Figure 4. Secular variation in d¹³C values of samples used for Sr isotope analysis, as well as additional samples, that passed our petrographic and geochemical screening criteria for retaining primary marine d¹³C values but did not pass the criteria for Sr isotope analysis. Black curve is a five-point running average of d¹³C values; blue curve is five-point running average of ⁸⁷Sr/⁸⁶Sr values. Inset curve shows best estimate of seawater secular C isotope curve for Cambrian Period based on compilation of data from this study (bold) and data presented in Brasier and Sukhov (1998). Carbon isotope analyses were conducted at University of Southern California, University of Texas at Austin, and University of California, Davis. External precision (1 s) for d¹³C was ±0.05‰.