Elsevier

Organic Geochemistry

Volume 102, December 2016, Pages 110-119
Organic Geochemistry

Using distributions and stable isotopes of n-alkanes to disentangle organic matter contributions to sediments of Laguna Potrok Aike, Argentina

https://doi.org/10.1016/j.orggeochem.2016.10.001Get rights and content

Highlights

  • Quantified relative contributions of n-alkane sources to lacustrine sediment.

  • Two model approaches were used based on n-alkane distributions and isotopes.

  • Both models provide similar estimates of major n-alkane sources in lake sediments.

  • Major sources in Laguna Potrok Aike, southern Patagonia are macrophytes and dust.

Abstract

When using biomarkers such as n-alkanes as tools for paleo-environmental reconstructions, it is imperative to determine their specific sources for each setting. Toward that goal, we analyzed a set of various potential organic matter (OM) sources such as aquatic and terrestrial plants, dust, and soils from Laguna Potrok Aike (LPA) and surrounding areas in southern Patagonia. We determined chain length distributions and hydrogen (δD) and carbon (δ13C) isotopic compositions of n-alkanes of different OM sources in order to quantify their relative contributions to lake sediments. Our results reveal that the mid-chain n-alkane, n-C23, is predominantly produced by submerged aquatic plants, whereas long-chain n-alkanes (n-C29n-C31) are derived from various terrestrial sources. We estimated their relative contributions to the sediment using two approaches, i.e., based on the n-alkane distributions and their δD and δ13C values. Both approaches result in similar estimates of aquatic and terrestrial contributions for mid- and long-chain n-alkanes to the sediment. 62–73% of the mid-chain n-C23 alkanes originate from aquatic sources while 66–77% of the long-chain n-alkanes originate from dust and 14–30% from terrestrial plants. Our study shows that mid-chain n-alkanes such as the n-C23 alkane in LPA are derived mainly from aquatic macrophytes and thus have the potential to record changes in lake-water isotopic composition. In contrast, the n-C29 alkane reflects the isotopic signal of various terrestrial sources from southern Patagonia.

Introduction

Southern Patagonia is a key location for paleo-environmental reconstruction since it is the southernmost ice-free landmass strongly influenced by the Southern Hemispheric Westerlies (SHW). Their position and intensity is thought to influence global deep-water circulation and thus other climate-controlling factors, such as exchange of CO2 with the atmosphere (Anderson et al., 2009, Mayr et al., 2013). To investigate past climatic changes in the southern mid-latitudes, the ICDP drilling campaign PASADO retrieved a 100 m long lacustrine sediment core from Laguna Potrok Aike (LPA) in 2008. The core spans the last ca. 51,000 years (Kliem et al., 2013) and enabled various multiproxy investigations to study past environmental and climate changes in southern Patagonia, such as pollen studies (Wille et al., 2007, Schäbitz et al., 2013), diatom studies (Massaferro et al., 2012, Recasens et al., 2015, Zimmermann et al., 2015), isotopic reconstructions (Mayr et al., 2009, Zhu et al., 2013, Zhu et al., 2014, Oehlerich et al., 2015) and others (Zolitschka et al., 2013). A previous study used bulk organic δ13C and δ15N to distinguish organic matter (OM) sources in and around LPA to identify the sources of sedimentary OM (Mayr et al., 2009). We followed a similar approach by using n-alkanes as specific tracers for OM sources.

The source of n-alkanes in sedimentary archives may be inferred from their distinct chain lengths. While mid-chain n-alkanes with 23 and 25 carbon atoms (n-C23, n-C25) are inferred to be predominantly derived from submerged aquatic plants (Ficken et al., 2000) and Sphagnum species (Baas et al., 2000, Bush and McInerney, 2013), long-chain n-alkanes with 27–31 carbon atoms (n-C27n-C31) are thought to be mainly produced by terrestrial higher plants (Eglinton and Hamilton, 1967). Some recent studies, however, have suggested that terrestrial higher plants also produce small amounts of mid-chain compounds (Gao et al., 2011, Liu et al., 2015), while aquatic plants might also contribute long-chain n-alkanes (Ficken et al., 2000, Aichner et al., 2010a, Liu et al., 2015).

