Controls on the hydrogen isotope composition in the pore water from the Dajiuhu Peatland, central China

Abstract

The hydrogen isotope composition of leaf wax n-alkanes (δ²H_alk) has been used to reconstruct hydroclimate conditions, yet the factors that influence it are not fully understood, particularly the control of soil pore water δ²H. This study monitored the temporal and vertical variations of peat pore water δ²H (δ²H_pw) from 2015 to 2019 in the Dajiuhu peatland, central China. Results showed that δ²H_pw was highly variable in the surface layers (0−10 cm; avg. −47‰, 1σ = 11‰) and remained almost constant in deeper depths (below 50 cm; avg. −56‰, 1σ = 2‰). The δ²H_pw of the 0−10 cm layer was strongly correlated with the preceding month’s precipitation δ²H (δ²H_p) in the adjacent area (r = 0.7, p < 0.01), indicating that δ²H_p is the main factor affecting the temporal variations of δ²H_pw in the upper layers. Moreover, the surface (0−10 cm) peat pore water slightly deviated from the local meteoric water line, suggesting that evaporation may also have an effect on the δ²H_pw. These findings emphasize the importance of precipitation isotope composition in interpreting the δ²H_alk in peat deposits under subtropical climates.

Graphical abstract

Keywords

pore water / hydrogen isotope composition / spatiotemporal variability / soil evaporation / leaf wax n-alkane / peat deposit

1 Introduction

Along with the advance of instrumental techniques over the past decades, the hydrogen isotope compositions (δ²H) of leaf wax n-alkyl lipids have been increasingly used as a powerful tool for paleohydrological reconstructions (e.g., Sachse et al., 2012; McFarlin et al., 2019; Liu et al., 2022a; Zhao et al., 2022). In the East Asian summer monsoon regions, the leaf wax n-alkane δ²H (δ²H_alk) sequences from lacustrine and peat deposits and loess-paleosol sequences have been extensively studied and interpreted as a reflection of variations in precipitation isotope compositions and associated monsoon activities (e.g., Liu and Huang, 2005; Rao et al., 2016; Wang et al., 2021; Zhao et al., 2021; Zhao et al., 2022; Huang et al., 2023). Nonetheless, the effects on the paleo applications of δ²H_alk in natural archives are not fully understood (Sachse et al., 2012; Liu and An, 2018).

Investigations into modern processes have revealed that the δ²H_alk is mainly affected by three factors: the δ²H of precipitation (δ²H_p) (Liu and An, 2018; McFarlin et al., 2019), the evaporation of soil water and the transpiration of plants (Feakins and Sessions, 2010; Kahmen et al., 2013), and the fractionation of isotopes during wax biosynthesis (Sachse et al., 2012; Liu and An, 2019; Liu and Liu, 2019). Significant attention has been focused on the impacts of δ²H_p (Sachse et al., 2012; McFarlin et al., 2019) and plant types (Liu and Yang, 2008; Liu et al., 2022a) on δ²H_alk; however, the influence of soil water evaporation has not been extensively studied. It is postulated that soil water evaporation leads to an enrichment of deuterium, thus causing a more positive δ²H_alk (Gibson et al., 2008). Some studies suggest that the effect of soil water evaporation on δ²H_alk is insignificant even in arid conditions due to the water absorption through deep roots (Dawson, 1993; Feakins and Sessions, 2010). In contrast, other studies indicate that the impact of evaporation on the δ²H of soil water (δ²H_sw) can reach deeper depths (even > 150 cm) in arid settings (Sprenger et al., 2016) and become weaker with increased depth (Dai et al., 2020, 2022). Furthermore, the δ²H_alk sedimentary sequence in peat deposits supports the notion that evaporation may have an effect during long-term drying periods (Huang et al., 2018; Wang et al., 2022). To date, the δ²H_sw variations over long periods of time have not been widely examined.

