Late Holocene foraminiferal record from mangrove reserve, Qi’ao Island, Pearl River Estuary and its implications for paleoenvironment

Intertidal foraminifera has been widely used in paleoenvironmental reconstruction, and especially, in researches on high-resolution relative sea-level fluctuation. Unfortunately, such researches are lacking in the South China coast. This time, a long core up to 300 cm in length was retrieved from the upper intertidal zone of the mangrove on the Qi’ao Island of the Pearl River estuary. AMS¹⁴C dating, foraminifera, grain size distribution and grayscale analysis were employed to ascertain the possibility to apply the mangrove foraminifera as a high-resolution proxy for sea level fluctuation and paleoenvironmental evolution. Two foraminiferal assemblages were acquired from the core QA, a calcareous group dominated by Ammonia becсarii (78.7%), with common species Proelphidium granosum (6.6%) and Elphidiella kiangsuensis (5.6%), from a clayey slit sediment, indicating an estuarine subtidal environment; and an agglutinated assemblage dominated by Arenoparella mexicana (50.6%) with common species Miliammina fusca (17.5%), Trochammia sp. (6.8%), from a layer of sandy silt deposits with abundant plants debris and residues, indicating an upper or middle mangrove intertidal environment. AMS¹⁴C dates suggest that, the sediments formed approximately 4 200 cal. aBP, when the study area was a subtidal or mudflats environments. It turned to a middle-high mangrove at ～2 400 cal. aBP, resulted from the falling of relative sea-level caused by deltaic progradation. Two fine grained layers were intercalated in mangrove sediments, which contain calcareous foraminiferal tests corresponding to 1 022 ～ 729 cal. aBP and 508 ～ 358 cal. aBP respectively, probably related to a sub-regional sea-level rise. The grayscale suggests three phases of high humification, corresponding to ～3 000～2 700 cal. aBP, 1 200～1 100 cal. aBP, 600～500 cal. aBP respectively, indicating a warm and humid climate, which are close to the sea-level rise periods.