• journal_title：Geology
• Contributor：Michael A. Urban ; David M. Nelson ; Gonzalo Jiménez-Moreno ; Jean-Jacques Châteauneuf ; Ann Pearson ; Feng Sheng Hu
• Publisher：Geological Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1130/G31117.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：38
• issue：12
• firstpage：1091
• section：Articles

C₄ plants are widely successful in the grass-dominated ecosystems of tropical, subtropical, and warm-temperate regions, largely as a result of their ability to limit photorespiration and improve water-use efficiency. A widely held paradigm is that low (<∼400 ppm) atmospheric CO₂ concentrations were an important factor selecting for the origin of C₄ plants, although support in geological records is limited. We determined the carbon isotopic composition of 686 individual grass-pollen grains preserved in eight samples of lacustrine and shallow-marine sediments from three basins spanning the Early Oligocene to Middle Miocene in southwestern Europe. Grasses composed <15% of the total abundance of terrestrial pollen grains, and 26%–62% of the grass pollen was from C₄ grasses. Thus C₄ grasses occurred on the landscape as early as the earliest Oligocene, ∼14 m.y. earlier than previous isotopic evidence of first C₄ plants and before pCO₂ fell during the Oligocene.