| Abstrakt: |
In the late Miocene, grasslands spread across the forested floodplains of the Himalayan foreland, but the causes of the ecological transition are still debated. Recent seafloor drilling by the International Ocean Discovery Program (IODP) provides an opportunity to study the transition across a larger region as archived in the Indus submarine fan. We present a multiproxy study of past vegetation change based on analyses of the carbon isotopic composition (δ13C) of bulk organic carbon, plant wax n‐alkanes and n‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX86 index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C4 grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C4 expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C4 expansion. Plain Language Summary: This project studied the mud and sand on the seafloor in the Indian Ocean, west of India and south of Pakistan. We drilled a core through the mud and focused on a section corresponding to 5.5 to 10 Ma. Much of the sand came from the Indus River, but when the river sediments went elsewhere, layers of chalky sediments formed from the shells of marine organisms. Those chalks contain the history of longer spans of time. In the sediments, we found molecular fossils from the waxy coating on plant leaves and the woody parts of plants as well as pollen and charcoal; these point to the types of plants that were growing on land whose remains were washed or blown out to sea. We also found molecular fossils of microbes that lived in the oceans, whose structures indicate ocean temperatures—although when soils washed out in large amounts, ocean temperature estimates are not available. The main finding is that grasses replaced forests across much of the Indian subcontinent and Indus catchment, accompanied by more grass fire and cooler ocean temperatures. Key Points: Multiproxy study of the Indus Fan organic matter reveals that terrestrial sources differ between turbidite and hemipelagic faciesCarbon isotopes, grass pollen, and charcoal indicate that C4 grasslands expanded between 7.4–7.2 Ma in hemipelagic faciesHydrogen isotopes in plant waxes differ between source regions but do not detect monsoon rainfall changes across the C4 grassland expansion [ABSTRACT FROM AUTHOR] |
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