Earth's Climate History, Ice Sheets, and Sea Level Over Last Four Million Years

 Published estimates of sea level in the mid-Pliocene (3 Ma), a time cited as potentially analogous to predicted future climate, range between 10 and 40 m above present, with +25 m typically adopted in numerical climate model simulations. A eustatic change of this magnitude implies complete deglaciation of the West Antarctic and Greenland Ice Sheets and a significantly diminished East Antarctic Ice Sheet. We modeled glacioisostatic effects on mid-Pliocene shorelines using a gravitationally self-consistent treatment of post-glacial sea level change. Our primary results (Raymo et al., Nature Geoscience, 2011) show that the predicted modern elevations of such shoreline features can deviate significantly from the eustatic signal even in the absence of post-depositional tectonic or epeirogenic movements. This non-eustatic sea-level change is due primarily to residual adjustments associated with late Pleistocene glaciation and the geographic fingerprints of sea-level change we derive can guide field efforts aimed at establishing sea level during the mid Pliocene warm period. The integration of GIA modeling results, such as these, with observational field data will greatly enhance our ability to assess ice sheet stability in a slightly warmer world.

Another major finding (Raymo and Mitrovica, Nature, 2012) presents a solution to a decade old debate about the significance of +20m shorelines in the Bahamas and Bermuda. These contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 meters higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11. We show that the elevations of these features are corrected downwards by ~10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to 6-13m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor.

 All the data we are collecting can be used to evaluate mantle viscosity models and the array of dynamic topography models now in the literature. This is significant as these are some of the same models used to forecast future sea level change.  In addition, both of the studies described above are cited in the current draft of the IPCC report (past sea level changes section).  A number of high school and undergraduate students gained research experience working closely on this project; in addition, three post-doctoral scholars were supported by, or collaborated on, this grant.   PI Raymo made numerous contributions to public outreach and student education through, for instance, being Keynote Speaker at the 23rd Annual Senior Leadership Conference of the Girl Scouts of America, lecturing each summer at the Urbino Summer School in Paleoclimatology, lecturing at the Bruce Museum, and presenting numerous talks at workshops, universities, and at national meetings such as the 2012 Planet Under Pressure Meeting.

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