Is the world’s largest fluvio-deltaic mangrove forest, the Sundarbans, keeping pace with relative sea level rise?

Abstract:

The fate of coastal wetlands is controlled by the maintenance of surface elevation with respect to rising water levels from eustatic sea-level rise, subsidence, and tidal amplification. Surface equilibrium is dictated by the balance of elevation gain from vertical accretion (organic and inorganic deposition) relative to elevation loss from rising water levels and subsidence (compaction, dewatering, oxidation). In general, deltaic floodplains aggrade to mean flood elevation and tidal landscapes build to approximately mean high water. In the Sundarbans mangrove forest of the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during semi-diurnal mesotides coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. Over the past several decades, widespread embankment (“polder”) construction in the GB tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. Here we analyze how surface elevation is maintaining pace with relative sea-level rise (RSLR) in the Sundarbans mangrove forest deltaplain. We utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide time series of sediment and elevation dynamics with respect to local platform elevation and associated hydroperiod in two hydro-geomorphic settings: proximal and distal to the GB river mouth. Preliminary results show that elevation gain is occurring in both settings, with the highest rates observed at elevated stream-bank zones. This elevation gain occurs primarily in response to seasonal sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution are minimal). Proximal to the GB mouth, the platform appears to be super-elevated from monsoon flooding or storm surges. Looking forward, rates of surface elevation gain in the Sundarbans greatly exceed RSLR and more closely follow rates of RSLR augmented with tide range amplification. This highlights that coastal mangroves are capable of adapting to human-induced environmental change, but in this setting it is reliant on sufficient sediment delivery to maintain platform elevation.