Vegetation and climate reconstruction in the forest–steppe of Mongolia is
still challenging regarding the pattern of forest and grassland distribution
during the Holocene. Different sediments containing paleosols and humic
layers provide geomorphological archives for landscape development in
Mongolia. n-Alkane and macro-charcoal ratios represent specific indicators to
distinguish the share between grasses and trees. In a preliminary study, we
investigated the applicability of these two paleo-proxies from soils for
vegetation reconstruction comparing different relief positions and site
conditions in the northern Khangai Mountains of Mongolia.
n-Alkanes that are deposited from leaf waxes in the soil have the potential to indicate vegetation composition on a local scale. Depending on
site-specific environmental conditions, n-alkanes are subjected to different
degrees of microbiological decomposition, which is more intensive in soils
of dry steppe than of forests. Mongolian forests are often underlain by
permafrost that may reduce microbiological activity. In steppe soils, the
decomposition of n-alkanes increases the quantity of mid-chain n-alkanes that adulterate the biomarker proxy signal to indicate more forest share. Macro-charcoals in soils have a site-specific component, but additional eolian input of macro-charcoals from long-distance transport can provide a distinct proportion in
sediments. Thus, eolian influx of wood-derived macro-charcoal can dominate
the proxy signal at sites where trees were few or had never existed.
Radiometric dating of several paleosols and humic layers has shown that
both proxies coincide as evidence for high grassland-to-forest ratios during the Early Holocene. By contrast, the proxy signals diverge for the Late Holocene.
For this period, n-alkanes generally indicate more grassland, whereas
macro-charcoals show increased wood-derived proportions. We imply that this
difference is caused by increased forest fires and simultaneously spreading
steppe area.
A main portion of leaf waxes and charcoal particles in soils directly derive from the covering and nearby vegetation, whereas large lakes and glacier may receive these biomarkers from a larger catchment area. Thus, we conclude that soil archives provide proxies on a more local and site-specific scale than other archives do. Although the temporal resolution of soil archives is
lower than for the other ones, biomarker proxies for paleosols and humic
layer can be related to periods of distinct geomorphological processes.
Further investigations comparing the multi-proxy data of different
geomorphological archives are necessary to improve the paleo-ecological
reconstruction for landscape development in Mongolia.
Kurzfassung
Die Rekonstruktion von Klima und Vegetation in der mongolischen Waldsteppe
stellt nach wie vor eine große Herausforderung in Hinsicht auf die
Verbreitungsmuster von Wald- und Grasslandschaften im Laufe des Holozäns
dar. Verschiedenartige Sedimente mit Paläoböden und organischen
Schichten können dafür als hervorragende Archive für die Analyse
der Landschaftsentwicklung in der Mongolei dienen. Die proportionalen
Verhältnisse verschiedener n-Alkane und Typen von Holzkohlepartikeln
lassen sich als Indikatoren zur Bestimmung der Grass und Baumanteile
heranziehen. Im Rahmen einer Vorstudie haben wir die Anwendbarkeit dieser
zwei Paläoproxies für die Vegetationsrekonstruktion aus
Bodenarchiven untersucht. Dafür wurden Profile mit unterschiedlichen
Reliefpositionen und Standortbedingungen aus einem Untersuchungsgebiet im Norden des Khangai-Gebirges in der Mongolei verglichen.
n-Alkane von der Wachsschicht der Blätteroberflächen gelangen in den Boden und bieten die Möglichkeit die Zusammensetzung der Vegetation auf lokaler Ebene zu analysieren. Abhängig von den standortspezifischen
Umweltbedingungen werden n-Alkane mikrobiell unterschiedlich stark abgebaut,
wobei der Abbau in Böden der Trockensteppen intensiver ist als in den
Wäldern. In den Wäldern der Mongolei ist häufig Permafrost im
Boden vorhanden, der die mikrobiologische Aktivität verringert. In den Steppenböden erhöht sich durch die Zersetzung von n-Alkanen den
Anteil an mittelkettigen n-Alkanen, was das Biomarker-Proxysignal in Richtung
eines höheren Waldanteils verfälscht. Holzkohlepartikel in Böden
haben eine standortspezifische Komponente. Allerdings kann ein
zusätzlicher äolischer Eintrag von Holzkohlepartikeln aus dem
Ferntransport einen deutlichen Anteil in den Sedimenten ausmachen. Durch den
äolischen Anteil können holzspezifische Kohlepartikel in den
Proxysignalen an Standorten dominieren, an denen es nur wenige oder niemals
Bäume gab.
Radiometrische Datierungen von Paläoböden und organischen Schichten
zeigen, dass beide Proxys einen hohen Anteil von Grasland gegenüber Wald
im frühen Holozän aufweisen. Im Gegensatz dazu divergieren die
Proxysignale für das späte Holozän. Für diesen Zeitraum
deuten n-Alkane allgemein auf mehr Grasland hin, während die
Holzkohlepartikel einen höheren Waldanteil erkennen lassen. Wir
vermuten, dass dieser Unterschied durch vermehrte Waldbrände und die
gleichzeitige Ausbreitung von Steppenflächen verursacht wurde.
Allgemein lässt sich folgern, dass Bodenarchive mehr lokale und
ortsspezifische Proxies liefern als es bei Archiven aus Seen und
Gletschereis zu erwarten ist. Diese Archive erhalten einen Materialeintrag
aus wesentlich größeren Einzugsgebieten. Obwohl die zeitliche
Auflösung von Bodenarchiven geringer ist als bei den anderen Archiven,
können Biomarker aus Paläoböden und humosen Schichten mit
Perioden spezifischer geomorphologischer Prozesse verknüpft werden.
