External forcing mechanisms controlling the Qiongdong upwelling in the northern South China Sea during the Holocene

摘要

Coastal upwelling along the northern South China Sea controls regional marine productivity and thereby in-fluences blue economies. However, centennial-scale upwelling variability and its associated forcing mechanisms are poorly understood, primarily due to the lack of high-resolution upwelling records beyond the instrumental and historical periods. Here we reconstruct monsoon-induced upwelling variations over the past 7000 years, based on a high-resolution (similar to 30 yr) delta N-15 record from the upwelling zone off the east coast of Hainan Island. delta C-13, delta N-15, C/N, TOC and biogenic opal are determined to verify the dominant source of marine organic matter in the GH6 core, and delta N-15 values are used to indicate the variations in nitrate utilization by phytoplankton. The results show distinct centennial-scale variability, which is generally synchronous with solar activity, latitudinal shifts of the Intertropical Convergence Zone (ITCZ), and El Nino-Southern Oscillation (ENSO) frequency. We conclude that the centennial-scale upwelling intensity was predominately induced by external solar forcing, with internal feedback such as latitudinal shifts of the ITCZ and ENSO variability as a mediator amplifying the solar variation signal. Our new findings indicate that external forcing mechanisms exerted a significant influence on upwelling variations before the Anthropocene and should be considered when producing future projections of upwelling changes at low latitudes.