eustasy

Eustasy is the term used to describe global changes in sea level that result from variations in the total volume of the world's oceans or changes in the capacity of ocean basins. It refers to a worldwide sea level signal, in contrast to local vertical movements of the land. Eustatic sea level is often distinguished from isostatic sea level, which arises from regional land uplift or subsidence.

The principal mechanisms of eustatic change include changes in the amount of seawater stored on land as ice (glacioeustasy) during glacial–interglacial cycles, and thermal expansion or contraction of seawater due to climate warming or cooling (steric effects). Long-term tectonic processes can also alter ocean basin volume, thereby affecting global capacity to hold seawater. The combination of these factors yields a global sea level curve that is superimposed on, and can be partially offset by, regional land movements.

Historically, eustatic fluctuations are best documented in the Quaternary, with sea level dipping by about 120 meters during the Last Glacial Maximum and rising as ice sheets melted. In the current interglacial, ongoing warming contributes to thermal expansion and glacier retreat, contributing to a rising global mean sea level, though regional rates vary with land movement and ocean dynamics. Eustasy is inferred from proxies such as oxygen isotope records, deep-sea sedimentology, and correlations of paleo-sea-level indicators, and it informs interpretations of paleogeography, sedimentation, and climate history.