Peat pore water in peatlands offers an interesting chance to investigate the spatial and temporal variations of δ²H_sw. Peat deposits have been widely studied to reconstruct paleoclimate through lipid proxies, such as δ²H_alk (e.g., Xie et al., 2000; Seki et al., 2011; Huang et al., 2018), and are suitable for exploring the effect of soil water evaporation on δ²H_alk (Huang and Meyers, 2019). Moreover, the high moisture content in peatlands makes it easy to obtain pore water even during periods of drought. The Dajiuhu peatland, a typical subtropical peatland, has a number of studies conducted on it to explore the influence of evaporation on δ²H_sw over a long-term series. These include sedimentary δ²H_alk (Huang et al., 2018; Yang et al., 2023), physicochemical properties of peat pore water (Zhang et al., 2022), and meteorological and hydrological monitoring (Yang et al., 2024).

This study seeks to examine the spatial and temporal features of δ²H_pw in Dajiuhu peatland, and explore the possible controlling factors (e.g., local conditions, δ²H_p, evaporation) that may influence the spatiotemporal variations of δ²H_pw, based on the monitoring program conducted in this peatland from 2015 to 2019.

2 Materials and methods

2.1 Study area

The Dajiuhu basin (31°28′ N, 110°00′ E, 1700 m above the sea level) is located at the western edge of the Shennongjia Forest District, Hubei Province, central China (Fig.1). This site was designated as an internationally important wetland by the Ramsar Convention in late 2013. Dajiuhu peatland spans an area of 7.5 km², with a typical peat thickness of 2 m (Huang et al., 2017). The dominant peat-forming plants in this peatland are Sphagnum palustre and graminoids (Zhao et al., 2018). The climate is mainly characterized by the Asian monsoon, with a mean annual air temperature of 7.2°C and a mean annual precipitation of 1560 mm (Huang et al., 2017).

2.2 Sampling

In 2014, 15 sites were selected for long-term monitoring of hydrological regimes and biogeochemical processes (Tab.1; Fig.1; Huang et al., 2017). These sites were chosen to represent a range of water levels and are less impacted by anthropogenic influences (Huang et al., 2017; Yang et al., 2024). MacroRhizon soil solution samplers (Rhizosphere Research Products B.V., The Netherlands) were installed at seven depths (0−10, 20−30, 50−60, 80−90, 100−110, 120−130, 150−160 cm) to collect peat pore water samples. From 2015 to 2019, peat pore water samples were taken in spring (April, May), summer (June, July, August) and autumn (September, October) of each year.

Three parallel samples of the river and lake water were collected directly from the lake and river connected to the peatland. The river site is located upstream, while the lake site is the water outlet of lake groups. The river and lake water samples were put into 50 mL polyethylene tubes and filtered with 0.45 μm filter membranes before being subjected to instrumental analysis. In 2017, no river and lake samples were taken.

2.3 Instrumental analysis

The isotope compositions of water samples were determined using a liquid water isotope analyzer (IWA-35-EP, Los Gatos Research, USA) at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan. The analytical precision is ± 0.6‰ for δ²H and ± 0.2‰ for δ¹⁸O. Isotope results were normalized against Vienna Standard Mean Ocean Water (V-SMOW).

2.4 Statistical analysis

The local meteoric water line (LMWL) and line-conditioned excess (lc-excess) were calculated to evaluate the effect of evaporation on δ²H_pw. The LMWL in the Shennongjia Forest District was determined to be δ²H = 8.25 × δ¹⁸O + 17.14 (Wang, 2021). The lc-excess represents the deviations between water samples and LMWL in the dual-isotope plots (Landwehr and Coplen, 2006). The expression is as follows:

(1)\mathrm{l}\mathrm{c}\text{-}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}={\delta }^2 \mathrm{H}–\mathit{a}\times {\delta }^{18}\mathrm{O}-\mathit{b},

where a represents the slope and b represents the intercept of LMWL. According to the LMWL in the Shennongjia Forest District, the lc-excess of collected water samples was calculated as

(2)\mathrm{l}\mathrm{c}\text{-}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}={\delta }^2 \mathrm{H}-8.25\times {\delta }^{18}O-\mathrm{17.14.}

Correlation analysis and the Pearson significance test were conducted in the R platform (R Core Team, 2022) with the “corrplot” package (Wei and Simko, 2021).

