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Microfossil and model constraints on the global climate during the Cambrian ‘explosion’

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The Neoproterozoic/Phanerozoic transition was characterised by one of the most fundamental phases in the evolution of life on Earth: the radiation and diversification of metazoans. The increasing abundance, mobility and mineralisation of early animals had profound consequences for Earth’s physical environment and geochemical cycles.

However, our understanding of the co-evolution of the physical environment and the biosphere throughout the Ediacaran/Cambrian radiation of animals is hampered by substantial uncertainties in global palaeogeography and palaeoclimate. Earth surface temperatures during this interval are particularly poorly constrained.

The stable oxygen isotope ratios (δ 18 O) is perhaps the most widely used proxy for quantitatively constraining Phanerozoic ocean temperatures. In particular, the δ 18 O data held in the biomineralised parts of foraminifera, molluscs, brachiopods and conodonts are routinely used to investigate palaeoenvironmental questions. Indeed, there is a near-continuous δ 18 O record back to the Early Ordovician.

The notable dearth of δ 18 O data in the Cambrian Period coincides with the global establishment of animal-rich ecosystems, and in part arises from a lack of traditionally targeted fossils in this interval.

Phosphatic ‘small shelly fossils’ (SSFs) provide a novel alternative source of biomineral δ 18 O data, but need to be carefully screened to exclude diagenetically altered specimens. Oxygen isotope data from well-preserved SSFs suggest that the Cambrian was a greenhouse interval in Earth’s climate history. This is consistent with the available geological (sedimentological) evidence, including palaeogeographically widespread carbonate and evaporite deposits.

Taking these δ 18 O data in concert with general circulation climate models (GCMs) of the early Cambrian Period, suggests that the early Cambrian climate was comparable to later, and better understood, Phanerozoic greenhouse climate states such as those of the late Mesozoic and early Cenozoic.

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