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

2013年，大气中的二氧化碳(CO2)浓度达到了400ppm，这引发了人们对地球将如何应对这一人类历史上前所未有的压力的疑问。一个主要的担忧是地球上剩余的冰盖对现在(和未来)的二氧化碳水平的反应，以及它们对海平面上升的贡献。在格陵兰冰盖(GIS)的情况下，模型提出了一个阈值行为，一旦超过一定的温度，GIS就会完全融化。然而，不同的模型提出了不同的阈值，因此目前还不清楚地球是否已经越过了无冰格陵兰的临界点。大气二氧化碳最后一次达到400ppm是在330万年前的上新世中期气候最佳时期，这提供了过去与地球现在气候的相似之处。气候和冰盖模型用较小的地理信息系统模拟地球变暖，但显示了一系列的响应。上新世海平面记录因地壳运动而从后来的冰盖进退中显著复杂化，从而阻碍了对全球冰量的清晰记录。该项目将利用保存在格陵兰岛南部埃里克漂流中的海洋沉积物档案，调查南部地理信息系统和邻近的拉布拉多海在上新世中期对二氧化碳升高的反应。需要解决的具体问题是：1)上新世中期拉布拉多海的最适温度是多少？2)格陵兰岛南部是否对这种温暖做出了完全的冰川消融？这项研究的结果将有助于确定地球是否已经越过了一个临界点，这将导致一个无冰的格陵兰。研究小组将使用为跟踪第四纪间冰期期间地理信息系统的行为而开发的方法来回答这些研究问题。问题1将通过国际海洋发现计划(IODP)站点U1307中记录的上新世中期埃里克漂流中浮游有孔虫d18O和镁/钙的测量数据和d18O和镁/钙的测量来解决。问题2将通过使用粉砂大小的锶-钕-铅同位素特征和粉砂和粘土大小的磁性来追踪埃里克漂流中上新世碎屑沉积物的来源来回答。该团队广泛的物源研究表明，具有前寒武纪同位素特征、高磁性浓度和粗磁性颗粒大小的粉砂颗粒仅在格陵兰通过地理信息系统对基岩的侵蚀而产生；冰岛具有明显不同的磁性和同位素特征。地理信息系统的融化提供了将这些泥沙颗粒输送到埃里克漂流的机制。如果格陵兰南部的地体消融，去除了侵蚀剂和搬运机制，那么该地体的同位素和磁性特征在埃里克漂移沉积物中基本上是不存在的。这些记录将确定拉布拉多海和南部地理信息系统对当今二氧化碳水平的反应，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的记录相结合，将测试格陵兰冰川消融所需的气候门槛是否在大气二氧化碳浓度~400ppm。该项目将支持一名博士后研究人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种替代方法，以解决与社会相关的气候/海平面问题。地学中代表性不足的群体将优先招收博士后研究员和本科生。

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.