--> Abstract: Isotopic Evidence From Terrestrial and Marine Biomarkers For Climatic Conditions in the Middle and Late Miocene: Implications for the Expansion of C-4 Plants, by K. H. Freeman, L. A. Colarusso, and M. Pagani; #90937 (1998)

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Abstract: Isotopic Evidence From Terrestrial and Marine Biomarkers For Climatic Conditions in the Middle and Late Miocene: Implications for the Expansion of C-4 Plants

FREEMAN, KATHERINE H., LEE A. COLARUSSO and MARK PAGANI, Department of Geosciences, The Pennsylvania State University, University Park, PA 16802.

The Middle Miocene was a time of pronounced change in the global climate system, including global cooling, expansion of Antarctic ice sheets and fundamental changes in deep-ocean circulation. Middle Miocene climatic conditions possibly set the stage for the evolution of C-4 plants, and several authors have suggested additional changes in climate during the Middle and Late Miocene directly resulted in the expansion of low-latitude C-4 plant grasslands on a global scale. We have employed the isotopic compositions of organic compounds preserved in ancient soils in Pakistan and Nepal to track the rise of C-4 plants in southwestern Asia. Our results from organic matter document a similar pattern as the d13C values for pedogenic carbonates and fossil tooth enamel at this site and other locations, confirming this molecular approach to ecosystem reconstruction. In addition, we have produced a record of the d13C of alkenones, compounds derived from haptophyte algae, from several sites in the global ocean. Based on the isotopic record of alkenones and coexisting planktonic formaminifera, we have reconstructed changes in the concentration of oceanic and atmospheric CO2 levels through the Middle and Late Miocene. Taken together, our marine and terrestrial biomarker data suggest that the expansion of C-4 plants was not directly triggered by a rapid decline in global CO2 during this time. We suggest that the role of other factors such as changes in low to mid-latitude temperatures and water availability, possibly related to shifts in vapor transport driven by ocean circulation, should be evaluated further.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah