Late Quaternary mangrove biogeography and paleoenvironments in the reef area of the South China Sea based on analysis of palynomorph assemblages

In this study, we analyse the development of mangroves around the South China Sea (SCS) over the last 40 kyrs BP. Findings are based on 208 palynological samples obtained from a sediment core (NS07–25) in the reef area of the SCS. These palaeo-records are interpreted in the context of 161 surface sediment palynological samples obtained from across the SCS that serve as a modern reference set. Employing the Near Analysis function in ArcGIS, mangrove pollen in modern surface sediment comprises two major assemblages: dominant Rhizophoraceae (subtropical, wind- and animal-pollinated taxa) in the northern SCS and diverse Sonneratia (tropical, animal-pollinated taxa) in the southern SCS. The distribution field of these modern mangrove pollen assemblages is bounded by the Nansha Islands and reefs, which may act to obstruct pollen transportation between the northern and southern parts of the SCS. In our late Quaternary core, concentrations of Rhizophoraceae and Sonneratia pollen assemblages gradually increase in the late stage of Marine Isotope Stage 3 and during the Last Glacial Maximum, as a result of falling sea level and mangrove colonization of the exposed marine shelf, whereas they rapidly decrease in the Deglacial period, coincident with an increase in sea level and coastal retreat. The occurrence of Rhizophoraceae pollen assemblages in the southern SCS during the Last Glacial Maximum indicates that these communities disperse southward in response to climate change. Our findings highlight the complex effects of growth conditions, varied pollinators, and transportation patterns on the preservation of Rhizophoraceae and Sonneratia pollen assemblages with implications for understanding change in climate and sea level since 40 kyr BP.