Palaeoseasonality in the Pliocene of the North Atlantic

A number of previous studies have shown that there is an inverse correlation between zooid size in cheilostome bryozoans and ambient water temperature, a relationship which can provide valuable information about the seasonality experienced by fossil bryozoan colonies during their life. A total of 274 colonies of Pliocene cheilostome bryozoans from Panama, the USA (Florida, South Carolina, North Carolina, Virginia) and the UK (Suffolk) provided estimates of the mean annual range of temperature (MART) for the fossil localities. Results show a general increase in seasonality (MART) from low to mid latitudes, as expected. The small MART for Panama (4.4^oC) suggests low seasonality in the Caribbean, consistent with the Central American Isthmus being either a shallow sill or nearly closed at this time (3.6-3.5 Ma). The larger than expected MART estimated for the Lower Tamiami Formation of Florida (7.4^oC) is suggestive of seasonal upwelling, in agreement with studies on other faunal groups. The mean MART (8.2^oC) estimated for Suffolk on the southern edge of the North Sea, combined with the presence of bryozoan genera that today inhabit warm waters, suggests warmer but less seasonal conditions than those observed in the North Sea at the present day (average MART ~12^oC). Comparison of MART estimates from bryozoans with outputs from a coupled ocean-atmosphere general circulation model (HadCM3) show that, for most localities, model predicted Pliocene palaeoseasonality is consistent with estimates derived from fossil bryozoa. This is the first study to apply the MART technique to localities spanning a large latitudinal range and demonstrates that the technique is an extremely useful tool for interpreting palaeoseasonality.