Sediment transport by tropical cyclones recorded in a submarine canyon off Bangladesh
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Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones.
BM acknowledges the support by the German Science Foundation (DFG), the German Federal Environmental Foundation (DBU), and the German Academic Exchange Service (DAAD). IM was supported by CARIMA/BMBF. WZ is supported by the research program “Marine, Coastal and Polar Systems” (PACES II) of the Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. Grain-size analyses were performed by BM at LIAG/Hannover. U. Röhl at MARUM/Bremen did the RFA scan. 137Cs concentrations were determined by the Bundesamt für Seeschifffahrt und Hydrographie/Hamburg. A former shorter version of the manuscript benefited from the comments of P. Puig, Institut de Ciencies del Mar, Barcelona/Spain, and J. P. Walsh, East Carolina, Greenville/USA. The present manuscript greatly profited from the suggestions of B. Flemming, Senckenberg am Meer, Wilhelmshaven/Germany, and two anonymous reviewers.
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The authors declare that they have no conflict of interest.
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