Calcareous Nannofossil Paleoecology of the mid-Cretaceous Western Interior Seaway and Evidence of Oligotrophic Surface Waters during OAE2
Abstract Analysis of calcareous nannofossils, total organic carbon(TOC) values, and carbon isotope records from outcrop and drill cores in the central (Colorado and Kansas) and southern (Texas) Western Interior Basin (WIB) indicates predominantly oligotrophic surface water conditions during the late Cenomanian–Turonian Oceanic Anoxic Event 2(OAE2). High-resolution bio-chemostratigraphy of this interval allows for detailed correlation of changes in nannofossil assemblages and TOC across the WIB and aids comparison with other global records. Prior to and at the onset of OAE2, the A interval in the δ13C positive excursion identified by Pratt and Threlkeld (1984), mesotrophic to eutrophic nannofossil taxa Biscutum and Zeugrhabdotus are relatively high in abundance and foraminiferal assemblages record a high diversity “benthonic zone”. TOC sharply declines at the start of OAE2 despite evidence of high surface productivity and benthic diversity, implying that the seafloor was briefly oxygenated during a period of enhanced mixing in the expanding Western Interior Seaway. During the latter two thirds of OAE2 (intervals B and C in the excursion) TOC periodically increases, suggesting that anoxic conditions returned at the substrate, despite persistently low abundances of Biscutum and Zeugrhabdotus, implying oligotrophic surface water conditions. The divergence of these two proxies suggests a highly stratified water column in the WIB, not higher productivity, which resulted in anoxic conditions during OAE2. If productivity was not the primary cause of increased carbon burial in the WIB it is probable that increased terrestrial runoff and incursion of warm, saline Tethyan waters from the south led to salinity stratification. In the absence of mixing, warm and saline bottom waters would have been isolated from freshened Boreal currents and become stagnant, allowing for preservation of greater amounts of organic material in the substrate. Nannofossil records from Cuba, KS are an exception, and reveal relatively high productivity throughout, suggesting that occasional mixing of nutrients to the surface may have continued along the eastern margin of the central WIB.