THE EARLIEST CAMBRIAN RECORD OF ANIMALS AND OCEAN GEOCHEMICAL CHANGE

The earliest Cambrian is characterized by the diversification of skeletal animals and animal burrows, the first appearance of metazoan reefs, and some of the largest amplitude δ¹³C_carbonate variability in the Phanerozoic (comparable in amplitude even to Neoproterozoic excursions). The δ¹³C_carbonate oscillations form the backbone of many earliest Cambrian intra- and inter-continental correlations constructed to address questions about timing and geographic patterns in animal evolution. Furthermore, the δ¹³C_carbonate data has been coupled to precise U-Pb geochronology to model the behavior of the earliest Cambrian carbon cycle in terms of the magnitude and isotopic composition of the carbon fluxes entering and exiting the ocean. However, recent challenges to the validity of carbon-isotope data from carbonate rocks as a primary record of the global inorganic carbon reservoir cast doubt on these correlations and carbon cycle models. We discuss sedimentological, U-Pb geochronological, and stable isotopic data from Morocco, Siberia, Mongolia and China to test competing models for the origin of earliest Cambrian δ¹³C_carbonate variability. In particular, we evaluate burial diagenesis and Holocene-Bahamas-style mineralogical and local water mass hypotheses for negative δ¹³C_carbonate excursions. We conclude that earliest Cambrian δ¹³C_carbonate variability primarily reflects changes in the isotopic composition of global dissolved inorganic carbon.