Asymmetric progradation of a coastal mangrove forest, Cù Lao Dung, Vietnam: sediment dynamics to stratigraphy

Mangrove-colonized shorelines promote coastal resilience by encouraging sediment retention, limiting erosion, and damping the effects of waves and storm surge. As a result, there is significant interest in understanding the factors that promote the growth or retreat of coastal mangrove forests. Here we present new observations from a coastal mangrove forest on Cù Lao Dung, an island at the mouth of a Mekong distributary channel (Sông Hậu). Satellite observations reveal that this forest is prograding in an asymmetrical manner, growing ~60 m/y in the SW part of the forest, and staying roughly stationary in the NE despite an ample sediment supply from the adjacent river. This progradational asymmetry allows us to explore the processes that encourage or limit mangrove advance, and the sedimentary signatures that result.

Timeseries of wave, current, CTD, and suspended-sediment data were collected simultaneously in both the SW and NE study sites over a spring-neap cycle during the two dominant seasonal conditions: high river discharge during the SW monsoon, and low river discharge during the NE monsoon. These observations show that conditions in the SW mangrove forest are consistently more energetic than in the NE forest, and that the system as a whole is more energetic during the NE monsoon when winds and waves are more intense. Surface grain size reflects this gradient in energy between sites and seasons, with the coarsest sediment found in the SW area during the energetic SW monsoon. Net sediment import into the SW mangrove forest, and export from the NE forest was observed during both seasonal conditions. Profiles of ²¹⁰Pb in cores suggest that sediment accumulates roughly twice as quickly in the SW mangrove region as in the NE region. This dataset allows evaluation of the mechanisms that contribute to sediment accumulation or erosion including wave characteristics, circulation, and sediment availability, which control the trajectory of mangrove shorelines.