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，大气中二氧化碳（CO2）浓度达到400 ppm，这引发了人们对地球将如何应对人类历史上前所未有的压力的疑问。一个主要的问题是地球剩余的冰盖对现在（和未来）二氧化碳水平的反应，以及它们对海平面上升的贡献。在格陵兰冰盖的情况下，模型显示了一个阈值行为，一旦超过一定的温度，格陵兰冰盖将完全融化。然而，不同的模型给出了不同的临界值，因此目前还不清楚地球是否已经越过了格陵兰岛的无冰临界点。最后一次大气中的二氧化碳达到400 ppm是在330 - 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一个过去的模拟。气候和冰盖模型用较小的地理信息系统模拟了一个更温暖的地球，但显示了一系列的反应。上新世的海平面记录由于后期冰盖的前进和后退造成的地壳运动而变得非常复杂，这使得全球冰量的记录无法得到清晰的记录。该项目将利用保存在格陵兰岛南部Eirik漂移区的海洋沉积物档案，调查南部地理信息系统和邻近的拉布拉多海对上新世中期二氧化碳浓度升高的反应。需要解决的具体问题是：1）在上新世中期的拉布拉多海有多温暖？，2）南格陵兰岛是否因这种温暖而完全冰川消退？这项研究的结果将有助于确定地球是否已经越过一个临界点，将导致一个无冰的格陵兰岛。研究小组将回答这些研究问题，使用的方法开发的跟踪GIS的行为在第四纪间冰期。问题1将通过有孔虫普查数据和测量d18 O和Mg/Ca的浮游有孔虫在Eirik漂移整个上新世中期的最佳序列记录在国际海洋发现计划（IODP）网站U1307。问题2将通过使用粉砂大小的Sr-Nd-Pb同位素特征和粉砂和粘土大小的磁性追踪Eirik漂移中上新世碎屑沉积物的来源来回答。该团队广泛的来源工作表明，具有前寒武纪同位素特征、高磁性浓度和粗磁性粒度的粉砂大小的颗粒仅在格陵兰岛通过基岩的GIS侵蚀产生;冰岛具有明显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漂移的机制。如果格陵兰南部的冰川消融，去除侵蚀剂和运输机制，冰川的同位素和磁性特征在很大程度上是不存在的埃里克漂移沉积物。这些记录将确定拉布拉多海和南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。海洋气候和南部地理信息系统记录的结合将测试格陵兰冰川消退所需的气候阈值是否位于大气CO2浓度约400 ppm。该项目将支持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的多代理方法，以解决社会相关的气候/海平面问题。在招收博士后研究员和本科生时，地球科学领域代表性不足的群体将优先考虑。

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.