Rare earth element fractionations of the northwestern South China Sea sediments, and their implications for East Asian monsoon reconstruction during the last 36 kyr

摘要

Rare earth elements (REEs) fractionations in siliciclastic marine sediments are related to the parent rock properties, weathering degree, grain size, and heavy minerals. The relations between marine sediment REE compositions and regional paleoclimate are not fully understood. Here, the REEs for detrital components of a core retrieved from the northwestern South China Sea (SCS) were measured. Results show the REE (LREE/HREE, (La/Yb)(N)) and Light REE ((La/Sm)(N)) fractionation degrees have good responses to the regional paleoclimate over the past 36 kyr. Sea level played a key role in REE fractionations during similar to 36-24 kyr BP, manifesting as lower REE and LREE fractionation degrees correspond to the decreased sea level. The entire trends for LREE/HREE, (La/Yb)(N) and (La/Sm)(N) over the past 20.5 kyr are almost in phase with the regional East Asian summer monsoon (EASM) records. Because REEs are mainly concentrated in heavy minerals, the density difference of heavy minerals is probably the main controlling factor on REE fractionations. Clay minerals and feldspars released during the weathering process have a high content of Light REEs (LREEs). In the warm, wet periods (Holocene Optimum, and Bolling-Allerod), more precipitation promoted weathering and strengthened the estuarine hydrodynamic force, which respectively resulted in high content of clay minerals and feldspars, and high-density heavy minerals (e.g., monazite). The process led to LREE enrichment because the enriched minerals contain higher contents of the LREEs. In contrast, the dry, cold climate conditions during the periods Heinrich 1, Younger Dryas, and Last Glacial Maximum resulted in the reversed REE fractionations, as the weak hydrodynamic force can only carry the low-density heavy minerals (e.g., zircon) that are enriched in HREEs. This model does not seem to apply to 36- 24 kyr BP, probably because of the re-transport of ancient shelf sediments. The study facilitates the understanding of REE application in paleoclimate reconstruction within slope sediments.