Marine palynological record for tropical climate variations since the late last glacial maximum in the northern South China Sea

The evolution of the Asian summer monsoon (ASM) over the past 7500 years and regional human–environment interaction over the past 2000 years were investigated using high-resolution multi-proxy records from a well-dated sediment core (YJ Core) from the northern inner shelf of the South China Sea. The chemical index of alteration (CIA) of bulk samples were determined to infer changes in regional chemical weathering intensity, and δ¹³C_org and Total Organic Carbon (TOC) measurements were used to indicate changes in the strength of fluvial discharge. The results suggest intense chemical weathering and strong fluvial discharge during the interval 6800–3500 cal yr BP, followed by a weakening in both factors from 3500 to 2000 cal yr BP, and intensification again after 2000 cal yr BP. Comparison with climatic records from different monsoonal regions indicates that the ASM controlled both the chemical weathering and fluvial discharge intensity before 2000 cal yr BP. The enhanced chemical weathering and fluvial discharge during the past 2000 years coincide with an increase in both magnetic susceptibility (χ_lf) and frequency-dependent magnetic susceptibility (χ_fd), as well as an increase in sedimentary Cu and Pb concentrations. This interval also saw an increase in the population of Guangdong Province. Therefore, it is inferred that an increase in soil erosion and the use of metal tools were stimulated by a growing human population and the expansion of agricultural and mining practices. The present results suggest that enhanced human activity during the past 2000 years has overall overwhelmed the natural climatic controls on the environment and landscape of the Ling-Nan region (southernmost China).

For the first time, palynological records of terrestrial palynomorphs and dinoflagellate cysts are investigated in a sediment core from the northern South China Sea (SCS) covering the last 12,500 years. Both terrestrial and marine palynomorph records show strong signals of the sea-level change during the studied interval. The highest herb pollen content was associated with extensive grasslands on the exposed shelf at the low sea-level stand during the Younger Dryas and early Holocene. The increase in fern spores and decrease in concentrations of dinoflagellate cysts and terrestrial palynomorphs was observed during the sea-level rise interval from 12,500 to ~ 6800 (or 6000) cal yr BP. Then, the sea level became stabilized and consistently low dinoflagellate cyst abundances and high abundances of fern spores were recorded.

A high abundance of Impagidinium in the period ~ 12,000–10,400 cal yr BP possibly resulted from increased input of western Philippine Sea waters into the SCS and the branching of the Kuroshio Current. A short-term decrease of Impagidinium at ~ 11,700–11,000 cal yr BP corresponding to the MWP-1B event might be associated with input of the East China Sea waters through the Taiwan Strait.

The relationship between the sedimentation rates and the concentrations of terrestrial palynomorphs indicates a water-dominant transport for pollen and spore dispersal prior to ~ 6300 cal yr BP, whereas wind transport became more prominent thereafter. The timing of this change corresponds to the highest sea-level stand at ~ 6800–6000 cal yr BP, when the present oceanographic setting was formed. The mid-Holocene Optimum can be seen by the highest abundance of subtropical-tropical broad-leaved arboreal pollen and by the highest abundances of Dapsilidinium pastielsii. Three strengthened winter monsoon intervals at ~ 5500 cal yr BP, 4000 cal yr BP, and 2500 cal yr BP are reflected by increases in Pinus pollen content after the present oceanographic condition formed.

