Response of the Labrador Sea and south Greenland Ice Sheet to the mid-Pliocene climate optimum: sedimentary, magnetic and geochemical evidence from the Eirik Drift

拉布拉多海和南格陵兰冰盖对上新世中期气候最佳状态的响应：来自埃里克漂移的沉积、磁性和地球化学证据

• 批准号: 1352008
• 负责人: Anders Carlson
• 金额: $ 40万
• 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: 2014-08-01 至 2017-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1352008&HistoricalAwards=false
• 关键词: Response; Labrador; Sea; south; Greenland

In 2013, atmospheric carbon dioxide (CO2) concentrations reached 400 ppm, raising questions as to how the Earth will respond to this unprecedented forcing in human history. A major concern is the response of Earth's remaining ice sheets to present-day (and future) CO2 levels, and their attendant contribution to sea-level rise. In the case of the Greenland Ice Sheet (GIS), models suggest a threshold behavior where the GIS will completely melt once a certain temperature is exceeded. However, different models suggest different thresholds, so it is unclear whether the Earth has already crossed an ice-free Greenland tipping point. The last period when atmospheric CO2 reached ~400 ppm was during the mid-Pliocene climate optimum 3.3-3.0 million years ago, providing a past analogue to Earth's present climate. Climate and ice-sheet models simulate a warmer Earth with a smaller GIS, but show a range of responses. Pliocene sea-level records are significantly complicated by movement of the crust from advance and retreat of later ice sheets, preventing clear documentation of global ice volume. This project will investigate the response of the south GIS and adjacent Labrador Sea to elevated CO2 in the mid Pliocene using marine sediment archives preserved in the Eirik Drift off of south Greenland. Specific questions to be addressed are: 1) How warm was the Labrador Sea during the mid-Pliocene optimum?, and 2) Did south Greenland completely deglaciate in response to this warmth? Results of this study will help establish whether the Earth has already crossed a tipping point that will lead to an ice-free Greenland.The research team will answer these research questions using methods developed for tracking GIS behavior during Quaternary interglacials. Question 1 will be addressed through foraminifer census data and measurement of d18O and Mg/Ca on planktonic foraminifera in the Eirik Drift across the mid-Pliocene optimum sequence recorded in International Ocean Discovery Program (IODP) Site U1307. Question 2 will be answered through the tracing of the sources of mid-Pliocene detrital sediment in the Eirik Drift using silt-size Sr-Nd-Pb isotopic signatures and silt- and clay-size magnetic properties. The team's extensive provenance work has shown that silt-size particles with Precambrian isotopic signatures, high magnetic concentrations, and coarse magnetic grain-sizes are only produced on Greenland through GIS erosion of the bedrock; Iceland has distinctively different magnetic and isotopic signatures. GIS melting provides the mechanism to deliver these silt particles to the Eirik Drift. If a south Greenland terrane deglaciates removing the erosive agent and transport mechanism, the terrane's isotopic and magnetic signatures are largely absent in Eirik Drift sediment. These records will determine the Labrador Sea and south GIS response to present-day CO2 levels including the impact of long-term feedbacks necessary for assessing Earth-system sensitivity. The combination of ocean climate and south GIS records will test if the climate threshold necessary to deglaciate Greenland lies at ~400 ppm atmospheric CO2 concentration. This project will support a postdoctoral researcher and undergraduate students in multi-proxy approaches in the fields of paleoceanography, paleoclimatology, geochemistry, environmental magnetism, and glacial geology to address societally relevant climate/sea-level questions. Under-represented groups in the geosciences will take precedent in recruitment of the postdoctoral researcher and undergraduate students.

2013 年，大气中二氧化碳 (CO2) 浓度达到 400 ppm，引发了人们对地球将如何应对人类历史上前所未有的强迫的疑问。人们主要关心的是地球上剩余的冰盖对当前（和未来）二氧化碳水平的反应，以及随之而来的对海平面上升的影响。就格陵兰冰盖 (GIS) 而言，模型提出了一种阈值行为，一旦超过特定温度，GIS 将完全融化。然而，不同的模型提出了不同的阈值，因此尚不清楚地球是否已经跨越了无冰的格陵兰临界点。大气中 CO2 达到约 400 ppm 的最后一个时期是在 3.3-300 万年前的上新世中期气候最佳时期，这为地球现在的气候提供了过去的模拟。气候和冰盖模型使用较小的 GIS 模拟温暖的地球，但显示出一系列响应。由于后来冰盖的前进和后退造成的地壳运动，上新世的海平面记录变得非常复杂，从而阻碍了对全球冰量的清晰记录。该项目将利用格陵兰岛南部埃里克漂流中保存的海洋沉积物档案，调查上新世中期南部 GIS 和邻近拉布拉多海对二氧化碳浓度升高的反应。要解决的具体问题是：1）上新世中期最佳时期拉布拉多海有多温暖？2）格陵兰岛南部是否因这种温暖而完全消融？这项研究的结果将有助于确定地球是否已经跨越了格陵兰岛无冰的临界点。研究小组将使用为跟踪第四纪间冰期期间 GIS 行为而开发的方法来回答这些研究问题。问题 1 将通过有孔虫普查数据以及对国际海洋发现计划 (IODP) 站点 U1307 记录的上新世中期最佳序列的埃里克漂流中浮游有孔虫的 d18O 和 Mg/Ca 测量来解决。问题 2 将通过使用淤泥大小的 Sr-Nd-Pb 同位素特征以及淤泥和粘土大小的磁性来追踪埃里克漂移中上新世中期碎屑沉积物的来源来回答。该团队广泛的溯源工作表明，具有前寒武纪同位素特征、高磁性浓度和粗磁性颗粒尺寸的粉粒大小颗粒仅在格陵兰岛通过 GIS 对基岩的侵蚀而产生；冰岛具有截然不同的磁性和同位素特征。 GIS 融化提供了将这些淤泥颗粒输送到 Eirik Drift 的机制。如果格陵兰岛南部地体消融，去除了侵蚀剂和运输机制，则埃里克漂移沉积物中基本上不存在该地体的同位素和磁性特征。这些记录将确定拉布拉多海和南部 GIS 对当前二氧化碳水平的响应，包括评估地球系统敏感性所需的长期反馈的影响。海洋气候和南方地理信息系统记录的结合将测试格陵兰岛冰川消融所需的气候阈值是否位于~400 ppm 大气二氧化碳浓度。该项目将支持博士后研究员和本科生在古海洋学、古气候学、地球化学、环境磁学和冰川地质学领域采用多代理方法来解决与社会相关的气候/海平面问题。地球科学领域代表性不足的群体将优先招收博士后研究员和本科生。