Possible links between extreme oxygen perturbations and the Cambrian radiation of animals

Abstract

The role of oxygen as a driver for early animal evolution is widely debated. During the Cambrian explosion, episodic radiations of major animal phyla occurred coincident with repeated carbon isotope fluctuations. However, the driver of these isotope fluctuations and potential links to environmental oxygenation are unclear. Here, we report high-resolution carbon and sulphur isotope data for marine carbonates from the southeastern Siberian Platform that document the canonical explosive phase of the Cambrian radiation from ~524 to ~514 Myr ago. These analyses demonstrate a strong positive covariation between carbonate δ¹³C and carbonate-associated sulphate δ³⁴S through five isotope cycles. Biogeochemical modelling suggests that this isotopic coupling reflects periodic oscillations in atmospheric O₂ and the extent of shallow ocean oxygenation. Episodic maxima in the biodiversity of animal phyla directly coincided with these extreme oxygen perturbations. Conversely, the subsequent Botoman-Toyonian animal extinction events (~514 to ~512 Myr ago) coincided with decoupled isotope records that suggest a shrinking marine sulphate reservoir and expanded shallow marine anoxia. We suggest that fluctuations in oxygen availability in the shallow marine realm exerted a primary control on the timing and tempo of biodiversity radiations at a crucial phase in the early history of animal life.

Fig. 1. Carbonate carbon and carbonate-associated sulphate sulphur isotope records from Cambrian Stage 2 to Stage 4 of Siberian Aldan-Lena rivers sections.

Regional stage subdivisions are shown next to the global subdivision plan for comparison (F.: Fortunian Stage; N.–D.: Nemakit–Daldynian Stage; TST: Transgressive System Tract; Fm.: Formation; A.: archaeocyaths; SSFs: small shelly fossils). Names for the positive δ¹³C peaks (III, IV, V, VI, VII) are consistent with those of previously suggested δ¹³C curves. FAD: first appearance datum.

Fig. 2. Carbon and sulphur cycle model output.

a. This model takes measured δ¹³C values as an input parameter. b. Burial rates of organic carbon (C_org) are inferred from isotope mass balance and δ¹³C record, and burial rates of pyrite are assumed to be controlled by modelled organic matter availability. c. Comparison between analysed δ³⁴S data (green curve) and simulated seawater sulphate δ³⁴S values (pink); Dashed part of the green curve shows the sampling gap. d. Variations in modelled net oxygen production. For all plots, the uncertainty window represents an alteration of the δ¹³C values of carbon inputs between -5‰ and -8‰.

Fig. 3. Animal diversity, biological events and their correlation to the isotope records and oxygenation pattern across Cambrian stages 2-4.

Global oxygen production is inferred from isotope mass balance modelling, using inputs of δ¹³C only (light shade), or δ¹³C and δ³⁴S (dark shade). Archaeocyathan species (blue line) and total animal species (green line) diversity records are expressed as the mean number of species per sampling unit (grey box) in Siberia; OP: oxygenation pulse; BH: biodiversity high; F.: Fortunian Stage; N.–D.: Nemakit–Daldynian Stage. FAD: first appearance datum.