Reconstructing Chemical Weathering Intensity in the Mekong River Basin Since the Last Glacial Maximum

Abstract

High-resolution clay mineralogy and major element geochemistry of Core SO18383-3 collected off the Mekong River mouth in the southern South China Sea have been investigated, aiming to reconstruct sediment provenance and chemical weathering intensity since the last glaciation. The chronology is based on foraminiferal accelerator mass spectrometry 14 C dates. Clay mineral analysis suggests that the Mekong River is a major sedimentary source for the studied area. The values of chemical index of alteration (CIA) indicate moderate chemical weathering in the Mekong River basin. Smectite/(illite + chlorite) and smectite/kaolinite ratios coupled with TiO 2 /K 2 O ratio reveal the temporal variation in chemical weathering intensity in the Mekong River basin. The lower ratios around the Last Glacial Maximum (29.9-14.7 cal ka BP), the Heinrich Event 1 (~14.8 cal ka BP), the Younger Dryas interval (12.4-11.3 cal ka BP), and middle to late Holocene (7.6-1.1 cal ka BP) indicate weak chemical weathering intensity. In contrast, the higher ratios occurring during the Bølling-Allerød period (14.7-12.7 cal ka BP) and the period of main deglaciation (14.7-7.6 cal ka BP) indicate stronger chemical weathering in the Mekong River basin. The good correlations between these proxies and the available proxy records of the East Asian summer monsoon intensity suggest that the intensity of chemical weathering in the Mekong River basin over the last 30 ka is significantly controlled by the summer monsoon. This study greatly increases our understanding of the principal forcing factor on continental weathering in the Mekong River basin since the Last Glacial Maximum.

Reconstruction of Chemical Weathering Intensity and Asian Summer Monsoon Evolution in the Red River Basin Over the Past 36 kyr

“…Generally, Si is concentrated in coarse‐grained fraction (e.g., quartz), and Al is concentrated in fine‐grained fraction (e.g., clayey minerals). Thus, the SiO 2 /Al 2 O 3 ratio is widely used to indicate the grain size (Jiwarungrueangkul et al., 2019). The Al 2 O 3 , K 2 O, TFe 2 O 3 , and MgO contents were negatively correlated with SiO 2 /Al 2 O 3 ratio (Figures 5a–5d), indicating these elements held by fine‐grained fraction.…”

“…So far, the interpretation on the paleoenvironment meaning of silicate chemical weathering degrees documented by marine sediments is still controversial (Colin et al., 2006; D. Hu et al., 2012; C. Huang et al., 2018; Jiwarungrueangkul et al., 2019; Liu et al., 2020; Wei et al., 2006). The reported studies have shown that the monsoon‐related precipitation and temperature exert a major impact on the silicate chemical weathering degree after the late Pleistocene, when tectonic activities may have little impact; thus, the chemical weathering intensity indicates the summer monsoon in the source regions (M. Cai et al., 2019; Colin et al., 2006; J. Huang et al., 2016; Jiwarungrueangkul et al., 2019). However, on the orbital and millennium scales, chemical weathering displays a complex response to the environmental changes in certain monsoon regions (Liu et al., 2020; Wan et al., 2017).…”

Sedimentary Responses to Climate Changes and Human Activities Over the Past 7400 Years in the Western Sunda Shelf

“…CaO * in CIA is corrected by comparing the molar contents of CaO with Na 2 O, and the lower value is regarded as the CaO content in the silicate fraction (Singh et al, 2005;Liu et al, 2020). K 2 O is preferentially leached in aqueous fluids compared to the immobile Al 2 O 3 during the chemical weathering process (Nesbitt and Young, 1982); therefore, a lower K 2 O/Al 2 O 3 ratio could indicate increased chemical weathering related to strengthened monsoon rainfall, according to the basic principles of silicate weathering (Wei et al, 2004;Clift et al, 2014;Jiwarungrueangkul et al, 2019a). TiO 2 /CaO and Rb/Sr ratios were used to evaluate the variation of terrigenous sediment input related to erosion in this study (Jiwarungrueangkul et al, 2019b;Li et al, 2019).…”

“…Because of the relatively constant source of Core K17 (Figure 7), provenance changes could not significantly influence weathering and terrigenous indicators. Sea level change played an important role in controlling terrigenous sediment input to the southern SCS before or during the early Holocene (Jiwarungrueangkul et al, 2019a) but is relatively constant with a weak fluctuation since ca. 7-8 ka on the Sunda Shelf, when the coastline reached a modern position (Steinke et al, 2003;Hanebuth et al, 2011).…”

Chemical Weathering of the Mekong River Basin With Implication for East Asian Monsoon Evolution During the Late Quaternary: Marine Sediment Records in the Southern South China Sea

“…The southern South China Sea inputs huge weathering products of parent rocks from surrounding river basins in Southeast Asia under the strong influence of the East Asian monsoon and sea-level change, and this region becomes a valuable region for studying sediment source-to-sink and land-sea interactions (Hanebuth et al, 2002;Hanebuth et al, 2009;Hanebuth et al, 2011;Steinke et al, 2008;Huang et al, 2016) (Figure 1). Offshore from the Mekong River, clay mineralogy of Cores MD01-2393, MD97-2150 (Liu et al, 2004), and SO18383-3 (Jiwarungrueangkul et al, 2019b) and Sr-Nd isotopic compositions of Core MD01-2393 (Liu et al, 2005) were employed to indicate the Mekong River is the primary source of terrigenous sediments. Liu et al (2004) and Liu et al (2005) showed that strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods in the Mekong River basin, whereas weakened chemical weathering is associated with reduced sediment discharge and intensified physical erosion during glacial periods since 190 ka ago.…”