LARGE SEA, SMALL TIDES: THE UPPER CARBONIFEROUS SEAWAY OF NORTHWEST EUROPE

Finite element modelling of the European Upper Carboniferous epi-continental seaway predicts an exceedingly low lunar tidal range. Model predictions for the UK and Southern North Sea regions imply an a-tidal regime (c. 5 cm.). Essentially the model predicts that this part of the seaway was a-tidal and behaved more like a salty lake than a modern-day tidally-mixed coastal system.

The Imperial College Ocean Model (ICOM) uses finite element methods and a unique adaptive mesh that is very computationally efficient. The model accurately predicts the lunar tides in the modern Mediterranean. We tested the sensitivity of the model to bathymetric variations by running the model on a series of artificial "Mediterranean-basins" with variable depths. The Mediterranean varies in depth up to 5.4 km and has an average depth of 1.5 km. We found that a straight sided Mediterranean-basin with a uniform depth of 1 km essentially predicts truth. This demonstrates that the model is robust and defines the effect of bathymetric uncertainty on model output.

The extremely low tidal ranges predicted for the European Upper Carboniferous are thus deemed accurate. This has several important implications. Without tidal mixing, the tropical equatorial heat and fresh-water input from vast fluvial-dominated deltas would have led to stratification. The introduction of organic matter would likely cause anoxic conditions, carbon accumulation and biotic mortality. Putative Upper Carboniferous tidal deposits have been described in the UK and southern North Sea. However, these are limited to paleo-estuaries and represented by cyclic rhythmites. We modelled the effects of local funnelling on an embayed coastline and found that the tidal range could have been amplified to c.1 m in estuaries. Simple calculations show that this is a sufficient tidal range to form cyclic rhythmites.