Reasons for the low sedimentation and slow progradation in the Pearl River delta, southern China, during the middle Holocene

Abstract

Holocene sea-level rise, monsoonal discharge and human activity have been the main drivers for the evolutionary history of Asian deltas. In theory, when sea level ceased to rise in around 7000 cal. a BP, deltas should have started to prograde seawards. However, previous studies revealed that the progradation of modern deltas was initially very slow, and more importantly, sediment accumulation within central deltaic basins was exceptionally low between 7000 and 4000 cal. a BP. In order to investigate the main reasons for such slow progradation and low sedimentation during the middle Holocene, this study reconstructed the sedimentary history of the head area of the Pearl River delta. The results show marine transgression from 9500 to 7500 cal. a BP, followed by marine regression starting from 7500 to 4000 cal. a BP, in response to sea-level change. During the latter period, sedimentation was concentrated in the head area. The amount of sediment entering the central basin was significantly reduced. This limited amount of sediment was spread thinly across the wide deltaic basin. A portion of it could have been transported out into the South China Sea by the strong tidal currents, resulting in an exceptionally low sedimentation in the central basin. However, in locations adjacent to former/present islands, locally supplied sediment helped sedimentation to continue from 7500 to 4000 cal. a BP. By about 4000 cal. a BP, the deltaic shoreline advanced into the central basin. This period saw an increase of human activity that caused soil erosion and sediment supply, leading to a marked increase in sedimentation rate. Active land reclamations also helped the acceleration of shoreline advance. This sedimentary evolution model in the Pearl River delta mirrors those of other deltas in the monsoonal Asia.