结果对各个学科都有潜在的影响，包括改进阿留申群岛东部最早人类考古遗迹的年代测定，以及了解那个时期的生态和气候变化带来的迁徙浪潮的时间。资金还支持来自威廉姆斯学院（RUI机构）的本科生的研究经历。更具体地说，最近发表的阿留申群岛东部萨纳克岛过去17天的温度年代学（Misarti et al. 2012）为重建亚北极太平洋的地表储层年龄和通风提供了一个新的独特机会，因为萨纳克岛靠近白令海东南部的乌姆纳克高原，该高原已被用于高分辨率的古海洋学工作。研究小组将使用电子探针和激光烧蚀感应耦合等离子体质谱法测量主要元素和微量元素的丰度，以匹配Sanak岛与附近三个海洋沉积物岩心之间的岩心，这些岩心的水深为1至2公里，并为岩心创建基于岩心的年龄模型。然后，他们将测量沉积物岩心中浮游生物放射性碳的时间序列，以产生白令海东南部当地地表储层年龄变化的记录。他们将以相同的分辨率测量底栖生物的放射性碳，并使用基于tephra的年龄模型来计算白令海1至2公里处水柱的Delta14C。通过这次重建，研究小组将能够评估北太平洋环流和白令海通风从最后一次冰川消融以来的变化，并约束过去北太平洋水团的末端成员特性。

Radiocarbon estimates of the past ocean are valuable because they have the potential to trace the distribution and transport of water masses, which are both important for understanding the role of ocean circulation in climate changes. A well-known problem with radiocarbon reconstructions from paired measurements of benthic and planktonic foraminifera is that most studies assume that the offset between the apparent radiocarbon age of the local surface water and the contemporary atmosphere (the "surface reservoir age") has been constant through time. Modeling simulations show that surface reservoir age probably changed by hundreds of years at centennial to millennial timescales in the past. Unaccounted-for changes in surface reservoir age are problematic, since they can introduce as large a signal into a radiocarbon reconstruction of the ocean interior as the changes associated with ocean circulation. In this collaborative project, the research team will match volcanic ash layers (tephras) from marine sediment cores in the southeast Bering Sea to independently-dated tephras from nearby Sanak Island. This will allow reconstruction of surface reservoir ages from this location and testing of the hypothesis that reservoir age varied by hundreds of years in the past. The team will also be able to re-evaluate the conclusions of studies based on foraminiferal radiocarbon where the authors assumed constant surface reservoir age, including the hypothesis that North Pacific intermediate water ventilation was antiphase with the North Atlantic during the last deglaciation. The results of this study have potential impact across disciplines, including improving the dating of archaeological remains from the earliest humans in the eastern Aleutians, and the understanding of the timing of waves of migration with ecological and climatic changes from that time period. Funding also supports research experiences for undergraduates from Williams College (an RUI institution). More specifically, the recently published tephrochronology from the last 17 ky from Sanak Island in the eastern Aleutians (Misarti et al. 2012) presents a new and unique opportunity to reconstruct surface reservoir ages and ventilation from the subarctic Pacific, since Sanak Island is near the Umnak Plateau in the southeast Bering Sea, which has been cored for high-resolution paleoceanographic work. The research team will measure major and trace element abundance with electron microprobe and laser-ablation-inductively-coupled-plasma mass spectrometry to match tephras between Sanak Island and three nearby marine sediment cores spanning 1 to 2 km water depth, creating tephra-based age models for the cores. They will then measure a time series of planktonic radiocarbon in the sediment cores to produce a record of local surface reservoir age changes from the SE Bering Sea. They will measure benthic radiocarbon at the same resolution and use the tephra-based age model to calculate the Delta14C of the water column at 1 to 2 km in the Bering Sea. With this reconstruction, the team will be able to evaluate proposed changes in North Pacific circulation and Bering Sea ventilation from the last deglaciation, and constrain the end-member properties of North Pacific water masses from the past.