CARBON CYCLING PERTURBATION THROUGH THE LOWER-MIDDLE CAMBRIAN BOUNDARY INTERVAL AND OLENELLOID BIOMERE EXTINCTION AS REVEALED BY d13CORG

We present the results of a high-resolution carbon isotope analysis of organic matter integrated with detailed trilobite biostratigraphic study of marine shales from the uppermost Lower Cambrian (Olenellus) through lower Middle Cambrian (Eokochaspis nodosa and Amecephalus arrojosensis biozones) boundary interval that more fully delineates the relationship between perturbation in carbon cycling and biomere extinction. The d¹³C values of organic matter (d¹³C_org) in shales from four correlated sections of the boundary interval along a shelfal to slope transect in the southern Great Basin show repeated short-lived fluctuations (from -28‰ to -21‰). The magnitude of the d¹³C_org fluctuations increases and their duration decreases upwards in the upper Olenellus biozone. The most positive d¹³C_org values in all three sections are observed proximal to the boundary. The Lower-Middle Cambrian boundary is characterized by 2 to 3 rapid oscillations; variability in the number of fluctuations that are developed in each section immediately below the biomere boundary likely records stratigraphic hiatuses that are of sub-biostratigraphic resolution. The d¹³C_org values in overlying, sparsely fossiliferous shales define multiple oscillations of smaller magnitude and lower frequency than those associated with the boundary. Preliminary analysis of organic matter in the Kaili Formation, Balang section in southeast China, documents the presence of similar magnitude and frequency d¹³C_org fluctuations in the boundary interval. These temporal trends in magnitude and frequency of d¹³C_org oscillations, coupled with trends in faunal diversity throughout the Lower-Middle Cambrian boundary interval, are interpreted to record repeated fluctuations in marine primary productivity and seawater [CO₂] possibly driven by changes in deep-water ventilation or introduction of O₂-poor, nutrient-rich deeper waters onto the Cambrian shelves during pulsed latest Cambrian transgression.