Holocene development and human use of mangroves and limestone forest at an ancient hong lagoon in the Tràng An karst, Ninh Binh, Vietnam

Abstract

How past environments and communities responded to episodes of coastal inundation can inform preparations for future resilience to predicted rises in sea level. Southeast Asia's extensive coastlines and expanding coastal populations mean vast natural and human capital is at risk from future sea level rise. Regional mangroves provide many ecosystem services that can help mitigate such risks, but deforestation has left them threatened and compromised. The present study examines the Holocene development and human use of mangrove forest in northern Vietnam, where existing palaeo-records derive from sedimentary archives in tidal flat, estuarine and deltaic settings. Here, we expand that coverage by describing conditions at an enclosed doline within the Tràng An limestone karst in Ninh Binh province that would have been sheltered from deltaic and marine processes. We present a multi-proxy assessment incorporating pollen analysis of the 8125-year-old discontinuous sediment core obtained from the doline floor, combined with inferences from erosional tidal notches in the enclosing limestone, and analyses of phytolith, vertebrate and mollusc assemblages from an adjacent archaeological cave site with deposits of comparable age. The results provide a detailed example of how enclosed coastal environments and communities responded to Middle Holocene marine inundation. High percentages of pollen from mangroves (17-57%) suggest their colonisation of the doline from ~8100 cal. BP and persistence until ~250 cal. BP, well after the intertidal zone regressed seaward beyond the massif. Archaeological assemblages dating to ~5500 cal. BP and containing palm and woody eudicot phytoliths and sponge spicules, neurocrania of the fish genus Pomadasys ('grunts', 'grunters' or 'javelins') and brackish-water molluscs Sermyla riqueti and Neripteron violaceum support the persistence of mangrove environments through the Middle Holocene high-stand, a period of hiatus within the core, and indicate human foraging and fishing activities took place in mangrove and lagoonal habitats alongside hunting in the surrounding limestone forest. Subsequent structural opening of this latter forest formation from ~1075 cal. BP (875 CE), evident in the pollen record, coincides with the Medieval Climate Anomaly and with the adjacent development of the ancient capital at Hoa Lu. We propose that given the long-term persistence of mangrove habitats and associated resources documented in this study, regional initiatives aimed at rehabilitating mangroves (with the positive consequences that this holds for biodiversity and socio-economic conditions) may wish to consider selective restorative measures within Tràng An and similar sheltered sub-coastal karst settings.

Geochemical fingerprinting of Pleistocene stone tools from the Tràng An Landscape Complex, Ninh Bình Province, Vietnam

“…Archaeological investigations at cave and rockshelter sites throughout Tràng An have revealed consistent occupation of the complex for at least 37,000 years [ 36 ]. Palaeoenvironmental reconstruction [ 17 , 37 – 40 ] suggests that the karstic ecology of Tràng An remained relatively resistant to large scale environmental instability associated with coastal inundation and global climate change, making it a potential refugial area for prehistoric populations. This means that Tràng An may have served as a key area where stable resource distribution encouraged long term, consistent occupation.…”

Confirmed archaeological evidence of water deer in Vietnam: relics of the Pleistocene or a shifting baseline?

“…The Tràng An karst landscape and environs are located in Ninh Binh Province, Northern Vietnam (see [9,[53][54][55][56] for detailed descriptions). Hang Thung Binh 1 (TB1) is one of six caves within an isolated limestone hill of the same name, in the northwest corner of the Tràng An core zone, 1.5 km west of the main massif set within cultivated alluvial plains (figure 2).…”

Prehistoric pathways to Anthropocene adaptation: Evidence from the Red River Delta, Vietnam

“…In addition to these data, one of the sources, Hoang et al [ 108 ], incorporated geomorphological proxies and shallow seismic sections into their model. Our own data from Tràng An [ 100 ] ( see Methods : 1) similarly, relies on geomorphological proxies, particularly corrosion notches ( Fig 2 ), supplemented by archaeological and coring data [ 109 ].…”

“…Based on current modelled expectations, a rate of sea level rise over 6.1 mm/year (i.e., that expected under the Representative Concentration Pathway (RCP)8.5 scenario for 2050) may exceed the tipping point at which mangroves are able to build vertically through sediment accretion [ 139 , 140 ]. Hydrological and microclimate conditions affecting the prehistoric establishment and long-term persistence of back-mangrove forest have been reported for the Tràng An massif [ 100 , 109 ]. This record runs contra to the wider deltaic observed trend for mangrove decline during the 6500–5000 cal.…”

“…While it is widely accepted that historical processes and changing coastal dynamics are important to the development of mangrove management and restoration [ 181 , 185 – 187 ], and the relevance of palaeoecological data has been flagged [ 188 , 189 ], with few exceptions [ 190 – 192 ], application to coastal or estuarine ecosystem restoration has lagged behind research in lacustrine and terrestrial environments [ 192 ]. Here we reference published results from Tràng An [ 109 ] to illustrate the role that palaeoecological data can play in such programmes, particularly with respect to site selection criteria and baseline mangrove community composition. Our findings show the importance of sub-coastal karst locations to the reestablishment of viable mangrove reserves and how palaeo-data permits restorative actions to be pursued pro-actively in protected locations where they might not otherwise be considered; an asset that enhances the existing positive gains and climate-change related benefits from forest restoration in an area of Vietnam’s coastline that is particularly vulnerable to erosion, storm, and inundation risks [ 193 , 194 ].…”