Mid-to late-Holocene fluvial and coastal evolution in the Aceh River delta, Sumatra: effects of sea-level change, sediment supply, wave climate, tectonics and tsunamis

The Aceh River delta, northern Sumatra, is a subject of interest since the coastline was struck, more than any other, by the 2004 Indian Ocean Tsunami, the largest recorded in human history. Thereafter, significant scientific efforts focused on short-term dynamics to address the environmental effects of the tsunami, but the long-term evolution of the delta in this specific context of volcanic eruptions, megathrust earthquakes and tsunami landing, however, remains to be understood. This study investigates the subaerial delta, based on shallow sediment borehole stratigraphies and C 14 ages, in order to provide a partial reconstruction of the western and eastern fluvial and coastal evolution over the past 7 ky. It also benefits from comprehensive resistivity maps produced during recent helicopter-borne electromagnetic (HEM) surveys, and from earlier geomorphological mapping. By prograding, the delta expanded seaward, exposure to swell increased, and a large strandplain accreted along the eastern delta front from 4 to 1 ky BCE. The delta thus evolved asymmetrically, with higher, tightly-stacked beach ridges in the east, which accreted at ∼6 km 2 /ky. Meanwhile, the Aceh River remained stable, along the western side of the delta, burying the western strandplain under its floodplain. After 0.5 ky BCE, delta progradation increased to ∼14 km 2 /ky, generating low-lying and wider spaced beach ridges to the east. A series of river avulsions between 0.2 ky BCE and 1.6 ky CE shifted the river course from the west to the center of the delta. An asymmetric cuspate promontory grew at 23 km 2 /ky after 0.5 ky in front of the current mouth of the river, projecting 1 km offshore of the current coastline, before undergoing erosion in the past few centuries. Here, we discuss which combination of global and local factors, including sea level change, sediment supply, wave climate, tectonics, land use and tsunamis may explain the most salient processes during the growth of the Aceh River delta. Beyond its local interest, this study provides clues for a wider understanding of the complexity of subaerial delta development.