EGQSJE&G Quaternary Science JournalEGQSJE&G Quaternary Sci. J.2199-9090Copernicus PublicationsGöttingen, Germany10.5194/egqsj-66-91-2017Late Quaternary climate and environmental reconstruction based on leaf wax
analyses in the loess sequence of Möhlin, SwitzerlandWüthrichLorenzloeru@live.comBliedtnerMarcelSchäferImke KathrinZechJanaShajariFatemehGaarDorianhttps://orcid.org/0000-0003-3000-5980PreusserFrankhttps://orcid.org/0000-0002-5654-1346SalazarGarySzidatSönkehttps://orcid.org/0000-0002-1824-6207ZechRolandGeographical Institute, University of Bern, Bern, SwitzerlandOeschger Centre for Climate Change Research, University of Bern, Bern, SwitzerlandGeo and Environmental Engineering, Technical University of Munich, Munich, GermanyInstitute of Geology, University of Bern, Bern, SwitzerlandInstitute of Earth and Environmental Sciences, University of Freiburg, Freiburg, GermanyDepartment of Chemistry and Biochemistry, University of Bern, Bern, SwitzerlandLorenz Wüthrich (loeru@live.com)21December201766291100This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://egqsj.copernicus.org/articles/66/91/2017/egqsj-66-91-2017.htmlThe full text article is available as a PDF file from https://egqsj.copernicus.org/articles/66/91/2017/egqsj-66-91-2017.pdf
We present the results of leaf wax
analyses (long-chain n-alkanes) from the 6.8 m deep loess sequence of
Möhlin, Switzerland, spanning the last ∼ 70 kyr. Leaf waxes are
well preserved and occur in sufficient amounts only down to 0.4 m and below
1.8 m depth, so no paleoenvironmental reconstructions can be done for marine
isotope stage (MIS) 2.
Compound-specific δ2Hwax analyses yielded similar values
for late MIS 3 compared to the uppermost samples, indicating that various
effects (e.g., more negative values due to lower temperatures, more positive
values due to an enriched moisture source) cancel each other out. A
pronounced ∼ 30 ‰ shift towards more
negative values probably reflects more humid conditions before
∼ 32 ka. Radiocarbon dating of the n-alkanes corroborates the
stratigraphic integrity of leaf waxes and their potential for dating
loess–paleosol sequences (LPS) back to ∼ 30 ka.
Wir präsentieren die Ergebnisse von Blattwachsanalysen (langkettige
n-Alkane) aus einer 6.8 m mächtigen Löss-Sequenz bei Möhlin,
Schweiz, welche bis etwa 70 ka zurückreicht. Nur bis 0.4 m Tiefe und
unterhalb von 1.8 m sind die Blattwachse gut und in ausreichender Menge
erhalten, so dass Aussagen bezüglich der Umweltbedingungen während
der marinen Isotopenstufe (MIS) 2 nicht gemacht werden können. Die
Muster der n-Alkane zeigen für alle Proben einen dominanten Input von
Gras-Alkanen, jedoch ist es nur anhand der n-Alkane nicht möglich, die
Existenz von Koniferen vor allem während des MIS 3 auszuschliessen.
Komponenten-spezifische δ2Hwax Analysen zeigen für MIS 3 ähnliche Werte wie für die obersten Proben aus der Sequenz.
Offensichtlich heben sich mehrere Effekte gegenseitig auf (z.B. negativere
Werte aufgrund niedrigerer Temperatur, positivere Werte aufgrund
angereicherter Feuchtigkeitsquellen). Ein auffälliger ∼ 30 ‰ Wechsel zu negativeren Werten zeigt vermutlich
feuchtere Bedingungen vor ∼ 32 ka an. Komponenten-spezifische
Radiokohlenstoff-Datierungen bestätigen die stratigraphische
Integrität der Blattwachse und untermauern ihr Potential für die
Datierung von Löss-Paläoboden Sequenzen bis etwa 30 ka.
citationstatementWüthrich, L., Bliedtner, M., Schäfer, I. K., Zech,
J., Shajari, F., Gaar, D., Preusser, F., Salazar, G., Szidat, S., and Zech,
R.: Late Quaternary climate and environmental reconstruction based on leaf
wax analyses in the loess sequence of Möhlin, Switzerland, E&G
Quaternary Sci. J., 66, 91–100, https://doi.org/10.5194/egqsj-66-91-2017,
2017.Introduction
Vast parts of Switzerland were repeatedly covered by glaciers (Graf, 2009).
