ISOTOPIC CORRELATION TOOLS IN THE MID-PALAEOZOIC AND THEIR RELATION TO EXTINCTION EVENTS.

Abstract

At least nine and possibly as many as twelve extinction events of global or near-global extent can be discriminated in the mid-Palaeozoic (earliest Silurian to Early Carboniferous), on the basis of brachiopod, coral, conodont and ammonoid data, and the history of carbonate build-ups. From a review of new and available carbon and oxygen isotopic data from whole-rocks and brachiopods from Australian carbonate sequences precisely constrained by conodont data, distinct isotopic responses to these extinction events may be discerned. There is a good correlation between extinction events and isotopic anomalies with the sense of isotopic shift different for different events. In the Silurian sections there are relatively constant 13C values with sharp and sustained enrichments in 13C at recognized extinction events. The 13C values for the Silurian sections are generally enriched in 13C compared with other low-mid Palaeozoic data. In the Devonian sections, the extinction events correlate with a short-lived depletion in 13C values. The different isotopic responses to periods of marked reduction of biomass and biodiversity in the Silurian and the Devonian implies fundamentally different causes and/or responses to the environmental factors leading to the events. The isotope data across recognized extinction events in the mid-Palaeozoic point to their use in inter-and intra-basin correlation.