Isotopic evidence for temperate oceans during the Cambrian Explosion

Area of Science:

• Paleoclimatology
• Paleontology
• Geochemistry

Background:

• The Cambrian Explosion marks a pivotal period in Earth's evolutionary history.
• Previous estimates suggested extreme sea surface temperatures (~60°C) during this time, posing a challenge to known thermal limits of marine life (~38°C).

Purpose of the Study:

• To reconstruct accurate sea surface temperatures during the Cambrian Explosion.
• To resolve the discrepancy between proposed high temperatures and the biological feasibility of a major evolutionary event.

Main Methods:

• In situ oxygen isotope (δ¹⁸O) analyses of Cambrian phosphatic brachiopods using secondary ion mass spectrometry (SIMS).
• Evaluation of diagenetic alteration to identify pristine δ¹⁸O_phosphate signatures, reflecting primary δ¹⁸O_seawater.
• Temperature calculations assuming ice-free conditions and stable δ¹⁸O_seawater values.

Main Results:

• Initial temperature estimates ranged from 35°C ± 12°C to 41°C ± 12°C, still above modern and lethal limits.
• Inferred a minimal depletion of approximately -3‰ in early Cambrian δ¹⁸O_seawater compared to present-day values.
• Revised temperature estimates, considering this depletion, indicate habitable subequatorial ocean temperatures around 30°C.

Conclusions:

• Early Cambrian sea surface temperatures were likely habitable for marine invertebrates, supporting the occurrence of the Cambrian Explosion.
• The study refines our understanding of paleoclimates and the environmental conditions conducive to major evolutionary radiations.
• Phosphatic brachiopod oxygen isotopes provide a reliable proxy for reconstructing ancient ocean temperatures.