Fluvial terraces of the lower Mekong River reflect quaternary global sea level fluctuations as a likely response to Himalayan glacial/deglacial runoff

Knowledge of the Quaternary history of the lower Mekong, the major river within Cambodia, is basic. Herein we advance understanding by investigations of river terrace topographic expression and stratigraphy. Satellite images, digital elevation models and fieldwork have been used to define the terrace elevations and extent. Three terrace levels can be recognized, separated in the vertical, lateral and temporal dimensions by distinctive sedimentary signatures. Strath surfaces and alluvial cover have been dated using terrestrial cosmogenic and optical luminescence protocols. The highest level (T1: notionally +100 m above present sea level) is a discontinuous, degraded, bedrock strath with a patchy veneer of well-weathered fluvial cobble gravel. T1 is younger than a regionally significant meteorite impact ~800 ka (Marine Isotope Stage² 20), and older than basalt flows on its surface (600 ka?). The T1 level was abandoned before 99.42 ± 7.52 ka (the end of the glacial MIS 5d), as the river incised in response to a rapidly falling sea level, to form a broad continuous strath terrace (level T2) exhibiting a thin alluvial cover, between 70 m and 40 m above sea level. The T2 terrace is composed of partially lateritic, interlayered, sand and gravel beds lying above weathered bedrock (blue/red clay). The basal deposits on the T2 level date to 70.65 ± 5.13 ka, following a sea level rise to a short-lived elevation of around +30 m around 80 ka (MIS 5a). The T2 level was progressively down cut between 57.73 ± 5.31 ka and 38.66 ± 2.40 ka (MIS 3). Steadily falling sea level sustained MIS 3 incision which reached c., 10 m above the modern river level c., 33.03 ± 3.09 ka, before the offshore minimum in sea level, c., 23 ka, i.e., towards the end of the Last Glacial Maximum. A loam-rich sandy terrace (T3; c., 0.45 ka (MIS 1)) is developed locally at c., +20 m above sea level. The timing of abrupt incisions, leading to the abandonment of the T1 and T2 levels, coincide with the onset of cool glacial stadials and falls in global sea level, whilst initial aggradation on the T2 level broadly can be associated with MIS 4. Despite a reduction in the contribution of glacial runoff from the Himalaya and Tibet towards the end of the Pleistocene, channel narrowing from T1 onwards has sustained the erosive power of the river, such that the rate of incision has only slowed within the Holocene.