The carbon stable isotope composition (δ13C) of n-alkanes reflects the different carbon sources, their isotopic compositions, and biosynthetic pathways of their producers. For instance, the availability of dissolved CO2 and HCO3 in lake ecosystems is reflected in the δ13C values of mid-chain n-alkanes from aquatic plants (Ficken et al., 2000), whereas δ13C values of the long-chain n-alkanes enables relative OM contributions by C3 and C4 plants to be distinguished in regions with both photosynthetic carbon fixation pathways (e.g., Magill et al., 2013). Vegetation in southern Patagonia, however, consists solely of C3 plants (Mayr et al., 2009, Pfadenhauer and Klötzli, 2015). Therefore, changes in δ13C ratios of long-chain n-alkanes likely reflect changes in water stress of C3 plants (Dawson et al., 2002). The hydrogen stable isotope (δD) composition of mid-chain n-alkanes from aquatic plants is presumed to reflect the lake water hydrogen isotopic composition (Mügler et al., 2009, Aichner et al., 2010a, Schmidt et al., 2014), whereas the δD values of long-chain n-alkanes serve as indicators for isotopic composition of precipitation and contain information about changes in humidity and evapotranspiration (Sachse et al., 2004, Schefuß et al., 2005, Mügler et al., 2008, Aichner et al., 2010b, Sachse et al., 2012).

For a reliable reconstruction of climate changes based on n-alkane isotope compositions, a well-constrained understanding of their sources in each setting is required. Therefore, we analyzed terrestrial plants, material from dust traps, topsoil samples from the LPA catchment and from an E–W transect, aquatic plant samples, and exposed and recent lake sediments from LPA, for their n-alkane distributions and compound-specific (δ13C and δD) isotopes. The goal of this research is to evaluate the reliability of the individual sedimentary n-alkanes as source-specific paleo-environmental indicators and for future investigation on the PASADO core. In addition, we compare two quantitative methods, which may be a useful approach to constrain the determination of the source of sedimentary n-alkanes and thereby also paleo-environmental reconstructions based on them.

Section snippets

Study site

Laguna Potrok Aike (51°58′S, 70°23′W; Fig. 1) is an almost circular maar lake located in the Pali Aike Volcanic Field in southern Patagonia. The 0.77 Ma old crater hosts a lake with a maximum diameter of 3470 m and a catchment area of about 200 km2 (Zolitschka et al., 2006, Zolitschka et al., 2013). In 2002, the lake level was recorded at 112 m above sea level and the maximum water depth was 100 m (Wille et al., 2007). Monitoring data of lake-level fluctuations show inter-annual differences of 1–2 m.

Sample collection

The plant samples and five topsoil samples were collected during several field campaigns in February and March 2002, 2003 and 2004 (Mayr et al., 2009). The plant samples analyzed in this study represent the predominant vegetation in the LPA catchment. Topsoil samples were collected from the uppermost cm of the soils. Additionally, 16 topsoil samples were taken every 20 km on an E–W transect along Route 40 from Rio Gallegos to Rio Turbio (February and March, 2011), (Fig. 1) (Schäbitz et al., 2013

n-Alkane distributions

The n-alkanes of all sample types show a dominance of odd over even numbered homologues (Supplementary Table 1). ACL values range between 24 and 30 (Supplementary Table 1). The lowest ACL values were observed for submerged aquatic macrophytes (ACL of 24) and highest values were found for topsoil and terrestrial plant samples (both ACL of 29–30).

The n-alkane distributions of terrestrial plants (belonging to Poaceae, Juncaceae and a dwarf-shrub) show a predominance of the n-C29 and n-C31 alkanes (

Identification of n-alkane sources

With regard to the n-alkane distribution of our samples (Fig. 2, Supplementary Table 1), the absence of Sphagnum mosses in the LPA catchment, and in accordance with other studies (Ficken et al., 2000, Mügler et al., 2008, Aichner et al., 2010a, Gao et al., 2011, Liu et al., 2015), we use the mid-chain n-alkane, n-C23, as the representative for aquatic derived n-alkanes in the LPA sediment. The long-chain n-C29 alkane, being the most abundant homologue in the sediment, represents the terrestrial

Summary and conclusions

This study presents distributions as well as carbon and hydrogen isotope compositions of n-alkanes from different organic matter sources in and around LPA and its surrounding area in southern Patagonia and evaluates their use for paleo-environmental reconstructions. The investigated samples consist of terrestrial and aquatic plants, dust traps, topsoil samples, sediment traps as well as exposed and recent lake sediments from LPA. The n-C23 and n-C29 alkanes are used as indicators for

Acknowledgements

We are grateful for research funding by the Deutsche Forschungsgemeinschaft (DFG) within the IODP/ICDP priority program (Sche903/12 and Mo1416/7). This research was also supported by the International Continental Scientific Drilling Program (ICDP) in the framework of the “Potrok Aike Maar Lake Sediment Archive Drilling Project” (PASADO). Funding for drilling was provided by the ICDP, the German Science Foundation (DFG), the Swiss National Funds (SNF), the Natural Sciences and Engineering

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