Clay mineralogical and geochemical proxies of the East Asian summer monsoon evolution in the South China Sea during Late Quaternary

Abstract

The East Asian summer monsoon controls the climatic regime of an extended region through temperature and precipitation changes. As the East Asian summer monsoon is primarily driven by the northern hemisphere summer insolation, such meteorological variables are expected to significantly change on the orbital timescale, influencing the composition of terrestrial sediments in terms of both mineralogy and geochemistry. Here we present clay mineralogy and major element composition of Core MD12-3432 retrieved from the northern South China Sea, and we investigate their relationship with the East Asian summer monsoon evolution over the last 400 ka. The variability of smectite/(illite + chlorite) ratio presents a predominant precession periodicity, synchronous with the northern hemisphere summer insolation changes and therefore with that of the East Asian summer monsoon. Variations in K2O/Al2O3 are characterized by eccentricity cycles, increasing during interglacials when the East Asian summer monsoon is enhanced. Based on the knowledge of sediment provenances, we suggest that these two proxies in the South China Sea are linked to the East Asian summer monsoon evolution with different mechanisms, which are (1) contemporaneous chemical weathering intensity in Luzon for smectite/(illite + chlorite) ratio and (2) river denudation intensity for K2O/Al2O3 ratio of bulk sediment.

Responses of the East Asian Summer Monsoon in the Low‐Latitude South China Sea to High‐Latitude Millennial‐Scale Climatic Changes During the Last Glaciation: Evidence From a High‐Resolution Clay Mineralogical Record

“…As a result, the smectite/(illite + chlorite) ratio represents the relative proportion of clay minerals deriving from Luzon (smectite) and Taiwan (illite and chlorite) and can be adopted as the mineralogical proxy for establishing the East Asian summer monsoon evolution history. This clay mineralogical ratio, with advantage of eliminating the dilution effects, has been confirmed in several previous studies at the relatively longer timescales (e.g., Boulay et al, 2005;Chen et al, 2017;Liu et al, 2003;Yu et al, 2016).…”

“…During the prevailing cold and dry climate like the intervals of YD, Heinrich stadials, and glacial periods associated with the retreat of the East Asian summer monsoon (Sun et al, 2012;Y. This clay mineralogical ratio, with advantage of eliminating the dilution effects, has been confirmed in several previous studies at the relatively longer timescales (e.g., Boulay et al, 2005;Chen et al, 2017;Liu et al, 2003;Yu et al, 2016). Then, surface processes favor reinforced physical erosion at Taiwan but potentially restrain the chemical weathering at Luzon under relative weakened rainfall intensity .…”

“…Previous studies of the SCS sediments have demonstrated that contour currents is strengthened during Heinrich stadials deduced from the striking increase of both magnetic susceptibility and mean size of sortable silt Zheng et al, 2016), coinciding with a period of the weakened East Asian summer monsoon (Sun et al, 2012;Y. To further address this problem, results of the present study have been compared with several previous available clay mineralogical records in the northern margin of the SCS (Boulay et al, 2005;Chen et al, 2017;Liu et al, 2016; Figures 1b and 3). Then, the intensified deep contour currents have potentially resulted in more illite and chlorite derived from Taiwan to Core MD12-3434, decreasing the smectite/(illite + chlorite) ratio and tending to amplify the signals of weakened summer monsoon to an unclear certain degree.…”

“…To better understand this issue, a compilation of clay mineralogical records located along two transects (T1 and T2) on the northern margin of the SCS have been intercompared (Boulay et al, 2005;Chen et al, 2017;Liu et al, 2016;Figures 1b and 8). This would explain why several previous records of the SCS semipelagic sediments are less sensitive to the prevailing East Asian monsoon on millennial timescale.…”

“…As direct weathering products of continental bedrocks, clay minerals are the most important components of terrigenous sediments, providing one of the most powerful tools for reconstructing the long-term East Asian monsoon evolution in the SCS (Boulay et al, 2005;Chen et al, 2017;Clift et al, 2014;Liu et al, 2003Liu et al, , 2004Wan et al, 2007). During the typical glacial-interglacial cycles, increase of illite and chlorite contents under reinforced physical erosion is considered to be associated with strengthened winter monsoon, while increase of smectite and kaolinite contents is related to intensive chemical weathering and then indicates strengthened summer monsoon (Liu et al, 2003(Liu et al, , 2004.…”