Figure 2. Temporal variations in ⁸⁷Sr/⁸⁶Sr values of samples determined to provide best estimate of primary marine isotope composition. Selected published Cambrian data for comparison. To facilitate comparison of ⁸⁷Sr/⁸⁶Sr results with published data, all ⁸⁷Sr/⁸⁶Sr values have been normalized to ⁸⁷Sr/⁸⁶Sr value of 0.710250 for NBS-SRM 987. Analysis of diagenetic calcite cements indicates that ⁸⁷Sr/⁸⁶Sr compositions of postdepositional waters ranged from less to more radiogenic than estimates of primary marine ⁸⁷Sr/⁸⁶Sr values and showed no systematic trends through the study interval. We present a five-point running average (black curve) through the data to show predominant isotopic trends. Inset shows best estimate of seawater secular Sr isotope curve for Cambrian Period (<520 Ma) based on compilation of data from this study (bold curve) and data presented in Derry et al. (1994) and Denison et al. (1998). Shaded bands separating trilobite biozones reflect uncertainty in absolute stratigraphic position of each biozonal transition. Representative analytical uncertainty for all ⁸⁷Sr/⁸⁶Sr values is based on replicate analyses of samples (avg. 2 s_m = 21 × 10^–6, n = 19) and the NBS-SRM 987 standard (⁸⁷Sr/⁸⁶Sr = 0.710260; 2 s = 18 × 10^–6, n = 94). Strontium isotope analyses were conducted at University of Texas at Austin following procedures in Banner and Kaufman (1994). Procedural blanks for Sr (5 to 26 pg) were negligible for samples analyzed.