Devonian Reef Cementation, Extinctions, and Carbon Isotopic Signatures

(Dr. Nat Stephens did most of the work on this project. It was funded by the Petroleum Research Fund, ACS, for 1997-1999, and 2000-2002, and by the UREP program for field work in 2000.)

Late Devonian reefs in the Canning Basin, Australia, commonly contain abundant calcite marine cements and abundant microbial communities, an uncommon characteristic for Phanerozoic reefs. There was also a major metazoan extinction near the Frasnian-Fammenian boundary. What are the relationships among microbial expansion in reef environments, demise of reef-building metazoans, and ocean carbonate chemistry? These relationships have important implications for biological interactions among bacteria/cyanobacteria and metazoans, ocean chemistry, and porosity evolution in major hydrocarbon reservoirs. Carbon isotopic data, generated by Nat, show substantial variations in seawater delta 13C during Frasnian and early Famennian time suggesting major changes in biological cycling (see 1999 GSA abstract; Nat has also submitted a paper to Paleo3). Some of these shifts can be correlated to similar shifts in Canada and Europe. These variations contain clues to the causes of extinction as well as microbial expansion.
Studies of the ecology of the microbial reefs is also producing interesting results. Thrombolites grew first and were later encrusted by several different microbial communities including renalcids. Nat is documenting the succession of encrustations to constrain niches within the reefs using reef blocks whose porosity was infilled by quartz sand during a sea level low stand. The figure to the right is a tracing of a reef sample that is about 14 cm across. The black represents the thrombolite (clotted microbial growths), whereas the white part was infilled with quartz sand before the thrombolites were coated by other organisms. Samples from deeper in the reef contain one or more layers of encrusting microbial communities. UREP participants (see below) helped us map out the distribution of sand in the reef blocks, determine the tops of the blocks, and select samples to analyze. The relationships among the microbial communities from these samples will help us determine the growth morphology and structure of reefs lacking metazoans (see 2000 GSA abstract by Stephens and Sumner).

Nat is also looking at the small scale variations in carbon isotopes within the reef to constrain microbial processes. By using careful petrographic observastion and a 20 micron diameter computer controlled microsampling drill, Nat has demonstrated the presence of a slight negative carbon isotopic signature associated with the micritic walls of renalcids, compared to all other microbial components, marine cements, and micrite. Using comparisons to biofilms in modern reefs, Nat developed a model for the growth of renalcids (see in press paper abstract soon).

We had some great help with this research through the University of California Research Expeditions Program (UREP). Here are some memories!

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616