THE EFFECT ON PALEO-PRODUCTIVITY OF THE FIRST MAJOR DELIVERY OF MID-LAURENTIAN SAPROLITE-DERIVED MATERIAL TO PHANEROZOIC OCEANS: CONTINENT-WIDE MARINE RAVINEMENT DURING SUBMERGENCE-EMERGENCE OF LATE CAMBRIAN NORTH AMERICA, AND THE GLOBAL CARBON ISOTOPE SPICE EVENT

Stratigraphic, biostratigraphic and Carbon isotope data from four Upper Cambrian pan-Laurentian sections— Utah, Iowa, Missouri and Newfoundland, demonstrate that the global +~5‰ d¹³C excursion, or SPICE event, closely tracked the Dunderbergia zone late Sauk II regression. Sea level minimum was marked by a conspicuous influx of quartz sand onto carbonate platforms, and corresponds to the SPICE maximum, at each of the studied areas. In contrast, immediately preceding regression, during Aphelaspis zone time, seas appear to have covered more of Laurentia than during any time previous in the Phanerozoic and possibly more than during any time in the preceding ~0.5 Gy. Immediately following terminal Sauk II regression, in Elvinia zone time, sea level returned to Aphelaspis zone levels or higher. This suggests a causal link between submergence-emergence-submergence of the craton and the d¹³C perturbation that tracked it.

Pre- Aphelaspis central Laurentia was presumably thickly mantled by saprolite, the product of protracted weathering of Precambrian rock. We propose that this saprolite was stripped and remobilized by Aphelaspis zone marine ravinement that culminated the first major transgression of the Phanerozoic. Nutrients that had hitherto been concentrated in a terrestrial repository, were thus delivered to the marine realm, and dispersed to circum-Laurentian oceans by the immediately ensuing latest Sauk II regression. This implies that broad, sweeping, continent-scale, marine ravinement was more effective at eroding the thick rind of weathered material than were the day-to-day geomorphic processes active on the pre-macrovegetated craton. The Carbon isotope record suggests that circum-Laurentia oceanic productivity was in equilibrium with pre- Aphelaspis terrestrial runoff, but was significantly perturbed by the delivery of weathering-products from marine ravinement of the cratonic interior. This interpretation explains the uniqueness of the relationship between the SPICE and sea level change— it could happen only once, because of the historical contingency that saw a lower Phanerozoic marine fauna react to the delivery of a previously-untapped bonanza of continental nutrients (presumably a bonanza for some, a calamity for others).