3 Results

3.1 Meteorological data

Meteorological data between 2015 and 2019 in the Dajiuhu peatland have been published in previous studies (Huang and Meyers, 2019; Yang et al., 2024; Fig.2). On average, the air temperature was relatively high in summer (avg. 19°C) and low in winter (avg. −2°C). Rainfall mainly occurred between April and October. The relative humidity (RH) kept above 70% for the most time period. During 2015−2019, the mean annual air temperature ranged from 8.3°C to 11.2°C (Fig.2(a)). The mean annual precipitation ranged from 1497 to 1710 mm (Fig.2(b)). Notably, the RH dropped to 68% during the dry period in the summer of 2016 (Fig.2(c)).

3.2 δ²H of peat pore water

The δ²H_pw in the upper layers (< 50 cm) became more negative and less variable as the depth increased. The δ²H_pw in the uppermost layer (0−10 cm) had the most variation, ranging from −69‰ to −29‰, and had an average value of −47‰ (Tab.2; Fig.3(a)). The layer of 20−30 cm had slightly smaller fluctuations of δ²H_pw (−58‰ − −46‰, avg. −51‰; Tab.2; Fig.3(a)). In contrast, the lower peat layers (> 50 cm) displayed more negative δ²H_pw and remained relatively constant over the five-year study period (−58‰ − −54‰; Tab.2; Fig.3(b)).

Over the period from April 2015 to October 2019, the δ²H_pw at 0−10 cm depth (δ²H_{0−10 cm}) from the 15 monitoring sites displayed a variation of less than 12‰ (variance, 1σ) at the same sampling time (Tab.2). Additionally, strong positive correlations for the δ²H_{0–10 cm} (r > 0.6, p < 0.05) were observed among different sites (Fig.4). Regarding the layer of 20−30 cm, the δ²H_pw from the 15 monitoring sites fluctuated by less than 9‰ at the same sampling period (Tab.2).

Seasonal variations in the averaged δ²H_{0–10 cm} are evident. In Spring (April, May) and early Summer (June), the δ²H_{0–10 cm} ranged from −51‰ to −29‰ and were higher than in the following seasons. Exceptions appeared in September 2015 and 2016, with relatively positive values of −47‰ and −50‰ (Fig.3(a)). On the annual time scale, the averaged δ²H_{0−10 cm} for 2015 and 2019 were −43‰ and −37‰, respectively, which were higher than those in other monitoring years (−50‰ − −53‰) (Fig.3(a)). In comparison, the temporal variations of δ²H_{20−30 cm} were less pronounced (avg. −51‰, 1σ = 4‰).

3.3 δ²H of the river and lake water

During 2015−2019, the δ²H of river samples (δ²H_river) ranged from −79‰ to −39‰, whereas the δ²H of lake samples (δ²H_lake) fluctuated between −65‰ and −40‰ (Fig.3(c)). Generally, δ²H_lake was the highest in June (except for July 2015) yet the lowest in the last sampling month of each year. The temporal patterns of δ²H_river and δ²H_lake were similar across the sampling period and had a strong correlation with the averaged δ²H_{0−10 cm} (r > 0.46) (Fig.4).

4 Discussion

4.1 Controls on spatial and vertical variations of δ²H_pw

The δ²H_pw in the peat profiles of the Dajiuhu peatland decrease with increasing depth, with mean values ranging from −47‰ to −56‰ and deviation decreasing from 11‰ to 2‰ (Fig.3). This vertical pattern is likely the result of two factors: soil evaporation in shallow layers and long residence time in deeper layers (Sprenger et al., 2016; Dai et al., 2020). Surface δ²H_pw is usually affected by evaporation, leading to higher values due to kinetic effects (Gibson et al., 2008). With increasing depth, the influence of soil evaporation diminishes, resulting in less enrichment of ²H. This is evident in the dual-isotope plot, where the isotope data of soil water influenced by evaporation is plotted on the lower right of the LMWL, with slight deviations of surface (0−10 cm) peat pore water from the LMWL (Fig.5). The influence depth is limited in the top peat layers (0−10, 20−30 cm) in the Dajiuhu peatland and shallower than in other locations (Sprenger et al., 2016), which is likely due to the humid climate and water table close to the ground surface. The δ²H_pw in the Dajiuhu peatland does not show an abrupt change, indicating that preferential flow is not the primary factor for the vertical variations of δ²H_pw (Pu et al., 2020).

