MULTI-PROXY RECONSTRUCTION OF ANCIENT CO2 CONCENTRATION FOR EARLY MIOCENE OF INNER MONGOLIA BASED UPON FOSSIL REDWOODS

The current monsoonal climate in eastern Asia is thought to have initiated in the Neogene due to the Tibetan-Himalayan uplift. Located at the northern edge of the current summer monsoon boundary, the early Miocene megaflora from lacustrine sediments of the Hannuoba Formation in the central Inner Mongolia offers well preserved plant fossils for paleoclimate reconstruction of North China. The rapid accumulation of current atmospheric CO₂ has reached to the highest level in human history, and the concentration is rapidly heading toward levels last seen during the Miocene time. Marine based CO₂ proxies have encountered difficult issues and yielded conflicting results for early Miocene. Thus, a reconstruction of CO₂ level based upon plant mega-fossils during early Miocene in north China would provide critical information toward better understanding of Cenozoic climate system.

Our ongoing research focuses on well-preserved redwood genera, Metasequoia and Sequoia, two closely related plants in Sequoioideae of Cupressaceae which have their living populations restricted in South-Central China and North-West America. Molecular analysis through Py-GC-MS collaborated with detailed morphological and anatomical evidence indicates that these plant fossils contain labile biomolecules such as polysaccharides along with abundant lignin to support the three dimensional structure. The excellent preservation allows us to employ an improved chemical treatment that normally applied to modern plant leaves, yielding large pieces of cleared leaf fragments for the precise measurements of key stomatal parameters such as stomatal index, stomatal density, guard cell length and width, and pore length. These parameters are essentials for the applications of multiple proxies for the reconstruction of ancient CO₂ concentration.

We preformed empirical analysis on the variations of these stomatal parameters at both fossil population and individual specimen levels, leading to a better understanding of the sensitivity and accuracy of these stomatal-based CO₂ reconstruction methods. Based upon the stomatal index method, the leaf gas-exchange model, and the C₃ plant carbon isotope proxy using taxa from the same fossil locality, our ancient CO₂ reconstruction reveals the strength and shortcoming of each method. A better understanding of the source of errors and variations of stomatal parameters that influence the sensitivity of each method would yield more accurate reconstruction of ancient CO₂ for this important time period.