This project uses the distribution patterns and geochemistry of planktonic chitinozoans and graptolites to ground-truth Ordovician climate model (GCM) predictions of ocean state, and to reconstruct the climatic impact of the deflection of water masses during the onset of the end-Ordovician glaciation. It provides a test for the 'closing ocean gateway hypothesis', also postulated as a cause of Pleistocene glaciation.