Benthic calcareous algae as Jurassic marine temperature indicators in Western Europe

The determination of calcareous chlorophyte distributional limits is via temperatures---namely isocrymal limits, because the green algae are very susceptible to cold temperatures. The calcereous green algae are today and were in the past, one of the most sensitive and useful environmental indicators. As well as being useful in distribution analysis, they are a useful part of biotic analysis in general. Calcareous green algae are mostly, but not wholly tropical in nature---they are found in shallow marine environments up to their range extent of just into temperate water masses (12°C isocrymal). The Dasycladales were dominated form of calcareous alga in the Jurassic (today their role has been largely taken over by the Halimidaceae). It can be seen that in the Upper Jurassic (where there are enough datapoints to form a reasonable limit, as opposed to the Lower or Mid Jurassic) that the Halimedaceae appeared to terminate at a line slightly southwards of the Dasycladales (this compares with the modern situation of a shared termination limit). Modern calcareous Chlorophyta distributions can delimit in the 12°C isocrymal water temperature boundary, and this limit can be imposed on the Jurassic Chlorophyta distribution dataset for palaeothermometry purposes. Together with selected isotope values, it is possible to estimate Jurassic isocrymes across Western Europe. This suggests the possibility of creatures such as belemnites (which are used for isotopically derived palaeotemperatures) having recorded summer temperatures in their isotopes rather than annual average. Modern algal temperature value differences (isocrymal versus summer maximum) is by ∼10°C and this change in values for the Jurassic Chlorophyta produces temperature figures equivalent to a majority of the isotopically derived temperature values