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Abstract Detail



Campanian-Maastrichtian floras on Laramidia: vegetation trends west of the seaway

Matsunaga, Kelly K.S. [1], Smith, Selena [2], Sheldon, Nathan [3].

Paleoenvironmental insights into a Maastrichtian coniferous coastal ecosystem on the western margin of the Western Interior Seaway of North America.

The Western Interior Seaway occupied central North America during the mid to Late Cretaceous, extending as far north as Alaska during its highest stand, and beginning its retreat in the late Campanian or Maastrichtian. Calcareous concretions containing permineralized plant fossils from Maastrichtian deposits of the Pierre Shale Formation in Colorado provide a window into the vegetation and terrestrial environments on the western margin of the seaway. The concretions occur in shallow marine sediments and preserve fossilized fruits, seeds, spores, leaves, stems, and wood fragments that reflect a diverse flora comprising lycophytes, ferns, conifers, Bennettitales, and angiosperms. However, most fossils represent conifers, pointing to a conifer-dominated ecosystem at a time when many coeval floras in North American were angiosperm-dominated. Additionally, the frequent occurrence of charcoalified conifer wood suggests regular fires. To understand the paleoenvironmental context of this flora better, we analyzed the carbon isotopic composition of wood, charcoal, and bulk plant carbon. Our preliminary data yield a δ13C range of -26.50 – -22.46 (mean=-24.31) for wood. Similar values were observed for charcoalified wood, which exhibited a range of -25.21–-23.30 (mean=-24.47). In contrast, the δ13C of bulk detrital plant carbon was -27.48–-26.78 (mean=-27.08). Using a relationship derived from modern C3 plants that relates plant δ13C to atmospheric δ13C and the mean δ13C of the wood, we calculated a δ13C value for atmospheric CO2 of -5.20, which is consistent with a dominantly mantle/volcanic source of Cretaceous CO2. These data were then used to calculate two environmental indicators: pi/pa and Δleaf. pi/pa expresses the partial pressure of CO2 inside the leaf (pi) relative to that of atmospheric CO2 (pa), and increases with atmospheric CO2 concentration. Δleaf represents the offset between δ13C of the atmosphere and δ13C of plant leaf carbon, and generally decreases with water availability reflecting increased water-use efficiency by the plant. The mean pi/pa value for bulk carbon was 0.77, consistent with high Cretaceous atmospheric CO2 concentrations. For Δleaf, values for wood ranged from 21.29–17.26, with a mean of 19.11. In contrast, bulk carbon values ranged 22.28–21.58, and a mean of 21.88. The offset between Δleaf values of the bulk carbon and the wood, all or most of which was conifer wood, indicates the conifers were experiencing water stress. Taken together, the isotopic and macrofossil data indicate a relatively water-stressed, and possibly wildfire-prone, coniferous coastal ecosystem on the western margin of the seaway during the Maastrichtian.


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1 - University Of Michigan, Earth and Environmental Sciences, 1100 North University Avenue, Ann Arbor, MI, 48109, USA
2 - University Of Michigan, Department of Earth and Environmental Sciences, 1100 North University Avenue, 2534 CC Little Building, Ann Arbor, MI, 48109, USA
3 - University Of Michigan, Department of Earth and Environmental Sciences, 1100 North University Avenue, 2534 CC Little Building, Ann Arbor, MI, 48109, United States

Keywords:
Cretaceous
carbon isotope
fossil
conifer
western interior seaway
paleoenvironment
CO2.

Presentation Type: Colloquium Presentations
Session: CO3, Campanian-Maastrichtian floras on Laramidia: vegetation trends west of the seaway
Location: Sundance 3/Omni Hotel
Date: Wednesday, June 28th, 2017
Time: 10:45 AM
Number: CO3009
Abstract ID:375
Candidate for Awards:None


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