The distribution of the averaged δ²H_pw along depths is mainly shaped by evaporation, but the magnitude of the δ²H_pw changes at different depths is mainly a consequence of the mixing of multiple precipitations, as evidenced by the nearly constant δ²H_pw (~ −57‰) in deeper depths (50−60, 80−90, 100−110, 120−130, 150−160 cm). Studies have indicated that δ²H_p may vary with rain events (Dai et al., 2020) and differ significantly between seasons (Wang et al., 2020). This is why the δ²H_{0–10 cm} exhibits the highest variation (−69‰ to −29‰) due to the direct influence of precipitation. In the deeper peat layers (20–30, 50–60, 80–90, 100–110, 120–130, 150–160 cm), the δ²H_pw is affected by the mixing of prior precipitation (Allen et al., 2014; Xu et al., 2022) and thus becomes less variable. Jung et al. (2022) observed that the δ²H of groundwater across South Korea was mainly recharged by precipitation during the rainy season. In Dajiuhu peatland, most rainfall in a single year (around 80%) occurs during the rainy season (April to October). The mean δ²H_p in this period was −65% in Dajiuhu (Pu, 2018), which is close to the δ²H_pw beneath 50 cm. This indicates that the δ²H_pw at deeper depths (50−60, 80−90, 100−110, 120−130, 150−160 cm) is mainly affected by the mixing of the rainy season δ²H_p. However, the near-constant δ²H_pw at deeper depths over five years suggests that the peat pore water at deeper depths may be affected by multiple years instead of one year, which needs to be proved in future research. Therefore, only the characteristics of surface δ²H_pw are discussed in the following part due to the limited variations of the δ²H_pw in other peat depths.

The δ²H_{0−10 cm} among the different monitoring sites in the Dajiuhu peatland showed only minimal deviation, with the variance staying below 12‰ from 2015 to 2019 (Tab.2). The close correlations and slight variance among monitoring sites suggest similar controlling factors. A relatively small variance was also observed in the surface soil δ²H_sw in forest, karst hillslope and agricultural fields (Goldsmith et al., 2019; Liu et al., 2022c; Xu et al., 2022). However, the mean δ²H_{0−10 cm} (ranging from −12‰ to −32‰, with an average value of −19‰) in the Dajiuhu peatland is smaller than that in other locations, likely due to the similar microtopography and high moisture content. Liu et al. (2022c) observed that the ranges of surface δ²H_sw in dry seasons were larger than that in wet seasons. In contrast, the ranges of δ²H_{0−10 cm} in the Dajiuhu peatland do not show similar seasonal variations and large ranges may occur in both wet seasons and dry seasons (Fig.3, Tab.2), likely due to the high water table in peatlands.

4.2 Controls on seasonal variations of δ²H_pw

Precipitation is typically the main source of peat pore water. In Dajiuhu, the seasonal variations of δ²H_{0−10 cm} are reflective of the fluctuations in precipitation, with higher values in the spring (April, May) and lower values in the summer (June, July) (Fig.6). This decrease is linked to the summer monsoon bringing in moisture with depleted deuterium from the tropical Indian Ocean and the western Pacific Ocean to central China (Tan, 2014). Autumn precipitation also contributes a moderate amount of rain to the area (Ding and Wang, 2008), resulting in relatively lower δ²H_p and δ²H_{0−10 cm}.

The influence of precipitation on δ²H_{0−10 cm} is not immediate. A previous study found a time lag of 1−3 months between δ²H_sw in shallow soil (0−60 cm) and δ²H_p in Changsha (Dai et al., 2020). In the Dajiuhu peatland, it is assumed that δ²H_{0−10 cm} lags δ²H_p by approximately one month. This is supported by the fact that δ²H_{0−10 cm} decreases after early summer (June), which is a month later than the variations of δ²H_p. Additionally, δ²H_{0−10 cm} is strongly correlated with the preceding month’s δ²H_p in the nearby area (δ²H_p near the Heshang cave, r = 0.73, p < 0.01, n = 15; δ²H_p near the Furong cave, r = 0.68, p < 0.001, n = 23) as opposed to the parallel-month δ²H_p (r < 0.29, p > 0.05 for both sites) (Wang et al., 2020; Qiu et al., 2021). Furthermore, the lc-excess_{0−10 cm} is significantly correlated with the subtraction of the preceding month’s δ²H_p from δ²H_{0−10 cm} in 2016 (r = −0.76, p < 0.05, n = 7) (δ²H_p collected from Pu (2018)).