Weitere Detailuntersuchungen zum Vergleich von Multiproxy-Daten aus
verschiedenen geomorphologischen Archiven sind notwendig, um die
paläoökologischen Rekonstruktionen zur Landschaftsentwicklung in der
Mongolei auf regionaler Ebene zu verbessern.
citationstatementLerch, M., Unkelbach, J., Schneider, F., Zech, M., and Klinge, M.: Holocene vegetation reconstruction in the forest–steppe of Mongolia
based on leaf waxes and macro-charcoals in soils, E&G Quaternary Sci. J., 71, 91–110, https://doi.org/10.5194/egqsj-71-91-2022, 2022.Introduction
The Mongolian forest–steppe represents a zonal ecotone at the southern
fringe of the Siberian boreal forests (Breckle et al., 2002; Erdős et
al., 2018). Due to the continental and semi-arid climate, the vegetation
pattern of forest patches and open grassland is controlled by local
geo-ecological conditions (Dulamsuren and Hauck, 2008; Hais et al., 2016;
Khansaritoreh et al., 2017). Paleo-environmental reconstruction based on
vegetation distribution is the key for interpreting climate and human impact
on the Holocene landscape development.
Pollen analyses are most commonly used to produce proxy data for
paleo-vegetation. Peat bogs and lake sediments represent efficient archives
for pollen analysis that can provide long-term and high-resolution
paleo-environmental records (Fowell et al., 2003; Rudaya et al., 2009;
Unkelbach et al., 2021). However, the pollen influx depends on near- and
long-distance transport and plant-specific pollen productivity. Thus, pollen
spectra represent a mixture of different vegetation types from a wide
catchment area (Odgaard, 1999; Zech et al., 2010b). Pollen analyses from
soil material may be hampered by selective pollen decomposition under
aerobic conditions. In contrast, charcoal particles from burned vegetation
and long-chain n-alkanes originating from leaf waxes (Eglinton and Hamilton,
1967; Kolattukudy, 1976) are relatively resistant against physical, chemical
and biological degradation. The long-lasting persistence of charcoal and
long-chain n-alkanes in soil provides a valuable archive tool for
reconstruction of local paleo-vegetation (Patterson et al., 1987; Bliedtner
et al., 2020; Thomas et al., 2021).
Few investigations on charcoal and n-alkane distribution exist for different
paleo-environmental archives in Mongolia and central Asia. Charcoal records
from Mongolia have been reported from lacustrine archives (Umbanhowar et al.,
2009; Unkelbach et al., 2019, 2021) and glaciers (Brugger et al., 2018).
These records have predominantly been used for the reconstruction of fire
chronologies. Furthermore, Miehe et al. (2007) found charcoal pieces from
birch, willow and coniferous trees in soils of the Gobi Altai Mountains as evidence
for forest distribution in southern Mongolia before 4 ka. In addition to
charcoal, long-chain n-alkane patterns and ratios were used for the
reconstruction of past vegetation composition (Zech et al., 2010b; Tarasov
et al., 2013). In a case study along several transects in Mongolia, Struck
et al. (2020) have shown that the total n-alkane concentration (TAC) and the odd-over-even predominance (OEP) from topsoils significantly correlate with climate parameters. OEP and TAC increase with low temperatures and high
precipitation. The authors explain low n-alkane concentrations in topsoils by
low biomass production, enhanced alkane degradation and livestock grazing
intensity. Furthermore, Struck et al. (2020) found no climate effect on the
average chain length (ACL) and the n-alkane ratio
(nC31/(nC29+nC31)) of plants and topsoils. Thus, the authors
proposed the applicability of these indices to distinguish between
vegetation dominated by grasses and woody shrubs.
According to literature data, vegetation composition consisting of deciduous
trees and shrubs is characterized by dominant n-alkane chain lengths of
nC27 and nC29, whereas grasses and herbs show dominant n-alkane chain lengths of nC31 and nC33 (Zech et al., 2010a, b; Schäfer et
al., 2016). Apart from leaf waxes, charred biomass and soil, microbial
biomass may also contribute to the n-alkane fractions in paleosols (Zech et
al., 2017). Wiesenberg et al. (2009) investigated the thermal degradation of
rye and maize straw and found that short-chain n-alkanes in soils may serve as a marker for charred grass biomass. Hence, Zech et al. (2017), who evaluated post-depositional contamination of n-alkane biomarkers in paleosols, interpreted nC18 as a proxy for charred sedimentary organic matter. In addition, decomposition by soil microorganisms generates short- and mid-chain n-alkanes (Buggle et al., 2010; Zech et al., 2017). In semi-arid
regions of central Asia, different relief positions and topographic
asymmetries in soil moisture and vegetation cover induce specific variations
in soil ecological conditions (Iijima et al., 2012; Kopp et al., 2014; Hais
et al., 2016; Pelletier et al., 2018) and control the distribution of
discontinuous permafrost (Klinge et al., 2021). The differing geo-ecological
conditions lead to differences in the soil formation rate and microbiological
activity. Thus, the degree of n-alkane decomposition varies between different topographic sites and soil conditions that may influence effectively the n-alkane pattern and its relevance for paleo-environmental interpretation in
terms of paleo-vegetation.
Geomorphological mapping and investigation of soil profiles for
reconstruction of Holocene landscape development comprise a general aim of the
research project (Klinge et al., 2022). As part of the general aim, we
intended to evaluate the applicability of macro-charcoal and n-alkanes for
reconstruction of past vegetation composition from paleosols and humic
layers in the semi-arid forest-steppe region of Mongolia by executing two
different approaches for proxy-data analysis. We calculated macro-charcoal
and n-alkane ratios for data interpretation. Furthermore, we evaluated
mid-chain n-alkane contents related to soil properties to estimate
microbiological activity and biomass decomposition in humic layers and
paleosols and distinguish their paleo-environmental relevance between
different site conditions.
Material and methodsStudy area
The study area is located near the town of Tosontsengel (48∘46′ N, 98∘16′ E;
1670 m a.s.l.) in the northern Khangai Mountains of
central Mongolia (Fig. 1). At Tosontsengel, the mean monthly temperatures
range between -31.7 ∘C in January and 14.7 ∘C in July
(National Agency for Meteorology and Environment Monitoring of Mongolia,
Ulaanbaatar). The annual precipitation amounts to 220 mm and reaches up to
500 mm in higher altitudes (Academy of Sciences of Mongolia and Academy of
Sciences of USSR, 1990). Most of the precipitation occurs in summer during
the growing season, whereas winters are mostly dry. Forest–steppe is the
dominating vegetation under the continental and semi-arid climate. Cold
conditions lead to the distribution of discontinuous permafrost. Permafrost
mainly occurs in valley bottoms, on upper mountains and partially on
north-facing slopes under large forest stands (Klinge et al., 2021). In the
central Khangai Mountains in the south, the mountains reach up to 3200 m a.s.l., whereas the main river in the north, the Ider Gol, is situated around 1600 m a.s.l. The upper treeline rises southward from 2400 to 2600 m a.s.l., while a lower treeline occurs at around 1800 m a.s.l. (Klinge et al., 2018).
