Evolution of the Calcareous Nannofossil Genus Biscutum in the Cretaceous

Brace, Bobbi; Watkins, David

The calcareous nannofossil genus Biscutum is ubiquitous in Upper Cretaceous pelagic sediments. It is a member of the oldest known family of placolith coccoliths, Biscutaceae, which first appears at the Sinemurian-Pliensbachian boundary and persists in to the Paleocene. The abundance of Biscutum constans has also been used frequently as a paleoceanographic proxy for surface water fertility which has important ramifications for the study of hydrocarbon source bed deposition. Species of Biscutum are also used in several biostratigraphic zonation schemes. Unfortunately, species concepts within Biscutum have been inconsistently applied and interpreted owing to a convoluted taxonomic history, thus limiting its utility. For example, Biscutum constans is often referred to interchangeably as B. ellipticum, although the true nature of this taxon is uncertain due to poor definition. This study presents a revision of Biscutum taxonomy in order to increase its efficacy as a biostratigraphic and paleoceanographic indicator. This taxonomic revision allows investigation into the ways in which oceanographic changes during the Late Cretaceous affected the mode and tempo of Biscutum evolution. Samples from five localities were examined: three in the Western Interior Basin (Washington Co., KS; Smoky Hill type area, KS; Sisseton, SD) and two North American deep water localities (DSDP Leg 10; ODP Leg 171b), spanning the upper Albian through Maastrichtian. Morphometric and relative abundance data are presented with respect to oceanic anoxic events (OAEs), including OAE2, one of the most prominent perturbations in the Mesozoic carbon cycle, occurring at the Cenomanian-Turonian boundary. A redefinition of Biscutum species concepts of the Late Cretaceous is also presented based on a review of original type descriptions, new observations via light microscopy and biometric analyses.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013