Elsevier

Organic Geochemistry

Volume 150, December 2020, 104121
Organic Geochemistry

Carbon isotope systematics of leaf wax n-alkanes in a temperate lacustrine depositional environment

https://doi.org/10.1016/j.orggeochem.2020.104121Get rights and content

Highlights

  • Lacustrine sediments incorporate mixed aquatic and terrestrial plant inputs.

  • Mixing influences n-alkane δ13C values differently for different homologues.

  • C27 and C29 reflect mixed sources in a temperate palaeo-lake.

  • n-Alkanes ≥ C31 show the lowest degree of mixing in a temperate palaeo-lake.

  • Very long-chain (≥ C31) n-alkanes more robustly reflect the photosynthetic pathway.

Abstract

The carbon isotope ratio (δ13C) of plant-derived organic carbon preserved in geological archives can be a valuable proxy for the relative abundance of terrestrial plants using C3 and C4 photosynthesis. In certain sedimentary archives, however, mixing of terrestrial C3- and aquatic macrophyte-sourced carbon will result in sedimentary organic matter (OM) δ13C signatures that could be misinterpreted as shifts in the abundance of C3 and C4 vegetation. There is potential for this problem to be mitigated using leaf wax n-alkane compound-specific δ13C measurements because n-alkane production differs between terrestrial vegetation and aquatic macrophytes. This approach requires an increased understanding of how mixing of terrestrial plant and aquatic macrophyte n-alkane inputs to lacustrine sedimentary archives manifests in the δ13C values of different n-alkane homologues in diverse environmental settings. This study examines a Pleistocene lacustrine sequence in southeastern Australia in which the inputs from terrestrial and aquatic macrophytes vary naturally through time, enabling the characterization of the mixing dynamics for different n-alkane homologues. Relative contributions of terrestrial vegetation and aquatic macrophytes were estimated using the relative abundance of mid-chain to long-chain n-alkanes and compared to the δ13C values of discrete n-alkane homologues. We find that δ13C values of mid- and some long-chain n-alkanes (C23–C29) are strongly impacted by mixing between C3 terrestrial- and non-emergent aquatic macrophyte-derived n-alkanes. In contrast, δ13C values of very long chain (C31–C35) n-alkanes integrated in sediments are the least affected by isotopic mixing. These results indicate that aquatic macrophyte inputs can significantly influence C29 isotopic signatures and thus the δ13C values of the very long chain n-alkanes ( C31) will provide the most robust quantification of n-alkane inputs from terrestrial plants and will be most useful for reconstructing the abundance C3 and C4 vegetation from temperate lake sediments.

Introduction

δ13C values of plant-derived carbon preserved in sedimentary archives hold great potential as a tool for reconstructing past vegetation dynamics from a range of sedimentary archives (see Castañeda and Schouten, 2011, Leng and Henderson, 2013, Diefendorf and Freimuth, 2017 and References therein). Mixing of inputs from terrestrial vegetation using different photosynthetic pathways will affect the δ13C signature of plant-derived carbon in sedimentary records as a result of C4 plants being enriched in 13C relative to C3 plants (Collister et al., 1994). However, another possible control on the δ13C signature of plant-derived carbon in sediments is mixing of inputs from C3 terrestrial plants and non-emergent aquatic macrophytes (hereafter aquatic macrophytes) (Aichner et al., 2010a, Liu et al., 2015, Hockun et al., 2016, Liu and Liu, 2016, Naafs et al., 2019). Aquatic macrophytes are generally enriched in 13C as a result of the complexity of the carbon assimilation pathway of these plants. Many aquatic macrophytes have evolved the ability to assimilate carbon from dissolved HCO3 in conjunction with assimilation of CO2 during photosynthesis, with HCO3 being more 13C enriched than CO2 by ∼7–11‰ (Keeley, 1990, Chappuis et al., 2017). In addition, assimilation of carbon from CO2 by aquatic macrophytes is likely to be affected by the greater diffusional resistance of an aquatic compared to an aerial environment, influencing the δ13C signature of sedimentary carbon derived from aquatic macrophytes (Keeley and Sandquist, 1992, Chappuis et al., 2017). As such, problems may arise in the interpretation of proportional changes in C3 and C4 photosynthetic pathway from the carbon isotope ratio of geological archives where there is a significant contribution of plant-derived carbon from aquatic macrophytes (Aichner et al., 2010a, Liu et al., 2015, Hockun et al., 2016, Liu and Liu, 2016).

