Comparison of molecular distributions and carbon and hydrogen isotope compositions of n-alkanes from aquatic plants in shallow freshwater lakes along the middle and lower reaches of the Yangtze River, China
Introduction
n-Alkanes are important components of plant waxes. They tend to be well preserved in diverse geological archives, including lacustrine and marine sediments, peat deposits, and paleosol-loess sequences (e.g., Meyers, 2003, Eglinton and Eglinton, 2008, Naafs et al., 2019). Their molecular distributions and carbon and hydrogen isotope compositions have become important tools for paleoclimate and paleoecology reconstructions in the Quaternary epoch (Castañeda and Schouten, 2011, Sachse et al., 2012, Diefendorf and Freimuth, 2017). In lacustrine environments, many studies have explored the paleoenvironmental significance of leaf wax n-alkanes (e.g., Huang et al., 2004, Sachse et al., 2004, Mügler et al., 2008, Aichner et al., 2010a, Garcin et al., 2012, Rao et al., 2014, Liu et al., 2015). In such settings, sources of n-alkanes include not only autochthonous contributions of aquatic plants, bacteria, and algae, but also allochthonous contributions of terrestrial plants brought in by incoming runoff (Meyers, 2003, Castañeda and Schouten, 2011). Due to diverse sources of n-alkanes in lake sediments, the environmental significance of n-alkanes is not well understood.
Previous studies have revealed distinctive patterns of n-alkanes from different sources. Short-chain n-alkanes (<C21) mainly derive from bacteria and algae (Cranwell et al., 1987, Meyers, 2003), mid-chain n-alkanes (C21-C25) are mainly produced by floating and submersed plants (plants growing or adapted to grow underwater), and long-chain n-alkanes (C27-C33) are usually interpreted to be terrestrial in origin (Ficken et al., 2000, Aichner et al., 2010a). The contributions of aquatic plants (submersed versus floating) relative to emersed and terrestrial plants are captured by the Paq ratio [Paq =(C23 + C25)/(C23 + C25 + C29 + C31)] (Ficken et al., 2000). This proxy has then been widely utilized for paleoenvironmental reconstructions based on lacustrine sequences (e.g., Das et al., 2009, Aichner et al., 2010a; Oritz et al., 2013; Sun et al., 2018).
Recently, some studies have argued that submersed plants can also contribute a moderate proportion of long-chain n-alkanes, which would complicate the application of the Paq ratio in lacustrine sequences (Aichner et al., 2010a, Liu and Liu, 2016, Liu et al., 2016). In addition, some terrestrial plants contain significant amounts of mid-chain n-alkanes, adding complexity to the application of the Paq ratio (e.g., Ladd et al., 2018, Berke et al., 2019, Dion-Kirschner et al., 2020, He et al., 2020). Furthermore, due to the similar n-alkane compositions of floating and submersed plants, the Paq ratio does not have the ability to differentiate contributions from these two types of aquatic plants. In many shallow lakes, submersed plants have important ecological functions that are distinct from those of floating and emersed plants. Submersed plants are often major contributors to primary productivity and have important influences on water quality and other biogeochemical processes in lakes (e.g., Carpenter, 1981, Bayley and Prather, 2003). Thus, it would be valuable to develop proxies to track the dynamics of different components of aquatic plants in shallow lakes.
To distinguish the sources of n-alkanes in lake sediments, the carbon isotope compositions of n-alkanes (δ13Calk) have been investigated in previous studies (e.g., Meyers, 2003, Castañeda and Schouten, 2011; Holtvoeth et al., 2019). Some of these studies commonly observed higher δ13Calk values in submersed plants than in terrestrial plants (e.g., Chikaraishi and Naraoka, 2003, Aichner et al., 2010a, Liu et al., 2015, Liu et al., 2018). These studies indicate that δ13Calk values could act as an effective proxy to distinguish the sources of n-alkanes in lake sediments. Combining the Paq and δ13Calk values has the potential to improve the source interpretation of n-alkanes in lacustrine sediments; however, this potential has not yet been tested in shallow lake settings.