This explains the lack of widespread thick loess deposits as they exist, for
example, in southeastern Germany, southeastern and eastern Europe (Haase et
al., 2007). Nevertheless, there are a few sites in Switzerland where loess
has been found (Gouda, 1962; Preusser et al., 2011; Gaar and Preusser,
2017), and they might have potential for paleoenvironmental reconstruction.
One of these sites is the Möhliner Feld in northwestern Switzerland
(Fig. 1). Until recently, there was a subdued ridge considered to be a
terminal moraine of the most extensive glaciation (MEG;
> 300 ka) in the Swiss Alps (Gutzwiler, 1894; Penck and
Brückner, 1909; Keller and Krayss, 2011). However, the ridge cannot be
linked to the MEG, because till containing Alpine material is only found
30 m below the surface and covered by gravel and loess (Preusser et al.,
2011; Gaar and Preusser, 2017). While the till can be attributed to the
MEG, although no numerical age control is available, the ridge has to be
interpreted as a loess dune, not as a moraine, as it consists of loess (Gaar
and Preusser, 2017). The dune might have been deposited by strong winds,
called the “Möhlin jet”. The jet nowadays mostly occurs in winter, when
the air fills up the Swiss Plateau and overflows the eastern Jura Mountains.
This results in a dry wind with a speed of up to 10 m s-1
(Schüepp, 1982; Müller, 2001; Gaar and Preusser, 2017). In
2011, a rotary drill core was recovered from the ridge, consisting entirely
of loess sediment down to 6.8 m depth. According to luminescence dating (OSL
and IR50), most of the loess was deposited during marine isotope stage
(MIS) 2 and late MIS 3, while the lowermost part of the sequence probably
dates back to MIS 4 (Gaar and Preusser, 2017).
For the present study, we analyzed long-chain n-alkanes, i.e., leaf waxes, in
the drill core from Möhlin in order to evaluate their potential for
paleovegetation reconstruction. Moreover, we applied compound-specific
δ2H analyses on the most abundant n-alkanes (n-C29 and
n-C31) to reconstruct paleoclimate and hydrology. In addition, we
performed radiocarbon dating on some n-alkane samples to test the
stratigraphic integrity of the leaf waxes.
Research area. Orange fields mark loess deposits, drawn after
Gouda (1962). Source hillshade: Federal Office of Topography.
Leaf waxes – a novel tool in Quaternary research
Long-chain n-alkanes (> n-C25) are produced by higher
terrestrial plants (de Bary, 1871; Eglinton and Hamilton, 1963, 1967;
Kunst, 2003; Shepherd and Wynne Griffiths, 2006; Samuels et al., 2008).
They are straight, saturated carbon–hydrogen chains (CnH2n+2),
stable over geological timescales (Eglinton and Eglinton, 2008;
Schimmelmann et al., 2006) and thus preserved in various sedimentary
archives. Their homologue pattern depends on the type of vegetation
(Cranwell, 1973; Marseille et al., 1999; Schwark et al., 2002) and can
provide information about whether deciduous trees or grasses were dominant in the
research area, whereas deciduous trees produce mainly n-C27 and
grasses predominantly n-C31 and n-C33 (Zech et al., 2010;
Schäfer et al., 2016b; Schwark et al., 2002). Although controversy
exists as to whether this approach can be used universally to reconstruct
paleovegetation (Bush and McInerney, 2013; Wang et al., 2015), there is
good evidence that leaf wax patterns can be used on a local (Schwark et
al., 2002) and regional (Schäfer et al., 2016a) scale in Europe.
Schwark et al. (2002) investigated n-alkanes in a lake in southern
Germany and found a remarkable accordance of pollen records and n-alkanes.
Schäfer et al. (2016a) measured leaf wax patterns along a transect from
southern to northern Europe in grasslands, deciduous forests and coniferous
forests. They analyzed samples from litter and two depths of the uppermost
soil horizon. The results of the Schäfer et al. (2016a) study are that
(1) the chain lengths correlate significantly along the transect with the
grassland and deciduous vegetation, (2) n-alkanes from conifer forests
show the understorey and (3) a correction for degradation of the n-alkanes
is needed to get reliable information about the vegetation. n-Alkane
patterns are thus a particularly valuable tool for the reconstruction of
paleovegetation when pollen are absent, which is often the case in
loess–paleosol sequences (LPS).
