Brett J. Tipple and Mark Pagani
The C4 photosynthetic pathway is a series of structural and biochemical modifications around the more primitive C3 pathway that improve the photosynthetic efficiency under specific climatic conditions. Hence, the origin and subsequent geographical expansion of the C4 plants likely reflects a record of climate change. Multiple paleoatmospheric pCO2 proxies indicate a critical CO2 threshold was breached ~30 Ma, that potentially selected for CO2-concentrating mechanisms to overcome photorespiratory stresses imposed on the basic C3 pathway. Details of the C4 pathway’s earliest origins remain enigmatic given the paucity of the geologic record. Nonetheless, d13C proxy records from paleosol carbonates, ungulate teeth, and plant-derived compounds indicate C4 plants likely represented an important component of plant biomass by the Early Miocene. Low CO2 levels appear to be a precondition for the development of the C4 photosynthetic pathway; however, comparisons of δ13C proxy records indicate that the timing of C4 geographical expansion was not globally synchronous, and thus point toward more regional controls on the development of C4-dominated ecosystems. Terrestrial and marine records indicate that continental aridity and wind strength increased during the Late Miocene. These conditions would have likely increased fire occurrence and fire intensity leading to the clearing of large tree stands and the expansion of C4 grasses in warm-season precipitation regimes.
Brett J. Tipple and Mark Pagani
Hydrology, source region, and timing of precipitation are important controls on the climate of the Great Plains of North America and the composition of terrestrial ecosystems. Moisture delivered to the Great Plains varies seasonally and predominately derives from the Gulf of Mexico/Atlantic Ocean with minor contributions from the Pacific Ocean and Arctic region. For this work, we evaluate long-term relationships for the past ~35 million years between North American hydrology, climate, and floral change, using isotopic records and average carbon chain lengths of higher plant n-alkanes from Gulf of Mexico sediments. We posit that hydrological changes were linked to tectonic and oceanographic processes including the shoaling and closure of the Panamanian Seaway, amplification of North Atlantic Deep Water Production and an associated increase of meridional winds. The southerly movement of the Intertropical Convergence Zone ~4 Million years ago allowed for the development of a near-modern pressure/storm track system, driving increased aridity and changes in seasonality within the North American interior.
Mark Pagani, Katherine H. Freeman, Michael A. Arthur
Trends in carbon isotope compositions of soil carbonates and equid teeth enamel support a global expansion of C4 grasses in the late Miocene. This event was originally attributed to a large-scale decrease in pCO2 which would have favored C4 over C3 plant metabolism. Here we present alkenone-based pCO2 estimates for the late Miocene from Deep Sea Drilling Project (DSDP) Site 588. Our results indicate that there was no major change inpCO2 during this time (10-5 Ma). Instead, pCO2 steadily increased from ca. 14 to 9 Ma and stabilized at approximately pre-industrial values. We propose that C4 expansion was triggered by an episode of regional aridity on a global scale caused by both existing low pCO2 conditions and a late Miocene phase of Asian orogeny.