There is potential for this problem to be overcome through measurement of δ13C values from cuticular leaf wax compounds preserved in sediments (Mead et al., 2005, Aichner et al., 2010a, Liu et al., 2015, Hockun et al., 2016). Among these organic compounds are highly recalcitrant and long-lived n-alkyl lipids including leaf wax n-alkanes (Eglinton and Logan, 1991; Diefendorf and Freimuth, 2017). Mid- to long-chain (C21–C35) leaf wax n-alkanes are biosynthesized by both terrestrial plants and aquatic macrophytes (Eglinton and Hamilton, 1967, Chikaraishi and Naraoka, 2003, Aichner et al., 2010a, Liu and Liu, 2016). There is, however, strong evidence to suggest variation in n-alkane distributions between these groups (Fig. 1). In general, terrestrial plants produce n-alkane distributions with long-chain (C29–C33) abundance maxima (Ficken et al., 2000, Chikaraishi and Naraoka, 2003, Bush and McInerney, 2013, Diefendorf and Freimuth, 2017). Conversely, aquatic macrophytes in lacustrine environments are found to produce n-alkane distributions with mid-chain (C21–C25) abundance maxima (Ficken et al., 2000, Aichner et al., 2010a, Gao et al., 2011, Liu and Liu, 2016) (Fig. 1). Still, there is some crossover in n-alkane production between terrestrial plants and aquatic macrophytes (Liu and Liu, 2016, Pu et al., 2018).

Total n-alkane production between terrestrial plants and aquatic macrophytes is found to be relatively similar on average, albeit with a larger range of values in terrestrial plants (Fig. 2). In general, inputs to sedimentary records should be minimally biased by n-alkane production between these plant groups. As a result, the relative abundance of mid-chain (i.e. C23 and C25) and long-chain (i.e. C29 and C31) n-alkanes, designated as proportion aquatic (Paq) (Ficken et al., 2000), is considered a robust means for quantifying the relative input of organic matter from terrestrial plant and aquatic macrophyte sources.

The distinct molecular distributions and δ13C values of aquatic macrophytes and C3 terrestrial plants should result in different n-alkane homologues displaying different sensitivity to isotopic mixing in sedimentary environments where their inputs are combined, such as in lake sediments. δ13C values of mid-chain (C23, C25) n-alkanes in a record of mixed inputs should predominantly reflect the aquatic macrophyte component. As such, mid-chain n-alkanes should be systematically enriched in 13C, even where the proportion of C3 terrestrial vegetation-derived n-alkane inputs to the sediment are high (e.g. with low Paq values; Fig. 3). Conversely, δ13C values of long-chain (C29–C35) n-alkanes should predominantly reflect C3 terrestrial vegetation inputs, with these homologues depleted in 13C even where aquatic macrophyte inputs and Paq are high (Fig. 3). δ13C values of the C27 n-alkane should display the strongest isotopic mixing (Fig. 3), given the similar production by both aquatic macrophytes and C3 terrestrial vegetation.

Studies of modern lakes demonstrate interesting deviations from the paradigm illustrated in Fig. 3. On the Tibetan Plateau, aquatic macrophytes dominated the δ13C signals of C23 and C25 (Aichner et al., 2010a) and a mixture of terrestrial plants and aquatic macrophytes contributed to the C27 δ13C signal, as expected (Aichner et al., 2010a, Liu et al., 2015). Counter to expectations, however, δ13C values of C29 were also influenced by aquatic macrophytes rather than being derived predominantly from terrestrial plants (Liu et al., 2015). These finding demonstrate the potential for both C27 and C29 δ13C records to overestimate the proportion of C4 vegetation because of the influence of 13C-enriched aquatic macrophytes. In these lakes, C31 more faithfully records the δ13C values of terrestrial vegetation (Liu et al., 2015). A different pattern is observed at Laguna Potrok Aike in the Patagonian Steppe of Argentina. There δ13C values of sedimentary n-alkanes indicated that C27, C29, C31 and C33 were all derived predominantly from C3 terrestrial vegetation, either directly or through dust transport, with no significant aquatic macrophyte inputs (Hockun et al, 2016).

The differences in the sources of C27–C33 homologues between these lake systems could be the result of different relative fluxes from terrestrial vegetation and aquatic macrophytes. The lakes on the Tibetan Plateau contain abundant submerged aquatic macrophytes and are surrounded by treeless alpine meadows and steppe, with very low net primary productivity (Aichner et al., 2010c, Liu et al., 2015). Thus, the greater sensitivity of C27 and C29 to submerged aquatic macrophytes could reflect their greater flux relative to the surrounding sparse terrestrial vegetation. Laguna Potrok Aike is surrounded by the treeless Patagonian Steppe, but it is also downwind and within 100 km of the Magellanic subpolar forests. The forests, with their higher net primary productivity, represents a large source of terrestrial plant n-alkanes, that can accumulate and be transported with dust. Thus, the negligible influence of aquatic macrophytes on δ13C values of C27–C33 could reflect a larger flux of terrestrial plant-derived alkanes relative to aquatic macrophyte-sourced n-alkanes at this lake (Hockun et al., 2016).

