Monsoonal climate evolution in southern China since 1.2 Ma: New constraints from Fe-oxide records in red earth sediments from the Shengli section, Chengdu Basin

Abstract

Southern China has experienced a dominantly monsoonal climate since the early Quaternary, owing to the influence of Asian monsoons from several directions during that period. Pedogenic Fe-oxides (hematite and goethite) are regarded as sensitive indicators of monsoonal paleoclimates. To obtain a better understanding of monsoonal climate changes in the subtropical zone of southern China, samples from the Shengli section (Chengdu Basin) were investigated using diffuse reflectance spectroscopy (DRS), high-resolution transmission microscopy (HRTEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS). TEM results show that both hematite and goethite occur as fine particles with an irregular morphology. Most hematite grains have low crystallinity and nanoporous structure, suggesting that they formed from a ferrihydrite precursor. Uniformity of trace-element and REE distribution patterns, Eu and Ce anomalies, and (La/Yb)N ratios throughout the section suggests that the Shengli deposits were probably well mixed in the source area prior to transport and deposition, and that there has been little change in the source areas of primary materials since 1.2Ma. The high chemical index of alteration (CIA) of the red earth (79–88) reflects strong chemical weathering processes in southern China. Besides, DRS analyses of Fe-oxides show that most red earth samples at Shengli contain more hematite than goethite, which is opposite to typical loess-paleosol profiles from northern China. These results confirm that southern China generally experienced more intense weathering processes than northern China since 1.2Ma.Large fluctuations in major-element oxide concentrations and clay-mineral assemblages in the lower part of the section indicate climatic fluctuations between perennially humid and seasonally humid conditions in the mid-Pleistocene. Monsoonal climate evolution, as proxied by the hematite/(hematite+goethite) [Hm/(Hm+Gt)] ratio, shows a long-term but stepwise cooling and drying pattern in the Chengdu Basin with major transitions at ~1.0, 0.45, and 0.1Ma. These transitions were probably triggered by episodes of tectonic uplift of the Tibetan Plateau and global climate change, with consequent effects on the Asian monsoons. The transitions in our paleoclimate proxy records for southern China match well with shifts in the marine oxygen and carbon isotope records since the mid-Quaternary, providing evidence of strong teleconnections between global and regional climates, terrestrial weathering, and the global carbon cycle.