Small mammal carbon isotope ecology across the Miocene-Pliocene boundary, northwestern Argentina

The late Miocene expansion of plants using the C-4 photosynthetic pathway in South America has been documented by tooth enamel carbon isotope ratios (delta C-13(en)). However, a more detailed understanding of this ecological event is hampered by poor chronological control on the widespread fossil localities from which isotopic data are derived. This study develops a delta C-13(en) record from a single 2500 m-thick stratigraphic section in subtropical South America. Strata at Puerta de Corral Quemado (PCQ), northwestern Argentina, span 9 to 3.5 Ma in age, and existing paleosol carbonate data (delta C-13(pc)) document C-4 expansion across the Miocene-Pliocene boundary. Comparison of delta C-13(en) data with delta C-13(pc) data at high stratigraphic resolution refines understanding of this ecological event in South America. Small mammal delta C-13(en) data in particular are complementary to that of large mammal and paleosol delta C-13(en) data. Small mammal teeth integrate isotopic data over much shorter temporal and spatial scales than large mammal teeth, providing a sensitive measure of local vegetation and placing constraints on the landscape distribution of C-3 and C-4 plants. Explicit consideration of the distinctive carbon isotope enrichment factor between enamel and diet for rodents (epsilon*en-diet =11 parts per thousand, as opposed to 14 parts per thousand for large mammals) allows for unequivocal inference of C-4 vegetation similar to 1 Ma prior to that inferred from large mammal delta C-13(en), data, and similar to 2 Ma prior to delta C-13(pc) data. This multiproxy record demonstrates that C-4 plants were a stable component of the ecosystem hundreds of thousands of years prior to their major ecological expansion, and that the expansion of C-4 plants was pulsed at PCQ. Two periods of ecological change are demonstrated by delta C-13 and delta O-18 data at similar to 7 Ma and 5.3 Ma (coincident with the Miocene-Pliocene boundary). Development of small mammal delta C-13(en) records on other continents may provide similar insight into the early stages of the global C-4 event. (C) 2011 Elsevier B.V. All rights reserved.