Geochemical record of chemical weathering and monsoon climate change since the early Miocene in the South China Sea

Abstract

[1] The chemical index of alteration (CIA) and elemental ratios that are sensitive to chemical weathering, such as Ca/Ti, Na/Ti, Al/Ti, Al/Na, Al/K, and La/Sm, were analyzed for detrital sediments at Ocean Drilling Program Site 1148 from the northern South China Sea to reveal information of chemical weathering in the source regions during the early Miocene. High CIAvalues of $80, coupled with high Al/Ti and Al/Na and low Na/Ti and Ca/Ti, are observed for the sediments at $23 Ma, indicating a high chemical weathering intensity in the north source region, i.e., south China. This was followed by gradual decreases in Al/Ti, Al/Na, La/Sm, and Al/K ratios, as well as the CIAvalues, and increases in Ca/Ti and Na/Ti ratios. These records together with other paleoclimate proxies, such as black carbon d 13 C and benthic foraminifer d 18 O, give reliable information on the climate changes in south China. Our results show that the climate in south China was warm and humid in the early Miocene ($23 Ma) according to the chemical weathering records. The humidity in south China decreased from the early Miocene to Present with several fluctuations centering at approximately 15.7 Ma, 8.4 Ma, and 2.5 Ma, coincident with the global cooling since the middle Miocene. These climate changes implied that the summer east Asian monsoon has dramatically affected south China in the early Miocene, whereas the influence of the summer monsoon on this region has decreased continuously since that time, probably because of the intensification of the winter monsoon. Such an evolution for the east Asian monsoon is different from that for the Indian monsoon.

Major and trace elements of a peat core from Yunnan, Southwest China: Implications for paleoclimatic proxies

“…Chemical and mineral components in soils and sediments are records of the intensity of chemical weathering in their source regions (Kronberg et al, 1986;Nesbitt and Young, 1982;Wei et al, 2004Wei et al, , 2006.…”

Evolution of the East Asian monsoon and its response to uplift of the Tibetan Plateau since 1.8 Ma recorded by major elements in sediments of the South China Sea

“…Therefore, three factors were associated with sources according to the geochemical characteristics of major elements and the exclusivity of element association in the geological process. Titanium is a conservative element, and K, Fe, Mg, Al are closely related to the terrestrial source [18][19][20]; thus, the association reflected by F1 is of terrene nature (i.e. F1 is a terrestrial factor).…”

“…Because biogenic carbonate has been removed from the fine grain-sized sediments, it is generally accepted that residual carbonate-free detritus consists of silicate, a small quantity of ferromanganese oxide, and biogenic silicon [12,17], whereas the fine grain-sized silicate detritus is predominantly composed of terrestrial materials from rivers and volcanic ash [14,[18][19][20]. Therefore, three factors were associated with sources according to the geochemical characteristics of major elements and the exclusivity of element association in the geological process.…”

“…Contrarily, the variation in the terrestrial factor for marine sediments could reflect the intensity of chemical weathering in the source region and thus reveal the evolution of the summer monsoon. An increase or decrease in the fluvial contribution to marine sediments implies the enhancement or weakening of chemical weathering and consequently the summer monsoon [18][19][20]24,25]. Concerning the fine grain-sized carbonate-free sediments of the upper splice of ODP Site 1146, because the factor loads of the terrestrial elements and oxides P, Ti, TFe 2 O 3, MgO, K 2 O, Al 2 O 3 , and CaO in F1 are all positive, an increase or decrease in the F1 factor score would mean an increase or decrease in the fluvial contribution to the total sediments.…”

Sensitivity of Sediment Magnetic Records to Climate Change during Holocene for the Northern South China Sea

“…The SCS can be subdivided into northwestern, southwestern, and eastern sub-basins (He and Chen, 1987;Liu, 1992). As the most frequent activity area of East Asian monsoon, it has become a hot area of climate research Wei et al, 2006;Wan et al, 2007;Hu et al, 2012). As suggested by Han and Fyfe (1995) and Wang (2009), it is an important place for paleoenvironment and paleoclimate research and can be considered as another natural laboratory besides the Chinese Loess Plateau for the study of regional response to global climate change and its driving forces.…”