From snowball earth to the cambrian explosion: the interpretative potential of the isotope record

The interval in Earth history between 750 Ma and 530 Ma represents the erratic transition between the distinctly different "Earth systems" of the Proterozoic and the Phanerozoic. Geologists have developed several indirect methods to study this key interval but perhaps none with as much wanton enthusiasm as isotope geochemistry. Here I attempt a review of the isotope record across this key transition period, with special emphasis on the interpretative role that isotopes can play in understanding events such as the "Snowball Earth" glaciations and the "Cambrian Explosion". The marine isotope record reveals the Proterozoic-Phanerozoic transition to have been a unique period in Earth history with first-order features in all three of the major isotope proxies: ⁸⁷Sr/⁸⁶Sr, ³⁴S/³²S and ¹³C/¹²C ratios. Seawater ⁸⁷Sr/⁸⁶Sr, increased sharply between about 900 Ma and 500 Ma from 0.704 to 0.709, the largest rise ever. Although attributed to increasing rates of continental erosion, this remains only the most plausible explanation because few aspects of the rise have been satisfactorily linked to particular tectonic events. Much of the uncertainty surrounding the Proterozoic ⁸⁷Sr/⁸⁶Sr record is due to poor age dating and discrepancies in stratigraphic correlation schemes, which together with diagenetic alteration have led to the current inability to realise the potential of the ⁸⁷Sr/⁸⁶Sr record for global stratigraphic correlation. Carbon isotopes have played a key role in recent palaeoenvironmental interpretations of the Proterozoic-Phanerozoic transition, most of which have already been discarded or merely forgotten. Nevertheless, it is clear that the extraordinary δ¹³C values of the later Neoproterozoic (and the earliest Cambrian) demand extraordinary explanations, for which there is still much scope for future invention. Carbon isotopes may also play an important supporting role in correlation, however, the role that δ¹³C may play in global stratigraphic correlation has generally been overstated in the literature. Sulphur isotopes have been a less visible but just as important component of Proterozoic-Phanerozoic interpretations. After all it is partly δ³⁴S data that have provided crucial, non-palaeontological supporting evidence for a rise in surface oxygenation during the mid-Neoproterozoic. Intriguingly, sometime during the latest Neoproterozoic, seawater sulphate δ³⁴S rose to all-time highest values by the Precambrian-Cambrian boundary. Some proponents of the Snowball Earth hypothesis have surmised that global glaciations would have caused massive fluctuations in seawater sulphate δ³⁴S and recently published carbonate-bound sulphate δ³⁴S data appear to support this idea. However, it first needs to be demonstrated that such data indeed represent faithfully seawater isotopic composition. Isotope stratigraphy across the Proterozoic-Phanerozoic transition is in a state of exciting flux with many mutually conflicting correlation schemes and interpretations. Plainly more work is needed in order to reach a satisfactory consensus. However, what is also needed, is more focus, perhaps within the auspices of a global working group, keeping well in mind that although more data would be welcome, the single most important advance would be more secure age dating.