MIDDLE PALEOZOIC EXTINCTION EVENTS - FAUNAL AND ISOTOPIC DATA

At least 9 and possibly as many as 12 extinction events of global or n ear-global impact 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. Is otopic data from whole-rock samples are presented for three of these e vents, based on Australian carbonate sequences constrained by conodont data. These data represent the initial phase of a more extensive inve stigation using C, O, S, Sr and Nd isotopic signatures derived from co nodonts, articulate and inarticulate brachiopod shell, and fish remain s from numerous Australian and European sequences. The aim of the proj ect is to identify isotopic responses to extinction events, and addres s causes for these changes. In limestone sections analysed so far, var iations in carbon isotope compositions on a whole-rock scale are most marked at horizons that can be correlated with times of significant re duction in biomass and diversity. This is despite the fact that the wh ole rocks are in fact multicomponent systems with respect to carbon, i n part arising from diagenetic rearrangements of carbon distribution w ithin the scale of the whole rock. Thus, the carbon isotope data do pr ovide evidence on a whole-rock scale for fundamental changes in the gl obal carbon cycle that are correlative with extinction events. It is, however, unlikely that the magnitude of the isotopic shift will be pre cisely documented from such whole-rock analyses. The oxygen isotope re sults are less obviously related to the times of carbon isotope excurs ion; this contrast in apparent resilience to post-depositional modific ation of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates.