Estimating current velocities from ripples in tidal rhythmites: validation of deep-time tidal simulations

Tides drive ocean mixing, regulate nutrient upwelling, help regulate climate, and influence evolutionary events over geological time. Knowledge of tidal impacts has come from simulating tides and investigating tidal deposits, i.e. tidalites. However, existing numerical tidal model simulations and the paleogeographic reconstructions that underpin them are poorly constrained for most locations over most of Earth’s history because of a scarcity of direct paleotidal proxy data. This study addresses this issue by: (i) using relationships between current ripple dimensions and tidal flow to estimate paleotidal current velocity from Carboniferous (c. 318 Ma) tidal rhythmites in Wisemans Bridge, South Wales, U.K.; and (ii) comparing this paleoenvironmental proxy with regional tidal current velocities predicted using a numerical tidal model for the same period and paleogeographic setting. Dimensional ripple data collected from the Wisemans Bridge rhythmites suggest that the bedforms formed at tidal flow velocities of 0.25 – 0.32 m s-1. These values closely match with the modelled tidal current velocities of 0.15 – 0.38 m s-1 based on a combination of the principal solar, principal lunar, and principal lunar diurnal constituents. This study demonstrates how ripples in tidal deposits can be used to determine paleotidal current velocity and validate numerical tidal models. This approach can be extended to other locations and time intervals, improving confidence in models used to reconstruct tide-driven impacts on Earth's history.