Phytoplankton productivity and Milankovitch Cycles in the Cenomanian-Turonian Bridge Creek member of the Greenhorn Formation in southeastern Colorado

Abstract

Statistical analyses of palynomorph assemblages from the Upper Cretaceous Bridge Creek Member (Greenhorn Formation) near Pueblo, Colorado suggest that the micritic limestone/organic rich-shale cycles of the Bridge Creek may have been caused by a combination of fluctuating primary productivity and humid/arid climate cycles.

Species richness and evenness indices for palynomorph assemblages from 24 Bridge Creek beds were statistically analyzed using Analysis of Variance. The results indicate that assemblages for the limestone beds exhibit greater evenness and richness indices than do assemblage from the shale beds. Because phytoplankton communities typically exhibit lower evenness and richness values in eutrophic conditions, these results are consistent with possible surface water eutrophication during times of shale deposition. During times of high primary productivity, the aerobic oxidation of large amounts of organic matter settling to the sea floor may have consumed the oxygen available at depth creating an anaerobic environment and facilitating the preservation of organic carbon.

Riverine input to the seaway also contributed to cycle production. It appears that shale was deposited during humid periods when riverine runoff provided terrigenous material necessary for shale deposition. The flow of isotopically-light fresh water to the seaway during times of shale deposition is supported by lighter 𝛿¹⁸O values in the shale beds.

The fluctuating primary productivity and humid/arid cycles may have been caused by Milankovitch Cycle-driven climate change. Climate models indicate that insolation fluctuations driven by the precessional or obliquity cycle may have periodically increased upwelling along the eastern margin of the Cretaceous Interior Seaway and influenced rainfall patterns. At appears that organic rich shale was deposited when upwelling, nutrient-rich bottom water stimulated planktonic productivity and rainfall transported terrigenous material to the seaway.