Lipid Biomarkers of the Maquoketa Formation, Iowa: Transect of a Paleobathymetry Gradient in the Lead-Up to the Late Ordovician Mass Extinction
Abstract
The Late Ordovician (~450-44 Ma) was a period of drastic environmental change, beginning in a hothouse climate with epeiric seaways near a Phanerozoic high and concluding with the Hirnantian glaciation, large positive carbon isotope excursion(s) (Hirnantian isotopic carbon excursion, HICE) and one of the Big Five mass extinctions. The two-phased expression of the Late Ordovician mass extinction has been attributed to regression-driven habitat loss and the consequences of cooling climate, followed by transgression of oxygen-deficient bottom water onto previously oxygenated shelves. Lipid biomarker records indicate substantial changes in microbial communities during the glacial maximum and mass extinction (Rohrssen et al., in press); to fully uncouple the effects of sea level-driven facies change from more regional or global factors we have analyzed lipid biomarkers along a shallow to deep water paleobathymetry gradient in central Laurentia across a transgressive-regressive cycle. We compare results from the Maquoketa Formation to previous work on Hirnantian- and Katian-age rocks to develop a better understanding of the association of microbial communities with Late Ordovician-age epeiric sea and upwelling environments. During deposition of the Katian-age Maquoketa Formation, Iowa was bounded to the north by exposed highlands of the Transcontinental Arch and separated from the southeastern half of the Laurentian epeiric seaway by a northeast-southwest trending shelf-break into the deeper waters of the Seebree Trough, a depression thought to have connected central and eastern Laurentia to the open ocean. As a result of this paleotopography, samples of the Maquoketa Formation collected from drill cores BS5 (Clayton County), SS-15 (Jackson County), and H33 (Des Moines County) in a transect from northeastern to southeastern Iowa capture the change in facies from carbonate-rich platform to shale with phosphatic intervals at the shelf-break in contemporaneous deposits (Raatz and Ludvigson, 1996). Lipid biomarker records obtained from these three cores constrain the extent to which changes in microbial communities that took place during the Late Ordovician mass extinction are attributable to local facies effects as opposed to more regional or global drivers. Maquoketa Formation bitumens have low thermal maturities consistent with the tectonic history of the region, and lack compounds derived from contamination (e.g. oleanane from angiosperms). Maquoketa Formation bitumens show moderately high hopane/sterane ratios, reflecting greater contribution of bacteria than eukaryotes to sedimentary organic matter, a property that appears to be typical of Late Ordovician-age epeiric deposits. As previously shown for other latest Ordovician rocks, Maquoketa Formation bitumens have high 3beta-methylhopane indices, attributable to high relative contributions from Type 1 methanotrophic proteobacteria, suggesting that conditions suitable to proliferation of those organisms preceded the mass extinction by several million years.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP21D..02R
- Keywords:
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- 0473 BIOGEOSCIENCES / Paleoclimatology and paleoceanography;
- 1055 GEOCHEMISTRY / Organic and biogenic geochemistry;
- 4912 PALEOCEANOGRAPHY / Biogeochemical cycles;
- processes;
- and modeling;
- 9614 INFORMATION RELATED TO GEOLOGIC TIME / Paleozoic