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 (δ ¹³C _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 δ ¹³C _en record from a single 2500m-thick stratigraphic section in subtropical South America. Strata at Puerta de Corral Quemado (PCQ), northwestern Argentina, span 9 to 3.5Ma in age, and existing paleosol carbonate data (δ ¹³C _pc) document C ₄ expansion across the Miocene-Pliocene boundary. Comparison of δ ¹³C _en data with δ ¹³C _pc data at high stratigraphic resolution refines understanding of this ecological event in South America. Small mammal δ ¹³C _en data in particular are complementary to that of large mammal and paleosol δ ¹³C 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 (ε*en-diet=11‰, as opposed to 14‰ for large mammals) allows for unequivocal inference of C ₄ vegetation ~1Ma prior to that inferred from large mammal δ ¹³C _en data, and ~2Ma prior to δ ¹³C _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 δ ¹³C and δ ¹⁸O data at ~7Ma and 5.3Ma (coincident with the Miocene-Pliocene boundary). Development of small mammal δ ¹³C _en records on other continents may provide similar insight into the early stages of the global C ₄ event.