Thus far, studies on the sensitivity of n-alkane δ13C values to aquatic macrophyte and terrestrial plant inputs have been restricted to cold, high-altitude or high-latitude environments. Broader characterisation across a wider diversity of climatic and vegetation zones is needed to improve our understanding of lake carbon dynamics and facilitate palaeoenvironmental interpretations. This study explores the carbon isotope variations among n-alkane homologues from the Pleistocene lacustrine sedimentary record of Garvoc palaeo-lake in south-eastern Australia. This record enables us to examine the validity of the paradigm illustrated in Fig. 3 in a temperate lake that was set in what was likely predominantly an open woodland. Values of δ13C for discrete n-alkane homologues are quantified along with Paq estimates of varying contributions of terrestrial plants and aquatic macrophytes over time in order to elucidate the degrees of carbon isotopic mixing for different n-alkane chain lengths in this lake system.

Section snippets

Site information

Garvoc palaeo-lake (38°17ʹS, 142°47ʹE) is an extinct volcanic maar crater, 2.5–3.0 km in diameter, incised into the surrounding volcanic plain, and lacking fluvial inputs. The surface of the infilled lake is flat, ca. 95 m asl, and is surrounded by a rim ca. 110 m asl. The maar lake has infilled with organic rich (> 20% total organic carbon) clays, sands and tuff beds (Kershaw, 1997, Kershaw et al., 2014). Garvoc (in some sources referred to as ‘Yaloak Swamp’; Boyce 2013) is one of hundreds of

n-Alkane quantification

n-Alkanes are abundant throughout the record, with the summed concentration of mid-chain homologues (C23–C25) ranging from 0.098 to 9.31 μg/g dry sediment and that of long-chain n-alkane homologues (C27–C35) ranging from 0.54 μg/g to 41.9 μg/g dry sediment. Values of CPI are markedly high, ranging from 12.3 to 34.7, indicating a high odd-over-even predominance of long-chain n-alkanes throughout the record. These values of CPI indicate a predominantly higher plant source of n-alkanes in this

Assessment of C3 vs C4 vegetation

This study seeks to investigate the impact of mixing of C3 terrestrial vegetation and aquatic macrophytes on the δ13C values of different n-alkanes in sediments of Garvoc palaeo-lake. The aim is to evaluate the paradigm illustrated in Fig. 3, in which the terrestrial vegetation surrounding a lake is composed of C3 plants without any significant C4 cover. The predominance of C3 vegetation around Garvoc palaeo-lake can be established by evaluating whether δ13C values of the longer chain length n

Conclusions

δ13C values of most mid- and some long-chain n-alkanes (C23–C29) in the lacustrine sediments of our temperate lake sediment record indicate significant degrees of isotopic mixing between C3 terrestrial plant- and aquatic macrophyte-derived n-alkanes. However, δ13C values of mid-chain n-alkanes in lacustrine systems could also reflect variability in DIC pool δ13C values. δ13C values of the longest chain (≥C31) n-alkanes biosynthesized by plants and integrated in sediments will be least affected

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

We would like to thank Bernhard Aichner, an anonymous reviewer and the editors for their insightful and constructive feedback. We thank Peter Kershaw and Barbara Wagstaff for access to the cores and supporting information. Thank you to Andrew L. Masterson (Northwestern University) for leaf wax n-alkane compound-specific carbon isotope analysis. Funding for this research was provided by a Royal Society of South Australia Small Research Grant awarded to J.W.A., F.A.M. and J.M.K.S., an Australian

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      The relationship of δ13C Equil-CO2 to log PCO2 (Fig. 8) shows that most of the river water samples of our study lie midway between the compositions of atmospheric CO2 and groundwater sample UW27, which was influenced by C3-OM; our results are therefore consistent with the above-referenced studies, which showed that DIC in river water contains both atmospheric CO2 and carbon derived from OM. On the other hand, a lot of previous studies have used carbon isotopic compositions of OM in sedimentary archives as indicators in evaluations of both modern and paleo-environments (Castañeda and Schouten, 2011; Leng and Henderson, 2013; Diefendorf and Freimuth, 2017; Andrae et al., 2020). According to these works, the OM in terrestrial sediments (such as in lacustrine deposits) is generally enriched in 13C, likely because most aquatic macrophytes have higher δ13C values (–30‰ to –12‰) than C3 plants.

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