Tides, tidalites, and secular changes in the Earth–Moon system

Abstract

Tides have been a source of inquiry since the dawn of human civilization. It has been known for millennia that the Moon is a causative agent in the formation of tides, with the observation that lunar phases often correlated to changing tidal amplitudes. The precise mechanisms underlying the formation of tides and local tidal dynamics, however, have proven difficult to elucidate. Only with Newton's theory of gravitation in the 17th century was the correlation between lunar cycles and tides partially explained. Further work by Laplace and others resulted in a more dynamic theory that more closely matched observations and allowed for better prediction of local tidal behavior. Quantitative models derived from these methods have increased in precision and complexity (particularly with the advent of the electronic computer), and have allowed new insights into the nature of tidal dynamics and tidal dissipation. In more recent years scientists have analyzed deposits known as tidal bundles and tidal rhythmites in an attempt to extrapolate the history of tides from the geologic record. Tidal bundles are laterally accreted cyclic foreset beds separated by mud laminae. Tidal rhythmites are vertically accreted planar laminae that alternate between coarse and fine sediments forming couplets often composed of sands and muds. These deposits are characterized by bed/laminae thicknesses that vary rhythmically and preserve tidal periodicities, and are generally found in intertidal or subtidal depositional environments. The mode of deposition (e.g. sand or mud) is primarily determined by current velocity and tidal range, factors largely controlled by the tides in marginal marine settings. Quantitative analyses of tidal rhythmites may facilitate more precise elucidation of tidal periodicities encoded in the rock record. The partial reconstruction of the history of lunar recession from existing data and analyses indicates that the Earth is presently experiencing a high rate of tidal dissipation. Further data obtained from ancient tidal proxies may prove essential in constraining models of tidal dissipation, thereby revealing the mechanisms and dynamics present in the dissipation process controlling secular changes in the length of day and lunar orbit.