Fragmented forests with Siberian larch (Larix sibirica) are generally limited to
north-facing slopes. Steppe vegetation covers south-facing slopes and the
pediments in the basins (Artemisia spp., Chenopodiaceae, Poaceae, Potentilla fruticosa). Steppe-like grass
vegetation spreads into small forest patches and open forest fringes,
whereas the ground vegetation of large forests mostly consists of mosses,
herbs and shrubs (Lonicera altaica, Vaccinium vitis-idaea, Vaccinium myrtillus). Large forest stands are often accompanied by a
belt of broadleaf trees and bushes (Salix spp., Betula spp.) at toe slopes and in slope
depressions, where increased soil moisture is available. Along the rivers,
riparian woody vegetation including willow (Salix spp.), poplar (Populus spp.) and larch (Larix sibirica)
occurs. The semi-arid conditions promote frequent forest fires in this
region, which occur simultaneously with droughts and are often caused by
human fire setting (Nyamjav et al., 2007; Hessl et al., 2016). Due to these
conditions, tree regrowth on burned sites strongly depends on soil
hydrological properties and occurs irregularly (Schneider et al., 2021).
Forest distribution in the study area since 9.5 ka has been shown by Unkelbach et
al. (2021), and fire intensity increased after 4.5 ka (Klinge et al.,
2022). Epilobium angustifolium is a common fire indicator that spreads in burned forests (Khapugin et al., 2016).
Soil sampling and dating
Sampling of soil and vegetation was carried out during fieldwork in Mongolia
in the summers of 2018 and 2019. We took sediment samples from 12 soil
profiles for analyses. Organic matter from paleosols and humic layers was
used for 14C dating, whereas intermediate eolian layers were analyzed
by infrared stimulated luminescence (IRSL) dating. Details on dating methods and soil properties
and results for geomorphological processes were published by Klinge et al. (2022). All 14C ages are given as calibrated 14C ages in thousands of years before present (ka BP) here. Samples for biomarker and charcoal analyses were taken parallel to soil
samples by a separate sampling procedure.
Sample preparation and element analyses
Analyses of leaf-wax-derived n-alkanes were carried out on 35 soil samples
and 6 modern vegetation samples that were collected at the study site.
n-Alkane analyses as well as total carbon and total nitrogen measurements
were executed at the laboratory facilities of the Department of Soil Biogeochemistry, Institute of Agricultural and
Nutritional Sciences, Martin Luther University
Halle-Wittenberg, in Halle (Saale). Soil samples were air-dried and sieved
(< 2 mm) for preparation. Vegetation samples were dried, and whole
leaves were used for analysis.
The contents of total carbon (TC) and total nitrogen (TN) were measured
using a EuroVector EA3000 elemental analyzer (Hekatech, Wegberg, Germany)
coupled via a Conflow III interface to a Delta V Advantage isotope ratio
mass spectrometer (Thermo Fisher Scientific, Bremen, Germany).
Lipid biomarker analyses
n-Alkane preparation and quantification followed the procedure described by,
e.g., Lerch et al. (2018) and Zech et al. (2013). In brief, the total lipid
extracts (TLEs) of all soil and modern vegetation samples were gained using
Soxhlet extraction in 24 h and of ∼ 180 mL dichloromethane (DCM) : methanol (MeOH)
(9 : 1) as the solvent mixture. The necessary sample amount for n-alkane analysis depends on the total organic carbon content (TOC). The sample weight for the investigated soil material ranged from ∼ 3 to 8 g. A sample
weight of ∼ 1 g was chosen for plant material, and 5α-androstane was added as an internal standard to each vegetation and soil
sample. Additionally, we inserted ∼ 1 g of glass wool into
extraction thimbles for prevention of the splash effect before extraction
started. Afterwards, the TLE was dried with a rotary evaporator and
dissolved again with n-hexane. For separation of the TLE, aminopropyl silica gel (45 µm; Supelco) pipette columns were used (Struck et al., 2018).
The dissolved TLE was separated as well as eluted over these pipette columns
into (i) an aliphatic fraction (apolar) including the n-alkanes using
n-hexane (3 × 1 mL), (ii) an alcoholic fraction (polar) using DCM : MeOH (9 : 1, 3 × 1 mL) and (iii) an acid fraction using diethyl ether : acetic acid (95 : 5, 3 × 1 mL) for each sample. Afterwards, separated fractions were dried under
nitrogen. The aliphatic fraction was transferred into 1.5 mL gas chromatography vials with 2 × 0.5 mL n-hexane for measurements. n-Alkanes were identified and quantified
with a gas chromatograph coupled with a flame ionization detector (GC-FID,
2010 series, Shimadzu, Kyoto, Japan). External n-alkane standards with a
known concentration (n-alkane mix nC8–nC40; Supelco 49452-U) were repeatedly run with each sequence at different concentrations (25, 50 and 100 µg mL-1) for
quantification and identification (Hepp et al., 2017; Bliedtner et al.,
2018; Bittner et al., 2020). Detailed results of n-alkane analysis are shown in Table S2 in the Supplement.
