CAUSAL MECHANISMS FOR OAE'S II: THE ROLE OF WEATHERING?

It has been suggested that the formation of some Devonian black shales may be related to increased nutrient fluxes and enhanced production resulting from a change in the terrestrial weathering regime, in particular due to the rise of vascular plants. This interesting hypothesis has also been considered as a candidate to explain Cretaceous Oceanic Anoxic Events (OAE's). On one hand, it is well accepted that rates of volcanism were increased, long term CO2 levels were higher, and the world was warmer during the Cretaceous – under such conditions were the oceans sensitive to transient pulses in weathering (due to variable drivers) that increased the flux of terrigenous nutrients? In the Cretaceous these drivers may have included changes in uplift rates, short term warming events that enhanced chemical weathering rates, or changes in terrestrial floras, such as the rise of Angiosperms, that may have influenced soil forming processes. In this study we consider several lines of evidence in an effort to test the weathering rate hypothesis for the Cenomanian-Turonian OAE II in the Western Interior basin of North America. The basin is ideal for such a test as it preserves an excellent record of terrigenous inputs to the basin in the interval prior to and during OAE II. Because of OAE II's short duration (600-900 kyr) geochemical tracers of weathering such as Sr isotopes are impractical. We compile information on (a) changes in stratigraphic architecture that may indicate increases in sediment supply; (b) changes in clay mineral assemblages that have been used as a proxy for weathering regimes; (c) changes in geochemical proxies for riverine input, such as Ti/Al, Na/K, K/Fe+Mg, as well as changes in the mass accumulation rates of selected elements such as Ti and Fe; and (d) changes in floras as indicated by megafossils, preserved plant cuticle, and pollen. In considering the evidence for a change in the weathering regime during Cenomanian-Turonian time we will address two specific hypotheses: 1) did a change in weathering alter the flux of reactive Fe to the Western Interior sea, and 2) did a change in weathering alter the flux of reactive P to the Western Interior sea prior to or during OAE II?