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Project Progress

C and O isotope analyses of carbonates from the Proterozoic/Phanerozoic transition of the Siberian Platform: proxy for early life evolution?

Ongoing research revolves around the carbon and oxygen isotope signals of carbonate rocks from across the Precambrian/Cambrian boundary and the Cambrian of the Siberian platform. The Siberian platform is one of the few places in the world where the Cambrian section is present in relatively continuously and unmodified succession, making Siberia unique among localities where major depositional hiata occur throughout this important evolutionary time period. This transition from unfossiliferous to fossil-rich strata has long been recognized as one of the most significant boundaries in geological and biological history. For reference, the first major radiation of metazoa occurred between Vendian (Precambrian) and Tommotian (Lower Cambrian) times. C-isotope stratigraphy has proven exceedingly useful not only in correlating those strata, but also in elucidating the evolution of life during the “Cambrian Explosion” of life. Marine δ¹³C records are primarily controlled by changes in oceanic C concentrations and are thus useful proxies for paleoenvironments.

We are in the final stages of analyzing over 2000 samples from a profile spanning the uppermost Neoproterozoic (Vendian) to the Late Cambrian. To date we have two continuous profiles; one from the Vendian up to the lower Mayan (latest Middle Cambrian) covering the Precambrian/Cambrian transition, and one from the upper Mayan to the Upper Cambrian covering the SPICE anomaly (Steptoean Positive Carbon Isotope Excursion) (Figure 1). Stratigraphic correlation of the results is preliminary at this stage, and there is still a stratigraphic gap in sample coverage between the two profiles.

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The Precambrian/Cambrian transition is marked by a strong negative δ¹³C excursion of about —7‰ relative to the δ¹³C values in the uppermost Vendian. It is thought that mass extinction events are associated with negative δ¹³C excursions in general, and the Precambrian Ediacara fauna (=Vendian) became extinct during this time frame.

Previous work shows that throughout the Nemakit-Daldynian Stage, δ¹³C signals display a series of five major cycles with increasingly more positive peaks, reaching +6.4‰ towards the end of the Nemakit. Explosive diversification of marine invertebrates early in the Cambrian period may be correlated with the δ¹³C cycles. Whereas similar cyclicity during this time frame has been observed before, our profile shows clearly resolvable, sharp cycles which we detected also at the earliest stages of the Cambrian explosion. Our results will facilitate detailed and qualitative interpretations and correlations with biological events during the earliest Cambrian

The upper portion of our section spans the top of the Middle Cambrian to the top of the Ordovician. The δ¹³C signals are more stable than in the Lower Cambrian. We have detected a very brief and abrupt negative δ¹³C excursion (-3.3‰, arrow in Figure) immediately preceding SPICE. Similarly negative δ¹³C values have been observed in other profiles in different parts of the middle Cambrian (sequence boundaries?). We will test whether this excursion is a real signal of a pre-SPICE disturbance of the geochemical system that may be related to the mass extinction event or its underlying cause such as a change in ocean circulation, or whether diagenesis caused these signals.