ORGANIC CARBON ISOTOPIC RECONSTRUCTION OF C3/C4 PHOTOSYNTHESIS AND IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE SANTA MARíA BASIN, NORTHWEST ARGENTINA

Cenozoic climate change is characterized by a gradual decrease in atmospheric CO₂ levels punctuated by warming events. By the latest Miocene, many atmospheric CO₂ proxies indicate that levels had reached preindustrial values. These low values coincide with the rapid global spread of C₄ grasses between 8 and 3 Ma ago that also occurs during the late Miocene. This expansion has been attributed to a long-term decline in Cenozoic CO₂ and the consequential cooling and drying of the global climate, but recent work has shown that CO₂ may not be the primary driver of C₄ expansion, which may instead reflect local climatic and tectonic events. Previously published carbon isotopic records from pedogenic carbonates have suggested that the tectonic evolution of the Santa Maria basin in Northwestern Argentina controlled the climate and vegetation of the area during the late Miocene. Because pedogenic carbonates δ¹³C values may also reflect complicating factors including contamination from atmospheric δ¹³C in low productivity ecosystems, we choose to analyze organic carbon preserved in paleosols there as direct evidence of C₃/C₄ vegetation. We present a paleovegetation reconstruction from the analysis of paleosols from the Santa Maria Basin in Catamarca, Argentina from the late Miocene (7–3Ma ago). We will compare this organic carbon record to the pedogenic carbonate isotopic record from the same locality to determine if the isotopic shifts observed are due climatic changes from regional tectonic activity or are an artifact of the pedogenic carbonate method of paleovegetation reconstruction.