The Ethiopian Plateau, situated on the Western Escarpment of the East African Rift System, constitutes a part of a large igneous province that has experienced extensive flood basalt volcanism around 30 Ma related to the outpouring of the Afar mantle plume. This non-orogenic plateau with long-wavelength dynamic topography has been deeply incised by the Blue Nile River and its numerous tributaries. The plateau represents an excellent natural laboratory to study the interplay between bedrock river incision and uplift. Our analysis of a total of 202 convex upward and double-concave tributary longitudinal profiles reveals 244 knickpoints, the majority of which are not associated with lithologic boundaries or faults. The normalized steepness indices (k sn ) of these profiles vary in upstream versus downstream from knickpoints indicating variable uplift within the plateau. Our investigation of integrating mantle seismic P-wave velocity anomaly with k sn and knickpoints suggests ongoing incision of the plateau surface in response to Afar plume related mantle dynamics. Tributary reaches with higher k sn generally lie above the areas with negative velocity anomaly, thus hotter (than normal) mantle that are likely undergoing more uplift.Therefore, this study suggests that the transient landscape incision of the Ethiopian Plateau is largely controlled by the ongoing uplift of the plateau, indicating that the plateau physiography is dynamic, not stable.

The late Neogene vegetation change of C 4 plants replacing C 3 plants is widely documented across the world. This vegetation shift has been particularly well-documented in the Himalayan foreland based on δ 13 C isotopic data from palaeosol carbonates and bulk organic matter in the Siwalik sedimentary rocks of Pakistan, India and Nepal, showing asynchronous expansion of C 4 plants between 8 and ~5 Ma, 9 and 6 Ma, and around 7 Ma, respectively. In this study, compound-specific isotopic analysis of lipid biomarkers extracted from shale and palaeosols in the palaeomagnetically age-constrained Nepalese Siwalik is utilized to better understand this vegetation shift. This is the first comprehensive lipid biomarker study in the Nepalese Siwalik, with new isotopic results from the previously undocumented Karnali River section. The δ 13 C n-alkane (C 27-31 ) results from the Surai Khola section suggest C 3 plants were dominant between 12 and 8.5 Ma. A stepwise expansion of C 4 plants that started gradually at 8.5 Ma escalated rapidly after 5.4 Ma, so that by 5.2 Ma C 4 vegetation dominated the landscape. This dramatic ecological shift at the Miocene-Pliocene boundary was likely linked with an intriguing tectonic-climate coupling in the Himalayan-Tibetan region, prompting wetter summers and drier winters that drove C 4 grass expansion.However, in the Karnali River section, the data show no clear sign of C 4 plant expansion until 5.2 Ma (youngest sample age). In the two study locations separated laterally by ~200 km along tectonic-strike, this different trend of vegetation change likely indicates local controls like river-catchment influence. However, it is possible that, similar to the Surai Khola section, C 4 plants dominated after 5.2 Ma in the Karnali River section. This study also suggests that the past vegetation makeup of an area is better reconstructed using isotopic signatures of molecular markers than of bulk organic matter or pedogenic nodules.

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