Jurassic (Toarcian to Kimmeridgian) dinoflagellate cysts and paleoclimates

Abstract

Multivariate statistics have been used to identify the paleoclimatic‐paleoecological affinities of most European Jurassic (Toarcian to Kimmeridgian) dinoflagellate cyst taxa. These analyses were then used to construct dinoflagellate cyst diagrams (abbreviated to ‘cyst diagrams'), which are analogous to pollen diagrams. The marine and terrestrial records were then correlated using both pollen (miospore) and cyst diagrams. These plots appear to include many biostratigraphically significant paleoclimatic events which augment, and in some cases refine, the ammonite‐based biostratigraphy. The integration of the terrestrial spore‐pollen based correlations with the oceans, via dinoflagellate cysts, appears to prove that the oscillations in the palynomorph diagrams reflect widespread temperature changes. This is because the ‘cold’ group of dinoflagellate cysts includes several taxa with north‐polar paleogeographic distributions, and equivalent fluctuations can be matched in the terrestrial spore‐pollen diagrams, which in turn are correlated between the northern and southern hemispheres. In the early mid Jurassic, warming events were noted above the Toarcian‐Aalenian boundary and in the early Bajocian. The paleoclimatic signals in the Bathonian appear to be difficult to interpret consistently. The Callovian‐Oxfordian boundary is characterized by a marked cooling event. The Late Jurassic following the early Oxfordian appears to have been a time of increasing ambient paleotemperatures with a thermal zenith in the Kimmeridgian. This generally rising paleotemperature trend for the Callovian to Kimmeridgian interval, including specific variations, is consistent with other Jurassic paleoclimatic results derived using different methods such as clay mineral analyses, isotopic methods, miospore eco‐groups and macropaleobotany.