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Abstract C4 grassland ecosystems expanded across North America between ca. 8 and 3 Ma. Studies of ungulate enamel and environmental indicators from the middle Miocene Barstow Formation of southern California (USA) have demonstrated the presence of C4 vegetation prior to the late Miocene expansion of C4 grasslands. Fire promotes the growth of modern C4 grasslands and may have contributed to the Miocene expansion of C4 vegetation. We analyzed the concentration and accumulation rate (CHAR) of microscopic charred particles from sediment samples spanning the Barstow Formation in order to investigate the relationship between fire activity, canopy cover, and the presence of C4 vegetation. Concentration and CHAR were low throughout the formation then increased dramatically at 13.5 Ma. Enriched values of δ13C from soil organic matter and phytolith counts indicate the presence of C4 grasses and seasonally dry, open-canopy habitats at this time. The spike in concentration coincides with climatic cooling and drying in southern California after the Miocene Climatic Optimum. Increased fire activity may have contributed to habitat opening from forest to woodland and promoted the spread of C4 plants. This is the first charcoal record of fire activity from the middle Miocene of southwestern North America.
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Late Miocene expansion of grasslands in northwest Argentina linked to shifting hydroclimate: A complex interaction among tectonics, climate, and ecology
Factors driving the late Miocene expansion of C4 grasses remain widely debated. Here, we explored the role of climate and fire in controlling the abundance of C4 vegetation in the Angastaco Basin (Palo Pintado area) and La Viña Basin, NW Argentina, during the late Miocene (ca. 14−5.33 Ma). From paleosol horizons, we reconstructed paleoclimate and paleovegetation conditions using phytolith assemblages, geochemical and isotopic proxies, and polycyclic aromatic hydrocarbons (PAHs) to determine fire input. Our paleoclimate reconstructions suggest a stable mean annual temperature (MAT) of ∼10 °C and a gradual decline in mean annual precipitation (MAP) from 1100 mm yr−1 to 850 mm yr−1. Paleovegetation reconstructions from carbon isotopic composition and phytolith assemblages show a maximum of ∼15% C4 vegetation by 6 Ma. No significant increases in fire occurrence or establishment of fire feedbacks were identified from the PAH data. Though low in abundance (∼3% on average), our data identified the presence of C4 grass by the late Miocene. The lack of significant C4 expansion in this region was likely controlled by the changing hydroclimatic conditions associated with the Andes mountain range—increasing aridity and elevation constraints along with the lack of a fire feedback might have limited the distribution of C4 vegetation.
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- PAR ID:
- 10583735
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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