A high-resolution isotope and relative sea-level study of latest Early through earliest Late Cambrian carbonates of the U.S. and Canadian Cordilleran margin significantly refines the structure of the existing seawater Sr isotope curve by (1) constraining the highest seawater ⁸⁷Sr/ ⁸⁶Sr values (< 0.7093) over the past 2 b.y. to the latest Middle Cambrian to earliest Late Cambrian, and (2) by bridging the gap in the apparent long-term yet rapid increasing trend in seawater ⁸⁷Sr/⁸⁶Sr values beginning in Early Cambrian time (0.7082; Derry et al., 1994) and culminating in the latest Middle Cambrian to earliest Late Cambrian (0.7093). We interpret this overall trend to record the temporal evolution of mountain-building and attendant chemical weathering of the Late Proterozoic-Cambrian, Pan-African continental collision. Significantly, the magnitude (and rate) of the increase in seawater ⁸⁷Sr/ ⁸⁶Sr values during the Early through earliest Late Cambrian (0.00004 to 0.00005/m.y.) overlaps to exceeds the Tertiary rise in seawater ⁸⁷Sr/ ⁸⁶Sr values over the last 40 m.y., which is attributed to rapid exhumation and chemical weathering of the Himalayas and Tibetan Plateau. Higher-resolution (0.5 to 2 m.y.) changes in ⁸⁷Sr/ ⁸⁶Sr values are superimposed on the longer-term increasing Sr isotope trend; these higher-resolution variations in ⁸⁷Sr/ ⁸⁶Sr values covary with short-term sea-level events. We propose that eustatic changes, and their control on the amount of exposed pericratonic sediments and physical and chemical weathering rates, are a viable mechanism for producing short-term variations in the Sr isotopic composition of the oceans at the 105 to 106 year scale during greenhouse times. Higher resolution ⁸⁷Sr/ ⁸⁶Sr variations, defined over two stratigraphic intervals of latest Early to early Middle Cambrian and early Late Cambrian carbonates representing ~ 2 to 3 m.y., covary with both their carbonate d¹³C values and short-term relative sea-level events. C and Sr isotope trends show a consistent yet complex relationship with trilobite biomere boundaries throughout the stratigraphic interval. These relationships suggest a potential mechanistic link between processes controlling the Sr and C isotope composition of latest Early through earliest Late Cambrian oceans and the processes governing high-frequency sea level fluctuations during the Cambrian, as well as an indirect relationship to trilobite extinction events.

For details of this project, see our article in GSA Today, May 2000.

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