Eustatic

Eustasy refers to global changes in sea level caused by variations in the total volume of the world's oceans or in the capacity of ocean basins. It describes a worldwide shift in sea level that would be felt across coastlines around the globe, in contrast to relative sea-level change which results from local vertical land movements or crustal subsidence.

The main drivers of eustatic change are changes in ocean water volume and changes in the volume available to the oceans. Water volume varies with the growth and decay of continental ice sheets during glacial–interglacial cycles, as well as shifts in water storage on land. Seawater also expands or contracts with temperature, contributing to sea-level changes. Long-term tectonic processes can alter basin capacity and land elevation, but such effects usually operate on longer timescales. In practice, the most pronounced eustatic signals are associated with glacial cycles, where large ice sheets lock up water and lower global sea level, and rapid deglaciation returns water to the oceans, raising sea level.

Evidence for eustasy comes from marine sediments, fossil corals, isotopic proxies, and stratigraphic records that track global sea-level trends. A prominent example is the Last Glacial Maximum, when global mean sea level is estimated to have been about 120 meters lower than today.

In usage, eustatic describes global or worldwide sea-level shifts, while isostatic describes local or regional land movements that modify relative sea level. The concept is central to studies of transgressions, regressions, and paleoceanographic reconstructions.