Elsevier

Organic Geochemistry

Volume 120, June 2018, Pages 75-85
Organic Geochemistry

n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa

https://doi.org/10.1016/j.orggeochem.2018.03.001Get rights and content

Highlights

  • Mfabeni peatland ideally suited for n-alkan-2-one (n-ket) palaeoproxy testing.

  • n-Ket source via decarboxylation of n + 1-alkanoic acids and oxidation of n-alkanes.

  • Mid- and long chain n-ket/precursor ratios display basin water level controls.

  • The n-kets show promise as a palaeoclimate proxy and for reconstructing hydrology.

Abstract

The sub-tropical Mfabeni fen is the only continuous coastal peat deposit that documents glacial and interglacial palaeoenvironmental conditions since the late Pleistocene (ca. 47 cal kyr BP) in southern Africa. Published bulk geochemical, biomarker and leaf wax δ13C data, along with palynology and stratigraphic studies of the Mfabeni peat sequence, render it an ideal record for testing new palaeoreconstruction proxies. In this study, we aimed to establish the proxy potential of n-alkan-2-one (n-ket) compounds by tracing their source/origin and post-depositional diagenetic change, and if they preserve or not a robust palaeoenvironment signal that complements our understanding of palaeoclimatic variations. In the Mfabeni archive the most likely source for n-kets is via microbial decarboxylation of n + 1-alkanoic acids (n-FAs) and, to a lesser degree, oxidation of same chain length n-alkanes (n-alks). The n-ket average chain length (ACLket) and n-C23 and C25ket/precursor ratios displayed a statistical significant negative relationship with the n-alk aquatic plant proxy (Paq), suggesting the source of n-kets to be submerged aquatic plants during waterlogged conditions that suppressed microbial activity during the ensuing anoxic conditions. Both the mid-chain and long chain n-ket/precursor ratios displayed predominant water level fluctuation controls, with temperature as a secondary regulator. By comparing the n-ket data with published environmental and climate reconstructions from the same core, and with geomorphology and palynological studies of the Mfabeni basin, we conclude that the n-kets show promise as a palaeoclimate proxy and can be used in conjunction with other biomarker proxies to reconstruct ancient hydrological changes in sub-tropical peatlands.

Introduction

Palaeoclimate proxies have inherent limitations due to difficulties in quantifying geochemical relationships in modern systems and distinguishing between preservation and diagenetic changes after deposition (Sageman and Lyons, 2003). Therefore, it is useful to use a multi-proxy approach to reduce uncertainty in palaeoclimate reconstruction by providing improved prognostic data. Consequently, geochemists continually strive to add new and improved proxies to address these inadequacies and to advance understanding about past climate and environmental change archived in sedimentary records.

n-Alkan-2-ones (n-kets) are a relatively novel group of biomarkers compared with the other more commonly studied n-alkyl lipids, namely n-alkanes (n-alks), n-alkanoic acids (n-FAs) and n-alkanols (n-alcs, e.g. Xie et al., 2008). Even though studies have documented n-kets in soils (Huang et al., 1996, van Bergen et al., 1998, Bull et al., 2000, Naafs et al., 2004, Bai et al., 2006), lacustrine sediments (Cranwell et al., 1987, Meyers and Ishiwatari, 1993, Wenchuan et al., 1999), peat basins (Lehtonen and Ketola, 1990, Lehtonen and Ketola, 1993) and contemporary plants (Wenchuan et al., 1999, Baas et al., 2000, Hernandez et al., 2001, Nichols and Huang, 2007, Ortiz et al., 2011), their study is frequently part of a general characterisation of a wider suite of biomarkers. There has been little assessment of their effectiveness as a palaeoenvironmental proxy. The only studies dedicated to determining the proxy potential of n-kets are from the Hani mire, northeastern China (Zheng et al., 2011a) and the Hongyuan peatland on the eastern fringes of the Tibetan Plateau (Zheng et al., 2011b), where n-ket data were compared with regional geochemical and palynological studies.

Although some peat forming plants and phytoplankton are reported to contain low concentrations of n-kets (Lehtonen and Ketola, 1990, Hernandez et al., 2001, Nichols and Huang, 2007, Ortiz et al., 2011), other studies have reported that their principal origin in sedimentary environments is either via microbial oxidation of the same chain length n-alks and/or decarboxylation of the n + 1-FAs (Volkman et al., 1983, Bai et al., 2006, Ortiz et al., 2010, Ortiz et al., 2011). Furthermore, López-Días et al. (2013) reported that the distribution patterns of n-alks and n-kets in the Roñanzas peat bog did not justify a single source, and therefore suggested an additional secondary source of n-kets derived from bacterial input. Regardless, the predominant sources of these biomarkers appear to be governed by enzymatic microbial activity, which is usually linked to temperature (Schmidt et al., 2011).

Peat accumulates when net primary production (NPP) outstrips organic matter (OM) degradation (Chimner and Ewel, 2005). The relatively high ambient temperatures experienced in sub-tropical peatlands are therefore expected to produce an enhanced rate of OM decay driven by microbial processes. However, anoxic conditions generated by extended waterlogging events retard OM decomposition and result in peat accumulating in sub-tropical regions (Rieley et al., 1996). Consequently, physical peat forming indices, such as TOC concentration and accumulation rate (Baker et al., 2014) are useful for measuring the proportion of OM stored as peat and for providing insight into the palaeoenvironmental conditions prevalent at the time of sedimentation, which are ultimately controlled by climate (Baker et al., 2014, Baker et al., 2016, Baker et al., 2017).

