210 / 2025-04-15 01:15:57

Oscillatory behaviour in the Tonian – Cambrian climate system

Tonian, Cryogenian, Ediacaran, Cambrian, climate, Carbon isotopes, Carbon cycle, Feedbacks

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

Carbon isotope perturbations characterise the Tonian to Cambrian marine carbonate record, which exhibits uniquely high amplitude oscillations that also relate to other biogeochemical cycles, redox conditions, sea level and climate. Early work suggested that negative excursions marked glacial intervals (Kaufman et al., 1997), and it is now firmly established that the onset of Neoproterozoic global ice ages coincided with the recovery from negative anomalies. Our recent isotopic and geochronologic studies (Fairchild et al., 2018; Rugen et al., 2024) evidence this relationship through the pre-glacial Garvellach Formation in Scotland that is transitional into Sturtian glaciation, while a similar relationship has been proposed for the Marinoan ‘Snowball’ event (Rose et al., 2012). Although of more regional extent, Ediacaran ice ages (Gaskiers and Luoquan) also follow negative anomalies and much warmer intervals, suggesting a common causality. The largest of these negative anomalies, the late Ediacaran Shuram anomaly, with an amplitude of about 15‰, is the best catalogued, and has been identified globally in rocks deposited during a warm interval between about 580 and 560 Ma, respectively. Although the ‘Shuram’ (Wonoka / DOUNCE) anomaly has been repeatedly assigned to biased sampling, it is more parsimoniously explained as a sustained perturbation to the global carbon cycle that led to global warming, ocean oxygenation and consequential biological radiations. A primary origin for the Shuram anomaly implies low organic burial rates, which are consistent with independent isotopic evidence for highly efficient organic remineralisation, implying net transfer of oxidising power from weathered sulphate minerals to the global carbon cycle. (Shields et al., 2019). Here I will explore the possibility that such sink-switching between the coupled carbon and sulphur cycles led to oscillatory behaviour in the global carbon cycle whereby warmer intervals of excess pyrite burial (over pyrite weathering) were succeeded by cooler intervals of excess pyrite weathering (over pyrite burial). Environmental instability during the Tonian - Cambrian interval was likely driven by positive feedbacks that were exacerbated by relatively reducing, low sulphate conditions in the world’s oceans. Specifically, I argue that the exponential increase of bacterial respiration rate with temperature (Stanley, 2010) together with other earth system feedbacks involving episodic sulphate limitation, calcium cycling and sea level served to decouple climate from the compensatory organic and inorganic carbon cycle feedbacks that might normally be expected to regulate surface temperature.



References

Fairchild, I. J., Spencer, A. M., Ali, D. O., Anderson, R. P., Anderton, R., Boomer, I., Dove, D., Evans, J. D., Hambrey, M. J., Howe, J., Sawaki, Y., Shields, G. A., Skelton, A., Tucker, M. E., Wang, Z., Zhou, Y., 2018. Tonian-Cryogenian boundary sections of Argyll, Scotland. Precambrian Research, 319, 37-64.

Kaufman A. J., Knoll A. H., Narbonne G. M., 1997. Isotopes, ice ages and terminal Proterozoic earth history. PNAS, 94，6600-6605. 

Rose, C. V., Swanson-Hysell, N. L., Husson, J. M., Poppick, L. N., Cottle, J.M., Schoene, B., Maloof, A.C., 2012. Constraints on the origin and relative timing of the Trezona d¹³C anomaly below the end-Cryogenian glaciation. EPSL, 319-320, 241-250.

Rugen, E. J., Pastore, G., Vermeesch, P., Spencer, A.M., Webster, D., Smith, A. G. G., Carter, A., Shields, G.A., 2024. Glacially influenced provenance and Sturtian affinity revealed by detrital zircon U-Pb ages from sandstones in the Port Askaig Formation, Dalradian Supergroup. Journal Geol. Soc., 181, jgs2024-029.

Shields, G. A., Mills, B. J. W., Zhu, M., Raub, T. D., Daines, S. J., Lenton, S. M., 2019. Unique Neoproterozoic carbon isotope excursions sustained by coupled evaporite dissolution and pyrite burial. Nature Geoscience, 10.1038/s41561-019-0434-3.

Stanley, S. M., 2010. Relation of Phanerozoic stable isotope excursions to climate, bacterial metabolism and major extinctions. PNAS, 107, 19185-19187.