ONSET OF THE EARLY PALEOZOIC: GLOBAL HYPERWARMING AND PALEOGEOGRAPHY DURING THE CAMBRIAN EVOLUTIONARY RADIATION (CER)

Whether the terminal Ediacaran‒early Cambrian CER reflected intrinsic or extrinsic controls remains problematic, although the “retreat” of soft bodied trace fossil makers below the sediment-water interface and biomineralization have been suggested to reflect a common response to high UV-B episodes in the Ediacaran‒Cambrian boundary interval. The rise of increasingly Phanerozoic-aspect communities with deep burrowers, biomineralized metazoans, early conodont-like midwater predators, and metazoan-“algal” reefs (tropical archaeocyathans and coeval “worm” reefs with “Ladatheca” and Coleoloides in cool-water Avalonia) took place in a warm, almost completely ice-free world. Most continents show development of tropical carbonate platforms with local evaporites. Two continents (Baltica; the non-Gondwanan Avalon paleocontinent) had temperate, shallow-shelf siliciclastic platforms. Common depictions of west Gondwana at or near the South Pole through the Cambrian are belied by 1) early Cambrian carbonate platform development in eastern South America and Namibia‒southern Morocco‒Iberia and Sardinia‒southern France‒central Germany, 2) absence of late Ediacaran‒Cambrian ice-deposited sedimentary rocks (with exception of dropstone beds in Avalonian southern New Brunswick and Ireland), 3) late Ediacaran paleolatitudes shown by east African intrusives, and 4) the association of “ice house” conditions throughout the Phanerozoic only at times with a major polar continent or geographically isolated polar sea. The deep sea was well oxygenated, with well burrowed, red mudstones and with black mudstones limited to the upper slope and spreading onto the shelf with sea-level rises into the peritidal early Cambrian of east Laurentia (i.e, Moosalamoo Fm.) and in the post-early Cambrian of Baltica, Avalonia, and the southern Great Basin. Deep-water oxygenation, despite the apparent absence of dense, cold polar waters, may reflect sinking of salt-rich surface waters formed by evaporation under warm polar gyres.