Macro-charcoal analysis
For macro-charcoal analysis, 27 subsamples of 0.5 cm3 were taken from
organic layers and paleosols. All subsamples were processed based on the
method established by Stevenson and Haberle (2005), including KOH (10 %),
H2O2 (4 %) and wet sieving with low water pressure to avoid
further fragmentation of the charcoal material. All charred macro-particles
(> 125 µm) of each subsample were counted using a
binocular dissecting microscope. To reconstruct dominating vegetation types
(forest, steppe), all charred particles were divided into four different
morphological types while counting: wood (rather 3-dimensional, long), leaf (rather
2-dimensional, irregularly shaped), grass (rather 2-dimensional, long and
mostly rectangular) and other (including mosses, roots, seeds and
unidentifiably small fragments). Macro-charcoal
identification was based on the examples given in Umbanhowar and Mcgrath (1998) and Mustaphi and Pisaric (2014). Additionally, macro-charcoal
concentrations (particle sum per cubic centimeter) were calculated for each
subsample (Supplement File S1).
Data analysis
Macro-charcoals were classified as originating from wood, leaf and grass.
Approx. 25 % of particles could not be identified or had another source and
were classified as other and excluded from statistical analysis. The
share of charcoal classes was determined to evaluate steppe / forest ratios
(Csf) that could indicate vegetation composition:
Csf=grasswood.
The OEP (odd-over-even predominance) ratio considers long-chain n-alkanes and can be used as proxy for n-alkane degradation (Zech et al., 2012). High OEP values correspond to fresh plant material, whereas lower OEP values indicate an increased degradation of soil organic matter (Schäfer et al., 2016).
OEP=nC27+nC29+nC31+nC33nC26+nC28+nC30+nC32,
defined as odd-over-even predominance (OEP) according to Hoefs et al. (2002).
The C / N ratio serves as an additional soil parameter for biological
decomposition.
Based on n-alkane patterns of the analyzed vegetation samples and their
dominant n-alkane chain lengths, calculated n-alkane proxy data can be applied for soils regarding vegetation reconstruction (Zech et al., 2012). Here, we used the ACL (Poynter et al., 1989), defined as the average chain length of n-alkanes, in the modified version reported from Schäfer et al. (2016) to address the grassland–woodland composition:
ACL=27×nC27+29×nC29+31×nC31+33×nC33nC27+nC29+nC31+nC33.
To indicate fire marker and soil microbial degradation, we checked the
short-chain n-alkane fraction by elevated values of nC18 and ∑nC17–nC20 to be compared with OEP, C / N and macro-charcoal
distribution. Elevated nC18 contents along with higher OEP and C / N ratios indicate an input of charred biomass.
The relationship between charcoal and n-alkane parameters was assessed by
producing bivariate scatterplots and performing a series of correlation
analyses. Each relationship was assessed based on correlation coefficients
(Pearson's r) with a significance threshold (p value) set at 0.5.
Results and discussionSoil profiles and dating results
Paleosols and humic layers containing abundant charcoal were found in
various soil profiles in topographic positions on slopes under forest and
steppe, dunes, and alluvial plains (Fig. 1). Figure 2 illustrates the soil
profiles with positions of samples for soil analyses and dating, which are
described in detail in the following section. We classified the soil
profiles into three site groups. Group A contains all sites where trees exist
or have existed before. Group B comprises sites of exclusively steppe
vegetation. All sites with inconsistent dating results and where a mixture of
different solum material was found were assigned to group C, which
represents problematic profiles for correlation analyses.
Soil profiles with sampling positions, n-alkane and
charcoal ratios. Ages for radiometric dating were adapted from Klinge et al. (2022). Grey bars indicate parameters for enhanced biological decomposition, and light brown rectangles indicate parameters for decomposition by fire. Note the different scaling of the horizontal axes.
Profile P-B (Fig. 2, group A) was located in a toe-slope position inside
a dense larch succession at a formerly burned forest site. The profile was
composed of colluvial sediments with an imbricated layering that derived
from downslope transport of solum after repeated forest fires (Klinge et
al., 2021). The solum material was rich in soil organic matter (SOM), and
14C dating of charcoal from different layers provided upward consistent
calibrated 14C ages of between 3.8 and 2.0 ka. This dating indicates the
ages of the trees that burned in an upslope position during distinct fire
events. Two IRSL ages at around 10 ka from intermediate yellowish sand layers
pointed to the time of primary eolian deposition further upslope before this
sand was transported downslope by colluvial and/or periglacial processes
that occurred after 3.3 ka (Klinge et al., 2022). Permafrost occurred at a
depth of > 90 cm. Soil profile P-S (Fig. 2, group C) was located
at a valley bottom under dense broadleaf vegetation (Betula, Salix). Intensively
cryoturbated colluvial and eolian sediments with paleosols, humic layers and
charcoal were found, and permafrost ice occurred at a depth of 70 cm.
Carbon-14 dating of charred material and SOM provided calibrated ages of between
2.7 and 0.45 ka, whereas eolian material yielded an IRSL age of 12.1 ka.
The time–depth inconsistency of the ages may be due to colluvial,
cryoturbation and solifluction processes (Klinge et al., 2022).
Section P-C (Fig. 2, group B) was exposed in a gully in a basin under steppe
vegetation. The sediment consisted of two units. The lower unit rested on
Pleistocene fanglomerates and was composed of homogeneous eolian sand which
provided IRSL ages of between 11.0 and 8.9 ka. Two 14C dating analyses of SOM from
a distinct paleosol upon the lower sediment unit provided calibrated ages of
1.8 ± 0.1 and 0.9 ± 0.1 ka, whereas IRSL dating from the
upper sediment obtained an age of 1.1 ka (Klinge et al., 2022). In the
transition to the upper sediment unit, the paleosol layers diverged into
several sub-units, where a humpy upper boundary pointed to enhanced dust
trapping around tussock grass. The 60 cm thick upper sediment unit above the
paleosol consisted of layered sediments that derived from repeated slope
wash processes. A conspicuous black layer at 6 cm depth contained abundant
charcoal. Section P-R (Fig. 2, group A) was located at an alluvial fan under
steppe vegetation adjacent to an alluvial forest. The sediments consisted of
colluvial and slope wash layers including three paleosols with charcoal and
several dark brown humic layers. Carbon-14 dating of SOM provided time–depth-consistent calibrated ages of 3.1, 4.4 and 8.2 ka, which were
complemented by two IRSL ages of 7.4 and 13.5 ka. In contrast to the
profiles P-B and P-S that were located under forest, no permafrost was found
at sections P-C and P-R under steppe. This difference in permafrost
distribution may have an important influence on biological activity and
decomposition.
