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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


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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.