Date of Degree

10-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor(s)

Neil H. Landman

Subject Categories

Ecology and Evolutionary Biology | Paleontology

Keywords

carbonate, Cretacous, faunal communities, fluid flow rate, methane cold-seep, Pierre Shale

Abstract

Most investigations of ancient methane seeps focus on either the geologic or paleontological aspects of these extreme environments. In contrast, this thesis encompasses both disciplines to evaluate the paleoecology of these systems with greater detail than previously published either within the Western Interior Basin or elsewhere. This thesis addresses the following questions: 1) Are the changes in mineralogy of a seep discernable and predictable as a seep shifts from a clay-based environment to a carbonate-based environment? 2) What are the foundation organism(s) of Late Cretaceous methane cold-seeps? 3) Is there a correlation to the mineralogic changes and shifts in community structure? 4) What is the mediating factor of these shifts? and 5) Are there any spatial trends in seep formation and persistence?

To help resolve these queries, seep cement material and fossils were collected from 25 locations from the Baculites scotti - Didymoceras nebrascense, Didymoceras cheyennense, and Baculites compressus zones spanning ~2 million years in the Campanian of the Pierre Shale in South Dakota. The mineralogy of cements was determined through microprobe and electron dispersive spectroscopy. These data was spatially analyzed and suggest that there is a potential shift in mineralogy due to relative fluid flow rates over space and time: Baculites scotti - Didymoceras nebrascense Zone was of consistent moderate flow rates, Didymoceras cheyennense Zone was dominated by low fluid flow rates, and Baculites compressus Zone contained an unusually large number of high fluid flow seep assemblages compared to other biostratigraphic zones. Over 8000 individual organisms were counted and identified to the genus or species level (where possible) and these data were then processed with a series of diversity indices. A faunal pattern was found and shows that the foundation organisms, baculites, inoceramids, and lucinids, always dominate these seep assemblages, unlike the foundation organisms in modern methane cold-seep analogs. These fossil data were then paired up with each location's mineral data and a correlation was found between shifts in mineralogy and basic seep structure (general flow rate) and shifts in faunal compositions. Furthermore, specific fauna patterns and flow rates may indicate changes in oxygen or food availability. However, Akaike modeling techniques implemented found that the shifts in community composition within the cold-seeps of the Pierre Shale of South Dakota are most likely a reflection of sulfur availability according to the data.

 
 

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