Section P-J (Fig. 2, group C) was exposed at a dune field upon Pleistocene
gravel and under sparse vegetation cover. Carbon-14 dating of a distinct
paleosol obtained a calibrated age of 1.3 ka. IRSL dating from sand provided
ages of 0.29 ka above the paleosol and 9.2 to 11.7 ka below the paleosol.
The eolian sediment of section P-L (Fig. 2, group B) was located in the
valley bottom near the Ider Gol under dense steppe. IRSL dating of eolian
sand provided ages of 7.1 and 13.8 ka, whereas 14C dating of SOM and charcoal from paleosols obtained calibrated ages of 8.0 and 9.5 ka.
Sections P-F and P-K (Fig. 2, group A) were located in a valley bottom under
dense meadow steppe. A paleo-channel cut into alluvial sediments that were
composed of eolian and alluvial layers, paleosols, and humic layers,
containing charcoal and small pieces of wood. No permafrost was found,
whereas slight deformation of sediment layers indicated former weak
cryoturbation. At section P-F, several calibrated 14C ages of wood and
bone artifacts (Ovis) provided a consistent chronology between ca. 4.3 and 3.7 ka, which was complemented by two IRSL ages of 3.5 and 5.0 ka. At section
P-K, IRSL ages of 5.1 and 7.8 ka were somewhat older compared to section
P-F. However, the 14C ages of < 0.8 ka obtained from SOM of
three humic layers did not fit into the chronological framework based on the
other ages. Contamination by young organic material may have adulterated the
dating, and therefore P-K was assigned to group C.
Single samples were taken from three profiles that were not analyzed by
radiometric dating. Profile P-M (Fig. 2, group C) was a section in a
Pleistocene terrace near a river and was covered by eolian sediments, which
contained several cryoturbated paleosols. Profile P-N (Fig. 2, group B) was
situated in eolian sand that contained several cryoturbated paleosols above
basal moraine. Profile P-09 (Fig. 2, group A) was located at a burned forest
site in slope debris above till and contained many charcoals. The paleosol
of section P-E (Fig. 2, group A) was developed upon eolian sand and was
covered by colluvial sediments. Carbon-14 dating of SOM provided a calibrated
age of 12.8 ka. No n-alkane analysis was conducted on P-E.
Macro-charcoal distribution
Macro-charcoal occurred in all samples from paleosols and humic layers. The
total amount of macro-charcoal per sample varied between a minimum of 22 and a maximum of
338 particles cm-3, with a mean of 100 particles cm-3 (Fig. 3 and Supplement File S1). Charcoal of leaves generally had a lower proportion than charcoal of wood and grass. The ratios between charcoal of wood and grass per sample were significantly highly negatively correlated (r=-0.92), whereas charcoal of leaves occurred mostly
independently from the other two morphotypes (Table 1). The charcoal particle
concentration showed no distinct correlation pattern with the site type, sample
depth and charcoal type ratio (Fig. 3). At the dune profile P-J, the total
amount of charcoal was too low for further interpretations.
Charcoal concentration and morphotype distribution in
paleosols and humic layers at different study sites.
Correlation coefficient (r) between different
charcoal-morphotype distributions (n= 23, excluding profile P-J).
Charcoal typerp valueWood–grass-0.93< 0.001Wood–leaf-0.010.969Grass–leaf-0.360.088Patterns of long-chain n-alkanes in plants and topsoils
One sample of fresh plant material per species was analyzed for the n-alkane
distribution from the main species of the study area (Fig. 4). Siberian larch is
the dominant tree in the Mongolian forest–steppe, and its n-alkane pattern
shows dominance in nC27 and nC29 (Fig. 4), which was also
reported by Zech et al. (2010b). In contrast, Struck et al. (2020) stated a
dominance of nC25 for their larch samples from Mongolia, whereas
Diefendorf et al. (2011) stated a dominance of nC29 for Larix decidua from North
American samples. We detected a low total n-alkane concentration (TAC) of
19.7 µg g-1 dry plant material for our Siberian larch sample,
which is in agreement with literature data. Thus, larch is unlikely to
significantly contribute to the n-alkane patterns of the soils. Birch
(Betula platyphylla) and willow (Salix spp.), which often occur at moist and open sites in forest stands,
also have a predominance of nC27 and yielded TACs of 248.8 and 96.4 µg g-1, respectively (Fig. 4). Comparable
n-alkane distributions with a predominance of nC27 for birch species were also reported by Tarasov et al. (2013) (B. exilis, B. fruticosa) and Zech et al. (2010b) for eastern Siberia and by Strobel et al. (2021) (B. nana) for the Mongolian Altai.
Zech et al. (2010b) and Tarasov et al. (2013) also reported a predominance
of nC27 for willow, whereas the nC23 content in our sample was
exceptionally high. We assume that moist environmental conditions promote
the formation of a higher content of mid-chain n-alkanes such as nC23
and/or nC25 in leaf waxes of willow (Tarasov et al., 2013). Lonicera altaica, Vaccinium vitis-idaea and
Vaccinium myrtillus are common shrubs that are restricted to dense pristine forests and thus
may serve as secondary indicators for forests. We found a distinct
predominance of nC29 and high TAC of 493.3 µg g-1 for
Lonicera altaica (Fig. 4), whereas a predominance of nC31 was reported for both Vaccinium vitis-idaea and Vaccinium myrtillus (Tarasov et al., 2013). The authors stated that TAC for Vaccinium vitis-idaea was 4070 µg g-1 and for Vaccinium myrtillus was ∼ 3000 µg g-1.
Long-chain n-alkane patterns of plants from the study area at Tosontsengel (OEP, odd-over-even predominance; TAC, total n-alkane concentration of dry plant material; ACL, average chain length of n-alkanes; n= 1).
