Why does speleothem δ 18 O differ from groundwater δ 18 O in the late
Pleistocene? Insight from modern precipitation with their recharge
dynamics in the northern margin of the East Asian summer monsoon region
Abstract
Both δ 18O in groundwater and speleothem are inherited
from precipitation δ 18O, however, stalagmite δ
18O and groundwater δ 18O do not
simultaneously agree with each other in the monsoon regions, especially
in the late Pleistocene. The reason remains unclear due to the complex
stable isotope systematics in the monsoon affected regions with their
controlling factors as well as the possible different recharge dynamics
of groundwater and drip water. In this study, we have collected 539
daily precipitation samples throughout 2015 from seven monitoring
stations in Beijing, in the northern margin of the East Asian summer
monsoon region. Stable isotopes (δ 18O and δ
2H) of these precipitation samples are investigated
for the controlling factors of precipitation isotopes, e.g., moisture
source, temperature (T), and precipitation amount (P) effects. The δ
18O values in the precipitation decrease from the
south to the northwest, which is controlled by the monsoon from the
south and continental moisture from the northwest. Consistently, the
precipitation exhibits an apparent seasonal variation in δ
18O values. The P effect is significant during the
monsoon season, especially when the precipitation events are greater
than 5 mm and single moisture source is considered. In contrast, the T
effect is significant, with a gradient of 0.4‰ per °C during the
non-monsoon season. A Rayleigh distillation model indicates that the
moisture source and residual vapor fraction are the two most important
factors controlling the δ 18O precipitation patterns
in a monsoon region, independent of temperature. Isotopes in groundwater
in the late Pleistocene exhibit a significant T effect, implying that
the monsoon might not reach Beijing when the groundwater was formed at
that time. Furthermore, we highlighted the different recharge mechanisms
of groundwater and drip water: lateral flow and direct vertical
infiltration make diverse contributions of non-monsoon precipitations to
the groundwater and drip water respectively, resulting in that
speleothem δ 18O differ from groundwater δ
18O in the late Pleistocene.