2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 9:55 AM

C4 Ecosystems in Montana Throughout the Neogene

SHELDON, Nathan D., Department of Geological Sciences, University of Michigan, 1100 N. University Ave, Ann Arbor, MI 48109, nsheldon@umich.edu

The Cenozoic was a time of biotic innovation among both plants and animals, culminating in a new co-evolved grassland-grazer ecosystem. Many of the newly evolved plants, including most grasses, began to use a new photosynthetic pathway (C4) rather than the C3 mode of photosynthesis favored by earlier evolved groups of plants. Previous work using stable isotopes of paleosol carbonates and vertebrate teeth indicates that the transition to C4-dominant ecosystems in many areas occurred globally asynchronously in the latter part of the Miocene, but that in central North America, some C4 plants were present earlier on in the Neogene. Newly compiled δ13C values from paleosol organic matter and carbonate nodules in Montana demonstrate that plants using a C4 pathway had emerged there in the earliest part of the Neogene. Furthermore, the proportion of C4 photosynthesis was up to 20% at that time, suggesting that the origin of plants using it may be even earlier in Montana, perhaps pre-dating the central North American record. Though the proportion of C4 photosynthesis was variable throughout the Neogene, it was always <30%, including in the late Miocene, when other regions experienced significant radiation of grasses using the C4 pathway. Paleoclimatic data derived from physical and chemical properties of the same paleosol sequence indicate a long-term, stable, semi-arid to sub-humid climatic regime. This long-term climatic stability may explain both the early emergence of C4 photosynthesis there and the relatively invariant proportion of plants using that pathway.