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Pré-Publication, Document De Travail Année : 2022

Subsidence and relative sea level history in the Ganges Brahmaputra Meghna Delta during the Holocene

C. Grall
M. S. Steckler
  • Fonction : Auteur
M. Karptychev
  • Fonction : Auteur
J. Austermann
  • Fonction : Auteur
S. L. Goodbred
  • Fonction : Auteur
C. Wilson
  • Fonction : Auteur
S. H. Akhter
  • Fonction : Auteur

Résumé

Reconstructing the local Holocene sea level history still remains a challenge for many places, such for the world's largest delta plain, the Ganges-Brahmaputra-Meghna Delta (GBMD). To address this, relative sea level history and subsidence patterns have been determined from the Holocene stratigraphic archives, to produce the first millennial-scale map of subsidence of the GBMD (Grall et al., 2018). We construct a sedimentation model and calculate the effect of Holocene sediment loading on subsidence and the local sea level. The sedimentation model is based on the assumption that the delta plain is near or at base level, which is considered to be relative sea level, and includes the Holocene shoreline migration history. We ran two separated numerical models, one that considered viscoelastic adjustments (GIA) and another that considered the poroelastic deformation of the sedimentary column. GIA modelling includes the local effect of sediment loading, but also the load histories of seawater and ice sheets (ice-6g, with a 20 km resolution). Result suggests that GIA may account about 1/3 of the observed subsidence and that the high sedimentation rate in this delta plain appears to be a critical factor. We use an inverse approach to evaluate the hypothesis that the remaining 2/3 of subsidence, termed the residual subsidence, can be driven by the compaction of the sedimentary column in response to Holocene sedimentation. The effect of the Holocene sediment loading is computed (with a of 4 km resolution) with sediment mechanical properties and grainsizes determined by inverting the residual subsidence. Results are compared to a broad sediment database that includes 179 samples of Holocene and Pleistocene age from 400 tubewells where grainsizes have been determined. Field observations and modelling results converge and suggest that the residual subsidence can be the result of the compaction of underlying silico-clastic with low clay content sediments, under the load of Holocene sediments. Results suggest that sediment compaction, in area with high sediment inputs such as delta plains, is one important driving force that needs to be considered for a better understanding of rates of local relative sea-level change during periods of warming such as the Holocene.
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Dates et versions

hal-03878184 , version 1 (29-11-2022)

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C. Grall, M. S. Steckler, M. Karptychev, J. Austermann, S. L. Goodbred, et al.. Subsidence and relative sea level history in the Ganges Brahmaputra Meghna Delta during the Holocene. 2022. ⟨hal-03878184⟩
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