Seasonal and Inter-Annual Variations of Stable Isotopic Characteristics of Rainfall and Cave Water in Shennong Cave, Southeast China, and Its Paleoclimatic Implication

Hai Cheng,Yanjun Cai,Ye Tian,Fan Zhang,Rui Zhang,Haiwei Zhang,Zeyuan Liang

doi:10.3389/feart.2021.794762

Abstract

Speleothem calcite stable oxygen isotope (δ18OC) is one of the most widely used proxies in paleoclimate research, and understanding its seasonal-annual variability is very significant for palaeoclimate reconstruction. Five-year precipitation and karst cave water from 2016 to 2021 were monitored in Shennong cave, Jiangxi Province, Southeast China. The local meteoric water line (LMWL) is δD = 8.20 × δ18O + 13.34, which is similar to the global meteoric water line. The stable hydrogen and oxygen isotope (δD and δ18O) characteristics of precipitation and cave water were studied. δ18O and δD of precipitation and cave water show obvious seasonal variations. Lower precipitation δ18O and δD generally occur during summer and autumn compared with higher δ18O and δD values during winter and spring. Meanwhile, low precipitation δ18O values do not only appear in June–July when precipitation is the highest of the year but also appear in August–September when precipitation is limited. The back-trajectory analysis of monsoon precipitation moisture sources shows that the moisture uptake regions vary little on inter-annual scales; the water vapor of rainfall in June–July comes from the South China Sea and the Bay of Bengal, while the moisture source in August–September is mainly from the West Pacific and local area. The El Niño-Southern Oscillation is an important factor affecting the value of δ18O by modulating the percentage of summer monsoon precipitation in the annual precipitation and moisture source. The relationship between amount-weighted monthly mean precipitation δ18O and Niño-3.4 index shows that the East Asian summer monsoon (EASM) intensifies during La Niña phases, resulting in more precipitation in monsoon season (May to September, MJJAS) and lower δ18O values, and vice versa during El Niño phases.

Highlights

Speleothem calcite stable oxygen isotope (δ18OC) records in eastern China are used to reflect the characteristics of the EastAsian summer monsoon (EASM) on different timescales (Wang et al, 2001; Cheng et al, 2016; Cheng et al, 2019; Zhang et al, 2019)
In the northern and southwestern regions of monsoonal China, monsoon rainfall accounts for more than 70% of annual rainfall, while in southeastern China, especially the spring persistent rain region, where the East Asian summer monsoon (EASM) precipitation is equivalent to the non-summer monsoon (NSM) precipitation, the EASM precipitation only accounts for ∼50% of annual rainfall (Zhang et al, 2020)
We examined the correlations of δ18O and δD between amount-weighted monthly mean precipitation and cave water at Shennong cave in Southeast China over the period from 2016 to 2021

Summary

Introduction

Speleothem calcite stable oxygen isotope (δ18OC) records in eastern China are used to reflect the characteristics of the EastAsian summer monsoon (EASM) on different timescales (Wang et al, 2001; Cheng et al, 2016; Cheng et al, 2019; Zhang et al, 2019). On orbital timescales, the Asian summer monsoon intensity inferred from Chinese speleothem δ18OC records show a significant precession signal and closely follow Northern. Cheng et al (2021) and Zhang et al (2021) point out that a coherent orbital-scale speleothem δ18OC variability across most Asian monsoon regions (except southeastern China) stems from the NHSI-forced changes in overall monsoon intensity; speleothem and marine records are complementary rather than incompatible, and each record reflects a certain aspect of Asian monsoon dynamics. Speleothem δ18OC records in southeastern China are rather distinct and should not be linked directly to the overall monsoon intensity due to its distinct precipitation seasonality (Zhang et al, 2020; Zhang et al, 2021). In the northern and southwestern regions of monsoonal China, monsoon rainfall accounts for more than 70% of annual rainfall, while in southeastern China, especially the spring persistent rain region, where the EASM precipitation is equivalent to the non-summer monsoon (NSM) precipitation, the EASM precipitation only accounts for ∼50% of annual rainfall (Zhang et al, 2020)

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