A high-resolution pollen record from Huguangyan Maar Lake near the South China Sea spanning the past 20,400 yr BP has been presented, aimed to provide an improved understanding of the response of the regional vegetation in tropical–subtropical South China to climatic variations since the late Last Glacial Maximum. The results reveal the strong control of the Asian summer monsoon (ASM) strength on vegetation via orbitally-induced variations in Northern Hemisphere summer solar insolation. The trend of climatic evolution of the region consists of change from a warm temperate–northern subtropical climate (20,400–18,700 cal yr BP) to a southern subtropical climate (16,500–11,500 cal yr BP) during the last deglaciation, followed by a stepwise environmental shift from a tropical climate (11,500–8200 cal yr BP), to a southern subtropical climate (8200–6600 cal yr BP) and finally to a middle–northern subtropical climate. The Bølling–Allerød warming is characterized by the highest percentages of evergreen Quercus but the lowest percentages of xerophytic Gramineae throughout the record, suggesting intensified warm, wet conditions during 15,000–13,000 cal yr BP. By contrast, the Younger Dryas is characterized by a marked decrease in subtropical evergreen broad-leaved plants but a significant increase in herbaceous taxa, representing a brief cold reversal during 13,000–12,000 cal yr BP. The occurrence of two major cooling events at 8.2 and 6.6 kyr BP, both of which are characterized by dramatic decreases in tropical evergreen broad-leaved plants, is supposed to be linked to a distinct weakening of the ASM and southward migration of the Intertropical Convergence Zone associated with marked declines in Atlantic Meridional Overturning Circulation. The subsequent brief intervals of high herbaceous pollen percentages but low percentages of tropical taxa correspond to high El Niño/Southern Oscillation (ENSO) activity, supporting a link between the weakening of solar isolation-driven ASM strength and a gradual intensification of ENSO.

This study presents a marine palynological record of the Asian summer monsoon and sea level change in the Last Glacial Maximum (LGM) and the deglacial period in the northern South China Sea (SCS). A fossil core STD 235 (855 cm in length) and 273 surface sediment samples from the northern SCS were pollen analysed to reconstruct the paleoenvironment of the continental shelf during the last glacial period. Results from fossil pollen show that the main pollen source region fundamentally changed from the LGM to the deglacial period as sea level rapidly rose. The modern marine surface samples show that pollen concentrations in the estuary of the Pearl River are extremely high, and modern pollen assemblages are in good agreement with the regional vegetation. However, wind transport becomes more important in the deeper ocean as the percentages of Pinus, a taxon with very high pollen production and dispersal capacity, is highest in these sediments, which otherwise have very low pollen concentrations. The concentration of total pollen between surface and fossil pollen samples is compared in order to determine the possible vegetation sources areas for the marine core. Pollen concentration as high as >100 grains/g at the LGM suggested that the paleo-shoreline was located within 80 km of the core. Consequently, pollen would mostly have derived from the exposed continental shelf in the northern SCS. By contrast, pollen concentrations were very low due to a much greater transport distance (318 km at present, core STD 235) under higher sea levels, and windblown pollen played a more important role because of the limitation of riverine input into the deep ocean during this highstand period. Such alternation of pollen flux and source distance should be repeated during all glacial-interglacial cycles, reflecting closely sea level and climate dynamics. According to fossil pollen assemblages from Core STD 235, we conclude that wetland and/or grassland communities with sparse subtropical trees dominated most of the exposed shelf during the LGM rather than forest that characterizes the region today. The existence of a predominantly open landscape on the exposed continental shelf suggests lower precipitation during the LGM, which in turn indicates a weaker Asian summer monsoon. This finding is supported by other records from the Okinawa Trough and the East China Sea, suggesting that a weaker summer monsoon was a key characteristic of the LGM in East Asia.

High-resolution marine palynological records from the northern South China Sea (SCS) provide new information concerning the regional history of vegetation and climate conditions during the last glaciation (~ 70–19 cal ka BP). The preservation of rich pollen from tropical and subtropical broadleaf trees (largely composed of evergreen Quercus, Castanopsis, and Euphorbiaceae) suggests that a southern subtropical climate prevailed during the last glaciation, including the period of the Last Glacial Maximum. Relatively higher percentages of such pollen taxa occurred during Marine Isotope Stage (MIS) 3, when Phyllocladus pollen from the south also increased, indicating a warmer terrestrial climate and a strengthened East Asian summer monsoon comparing to MIS 4 and 2. Moreover, the maximum percentages of such pollen occurred during the early MIS 3, implying that an optimal climate prevailed in this period. The reconstructed terrestrial climate corresponds well to marine evidence such as δ¹⁸O values from planktonic foraminifer shell and records of sea surface temperature. This study confirmed that the regional terrestrial climate could be linked closely to changes in both global ice volume and the East Asian monsoon attributable to variations of the earth’s orbital parameters. Modern palynological investigations and statistical analyses of the pollen percentages in the profiles indicate that Pinus pollen and spores are easily transported by water flow in the northern SCS. Therefore, their relative abundance peaks suggest that several wet intervals occurred during the last glaciation.