Constraining The Marine Environments Of The Cambrian Metazoan Radiation

Major advances have been made in understanding the biological aspects of the Proterozoic/Phanerozoic transition, but Earth’s physical environment over this interval is poorly constrained. In particular, there are no quantitative constraints on early Cambrian sea temperatures. The stable oxygen isotope ratio (δ18O) from fossil biominerals is a widely used proxy for ocean temperatures, but a dearth of Cambrian biomineral δ18O data has left a substantial data gap coinciding with the evolution of animal-rich ecosystems. Carefully selected phosphatic ‘small shelly fossil’ (SSF) taxa from three coeval sites in the UK, Morocco, and Canada were examined as potential sources of early Cambrian biomineral δ18O data. A rigorous protocol for assessing the isotopic preservation of SSF biogenic phosphate was established that considered the microstructural, ultrastructural and chemical preservation of visually distinct SSFs. Subsets of SSFs identified as pristine or altered, distinguishable under optical microscopy, were found to be isotopically distinct, with pristine SSFs isotopically heavier than altered SSFs. Well-preserved SSFs yielded δ18O values consistent with geological evidence for a Cambrian greenhouse world, similar to Mesozoic and Cenozoic hothouse climates. Specifically, these data suggest that early Cambrian high latitude (Avaloian) sea surface temperatures (SSTs) were approximately 23 °C and low laititude (Laurentian) SSTs approximately 35 °C. These data provide the first quantitative constraints on early Cambrian sea surface temperatures. These isotopic SSTs were used to calibrate FOAM general circulation model (GCM) simulations set up to test a range of Cambrian pCO2 levels and continental configurations. The calibrated GCM simulations were compared to a new database of climatically sensitive lithologies, with data/model agreement scored according to the environmental conditions required for particular lithologies to form. This study places first-order quantitative constraints on Cambrian ocean temperatures from both proxy data and climate modelling approaches, and lays the groundwork for interrogating Cambrian environments with greater precision.