Studies of sea-level changes prior to the Pleistocene have focused on longer-term, cyclic events in the stratigraphic record that have been controlled by both global warming/cooling and large-scale tectonics.
Mean sea level is defined as the average height of the sea surface referenced to some fixed point. To determine the present mean sea level, hourly measurements of tidal height are taken at land-based stations using benchmarks and recording tide gauges. In the United States, the values for all stages of the tide are averaged over a nineteen-year period and used to publish tidal atlases that predict the tidal height of a daily basis at coastal sites throughout North and South America and the Caribbean Sea. These values are valid for only a limited distance (hundreds to thousands of kilometers) because of geoid variations, vagaries of basin geometry and local tectonic movements. In fact, fifty years is the minimum sea level record length that should be considered in an analysis of global sea level rise. More recently, data from Earth-orbiting satellites such as
TOPEX/Poseidon
(T/P) are being used to measure global sea level rise.
Still-stands of sea level prior to historic data can be inferred by features related to steady wave action over a long period of time (i.e., wave-cut benches. terraces and "nick points"). Fossil material deposited in nearshore sediments and in coral reefs may contain organisms of limited depth range, which can serve as relative indicators of water depth at the time of deposition.
Changes in sea level measured at a particular site may reflect either worldwide or local conditions, and changes have occurred many times in the past. We will examine the causes of sea-level variations, methods of measuring sea level, and then discuss the importance of sea-level change to marine deposition.