In addition to the molecular distributions and carbon isotope compositions, the hydrogen isotope compositions of n-alkanes (δ2Halk) have been widely applied to hydroclimate reconstructions in lacustrine sequences (e.g., Castañeda and Schouten, 2011, Sachse et al., 2012). The δ2Halk signals of individual compounds preserved in lake sediments could yield information about the evolution of the isotope composition of precipitation and its associated paleoclimate changes (e.g., Huang et al., 2004, Sachse et al., 2004). In addition, the δ2Halk differences between terrestrial and aquatic plants (εterr-aq) have been proposed as an effective indicator of paleohumidity (e.g., Mügler et al., 2008, Rach et al., 2014, Rach et al., 2017, Arnold et al., 2018). Interpretation of εterr-aq is based on the phenomenon that the isotope composition of the source water of aquatic plants is relatively unaffected by evaporation, whereas the isotope composition of the source water used for wax production in terrestrial leaves is sensitive to evapotranspiration (Mügler et al., 2008, Rach et al., 2014). However, downcore results from lake sediments from the Tibetan Plateau did not reveal a direct relation between εterr-aq and paleohumidity (Rao et al., 2014). In addition, the efficacy of this proxy might be limited by the source complexity of n-alkanes in lake sediments (Rao et al., 2014). To date, knowledge of δ2Halk variations of n-alkanes > C21 is limited for contemporary aquatic plants, particularly in regions with a humid monsoon climate (Chikaraishi and Naraoka, 2003, Aichner et al., 2010b, Liu et al., 2019).
There are many shallow lakes in the middle and lower reaches of the Yangtze River (MLYR), and aquatic plants flourish in many of them (Wang and Dou, 1998, Fang et al., 2006), providing an excellent setting to study the differences of n-alkane distributions and compound-specific isotope values in different types of aquatic plants. In this study, we collected aquatic plants from five representative lakes in the MLYR to establish a more effective method to evaluate the autochthonous sources of plant waxes in lake sediments and to interpret any variations of n-alkane δ2H values in the aquatic plants from these shallow lakes.
Section snippets
Study area and sampling
The MLYR region in eastern China has a humid subtropical monsoon climate. In this region, there are 108 lakes with surface areas>10 km2, including the three largest freshwater lakes in China, Lake Poyang, Lake Dongting and Lake Taihu (Wang and Dou, 1998). The warm and humid climate of the early- and mid-Holocene increased the water volume of the Yangtze River, which combined with the postglacial rise in sea-level, led to the development of lakes in the MLYR (Fang, 1991, Xu et al., 2019).
Concentrations and distributions of n-alkanes
In the aquatic plant samples in this study, n-alkane chain lengths range from C21 to C33 (Table 2, Fig. 2). The total n-alkane (C21-C33) concentrations vary among aquatic plants, with higher values in M. spicatum (range from 353.6 to 751.9 μg/g dry weight), Trapa sp. (range from 135.9 to 354.9 μg/g dry weight), and A. philoxeroides (361.9 μg/g dry weight) (Table 2), which are submersed, floating, and emersed plants, respectively. In all submersed and floating plant samples, the concentrations
Distinguishing submersed and floating aquatic plants by combining Paq values and δ13Calk values
For investigation of lake organic matter sources, the Paq ratio has been an effective indicator to distinguish contributions from submersed/floating aquatic macrophytes relative to subaerial emersed and terrestrial plant inputs (Ficken et al., 2000). As initially proposed by Ficken et al. (2000) and found in later studies, the Paq index can be used to distinguish emersed plant sources from the submersed/floating plant sources in shallow lakes in MLYR. However, Paq values alone cannot
Summary and conclusions
Aquatic plant samples were collected from five shallow lakes in the MLYR to analyze the molecular distributions and carbon and hydrogen isotopic compositions of their n-alkanes to differentiate the n-alkane sources among different plant types in shallow freshwater lakes and to improve interpretation of their δ2Halk values. The main findings are:
1. In the MLYR, the Paq ratio could distinguish the n-alkane contributions of submersed and floating plants from emersed plants. In addition, n-alkane δ
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (U20A2094, 41877317). Jiantao Xue and Meiling Zhao are thanked for helping in the sample collection and lipid analyses. The Associate Editor, Dr. Isla S. Castañeda, and the three anonymous reviewers are thanked for their constructive comments to improve the quality of this manuscript.
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