Accumulating evidence has shown that δ²H_p in the East Asian summer monsoon influenced areas is likely to be impacted more by regional conditions, such as the convective activities in the moisture source regions, rather than local meteorological conditions (Ruan et al., 2019; Wang et al., 2020). A similar trend may exist for the δ²H_{0−10 cm} in the Dajiuhu peatland, since correlations between δ²H_{0−10 cm} and local meteorological conditions (temperature, precipitation and RH) are weak (Fig.7).

The temporal variations of δ²H_{0−10 cm} are largely determined by the isotope composition of precipitation, but the effects of evaporation should not be overlooked. This is evidenced by the reduced values of lc-excess_{0−10 cm}, which suggests the enrichment of δ²H_sw due to evaporation (Sprenger et al., 2016; Liu et al., 2022b; Xu et al., 2022; Liu et al., 2023), from July to September. For example, the high temperature, low precipitation and RH in August and September 2016 caused a severe drought, which is supported by the lowest lc-excess_{0−10 cm} during 2016 and a quite positive δ²H_{0−10 cm} in September 2016. Furthermore, the influence of soil evaporation has been documented in sedimentary δ²H_alk (Huang et al., 2018).

The El Niño-Southern Oscillation (ENSO) is believed to be a major factor influencing the yearly variations of δ²H_{0−10 cm}. During El Niño years, the δ²H_{0−10 cm} tends to be more positive (avg. −43‰ in 2015; −38‰ in 2019) than in other years (avg. −53‰− −50‰ in 2016, 2017, 2018). Studies have suggested that El Niño activity has an effect on δ²H_p (Sun et al., 2018; Tian et al., 2021). This is because El Niño years tend to have a higher proportion of non-summer monsoon precipitation accounts (around 50% in Jiangxi province), leading to higher δ²H_p (Zhang et al., 2020; Tian et al., 2021). However, the percentage of monsoon precipitation (May to September) in the Dajiuhu peatland remained constant during 2015−2019 (60%−70%). This is likely due to the fact that the Dajiuhu peatland is located near the edge of persistent spring rain areas. Monitoring of δ²H_p near the Heshang cave, which is approximately 100 km from the Dajiuhu peatland, has suggested that the convection and precipitation in moisture source regions can be affected by ENSO (Wang et al., 2020). However, it should be noted that the limited time coverage of this study restricts the ability to compare yearly averaged δ²H_0−10cm to the ENSO activity on an annual timescale.

4.3 Implications for the application of δ²H_alk in peat deposits

Soil water is generally the main water source used by plants for leaf wax synthesis. Thus, the influence of evaporation on δ²H_pw may finally be recorded in the δ²H_alk (Hou et al., 2008; Yao and Liu, 2014; Herrmann et al., 2017). The spatial distributions of δ²H_pw in the Dajiuhu peatland suggest that the evaporative enrichment of δ²H_pw is mainly present in the surface peat. However, the effect of soil evaporation on the δ²H_alk of vascular plants is likely to be weak, as their root depths are usually deeper than 10 cm (Huang and Meyers, 2019). Furthermore, the leaf water δ²H of typical vascular plants was found to be close to the δ²H_pw below 50 cm, unaffected by evaporation (Huang and Meyers, 2019). As for Sphagnum, another dominant peat-forming plant in Dajiuhu and northern peatlands, although the exact mechanism is still unclear, it appears that soil evaporation does not significantly influence Sphagnum δ²H_alk.