Poaceae that prevail in the grassland reveal a predominance of nC31 and a TAC of 246.6 µg g-1 (Fig. 4), which was also shown by Bush and McInerney (2013) and Struck et al. (2020). The same is true for Potentilla fruticosa, which represents a dominant forb in the steppe with a TAC of 1200 µg g-1 (Fig. 4), as well as for Cyperaceae (Bush and McInerney, 2013; Struck et al., 2020). Furthermore, Potentilla fruticosa as a steppe-dominant plant has an
increased relative n-alkane contribution of nC33. By contrast, Artemisia spp., which contribute many prevalent genera of the steppe, as well as Caragana spp. have a predominance of nC29 (Struck et al., 2020).
Relative long-chain n-alkane content (%), OEP, TAC and ACL of topsoils and mean vegetation at different profiles in the study area (OEP, odd-over-even predominance; TAC, total n-alkane concentration of soil; ACL, average chain length of n-alkanes).
Most of the long-chain n-alkanes detected in the investigated topsoils show
similar patterns with increasing relative contributions of odd n-alkanes from nC25 to the predominant homologue nC31 (Fig. 5), which confirms the results from Schäfer et al. (2016) and Struck et al. (2020). We observed the highest relative abundances of nC33 in topsoils of the
profiles P-B, P-09 and P-C. This finding can be likely explained with the
input of litter from Potentilla fruticosa, being characterized by high nC33 contents. The OEP ratios of topsoils range between 7.5 and 16.1. A distinct exception of
the n-alkane pattern in the topsoils occurs at profile P-S. The dominant
carbon-chain length for this soil profile is nC27 (Fig. 5). Abundant
birch and willow trees at this site may have caused this high content of
nC27. TAC ranges from 4.5 to 26.7 µg g-1 soil and from 107 to 280 µg g-1 TOC for the investigated topsoils.
The ACL (Fig. 5) of different topsoils shows marginal relationships to the
modern vegetation. Currently, no trees exist at the sites P-C and P-F and
dense larch succession occurs at the site P-B. These sites are among those
with the highest ACL values in the topsoils. Leaf trees like willow and
birch occur in the vicinity of the steppe-dominated sites P-M and P-N and are
mixed with larch at the sites P-09, P-R and P-S. Topsoils of P-09 and P-S
are characterized by higher content of nC27 compared to the other
topsoils. The high production rate of nC27 of willow and birch produces
a distinct signal in the n-alkane distribution in topsoil, which decreases
the ACL compared to steppe- and larch-dominated sites. Strobel et al. (2021)
have shown comparable n-alkane patterns in topsoils with nC31
predominance at steppe-dominated sites and with nC27 predominance at
sites with Betula nana.
Correlation of n-alkane and macro-charcoal ratios in soil profiles
The correlation between n-alkane ACL and charcoal ratios as indicators for
steppe and forest distribution of all samples without site classification is
low (Table 2), although a common trend in the wood- and grass-dominated share
exists (r> 0.5).
Correlation (r) between the relative distribution of
charcoal types and ACL (average chain length of n-alkanes, n= 20, excluding profile P-K).
A detailed statistical analysis considering specific n-alkanes and charcoal
types (Table 2) has shown that correlations of wood-driven charcoal are
significantly positive with nC27 and significantly negative with
nC31 and nC33, whereas these correlations are inverse for grass
charcoal. Charcoals from leaves have no significant correlations, which
point to the minor relevance of these morphotypes for vegetation
reconstruction.
Correlation matrix between macro-charcoal and n-alkane
parameters (r). TAC denotes total n-alkane content; OEP denotes odd-over-even predominance; ACL denotes average chain length of n-alkanes.
A closer look at n-alkane and macro-charcoal ratios shows that grouping the
profiles by vegetation pattern and site conditions increases the statistical
correlations significantly (Table 3, Fig. 6). In group A, the profiles P-B
and P-R are located at forested sites. At profile P-F no trees occur at
present, but the analyzed sample at 151 cm depth included fragments of wood
alongside charcoal pieces and alluvial forest grows ca. 1 km upstream. Group A shows highly significant correlations between wood, grass, the grass / wood
ratio and C / N on the one hand with ACL and OEP on the other hand (Table 3).
Increasing OEP and ACL occurred parallel with the increasing share of grass,
which is caused by higher contents of long-chain n-alkanes (nC31) in
grasses than in other plants and is in agreement with results from Struck
et al. (2020) and Strobel et al. (2021). In addition, the C / N ratio
correlates positively with Csf, OEP and ACL, which indicates less
biological decomposition when the portion of grass vegetation increased at
forest-related sites. Permafrost distribution in forest stands, which
hampers microbial activity, is related to herb and moss ground
vegetation, whereas grass inside forest indicates less canopy closure
inducing warmer soil conditions.
Linear regressions between ACL and Csf grouped by different site conditions (ACL, average chain length of n-alkanes;
Csf, macro-charcoal grass / wood ratio).
Group B includes the profiles P-C and P-L, which are located under dry
steppe without any trees, and profile P-N occurs under dense meadow steppe
in the vicinity of riparian trees. Group B shows generally lower and
inverse correlations between charcoal and n-alkane ratios compared to group A (Fig. 6, Table 3). In addition, the total quantity of charcoal particles correlates positively with the short- and mid-chain n-alkanes nC18 and ∑nC17–nC20. These n-alkane chain lengths can be produced by microbial degradation or in the case of nC18 by charring (Wiesenberg et al., 2009; Zech et al., 2010a). The C / N and OEP values do not show significant correlation with charcoal and n-alkane ratios in group B, which may
point to less influence of microbial decomposition on these ratios.
Sediment stirring and inverse ages from radiometric dating indicate
disturbances and transformations in the organic matter of profiles P-K and
P-S, which become visible in the extreme outmost relations between
n-alkane and charcoal ratios (Fig. 6). Due to these inconsistencies, these
profiles were classified as group C, which is problematic for environmental
interpretation.
Figure 2 shows the vertical distribution of charcoal and n-alkane parameters in several soil profiles that were investigated in greater detail. Layers where low values of OEP and C / N and high values of nC18 occur are highlighted in grey to indicate the potential of increased microbiological decomposition. Layers where high values of OEP and C / N and nC18 occur are highlighted in light brown to indicate the potential of degradation by fire.