The Mfabeni fen on the southeastern coastline of Africa (Fig. 1), is an exceptional and continuous record of peat accumulation spanning the last ca. 47 cal kyr BP. Such rich sedimentary archives are rare in southern Africa due to the overall dry climate and steep topography of the region. Palynological (Finch and Hill, 2008) and geomorphological (Grundling et al., 2013) studies of the fen have been undertaken, and high resolution bulk geochemical (Baker et al., 2014), biomarker (Baker et al., 2016) and leaf wax 13C isotopic (Baker et al., 2017) investigations have been carried out on the same core (SL6), making the Mfabeni peat sequence an ideal archive for assessing the climate proxy value of the n-kets during the transition from glacial to postglacial conditions. Because the fen is at a relatively low latitude, Finch and Hill (2008) and Baker et al. (2016) found that the basin experienced subdued temperature fluctuation relative to higher latitudes since the Late Pleistocene. The biomarker (Baker et al., 2016) and leaf wax δ13C proxies (Baker et al., 2017) implied that plant OM sources in the Mfabeni were predominantly influenced by water level fluctuation, as opposed to temperature change in the basin.

Ficken et al. (2000) developed the n-alk aquatic plant proxy (Paq) on the premise that aquatic plants contributed predominantly mid-chain length (C23 and C25) n-alks, while emersed and terrestrial plant leaf waxes are dominated by long chain (C27–C33) homologues. Some of these compounds have been proposed as precursors for n-kets and could therefore help delineate the n-ket sources and ultimately the palaeoenvironmental conditions at times of peat deposition. Additionally, microbial respiration, the mechanism by which precursors may be converted to n-kets, can be influenced by several factors besides temperature, including OM chemistry and reactivity, soil pH, redox conditions and accessibility to potential decomposers (Schmidt et al., 2011). Considering that the Mfabeni basin geomorphology resulted in hydrology being the dominant control on plant-derived OM sources and rate of microbial alteration (Grundling et al., 2013, Baker et al., 2016, Baker et al., 2017), the bimodal distribution of n-ket homologues, the concentration correlations between n-alks, n-FAs and n-kets, and the ratios of the respective n-kets/n-ket precursors could help in delineating changes in past water levels which, according to Baker et al., 2016, Baker et al., 2017, can be directly linked to regional precipitation and to a lesser degree ambient air temperature.

In this study, we have explored the relationships between n-kets and published bulk geochemical and biomarker proxies from Mfabeni core SL6 to assess the potential of n-kets in reconstructing palaeoenvironmental conditions. The assessment involved determining their origin, post-depositional alteration, and corroborating the n-kets data with pollen and stratigraphic records from the Mfabeni fen to establish whether or not a climate signal is preserved in these novel compounds.

Section snippets

Site description

The shallow 350 km2 St Lucia Lake dominates the UNESCO World Heritage iSimangaliso Wetland Park on the northern shores of Kwazulu-Natal province, South Africa (Fig. 1). On the eastern shores of the lake, the Mfabeni fen lies within an interdunal valley (Botha and Porat, 2007) at ca. 11 m above sea level (a.s.l.; Finch and Hill, 2008); it measures 10 x 3 km (Clulow et al., 2012, Grundling et al., 2013) and has up to 10.8 m thick of sediment record that accumulated along the M8 transect (Fig. 1,

Methodology

A modified lipid extraction was carried out according to Wakeham et al. (2002) on 36 selected core intervals that were analysed for n-alks, n-FAs and n-alcs (see Baker et al., 2016 and references therein) and n-kets. Freeze-dried samples (ca. 2 g) were extracted in a Dionex automated solvent extractor with dichloromethane (DCM)/MeOH (9:1 v/v). An aliquot of the total lipid extract (TLE) was saponified with 0.5 N KOH (in MeOH) at 100 ⁰C for 2 h; then 5% NaCl was added and the mixture agitated

Interpreting biomarker trends

While Nichols and Huang (2007) found evidence of n-kets only in modern peatland Sphagnum species sampled across the Midwest and New York state in north America, Ortiz et al. (2011) recorded low concentrations in both Sphagnum (C23 max) and other peat forming plants in the Roñanzas bog. They documented high molecular weight n-ket homologues in terrestrial plants, more specifically sedge and grass species with predominant C27 and C31 n-alks, C29 and C31 n-kets and C22 and C24 n-FAs, respectively.

Conclusions

Our study shows that n-ket biomarker compounds have potential as a palaeoenvironmental proxy for interpreting past climatic conditions. Their predominant origin was established to be via microbial metabolic alteration of primary n-alk and n-FA compounds derived from higher plants. Because the Mfabeni fen falls within a sub-tropical climate, peat accumulation is predominantly controlled by waterlogging events, with temperature playing a secondary role. Consequently, we conclude that the n-ket

Acknowledgments

A. Clulow assisted with field access and site identification. iSimangaliso Authority and Ezemvelo KZN Wildlife granted park access and sampling permits. We thank the detailed reviews and comments from J.E. Nichols and an anonymous reviewer that significantly improved the manuscript. The project was supported through a bilateral funding agreement by the Swedish Research Link-South Africa program (Grant # 348-2009-6500). Student support was provided by the National Research Foundation (Grant #

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