n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa
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 #
References (57)
- et al.
A comparative study of lipids in Sphagnum species
Organic Geochemistry
(2000) - et al.
Distribution of aliphatic ketones in Chinese soils: potential environmental implications
Organic Geochemistry
(2006) - et al.
Geochemical records of palaeoenvironmental controls on peat forming processes in the Mfabeni peatland, Kwazulu Natal, South Africa since the Late Pleistocene
Palaeogeography, Palaeoclimatology, Palaeoecology
(2014) - et al.
Biomarker records of palaeoenvironmental variations in subtropical southern Africa since the late Pleistocene: evidence from a coastal peatland
Palaeogeography, Palaeoclimatology, Palaeoecology
(2016) - et al.
Climate variability in Mfabeni peatlands (South Africa) since the Late Pleistocene
Quaternary Science Reviews
(2017) - et al.
Soil chronosequence development in dunes on the southeast African coastal plain, Maputaland, South Africa
Quaternary International
(2007) - et al.
Organic geochemical studies of soils from the Rothamsted classical experiments—V. The fate of lipids in different long-term experiments
Organic Geochemistry
(2000) Monocarboxylic acids in lake sediments: indicators, derived from terrestrial and aquatic biota, of paleoenvironmental trophic levels
Chemical Geology
(1974)- et al.
Lipids of aquatic organisms as potential contributors to lacustrine sediments-II
Organic Geochemistry
(1987) - et al.
An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes
Organic Geochemistry
(2000)
A late Quaternary pollen sequence from Mfabeni Peatland, South Africa: reconstructing forest history in Maputaland
Quaternary Research
Development and persistence of an African mire: how the oldest South African fen has survived in a marginal climate
Catena
Origin and transport of n-alkan- 2-ones in a subtropical estuary: potential biomarkers for seagrass-derived organic matter
Organic Geochemistry
Post-glacial variations in distributions, 13C and 14C contents of aliphatic hydrocarbons and bulk organic matter in three types of British acid upland soils
Organic Geochemistry
Origin and transport of sedimentary organic matter in two subtropical estuaries: a comparative, biomarker-based study
Organic Geochemistry
Occurrence of long-chain acyclic methyl ketones in Sphagnum and Carex peats of various degrees of humification
Organic Geochemistry
Solvent-extractable lipids of Sphagnum, Carex, Bryales and Carex-Bryales peats: content and compositional features vs. peat humification
Organic Geochemistry
Different source of n-alkanes and n-alkan-2-ones in a 6000cal. yr BP Sphagnum-rich temperate peat bog (Roñanzas, N Spain)
Organic Geochemistry
Sediment and soil organic matter source assessment as revealed by the molecular distribution and carbon isotopic composition of n-alkanes
Organic Geochemistry
Lacustrine organic geochemistry: an overview of indicators of organic matter sources and diagenesis in lake sediments
Organic Geochemistry
Solvent extractable lipids in an acid andic forest soil; variations with depth and season
Soil Biology and Biochemistry
C23–C31 n-alkan-2-ones are biomarkers for the genus Sphagnum in freshwater peatlands
Organic Geochemistry
N-Alkan-2-ones in peat-forming plants from the Roñanzas ombrotrophic bog (Asturias, northern Spain)
Organic Geochemistry
Late Quaternary sea-level change in South Africa
Quaternary Research
The luminescence chronology of dune development on the Maputaland coastal plain, southeast Africa
Quaternary Science Reviews
Geochemistry of fine-grained sediments and sedimentary rocks
Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic
Geochimica et Cosmochimica Acta
Past and future reorganizations in the climate system
Quaternary Science Reviews
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2022, International Journal of Coal GeologyThe distribution of long-chain n-alkan-2-ones in peat can be used to infer past changes in pH
2020, Chemical GeologyCitation Excerpt :Our findings are consistent with previous studies which also observed that the concentration of n-alkan-2-ones in vascular plants is lower than that of Sphagnum spp. (Ortiz et al., 2011), and that many vascular peat-forming plants do not synthesize these lipids (Nichols and Huang, 2007). As discussed above, the large δ13C difference (>5‰) between KET27 in mosses and surface peat in the DJH peatland and the much higher concentration of n-alkan-2-ones in surface peat samples compared to Sphagnum moss suggest the microbial oxidation of n-alkyl lipids (e.g. Amblès et al., 1993; van Bergen et al., 1998; Ortiz et al., 2011; Zheng et al., 2011a; Baker et al., 2018) is also an important source of n-alkan-2-ones (pathways 1 and 2 in Fig. 5). This is further supported by the observation that in the REG peatland there are no living Sphagnum mosses, yet we find significant concentrations of n-alkan-2-ones in surface samples from this peatland.
Implications for peat formation in Maritsa-West Basin, SE Bulgaria: Insights from organic petrology, palynology and biomarker assemblage
2020, International Journal of Coal GeologyCitation Excerpt :In that case, the low amounts of n-ketones together with the low amounts of hopanes (Table 3) in the studied samples might reflect limited aerobic bacterial degradation of the organic matter. Furthermore, Baker et al. (2018) point out that the amount of ketones might be linked to the hydrological regime within the peat-forming environment. Since the extensive bacterial degradation of the organic matter (i.e. increase in n-alkanones concentrations) requires prolonged conversion of the precursor n-alkanes, n-alkanols and/or n-fatty acids under aerobic conditions, the low amounts of n-ketones in Maritsa-West lignite might also reflect rapid burial of the organic remains under water-logged, predominantly anoxic conditions.
Postglacial floral and climate changes in southeastern China recorded by distributions of n-alkan-2-ones in the Dahu sediment-peat sequence
2020, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :Long-chain ketones have been found to be ubiquitous in aquatic and terrestrial environments, including marine and lacustrine sediments (Cranwell, 1981; Volkman et al., 1983; Rieley et al., 1991; Wang et al., 2012; Rostek and Bard, 2013), soils (van Bergen et al., 1998; Bai et al., 2006), loess and paleosols (Xie et al., 2003b, 2004b; Zeng et al., 2011), and stalagmites (Xie et al., 2003a). Plenty of studies have also reported the occurrence of n-alkan-2-ones in peat deposits (Lehtonen and Ketola, 1990; Xie et al., 2004a; Nichols and Huang, 2007; Ortiz et al., 2011; Zheng et al., 2011a, 2011b; López-Días et al., 2013; Baker et al., 2018). In these peat deposits, n-alkan-2-ones almost extend over the carbon number range of C17–C35, with the distribution possessing a high odd-over-even carbon number predominance, and the most abundant homologues being C27, C29 or C31 (e.g. Xie et al., 2004a; Nichols and Huang, 2007; Zheng et al., 2011a).
Late Quaternary palaeoenvironmental change in the year-round rainfall zone of South Africa derived from peat sediments from Vankervelsvlei
2019, Quaternary Science ReviewsCitation Excerpt :These studies often complement and refine investigations of other archives, e.g., tree rings or lake sediments. However, palaeoenvironmental investigations of peatlands from South Africa are rather scarce and mainly cover the Mid to Late Holocene (Meadows, 1988), except for the Mfabeni peatland on the east coast (Baker et al., 2014, 2018; Finch and Hill, 2008; Miller et al., 2019). The past and present climate of southern Africa is known to be driven by variable interactions of oceanic, (the Benguela current on the west coast and the Agulhas current along the east and south coasts) and atmospheric circulation systems (Tyson and Preston-Whyte, 2000) (Fig. 1 A).