The dark-brown-colored humic layers (18–23 cm; 40 cm) of profile P-B show
low TAC, OEP and C / N and high nC18 and ∑nC17–nC20 values
(Fig. 2), which confirm the assumption of relocated topsoil from an upslope
position for these humic layers (Klinge et al., 2021). In contrast, the
bright yellowish sediment (23–38 cm) tends to have increased values for TAC,
OEP and C / N and low nC18 and ∑nC17–nC20 values, pointing to more parent material that was subjected to less soil formation. A similar n-alkane and C / N pattern occurs in profile P-C, where low OEP and C / N and high nC18 of a layer at ∼ 6 cm and a paleosol at 45–50 cm
indicated microbiological activity, whereas the other layers were less
affected. The inverse distribution of n-alkane and macro-charcoal ratios in
the pronounced paleosol at 45–50 cm depth in P-C is probably related to the
intense decomposition that led to a decrease in long-chain n-alkanes and
increase in short- and mid-chain n-alkanes. The same is true for profile P-L, where the differences in OEP and C / N between the upper layer (58 cm; OEP, 15.6; C / N, 20.1) and lower layer (116 cm; OEP, 9.3; C / N, 14.9) indicate more decomposition related to the decrease in long-chain n-alkanes in the latter,
which again explains the inverse charcoal ratio (Fig. 2).
At profile P-R, the n-alkane pattern is comparable with profile P-B. However,
the decomposition rate was already high in the topsoil of profile P-R in
contrast to P-B. Although decomposition may have influenced the n-alkane
pattern of the profiles P-B and P-R, the parallel pattern of macro-charcoal
ratios confirms the vegetation proxy. At the top and at the bottom of
profile P-F, the OEP, C / N and mid-chain n-alkanes indicate similar
microbiological activity and soil formation, but ACL points to more forest
in the lowest layer.
In the steppe profiles P-C and P-L (group B), higher macro-charcoal ratios
that indicate increased grassland share are combined with increased
degradation of n-alkanes, shown by a lower OEP. This phenomenon is reverse in the forest-related sites (group A, profiles P-B, P-R, P-F). An intensive
cryoturbation in profile P-S and substrate infiltration at the profile P-K
is underlined by the obtained ages, which diminished the analysis of the
n-alkane patterns, charcoal ratios and chronology.
Evaluating the impact of geomorphological processes, climate and
vegetation on n-alkanes and macro-charcoal distribution in soils for their suitability as proxy data
The horizontal distribution of n-alkane and macro-charcoal ratios in
different profiles show parallel patterns in group A and inverse patterns in group B (Fig. 2). A distinct environmental difference between both
groups is the occurrence of modern or past forest at the sites of group A,
whereas forest was probably absent at the sites of group B. Positive
correlations between both ratios may prove the delineation of
paleo-vegetation at the specific sample sites, whereas diverging ratios
derive from different sources, pathways and degradation degrees of elements.
Irregular biomarker and charcoal distributions in disturbed profiles show
inconsistent dating results too, which clearly points to problematic
samples that should be interpreted with caution or rejected.
Most of the n-alkanes in topsoils originate directly from the litter of the
covering vegetation. A paleosol most likely received n-alkanes that indicate
the local vegetation composition, whereas distinct humic layers as deposits
of translocated paleosol contain SOM that comes from distant positions.
Thus, n-alkanes of colluvial layers represent a mixture of solum material
from the upslope area, whereas alluvial layers may receive additional
organic material from a larger area of the fluvial catchment. Furthermore,
reworked leaf waxes and fossil n-alkanes may have deposited along with eolian influx and may have biased the biomarker signals (Haas et al., 2017).
However, it can be presumed that the eolian portion of n-alkanes in soils is negligible compared to those directly deriving from vegetation.
Paleosols and humic layers contain organic carbon from different sources
such as reworked solum material, charcoal, plant material, living roots and
dissolved organic matter, which produces uncertainties of > 100 years by 14C dating (Pessenda et al., 2001). Carbon-14 dating of
charcoal provides the age of tree growth before fire. Carbon-14 dating on
n-alkanes reflects the age of modern vegetation or paleo-vegetation (i.e., n-alkane-producing vegetation) and its input into soils. Therefore, we decided to focus the age
determination of our soils using 14C and IRSL dating on bulk material.
The n-alkane assemblage in the SOM covers the entire period of soil
development. Although soil formation may last for a long time, when no soil
erosion or sediment covering occurred, the high age frequency of humic
layers and paleosols found in our soil profiles (Fig. 2) limits most of the
soil formation periods to a few hundred years.
n-Alkanes are subjected to microbiological degradation, which is
controlled by different ecological factors relating to moisture and
temperature regimes in the investigated soils (Struck et al., 2020). Our
results have shown that decomposition of organic matter was generally higher
in dry steppe soils and lower under forest that is often underlain by
permafrost. The degradation of n-alkanes in Mongolian topsoils from semi-arid and arid regions was also investigated for two transects by Struck et al. (2020). OEP values for these topsoils range from 1.5 to 19 and agree with our OEP results (Struck et al., 2020) (Fig. 5).
Although small pieces of charred wood were found in many profiles, the
“macro-charcoals” that were analyzed here are particles of microscopic
size in the scope of pollen and dust. Thus, macro-charcoals of local origin
and eolian input were deposited in different shares. Increasing particle
concentration may serve as an indicator for a macro-charcoal source of nearer rather than long-distance fire. Treeless sites of the steppe area (profiles
P-C, P-L) must have received their wood-derived macro-charcoal
portion in particular by eolian influx. In contrast, the n-alkane patterns of steppe soils represent the local vegetation cover consisting of grasses and herbs. The increased share of short- and mid-chain n-alkanes (nC18–nC26) in
combination with a decreased OEP ratio suggests a more intense microbial
degradation of n-alkanes (Lerch et al., 2018). Furthermore, statistical
analysis (Table 3) proved that a combination of macro-charcoal and
n-alkanes yields inconsistent results for steppe soils (group B).
The information of macro-charcoal and n-alkane distribution (ACL) and the
dating results from the paleosols and humic layers provide a chronological
framework of the Holocene vegetation evolution in the study area (Fig. 7).
