Changes in Sea Level

Changes in sea level are well documented from studies of sedimentation. On short time scales (∼104 to 105 years) sea level changes can be explained by variations in the volume of the polar ice sheets. However, on longer time scales (∼107 to 108 years) the magnitudes of sea level changes are too large

to be understood in this way. If the polar ice sheets were completely melted, the water added to the oceans would increase sea level by about 80 m. Yet compared with its present level, the sea has been hundreds of meters higher during the last 550 Ma, as shown by the record in Figure 4–47 (the value at t = 0 of 80 m above present sea level accounts for the water in the polar ice sheets). Sea level in the Cretaceous (80 Ma) was 300 m higher than it is today, and water flooded about 40% of the present area of the continents. These large, long-term changes in sea level are attributed to changes in the average depth w̄ of the seafloor below the level of the ridge crests. If this average depth decreases as a consequence of a decrease in the average age τ of subduction, the volume of water contained in the deep ocean basins decreases and the height of the sea above the ridge crests h, that is, sea level, increases. This is supported by magnetic anomaly studies that show 324 Heat Transfer that the Cretaceous was a time when there were more ridges and the ridges were spreading more rapidly than at present. Since 75 Ma, an extensive ridge system has been subducted beneath western North America. Thus, on average, the present seafloor is older and deeper than the seafloor at 80 Ma. The present deep ocean basins hold more water than the basins during the Cretaceous, and the sea surface today lies well below its level in that period. Changes in the configuration of the seafloor cannot change the total volume of water in the oceans.