END-ORDOVICIAN CRISES: TIMING AND SIGNIFICANCE OF EVENTS

The end Ordovician was a time of paradoxical environmental and biotic crises. The extinction of an estimated 86% of species took place in 2 events separated by less than a million years, but there is little evidence of long term change in ecological structure. The extinctions are closely linked to major changes in the carbon cycle and the onset and demise of the Gondwanan glaciation: but that occurred during a time of supposed 'greenhouse' climate.

Stable isotopic data are proving critical in our understanding of the rapidity and magnitude of the changes and the links between environmental and biotic change. New high resolution oxygen and carbon stable isotope data based on the analysis of brachiopods and ostracodes demonstrate the precise synchroneity of environmental deterioration (significant cooling at low latitudes associated with polar ice-volume increase), changes in carbon cycling and the first phase of extinction. Carbon isotopic data from bulk sediments enable detailed correlation between different settings and a 'ruler' against which patterns of biotic change can be compared. In this way it is becoming possible to document a sequence of extinction events affecting different groups and to 'date' the first occurrence of post-extinction Hirnantian faunas.

Models to account for the glaciation and carbon isotope excursion need to be compatible with the sequence of events emerging from the high resolution isotopic studies. Whilst circulation models suggest that the 'glaciation in a greenhouse' is possible at relatively extreme settings of input parameters, published attempts to model the carbon cycle are not compatible with the observed pattern of oxygen and carbon excursions. Models of ocean circulation can provide useful insights into the carbon cycle but they cannot yet account for the magnitude of the observed change in the carbon isotope values.