The compound-specific isotopic composition of n-alkanes (δ2Hwax) is increasingly used as a tool for the reconstruction
of past climate conditions. Liu and Huang (2005) measured δ2Hwax in a Chinese loess–paleosol sequence and showed that
it recorded past climate changes. In principle, δ2Hwax
reflects the isotopic composition of precipitation (δ2Hprecip) (Sachse et al., 2012), which in turn depends on
(and is thus a proxy for) climate (temperature, precipitation amount,
evaporation) and geographical location (latitude, altitude, continentality,
moisture source) (Dansgaard, 1964). Additional factors that need to be
taken into account when interpreting δ2Hwax records are
biosynthetic fractionation, evapotranspirative enrichment, vegetation period,
degradation and type of vegetation (Sessions et al., 1999; Sachse et al.,
2006; M. Zech et al., 2011, 2015; Kahmen et al., 2013; Gao et al.,
2014; Hepp et al., 2015; Tipple et al., 2013). Evapotranspiration leads
to an enrichment, depending mainly on relative humidity (M. Zech et al.,
2013; Farquhar et al., 2007). Biosynthetic fractionation is often assumed
to be ∼ 160 ‰ (Sachse et al., 2012).
Compound-specific radiocarbon analyses on sedimentary n-alkanes is a new tool
in geochronology (Häggi et al., 2014; Haas et al., 2017; Zech et al., 2017). It allows us to test the synsedimentary
nature, and thus the stratigraphic integrity of leaf waxes, and can
complement other dating methods, for example based on luminescence.
Material and methods
The drill core from Möhlin is described in detail in Gaar and
Preusser (2017). In brief, the core consists of loess down to 6.8 m but
is decalcified and overprinted by pedogenesis down to 1.6 m depth and from
5.9 to 6.2 m. We sampled the core in continuous 20 cm intervals for leaf wax
analyses.
Leaf wax analyses
Approximately 40 g of each sample was extracted with an accelerated solvent
extractor using dichloromethane / methanol (9/1). The total lipid
extract was dried and passed over pipette columns with an aminopropyl-coated
silica gel as the stationary phase. Nonpolar compounds, including n-alkanes, were
eluted with hexane and then spiked with 5α-androstane. The
long-chain n-alkanes (nC25–nC35) were analyzed using a GC-FID (gas
chromatography–flame ionization detector) at the Department of Soil
Science at TUM-Weihenstephan, Freising, Germany. The Thermo Scientific Trace 1310 was equipped with a Zebron ZB-5HT column and operated in splitless
injection mode. The He carrier gas flow was set constantly to 1.2 mL min-1, and
the GC temperature was first held at 50 ∘C for 1 min, ramped to
250 ∘C at 30 ∘C min-1 and then to 340 ∘C at
7 ∘C min-1, and held for 11 min.
The most abundant n-alkanes, nC29 and nC31, were later targeted at the
Geographical Institute, University of Bern, for compound-specific δ2H analyses. Using an IsoPrime 100 mass spectrometer, coupled to an
Agilent 7890A GC via a GC5 pyrolysis–combustion interface operating in
pyrolysis mode with a Cr (ChromeHD) reactor at 1000 ∘C. Each
sample was measured three times. Precision of the measurements was checked
by analyzing a standard n-alkane mixture with known isotopic composition twice
every six runs. The H3+ factor was 3.4 and stable. The results are
given in delta notation (δ2H) versus Vienna Standard Mean Ocean
Water (VSMOW).
n-Alkane patterns
The total n-alkane concentration (µg g-1 dry weight, Atot) is here
defined as the sum of nC25 to nC35. The odd-over-even predominance
(OEP; Eq. 1) was calculated after Hoefs et al. (2002) and is an
indicator for degradation: values smaller than 5 are considered to be
strongly degraded (Zech et al., 2010; Schäfer et al., 2016a) and must thus be interpreted with care.
OEP=nC27+nC29+nC31+nC33nC26+nC28+nC30+nC32
Changes in the average chain length (ACL; Eq. 2) of the measured n-alkanes
indicate whether deciduous trees and shrubs (shorter ACL) or grasses and
herbs (longer ACL) were the dominant plant type (Poynter et al., 1989).
ACL=27⋅nC27+29⋅nC29+31⋅nC31+33⋅nC33nC27+nC29+nC31+nC33
Because degradation can influence the ACL, one needs to correct for those
effects (Zech et al., 2010). We performed a correction after
Schäfer et al. (2016a) (Eq. 3–6), which results in the
semi-quantitative %grass content for the samples.
