Holocene mangrove and coastal environmental changes in the western Ganga–Brahmaputra Delta, India

Abstract

A 50 m-long radiocarbon dated core was studied through sediment and pollen analysis to reconstruct the Holocene mangrove and environmental changes at a coastal site Pakhiralaya in the Sundarban Biosphere Reserve in the western Ganga-Brahmaputra Delta, India. This biosphere reserve harbours a diverse mangrove ecosystem and supports a large number of people living in the area. Pollen and stratigraphic data indicate the existence of a brackish water estuarine mangrove swamp forest in this area during the last 9880 cal yr B.P. The development of the mangrove forest is not shown continuously in the Holocene record. Rapid transgression of the sea (9240 cal yr B.P.) halted the development of the mangrove. After about 8420 cal yr B.P. mangrove recolonised the area and persisted until 7560 cal yr B.P. as a result of a balance between the sedimentation and sea level fluctuation. The mangrove disappeared again from the site until 4800 cal yr B.P. because of a high sedimentation rate and possible delta progradation with loss of habitats. The reappearance of mangrove at the study site occurred with a return of a brackish water estuarine environment and the site then gradually became supra tidal during the mid-late Holocene. The continuity of the mangrove development and dynamics was interrupted by the fluctuating sea levels. Climatic fluctuations were viewed as an indirect factor influencing the mangrove ecosystem.

Indian monsoon variations during three contrasting climatic periods: The Holocene, Heinrich Stadial 2 and the last interglacial–glacial transition

“…Between these two periods a maximum expansion of mangroves was recorded as a result of an increase in marine influence (Hait and Behling, 2009) with sea level rise documented in the Godavari area between 9,000-8,000 years BP. The highest percentage of Rhizophoraceae, between 8,500 and 7,500 years BP, was related to the start of the Godavari delta development between 8,400-8,000 cal years BP (Rao et al, 2012).…”

“…The ecological requirements of these different plants are presented in Table 1. In the fossil record, mangroves, mainly characterized by Rhizophoraceae represent humid and saline coastal vegetation; its biodiversity, growth and development influenced by both marine dynamics (i.e., sea level variation, flooding and salinity) and fluvial input (i.e., fresh water and nutrients) (Hait and Behling, 2009). …”

“…Suaeda sp. (Amaranthaceae) colonizes hypersaline zones close to mangrove vegetation (Hait and Behling, 2009). The pollen of this genus cannot be distinguished from that of the other genus included in the Amaranthaceae pollen morphotype, but considering the Godavari coastal ecological context (Selvam, 2003) we assume that the record of Amaranthaceae pollen percentages results from variations in the amount of Suaeda sp.…”

The Sundarbans and Bengal Delta: The World’s Largest Tidal Mangrove and Delta System

“…Sediment discharged by the Ganga, Brahmaputra and Meghna Rivers began filling the basin in the early Holocene, creating the enormous delta that now has a subaerial surface area of 110,000 km 2 . The onset of early Holocene delta formation is stratigraphically marked in the subsurface by the widespread deposition of intertidal muds containing wood fragments and estuarine shells (Hait and Behling 2009). This suggests the young delta front was occupied by an extensive coastal mangrove system that persisted for 2,000-4,000 years, despite the rapid rise in global sea level that was occurring at that time (Goodbred and Kuehl 2000).…”

Responses of the mangrove ecosystem to Holocene environmental change in the Sundarban Biosphere Reserve, India

“…Imbalances in this ecosystem have serious socio-economic and ecological implications. The delta was affected during the early and mid Holocene by natural factors such as an inclination towards the east and rising sea level (Blasco et al 1996, Stanley & Hait 2000a, Gopal & Chauhan 2006, Hait & Behling 2009 Barui 2011, Das 2014, Mishra et al 2016, Sarkar & Sen 2019. In recent times, increasing human activity in the form of land reclamation and withdrawal of river water upstream has been responsible for habitat loss and enhanced salinity of both the water and soil (Milliman et al 1989, Nazrul-Islam 1993, Alam 1996, Blasco et al 1996, Allison 1998, Stanley & Hait 2000a, Gopal & Chauhan 2006, Zaman et al 2013, Ghosh et al 2015.…”