A set of over 200 samples covering Cambrian strata all over the world has been analyzed for Sr-isotopic composition in order to set up a well defined global sea water curve. Taking into account the systematic studies by Diener et al. (1996) we were aware of the limiting conditions, particulary the fact that most of the invertebrates that provide appropiate sample material are just beginning to radiate in the Cambrian, they are relativy small and often subject to diagenetic alteration and overprinting of their geochemical signatures. In general, biogenic low magnesium calcite material of the secondary layer of articulate brachiopods is the most reliable recorder of Sr-isotope signatures, followed by biogenic apatite (euconodonts), other phosphatic materials (inarticulate brachiopods, fishteeth and paraconodonts), micrites and limestones with terrigeneous compounds. With all the above in mind, we have selected the middle Upper Cambrian to Ordovician time intervall for developping a high resolution Sr-isotope stratigraphy and the Lower to Upper Cambrian time intervall to define a global overall Sr-isotope trend.
The intervall was established by the International Working Group of the Cambrian-Ordovician boundary and spans about 10 Ma, that is in terms of conodont zones the base of the Proconodontus to the base of the Cordylodus angulatus zone. The members of this Working Group helped to sample 15 sections worldwide, including nearly all proposed stratotype sections.
Focussing on sections with abundant and unaltered conodont faunas, 6 sections with potential of defining high resolution strontium-isotope curves were selected. These are Black Mountain, Australia; Dayangcha, China; Djukte River, Siberia; Chandler Creek, Oklahoma; composite Threadgill Creek-, Lange Ranch- and Welge Ranch section, Texas; and Lava River, Estonia. For all sections higher order fluctuations are superimposed on a stratigraphical declining "area" Strontium-isotopic trend towards the Ordovician. At least the most distinctive higher order fluctuations coincide with eustatic sealevel events at conodont-zone boundaries, and they most likely represent times of nondeposition or erosion.
Two longranging sections in northeast and northwest Siberia provided micrite- and inarticulate brachiopod shell material for investigation of the general seawater curve. Other complementary sections worldwide were sampled in order to cover the entire of the Cambrian time intervall. The overall pattern of the curve shows a declining towards the Ordovician consistent with the already published data. Nevertheless, even the above "second choice" sample material of a relatively low sample density appears to indicate major breaks in the sedimentary record.
Diener, A., Ebneth, S., Veizer, J. & Buhl, D., Geochim. Cosmochim. Acta (1996, in press).