SEAWATER CHEMISTRY AND THE DECLINE OF BURGESS SHALE-TYPE PRESERVATION

Burgess Shale-type (BST) biotas occur globally in the Cambrian and offer unparalleled insight into the Cambrian Explosion. The prevailing hypothesis for the decline of BST biotas is that they were literally “burrowed away” by an increase in depth and extent of infaunal activity. Data now available indicate that a post-Cambrian increase in bioturbation cannot account for the apparent loss of BST biotas from the fossil record, because preservation at key localities occurred under anoxic benthic conditions that were sustained for meters of continuous section. Thus, other non-analogue conditions must have facilitated preservation. Recent work has focused on the role of seafloor carbonate cements in promoting BST preservation, by early isolation of the diagenetic environment from overlying seawater. An analysis of lithologic data from the Macrostrat database, in conjunction with petrographic and geochemical data, demonstrates that a period of enhanced weathering of continental crust led directly to elevated oceanic alkalinity during the Cambrian-Early Ordovician. A sustained flux of alkalinity to the ocean was caused by marine transgression across the Great Unconformity, a surface of unprecedented area of exposed continental basement rock. An elevated flux of chemical weathering products to the oceans led to widespread seafloor carbonate cementation in outer shelf mudstones, as well as to other patterns of anomalous chemical sedimentation. Seawater chemistry favorable to BST preservation persisted into the early Ordovician, however, outer shelf mudstones were prominently restricted from cratonic strata after the middle Cambrian due to widespread seaward progradation of carbonate platforms. Our results predict that younger BST biotas, which could bridge the evolutionary gap between the Cambrian Explosion and the Great Ordovician Biodiversification Event, should be present in rare outer shelf mudstones of late Cambrian and early Ordovician age (e.g., Fezouata biota).