C4 Ecosystems in Montana Throughout the Neogene

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.