The tidal proxy database: development, application, and a call for help

Recent numerical tidal modelling efforts strongly suggest that present day tides are anomalously large in comparison to the tides over the past 1.5 Gyr. Whilst these results can be qualitatively explained from dynamical principles, there are only a few quantitative validations of deep-time tidal simulations done using tidal proxies. One reason for this is a lack of easily accessible proxies for tides and something we are proposing to rectify here. Through extensive literature searches, we have identified over 600 publications containing potential tidal proxies and processed around 300 of them to date.

From the literature, we have identified proxies for three tidal properties. Under favourable circumstances, the geological record can provide direct estimates of the tidal range. These situations are rare (~10 papers have this information to date), but it is the best proxy for validation purposes. Tidal currents can be constrained by indirect methods. The presence of black shales indicates a poorly ventilated water column, which in turn is a sign of weak tides. By plotting the location of tidal mixing fronts and ensuring that they are located so black shales end on the stratified side of the front, we have a potential proxy for large-scale tidal current speeds. Tidal currents can also be constrained locally by investigating the dimensions of current ripples in the sediments. Finally, day-length, which is directly linked to global tidal dissipation rates, can often be inferred from the variation and cyclicity in layer composition and thickness in tidalites. These are vertically accreted laminated facies of a succession of couplets composed of sand and clay or silt and clay, with thicknesses of millimeters to centimeters, and they are at the heart of our inventory. Further potential proxies involve using paleobiology to track ranges of intertidal species (to obtain tidal ranges) and use microfossil assemblages as another mean of tracking tidal mixing from (and hence constrain current speed). As a proof-of-concept application, we revisit tidal model simulations from five deep-time slices showing that the methods we propose are viable as tidal proxies. The model simulations and proxies usually agree within the uncertainties of both methods.

The database will be made available to the community once the information currently in it has been quality controlled and used in our initial publications. Furthermore, any information that may be of use is welcome and we would love to hear about any potential tidal proxies you may have.