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

RUI：合作研究：末次冰消期期间白令海地表储层年龄和通风来自火山灰和放射性碳

• 批准号: 1435691
• 负责人: Alan Mix
• 金额: $ 14.75万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-10-01 至 2018-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1435691&HistoricalAwards=false
• 关键词: RUI; Collaborative; Research; Surface; reservoir

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.

对过去海洋的放射性碳估计是有价值的，因为它们有可能追踪水团的分布和运输，这两者对于理解海洋环流在气候变化中的作用都很重要。通过对底栖有孔虫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，大多数研究都假设当地地表水和当代大气的表观放射性碳年龄（“地表水库年龄”）之间的偏移量随时间推移是恒定的。模型模拟表明，在过去的百年至千年的时间尺度上，地表储层年龄可能变化了数百年。表面储层年龄的不明变化是有问题的，因为它们可以引入与海洋环流相关的变化一样大的信号到海洋内部的放射性碳重建中。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）与附近萨纳克岛的独立定年的tephras相匹配。这将允许从该位置重建地表储层年龄，并测试储层年龄在过去数百年内变化的假设。该小组还将能够重新评价基于有孔虫放射性碳的研究结论，其中作者假定表层水库年龄不变，包括北太平洋中层水通风在最后一次冰消期与北大西洋反相的假设。这项研究的结果具有跨学科的潜在影响，包括改善阿留申群岛东部最早期人类的考古遗迹的年代测定，以及了解该时期生态和气候变化的移民浪潮的时间。资金还支持威廉姆斯学院（RUI机构）本科生的研究经验。更具体地说，最近发表的阿留申群岛东部萨纳克岛最近17千年的火山灰年代学（Misarti等人，2012年）为重建亚北极太平洋的表层水库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，该高原已被取芯用于高分辨率古海洋学工作。研究小组将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和微量元素丰度，以匹配萨纳克岛和附近三个水深1至2公里的海洋沉积物岩心之间的火山灰，为岩心创建基于火山灰的年龄模型。然后，他们将测量沉积物岩心中的碳同位素放射性碳的时间序列，以记录白令海东南部的局部表面水库年龄变化。他们将以相同的分辨率测量海底放射性碳，并使用基于火山喷发的年龄模型计算白令海1至2公里处水柱的Δ 14C。通过这种重建，该团队将能够评估北太平洋环流和白令海通风从最后一次冰消期的拟议变化，并限制过去北太平洋水团的端元属性。