CALCAREOUS NANNOPLANKTON EVOLUTION AND CLIMATE CHANGE

A brief review of the Phanerozoic history of phytoplankton and its relation to long-term patterns in biodiversity will be followed by an investigation of the coccolithophore record and the possible controls on their evolution through the last 220 million years. At present, the global diversity of coccolithophores (and associated calcareous nannoplankton) is strongly influenced by diverse, K-selected communities living in oligotrophic, low-latitude water masses. Variation in the past extent of these environments, and the distribution and availability of nutrients in general, may provide the best explantion for the evolutionary patterns we observe. Climate, via its links with oceanography, most probably played a primary role in the development of these critical habitats, and we test this hypothesis by comparing the global diversity of calcareous nannoplankton with palaeoclimate records.

Cenozoic nannoplankton diversity trends are in close agreement with palaeoclimate trends, with high diversities recorded during warm and warming intervals (Late Paleocene-Early Eocene), and low diversities during cool or cooling periods (Late Eocene-Oligocene, Pliocene-Present Day). Diversity of the K-selected discoasters displays the same relationship, however, the r-selected Prinsiales group diversified during cool intervals (Middle Eocene-Oligocene), but displayed low, stable diversity trends through warm intervals (Paleocene-Middle Eocene, Miocene). The diversity histories of these palaeoecologically contrasting nannoplankton groups support the hypothesis that broad-scale changes in the relative development of oligotrophic and eutrophic water-masses influenced the evolution of marine plankton.

Mesozoic nannoplankton diversity shows less variablility, supporting the broad assertion that climates during this time were globally warm and stable. The overall trend is one of progressive diversity increase punctuated by short intervals of decrease or more rapid increase. The most significant diversity decline (not including the Cretaceous-Tertiary boundary extinctions), occurred in the Late Campanian when climate cooling occurred worldwide. Cooler intervals are characterised by enhanced biogeographic differentiation and diversification of the r-selected Biscutaceae group. The warm 'mid' Cretaceous interval saw the diversification of the enigmatic Incertae Sedis group, the nannoconids.