THE PRESENT-DAY ELEVATIONS OF ANCIENT MARINE SHELF SEDIMENTS AND LONG-TERM RATES OF SEDIMENT ACCUMULATION

The present-day elevations of ancient marine shelf sediments are, in principle, determined by both changes in the mean surface elevation of the ocean and subsequent vertical displacement of continental crust by deformation, isostatic adjustment, and mantle flow-induced dynamic topography. Efforts to constrain the history of absolute changes in global mean sea level have focused on identifying regions where vertical crustal motions are thought to be limited to isostatic adjustments caused by changing sediment and water loads. Recent work, however, shows that there may be no regions of such long-term vertical stability due to the spatially pervasive dynamic topographic expression of mantle circulation. Here we analyze data on the present-day elevations of 11,256 globally distributed sediments of Triassic to Recent age preserving sedimentological/paleobiological constraints on deposition within ~50 m of the sea surface. There is no relationship between the mean or median present-day elevation of these ancient shallow marine sediments and global eustatic sea level estimates, suggesting that either these sea level estimates are inaccurate or that there is no remaining signal of sea level in the modern vertical positions of globally distributed ancient shallow marine sediments. There is, however, a significant and strong statistical relationship between the mean/median and variance in elevation as a function of geological age. Global age-elevation distributions are well predicted by simple models of unbiased vertical diffusion, wherein sediments are deposited at approximately constant elevation on the continents and then undergo vertical displacements in both the upward and downward directions at stochastically constant rates. Incorporating models of erosional loss of sediment imposes an upper long-term stable boundary on mean elevation, but has little impact on model predictions. The mean rates of vertical uplift evidenced by the elevations of ancient shallow marine sediments are congruent with mean rates of subsidence documented by long-term sediment accumulation rates, a necessary condition of a diffusion model for global vertical continental crustal motion.