The relationship between the low-latitude hydrological cycle and regional-scale atmospheric circulations, such as the East Asian Summer Monsoon (EASM) and the Pacific Walker Circulation (PWC), over geological timescales (i.e., tectonic to orbital) remains unclear. In this study, we applied high-resolution XRF scanning analysis to samples from ODP Site 1143 in the southern South China Sea since the Late Miocene (ca. 9 Ma). According to Sr-Nd isotopes and clay assemblage, the terrestrial material at this site predominantly originates from the Mekong River Basin, which receives heavy (light) precipitation during the La Niña (El Niño) conditions along with the strengthened (weakened) EASM at decadal-interannual scale. 

The results reveal a long-term increase in the K/Al ratio, which suggests a reduction in precipitation, assuming that this ratio reflects chemical weathering intensity. In particular, the K/Al ratio increases from ca. 8 to 5 Ma, decreases from ca. 5 to 2.7 Ma, and then increases from ca. 2.7 Ma to the present. However, this pattern does not align well with other weathering indices, leading to the interpretation that the K/Al ratio may be more related to physical erosion and terrestrial input. Attributing sediment supply changes to EASM and/or PWC variations is uncertain, and tectonic forces, particularly the uplift and stabilization of the Central Highlands of Vietnam, may have been more influential. Additionally, sea-level drops after ca. 2.7 Ma likely boosted terrestrial material input.

Furthermore, power spectrums of high-resolution XRF-derived K/Al ratio show significant periodicities, such as eccentricity, obliquity and precession, which are well-known drivers of climate change at orbital timescales. The spectral patterns share similarities with the global benthic foraminiferal oxygen (δ18Obenthic) and carbon (δ13Cbenthic) isotopes, suggesting links between low-latitude hydroclimate, high-latitude climate, and the global carbon cycle, modulated by orbital forcing. Here we focus on climate-carbon cycle feedback on the 41-kyr obliquity timescale. Phase analyses suggest that δ18Obenthic and δ13Cbenthic have been in anti-phase since the Late Miocene. There is an interesting shift at ca. 2.7 Ma: prior to this, K/Al and δ18Obenthic were in-phase, while K/Al and δ13Cbenthic were anti-phase; afterward, this relationship reversed. The shift suggests that the climate-carbon cycle feedback mechanism transitioned from being regulated by the marine biosphere to the terrestrial biosphere. This change could provide new insights for the expansion of Northern Hemisphere Glaciation.