Sr, C, and O isotope geochemistry of Ordovician brachiopods: A major isotopic event around the Middle-Late Ordovician transition

摘要

Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater Sr-87/Sr-86 decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean delta(18)O values increase from -10parts per thousand to -3parts per thousand through the Ordovician with an additional short-lived increase of 2 to 3parts per thousand during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered delta(18)O in some samples, particularly those from the Lower Ordovician, maximum delta(18)O values, which are less likely to be altered, increase by more than 3parts per thousand through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite delta(18)O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater delta(18)O that was steadily increasing from less than or equal to-3parts per thousand standard mean ocean water (SMOW). By contrast, delta(13)C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician delta(13)C excursion up to +7parts per thousand. Nevertheless, an underlying trend in mean VC can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater Sr-87/Sr-86 and delta(18)O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event. Copyright (C) 2003 Elsevier Science Ltd. [References: 93]