Collaborative Research: Developing a high-resolution late Holocene sediment record of rapid Arctic climate change from the Beaufort Sea coastal zone

The PIs propose to develop a new, high-resolution (annual to subdecadal) paleoclimate record (0 to ~2,000 y) from sediment cores taken in Arctic lagoonal settings adjacent to rivers emptying into the Beaufort Sea. This effort is a follow-up to an EAGER award that allowed collection of initial core material and a seismic dataset in Simpson Lagoon near the Colville River delta in August 2010. Initial results suggest that these sediment cores contain a valuable history of system response to climate change on the adjacent continent (e.g., river drainage basins) and terrestrial-marine linkages. All cores will be analyzed for stratigraphy (X-radiography), bulk organic and mineral content, granulometry, and geochronology (239,240Pu/137Cs). Detailed analysis for paleoclimate proxies will be carried out: the age-depth relationship for the deeper part of these cores will be determined using radiocarbon. Climate indicators that will be applied are organic biomarkers (lignin-phenols, cutins, plant pigments, and ä13C) and mineral tracers (major and trace element chemistry, stable isotopes, clay mineralogy, heavy mineral assemblages, granulometry, event-layer stratigraphy). The central hypothesis is that lagoonal settings within the Beaufort Sea contain high quality sediment records because they receive sediment input from larger rivers that drain the Brooks Range, that, if exploited, will dramatically improve our knowledge of high Arctic paleoclimate over the last two millenia. The PIs hypothesize three distinct sources: 1) rivers that drain the Brooks Range and the Arctic Coastal Plain (ACP), 2) storms that erode shoreline permafrosted ACP deposits and overwash barrier islands, and 3) marine primary production by phytoplankton, ice algae, and benthos. After an initial micro-scale examination of recent event layer (annual and storm deposits) mineral and POC succession, necessary to interpret paleoclimate-induced fluctuations in composition, they will examine the downcore record of temporal variations in layers, contrasting sites to decouple spatial changes in sources. Since the three source end-members have distinct mineral/POC composition, they believe that temporal variations will provide proxy information about climate changes that influence their flux magnitude including 1) landfast ice extent, 2) river sediment supply from the glaciated uplands, 3) permafrost thawing, 4) summer-fall storminess, and 5) coastal primary productivity. Noting the rapid changes that have occurred in the Arctic during the latter half of the 20th century, the federal interagency Study of Environmental Arctic Change (SEARCH) program has cited the need to determine to what extent recent and ongoing climate changes in the Arctic can be attributed to anthropogenic forcing. A successful project will contribute to answering that question.

本项目首席研究员（Principal Investigators, PIs）拟从注入波弗特海（Beaufort Sea）的相邻河流周边的北极泻湖环境中获取沉积物岩芯（sediment cores），以此构建一套全新的、高分辨率（年际至亚十年尺度）的古气候（paleoclimate）记录，时间跨度为0至约2000年。本研究是对一项EAGER项目的后续工作——该项目于2010年8月在科尔维尔河（Colville River）三角洲附近的辛普森泻湖（Simpson Lagoon）完成了初始岩芯样品与地震数据集的采集。初步研究结果表明，这些沉积物岩芯蕴含了相邻大陆（如河流流域）系统对气候变化的响应历史，以及陆-海相互作用的宝贵记录。所有岩芯将开展以下分析：地层学（X射线成像（X-radiography））、总有机质与矿物含量测定、粒度分析（granulometry）以及地质年代学（geochronology）分析（采用239,240Pu/137Cs同位素体系）。本研究还将针对古气候代用指标（paleoclimate proxies）开展详细分析：岩芯深部的年龄-深度关系将通过放射性碳测年法确定。本次研究将采用的气候代用指标包括有机生物标志物（木质素酚类、角质素、植物色素以及δ13C）与矿物示踪剂（常量与微量元素化学组成、稳定同位素、黏土矿物学特征、重矿物组合、粒度分析结果、事件层地层学特征）。本研究的核心假说为：波弗特海周边的泻湖环境拥有高质量的沉积物记录，因为其沉积物来自布鲁克斯山脉（Brooks Range）流域的大型河流；若对该记录进行解译，将极大提升我们对过去2000年北极高纬地区古气候的认知水平。项目首席研究员提出了三类明确的物质来源：1）流经布鲁克斯山脉与北极海岸平原（Arctic Coastal Plain, ACP）的河流；2）侵蚀沿岸永久冻土覆盖的北极海岸平原沉积物并越岸冲积障壁岛的风暴；3）浮游植物、冰藻与底栖生物所贡献的海洋初级生产产物。在开展初步的微观尺度分析（用于解译古气候驱动的组成波动）——即对近期事件层（年沉积与风暴沉积）的矿物与颗粒有机碳（Particulate Organic Carbon, POC）序列进行研究——之后，团队将分析岩芯垂向上的层理时间变化记录，并通过对比不同采样点位，以区分物质来源的空间差异。由于三类源端元具有独特的矿物/颗粒有机碳组成特征，团队认为，物质通量的时间变化将提供与气候变化相关的代用信息——这些气候变化会影响物质的通量强度，具体包括：1）固定冰覆盖范围；2）冰川化高地的河流沉积物供给量；3）永久冻土融化程度；4）夏秋风暴活动强度；5）沿岸初级生产水平。鉴于20世纪后半叶北极地区发生的快速变化，跨联邦机构的北极环境变化研究（Study of Environmental Arctic Change, SEARCH）项目指出，亟需明确北极近期与当前的气候变化在多大程度上可归因于人为强迫（anthropogenic forcing）。本项目的顺利实施将有助于解答这一科学问题。