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.

中文翻译：

---

印度河扇记录的中新世C4草原扩张

在中新世晚期，草地遍布喜马拉雅前陆森林泛滥的平原，但生态过渡的原因仍在争论中。国际海洋发现计划（IODP）最近进行的海底钻探提供了一个研究跨大区域过渡的机会，如印度梧桐海底扇中存档的那样。提出了一种基于碳同位素组合物的分析（δ过去植被变化的多重代理研究¹³散装有机碳，植物蜡C）ñ -烷烃和Ñ链烷酸，以及木质素酚，木炭和花粉的定量分析。我们分析植物蜡的氢同位素组成（δD）以重建降水δD。我们使用支化和类异戊二烯四醚（BIT）指数来诊断浊度和半海相沉积物之间陆地和海洋脂质输入之间的变化。我们使用TEX ₈₆重建海洋温度仅在以海洋脂质为主的地方才进行索引。我们找到了两个相中新世晚期草地扩张的证据，证实这是区域生态系统的转变。浊积土主要来自印度河流域（贫D的植物蜡）中的陆地物质，通过木质素的存在通过河流的运输来传递。相反，与印度半岛风吹动的输入相一致，半海岸沉积物缺乏木质素并带有富含D的植物蜡。这些显示了印度次大陆上C ₄草原的7.4–7.2 Ma扩张。在每个相中，我们发现中新世C ₄晚期的δD值没有明显变化，这暗示着季风动力学对降雨的持续蒸馏作用。然而，阿拉伯海的降温恰逢C ₄ 扩张。