A diatom-based Holocene record of sedimentary and oceanographic environmental changes within the Beibu Gulf, NW South China Sea

A sediment core from a nearshore setting (10.8 m in water depth) of the inner Beibu Gulf, northwestern South China Sea, was analyzed in terms of diatom assemblage composition, grain size distribution and radiocarbon dating, with aim to describe sedimentary and oceanographic environmental changes during the Holocene. The fossil diatom record and changes in granulometry reveal that sea-level fluctuations had a strong influence on the evolution of hydrographic system, which changed significantly with rapid sea-level rise in the early Holocene ca. 10,000-7800 cal. yr. BP. These fluctuations continued in a series of stages dated to the middle Holocene ca. 7800-6200 cal. yr. BP, with sea-level increase, followed by a prolonged sea-level retreat during ca. 6200 to 3300 cal. yr. BP, and finally a relatively high sea level from ca. 3300 cal. yr. BP to the present. The resultant sedimentary facies could be divided into seven stages inferred by the diatom distribution and grain size characteristics: a river channel/beach environment at ca. 10000-9500 cal. yr. BP, a lagoonal environment at ca. 9500-7800 cal. yr. BP, a relatively broad embayment at ca. 7800-6200 cal. yr. BP, a bay at ca. 6200-3300 cal. yr. BP and a coastal environment setting up to present day. The coastal environmental setting has been subject to some shifts in climate and sea-level beginning with strong terrestrial influx at ca. 3300-1400 cal. yr. BP, which was then replaced by strong hydrological circulation at ca. 1400-800 cal. yr. BP, and a relative stable sedimentary environment approximate to the coast from ca. 800 cal. yr. BP to present. A very obvious increase in diatom abundance in the early and late Holocene reflected terrestrial influx transporting nutrients, driven by precipitation changes influenced by the Asian monsoon circulation pattern, particularly in the early Holocene. Increased anthropogenic impact, i.e. an enhanced nutrient condition, was inferred from higher abundances and reasonably high diatom diversity in this local record from 1400 cal. yr. BP, especially in the last 800 years. Rapid shifts in the diatom composition and grain size distribution at about 1400 and 1000 cal. yr. BP were observed that could be interpreted as overlapping influences of a global scale paleoclimate anomaly and local human activities. This study demonstrates the effectiveness of using diatom for understanding regional paleoenvironment change driven by both global climate changes and anthropogenic impact in coastal area of the Beibu Gulf, northwestern South China Sea.