Both indices show that grassland dominated in the period before 7.5 ka,
although forests had already existed in the Khangai Mountains since at
least 9.5 ka (Gunin et al., 1999; Wang et al., 2009; Unkelbach et al.,
2021). After 5.0 ka, the indices are more diverse and the vegetation pattern
becomes more complex. The site-specific n-alkane proxy tends to indicate more grassland compared to the macro-charcoal proxy, which was probably influenced by eolian deposition of allochthonous charcoal particles upon topsoils. The
widespread dispersal of charcoal particles after forest and steppe fires
represents a distinct sedimentation factor that can even be traced to the
high mountains as dust in glacier ice (Eichler et al., 2011; Brugger et al.,
2018). Thus, the higher share of wood-derived macro-charcoal in soils points
to increased forest fires in the region. In addition, the n-alkane patterns
that represent more local proxies in soils indicate an expanded steppe area
for most of the soil profiles, established by a high content of nC31,
since the beginning of the Late Holocene. The observed change in the pattern of
forest and steppe distribution as well as the increased fire activity in
Mongolia since the Late Holocene is related to climatic and/or anthropogenic
impact (Klinge and Sauer, 2019). Furthermore, desertification processes and
increased geomorphological activity occur concordantly with a cultural
change since the early Bronze Age when humans developed a pastoral economy
(Fernández-Giménez et al., 2017; Klinge et al., 2022). Nevertheless,
these initial findings need further evaluation due to the limited number of
samples presented here.
Chronological distribution of macro-charcoal ratio and
ACL (ACL, average chain length of n-alkanes; Csf, macro-charcoal grass / wood ratio).
Conclusions
Based on our results of comparing two different paleo-proxies, n-alkanes and macro-charcoals, we have shown that it is essential to consider their
different element sources, paths and catchment areas for interpretation of the
local and regional paleo-environment. Additional information about soil
properties as well as radiometric dating provides further evidence. The
analysis of lipid biomarkers, such as n-alkanes, on samples from soil
profiles of central Mongolia gives insight into local vegetation conditions.
n-Alkanes, which are organic molecules in leaf waxes, are directly
accumulated in the soils by local vegetation (Zech et al., 2010b).
Translocation of soil material may occur by slope wash and solifluction
processes. Decomposition of organic matter and n-alkanes is generally more
intense in dry steppe soils and less intense under forest that is often underlain
by permafrost. Macro-charcoals in soils represent a mixture of both local
and long-distance sources. Due to the eolian transport, the dominating wind
direction frames the catchment area of macro-charcoal influx.
We have shown that paleosols and humic layers represent a valuable archive
for reconstruction of the detailed paleo-environment at a high site-specific
spatial resolution. The application of lipid biomarker analysis to paleosols
can help to prove local differences in topographic site conditions over time
such as the aspect-dependent vegetation differentiation of the Mongolian
forest–steppe. Charcoal in paleosols indicates periods of increased fire
impact on landscape evolution and geomorphology. However, the temporal
resolution of paleosol and sediment archives is often low and discontinuous
due to irregular deposition intensity and soil-forming periods, but the
biomarker proxies are related to distinct geomorphological periods in a
local scale. In contrast, lake sediments may provide a more continuous
sedimentation chronology, but the sediment input depends on fluvial and
eolian catchments. Thus, these archives contain proxy data that represent a
larger area than soil archives do. Furthermore, stratified charcoal
particles in glacier ice and snow fields solely derive from long-distance
eolian transport. Their paleo-proxies depend on the main wind direction that
frames the catchment area and thus the region for the paleo-environmental
representation of their proxies. In summary, there is further need for
multi-proxy analysis (biomarkers, macro-charcoal, pollen, microorganisms,
ancient DNA, sediment properties) comparing the different soils, lake and
glacier archives in combination with radiometric dating to enhance and
refine the paleo-environmental records in Mongolia.
Data availability
All raw data can be provided by the corresponding authors upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/egqsj-71-91-2022-supplement.
Author contributions
FS and ML developed the project idea in collaboration with MK and JU. Fieldwork was carried out by FS and MK. ML and JU carried out most of the laboratory work. MK analyzed the data. MK prepared the manuscript, and ML, JU, MZ and FS contributed to the discussion of the results and read and
approved the manuscript.
Competing interests
At least one of the (co-)authors is a member of the editorial board of E&G Quaternary Science Journal. The peer-review process was guided by an independent editor, and the authors have also no other competing interests to declare.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank our project partners Choimaa Dulamsuren and Markus
Hauck (Faculty of Environment and Natural Resources, University of Freiburg)
and Uudus Bayarsaikhan (Department of Biology, National University
of Mongolia, Ulaanbaatar) for their great support during the fieldwork in
Mongolia.
We thank Daramragchaa Tuya (Tarvagatai Nuruu National Park, Tosontsengel
Sum, Zavkhan Aimag, Mongolia) for her invaluable support of our research. We
wish to express our gratitude to our Mongolian colleagues Amarbayasgalan, Enkhjargal and Enkh-Agar. We greatly appreciated
their hospitality and help in the field. Our thanks also go to the German
students Jannik Brodthuhn and Tim Rollwage for their assistance during soil
sampling in Mongolia.
In particular, we thank Daniela Sauer (Institute of Geography, University of Göttingen) and Manfred Frechen
(Leibniz Institute for Applied Geophysics, Hanover) for their professional support and project collaboration. Furthermore, we thank Bruno
Glaser and Tobias Bromm (Department of Soil Biogeochemistry, Martin Luther University Halle-Wittenberg) for providing access to laboratory facilities and support of laboratory analyses. We thank
Hermann Behling (Albrecht-von-Haller-Institute for Plant Sciences, University of Göttingen) for providing laboratory access.
We also thank Jens Holtvoeth and the anonymous reviewer for their valuable
recommendations to improve the manuscript.
Financial support
This research has been supported by the Deutsche Forschungsgemeinschaft (grant no. 385460422).This open-access publication was funded by the University of Göttingen.
Review statement
This paper was edited by Ingmar Unkel and reviewed by Jens Holtvoeth and one anonymous referee.
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