EARTH'S CONSTANT MEAN ELEVATION: IMPLICATIONS OF CHANGES IN DYNAMIC TOPOGRAPHY, CONTINENTAL CRUSTAL AREA AND MEAN DEPTH OF THE OCEANS AS JOINT DRIVERS OF LONG-TERM SEA LEVEL

A model is described of changes in global hypsometry associated with changes in fractional area of the continental crust and its impact on long-term sea level. The model explicitly maintains Earth’s constant mean radius, constant volume of seawater, and uses global paleogeographic reconstructions of continental flooding history to constrain global long-term sea level. Large-scale (±5%) changes in areal extent of continental crust are modeled by distorting a GIA-corrected global hypsometry, within which oceanic, arc-modified, and continental crustal contributions are distinguished. Continental crust is further separated into continental margin (elevations ≤500m) and mountains (elevations >500m) that are treated differently during distortion of the cumulative hypsometry. Decrease in fractional area of the continental crust is modeled by increasing the areal extent of mountains, decreasing margins, and increasing oceanic crust by multiplying the differential hypsometry of each component by corresponding constants that maintain constant surface area of the Earth to simulate the resultant cumulative hypsometry. Integration from the maximum depth upwards until the volume of the basin equals the volume of seawater defines the areal extent of continental flooding and paleo-height of the shoreline on the distorted cumulative hypsometry. The height on the GIA-corrected hypsometry corresponding to the paleo-height of the shoreline defines the present height of the paleo-shoreline. Both paleo- and present heights of the shoreline, and areal extent of flooding, decrease with decreasing areal extent of continental crustal area. Global paleogeographic reconstructions of paleo-shorelines are used to define the areal extent of flooding integrated with change in continental crustal area from (Ingalls et al., 2016) allow estimates to be derived of the paleo- and present heights of the paleo-shoreline, and change in mean depth of the oceanic crust. These estimates imply a maximum sea level height between 80 and 90 Ma of 100±48m, only about 60 m higher than that associated with a GIA- and ice-free Earth today. This sea level history is associated with minimal changes in mean depth of the oceans over the past ~165 Ma implying quite limited (±3-5 MY) changes in mean age of the oceanic lithosphere over this same time interval.