In the Dajiuhu peatland, the δ²H_wax of peat-forming plants was investigated in two batches of samples, the first collected in September 2016 (Zhao et al., 2018) and the second in August 2017 (Huang and Meyers, 2019). Previous research has indicated that leaf wax synthesis in deciduous trees occurs during the initial period of the growing season (Kahmen et al., 2011; Tipple et al., 2013). This is further evidenced by the fact that, after the initial synthesis, the δ²H_wax remained constant despite any damage to the leaf waxes (Tipple et al., 2013). However, it has been observed that leaf wax can be synthesized throughout the growing season of grasses (Gao et al., 2012), as supported by the grass δ²H_wax data from Dajiuhu (Xue et al., 2022). Thus, it is valid to compare the δ²H_wax with δ²H_pw during the leaf wax synthesis period. The δ²H_wax results of Euphorbia esula and Sanguisorba officinalis suggest that evapotranspiration has an influence, as the δ²H_wax (~ −204‰) in 2017 was more negative than in 2016 (~ −170‰; Zhao et al., 2018; Huang and Meyers, 2019). The differences in δ²H_wax exceeded the differences in δ²H_pw between September 2016 and August 2017 by about 23‰ for δ²H_{0−10 cm} and 30‰ for δ²H_{20−30 cm} (Fig.3). This indicates that the variations in δ²H_pw could not explain the large variations in δ²H_wax. One possible reason is the influence of transpiration. The intense drought in September 2016 (Fig.2) may have caused strong evapotranspiration, leading to the enrichment of leaf water δ²H and more positive δ²H_wax. The high water table in the Dajiuhu peatland appears to limit the depth at which the isotope composition of pore water is affected by soil evaporation, and the δ²H_alk of both vascular plants and Sphagnum in peatlands may be less affected by soil evaporation. However, long-term drying and severe drought may still affect δ²H_alk through evapotranspiration in peat deposits (Huang et al., 2018; Wang et al., 2022).

The seasonal variation of δ²H_{0−10 cm} in Dajiuhu reflected the seasonal variation of δ²H_p, though this characteristic was weakened with increasing depth due to the mixing of precipitation from the rainy season. δ²H_pw may capture δ²H_p changes over a period of months or even years (Dai et al., 2022). Similarly, δ²H_alk can record changes over longer timescales when water is taken from deep peat. Nevertheless, this does not impede the interpretation of sedimentary δ²H_alk in peat deposits, since the temporal resolution is usually decades or even centuries for most studies.

5 Conclusions

To explore the spatiotemporal variations of δ²H_pw and the possible controlling factors, peat pore water samples were collected from the Dajiuhu peatland from 2015 to 2019. The main findings are as follows.

1) The amplitude and values of δ²H_pw decrease significantly from −47‰ to −56‰ with increasing depth, and remain relatively constant in the deeper layers with a variance ranging from 11‰ to 2‰. This depth pattern is likely due to the effect of evaporation in the upper layers (0−10, 20−30 cm) and the mixing signal of multiple precipitation events in deeper depths (> 50 cm), which is further highlighted by the deviation between δ²H_{0−10 cm} and the LMWL in the dual-isotope plot.

2) Precipitation isotope composition is the primary factor influencing the temporal variations of δ²H_{0−10 cm}. The seasonal variations of δ²H_{0−10 cm} are in agreement with the δ²H_p in the adjacent areas, as evidenced by a strong correlation (1-month preceding δ²H_p near the Heshang cave, r = 0.73, p < 0.01, n = 15; 1-month preceding δ²H_p near the Furong cave, r = 0.68, p < 0.001, n = 23).

3) The δ²H_{0−10 cm} was relatively high (averaging −43‰ in 2015 and −38‰ in 2019) during El Niño years, indicating that the interannual variations of δ²H_{0−10 cm} may be associated with ENSO activities; however, a more extended time series is necessary to verify this possibility.

Leaf wax δ²H primarily reflects regional precipitation isotope signals, along with the impact of soil evaporation in the Dajiuhu peatland. Therefore, it can be used to monitor hydroclimate changes in subtropical climates, as evidenced by the large positive excursions of leaf wax δ²H during the dry period in the mid-Holocene in central China (Huang et al., 2018) and the mid-to-late Holocene transition on the south-east coast of China (Wang et al., 2022).

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