Cambrian strontium isotope ratios

This further emphasizes the importance of sedimentary processes in the development of such fractionated Rb/Sr and Eu/Sr ratios. Second, in the context of the LFB, the Ordovician turbidites have progressively lower Eu/Sr ratios than the feldspathic Cambrian sediments from which they were partly derived. Thus, it also appears that the displacement to higher Eu/Sr is linked to the number of weathering and erosion cycles involved in the formation of particular sedimentary rock suites. In the examples in Figure 20, the reduction in strontium content for the most evolved Ordovician sediments is nearly 10-fold, and this clearly results in an associated increase in Rb/Sr, and hence with time in strontium isotope ratios. 

Friedman G. M. (2002) Highest Phanerozoic strontium isotopic ratios of pre-rift late Cambrian passive margin in New York State USA products of continental weathering and orogenesis. Sedim Geol. 147, 143 153. 

For the strontium isotope record, the present is the key to the Cambrian in between, the Sr/ Sr ratio is seen to fall in roller-coaster fashion through the Paleozoic and early Mesozoic to a minimum in the Late Jurassic, and then to rise in steps to the present (Figure 3(a)). The Late Ordovician, Late Devonian (Figure 3(b)), and P-Tr events occurred just after local minima in Sr/ Sr and thus mark times of significant change in the strontium cycle. In contrast, the Tr-J event follows a local maximum value in the ratio, and marks a time of declining Sr/ Sr. The K-T event occurred contemporaneously with a local maximum in the ratio, on both the multimillion and 10 -year timescales (MacLeod et al., 2001 Figures 3(a) and (c)). 

The isotope compositions of oxygen and carbon identify this specimen as amarine limestone of Cambrian age most likely derived from the Shackleton Limestone (Veizer and Hoefs 1976). However, the Sr/ Sr ratio is lower than Sr/ Sr ratio of marine strontium of... 

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