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Dietmar Muller, The University of Sydney (Australia)
Maria Sdrolias, School of Geosciences (Australia)
Carmen Gaina, Geological Survey of Norway (Norway)
Bernhard Steinberger, Geological Survey of Norway (Norway)
Christian Heine, School of Geosciences (Australia)
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Enormous sea level variations have occurred in "hothouse" climates in the geological past, when neither humans nor inland ice caps existed, raising the question what drove these changes. However, there is enormous disagreement on how much sea level has actually varied. Miller et al's (Science, 2005) preferred sea level curve is derived from the sedimentary record of the US New Jersey margin and results in a low (∼40 m) amplitude of sea level change since the Late Cretaceous sea level highstand (80 Ma). In contrast many other methodologies suggest sea level change of the order of 200-250 m since the Cretaceous. We show how one can unravel all major factors contributing to sea level change over geological time. These include the creation and destruction of mid-ocean ridges, seafloor spreading rate fluctuations, the formation and subduction of large igneous provinces, and marine sedimentation. We reconstruct paleo-oceans by creating "synthetic plates", the locations and geometry of which is established on the basis of preserved ocean crust (magnetic lineations and fracture zones), geological data, paleogeography, and the rules of plate tectonics.
Based on this approach we have created a set of global oceanic paleo-isochrons and paleo-oceanic age grids. The grids provide the first complete global set of paleo-basement depth maps, including now subducted ocean floor, for the last 140 million years based on a depth-age relationship. After adding effects of oceanic plateaus, sedimentation, and changes in oceanic area, we derive a best estimate for the late-Cretaceous sealevel highstand of 170 m above present sea level. The subsequent long-term drop in sea level was primarily caused by the changing age-area distribution of Pacific ocean floor through time, and to a much lesser extent by the "supercontinent breakup effect", which resulted in the creation of the mid-Atlantic and Indian Ocean ridges at the expense of subducting old ocean floor in the Tethys. We use regional outputs from a global geodynamic model to demonstrate that the New Jersey margin has subsided by at least 100 m over the last 70 million years while global sea level dropped. The gradual sinking of New Jersey was driven by the westward motion of North America over the subducted Farallon plate, now descending in the lower mantle underneath the east coast of North America, drawing the surface down. A corrected New Jersey sea level curve is in reasonable agreement with other sea level estimates, resolving the controversy over seemingly conflicting sea level estimates.
Our results imply that long-term sea level fluctuations have been over two times larger than the 54 m sea level rise that would follow the melting of all present-day ice sheets. This adds a geological dimension to understanding the nature and magnitude of planetary climate change, especially considering that this debate tends to consider change on a human timescale only.
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