4. DISCUSSION
4.1 Influential
factors of seasonal variation of precipitation isotopic compositions
In the summer monsoon period (May-September), the mostly
ocean-originated moisture with high humidity and weak evaporation
results in low δ value of the precipitation (Peng et al., 2010; Uemura
et al., 2012; Cai et al., 2017). In the winter monsoon period
(October-February), there is relatively less precipitation due to the
influence of continental dry air masses carried by West Wind
Circulation, hence, the precipitation δ value is significantly higher
than that in summer (Liu et al., 2009; Peng et al., 2010). However, due
to the blockage of the Mountains in western Fujian, the continental air
masses tend to enter the East China Sea eastward and then transport
southward to Xiamen (Chen et al., 2016), so the wetter moisture (humid
air with higher relative humidity) brought from the sea makes the δD and
δ18O of winter precipitation in Xiamen lower than that
in the same latitude (Guo et al., 2021), showing significant marine air
masses characteristics. The period between winter and summer monsoon
periods is called seasonal shift period. During this period, the warm
and humid air masses from the ocean meet with cold and dry air masses
from the land, and then convective activity occurs in the study area,
resulting in frontal rainfall for long duration. The isotopic
composition of precipitation occurred during this period falls in
between summer and winter monsoon periods, with a significant signal of
mixed air masses from ocean and land. In addition, the study area is
significantly affected by typhoon processes produced in the Western
Pacific Ocean from June to November each year, and heavy precipitation
often occurs under the influence of typhoons, which show significant
lower δD and δ18O than the normal precipitation
(Fudeyasu et al., 2008; Xu et al., 2019).
Five environmental effects could affect the variation of atmospheric
precipitation isotopes, namely, latitude, temperature, altitude,
precipitation amount, and continental (Dansgaard 1964), among which the
most significant effects are the temperature and the precipitation
amount in the coastal region (Wolf et al., 2020). The lower temperature
will result in a lower δ value of precipitation due to the larger
isotopic fractionation coefficient at low temperatures, which is
generally positive in high latitude region (Merlivat and Jouzel, 1979).
The higher precipitation amount will also result in a lower δ value due
to the evaporation and isotopic exchange of rain drops with ambient
water vapor during the descent (Gedzelman and Lawrence, 1982). In this
study, the monthly mean δ18O values of atmospheric
precipitation in the Xiamen for 15 months were correlated with the
monthly mean temperature and monthly total precipitation amount to
analyze the temperature and precipitation amount effects during the
study period (Fig. 6). The results
show that the δ18O value is significantly negatively
correlated with the temperature, showing the characteristic of an
inverse temperature effect. This
may be due to the influence of monsoon climate in the study area, in
that the influence of moisture sources on δ18O values
overrides the influence of precipitation fractionation process on
δ18O values which leads to an inverse temperature
effect (Liu et al., 2009; Xie et al., 2011). In addition,
δ18O values were negatively correlated with the amount
of precipitation, but the correlation was not significant, and the
points with larger deviations were March and April 2019 (seasonal shift
period) (Fig. 6), which appears to be due to the influence of longer
duration of frontal rainfall compared to previous years in March to May
2019. This ultimately led to a less pronounced precipitation amount
effect because the δ18O of precipitation are mostly
larger than -4‰ during seasonal shift period (Fig. 5 e).
[Insert Figure 6]
The re-evaporation that occurs during precipitation is an important
physical process that affects precipitation isotopes (Salamalikis et
al., 2016; Zongxing et al., 2016). Evaporation during the precipitation
of raindrops in conditions with small amounts of precipitation and low
humidity will cause the meteoric water line slope to decrease (Vodila et
al., 2011). Therefore, higher temperature, stronger evaporation and
lower atmospheric humidity will result in a smaller slope of the
meteoric water line and a consequent change in the intercept (shown in
Fig. 5 d). The slope and intercept of the meteoric water line in
different seasons indicates differences in the influence of evaporation
processes on isotopic composition of atmospheric precipitation (Peng et
al., 2007; Vodila et al., 2011). The
higher slopes of the meteoric
water lines in Xiamen in summer monsoon period indicate a weaker
precipitation re-evaporation process, whereas the lower slope in winter
monsoon period indicate a stronger precipitation re-evaporation process
(Fig. 5 c and d). Although summer
temperatures are higher than winter and high temperatures promote
re-evaporation, high humidity and abundant precipitation in summer will
inhibit re-evaporation during the precipitation process (Guo et al.,
2021). On the contrary, the dry climate in winter, with scarce and slow
precipitation, facilitates the re-evaporation of rainwater during the
precipitation process (Peng et al., 2007). This may be an important
reason why the slope of meteoric water line is significantly lower in
winter than in summer. The slope of seasonal shift period is between
summer and winter monsoon period, indicating a transitional climate
condition (Fig. 5 e). In addition, the D-excess of precipitation
reflects the isotopic composition of the moisture and contains important
information about the source regions, including the state of the
evaporation process and the evaporation rate (Kong and Pang, 2016). The
evaporation effect of moisture originating from warm and humid marine
areas at low latitudes is smaller, and the evaporation process is
slower, resulting in a smaller D-excess; whereas the evaporation effect
of moisture originated from local or dry and cold inland areas at high
latitudes is larger, resulting in a higher D-excess (Frohlich et al.,
2002; Vreča et al., 2006; Xie et al., 2011).