Treeendmember=0.09⋅lnOEP+0.66Grassendmember=-0.17⋅lnOEP+0.75Ratio=n-C31+n-C33n-C27+n-C31+n-C33%Grass=ratio-treeendmembergrassendmember-treeendmember
Radiocarbon dating
For radiocarbon analyses of the n-alkanes, four samples were selected and
the nonpolar fraction passed over two pipette columns filled with AgNO3
impregnated silica gel and zeolite, respectively. The zeolite was dissolved
in HF, and the purified n-alkanes were recovered via liquid–liquid
extraction with n-hexane. Finally, the purified n-alkanes were
transferred with dichloromethane into tin capsules. The 14C measurements
were performed on the MICADAS accelerator mass spectrometer, coupled to an
element analyzer (Ruff et al., 2010; Salazar et al., 2015), at the LARA
AMS Laboratory, University of Bern (Szidat et al., 2014). 14C results
are reported as fraction modern carbon (F14C) and were corrected for
constant and blank contamination (Salazar et al., 2015; Haas et al.,
2017). The blank contamination was 0.4 µg C for a single tin
capsule with a F14C value of 0.734 ± 0.19. The calibrated
radiocarbon ages were calculated using OxCal (Ramsey, 2009) and IntCal13
(Reimer et al., 2013).
Radiocarbon data and ages for the four selected samples.
Most samples from Möhlin are characterized by a dominance of odd,
long-chain (> nC25) n-alkanes typical for leaf waxes; additionally, other compounds and an unresolved matrix complex occur in
variable amounts (Fig. 2).
However, samples from 0.4 to 1.6 m depth have very low concentrations and
quantification of the target compounds is difficult. Total n-alkane
concentrations for these samples can only be estimated as
< 0.4 µg g-1, and the OEP values are less than 5,
indicating enhanced degradation. These samples are therefore not plotted in
Fig. 3. All other data are illustrated in Fig. 3, Table 1 and the supplement
table (Wüthrich et al., 2017b).
The two uppermost samples have n-alkane concentrations of 1.2 and
0.4 µg g-1, OEP values of 8.3 and 7.5, and ACL values of 30.1,
respectively. Samples from depths of 1.6 to 6.8 m have highly variable
n-alkane concentrations, ranging from 1 to 5.3 µg g-1. Their OEP
values range from 9.9 to 18.6, and their ACL is between 30.5 and 31.1. The
uppermost two samples and the samples below 1.6 m depth had sufficient
amounts of nC29 and nC31 for compound-specific δ2H
analyses. δ2Hwax values range from
-215.1 to -143.5 ‰ and from
-210.4 to -146.9 ‰, respectively.
Down-core patterns are very similar and correlate well with an R value of
0.68.
All four samples selected and purified for radiocarbon dating yielded
sufficient carbon masses for AMS analyses (Table 4.1). Fraction modern
(F14C) ranges from 0.023 to 0.055, yielding calibrated 2σ ages
between 26.4 and 38.0 cal kyr BP. As we did measure the whole n-alkane
fraction and not single compounds, a contamination of post-sedimentary,
especially short-chain and even-numbered, n-alkanes is possible, leading to
too-young ages. But as three samples are in very good agreement with the ages
published by Gaar and Preusser (2017), a contamination can most probably be
excluded for the uppermost three samples.
Example of a chromatogram (M13 from 2.4 m depth), showing the unresolved matrix complex
(green) and the n-alkane compounds produced in leafs.
n-Alkane dates for the Möhlin loess sequence. OSL and IR50
(italic) ages have previously been published in Gaar and Preusser (2017);
radiocarbon ages are given in bold letters.
DiscussionChronology
The uppermost IR50 (infrared stimulated luminescence at 50∘) and OSL
(optically stimulated luminescence) ages from Gaar and Preusser (2017)
suggest that at least the upper ∼ 1 m of Möhlin sequence was
deposited ∼ 19 ka, i.e., MIS 2, and just before final deglaciation of
the Swiss Plateau (Wirsig et al., 2016; Wüthrich et al., 2017a). Soil
formation and decalcification down to 1.6 m depth must have occurred during
the Late Glacial and Holocene. The next three IR50 and OSL ages from depths
of 3.4 to 5 m
document rapid sedimentation between 34 and 31 ka, i.e., late MIS 3.
Our radiocarbon ages from depths of 2.0 and 3.4 m are slightly younger (26.4
to 30.0 cal kyr BP) but are in reasonable agreement in view of the
limitations and uncertainties related to both the luminescence and the
radiocarbon dating methods. The radiocarbon age from 5.8 m depth is 31.7 to
38.0 cal kyr BP and with only 0.023 F14C even closer to the lower
limit of radiocarbon dating. Nevertheless, the age is also in good agreement
with the OSL and IR50 ages of 31.0 and 41 kyr, respectively, from the same
depth, so all of these ages document rapid loess accumulation during late
MIS 3 (Fig. 3).
The weak paleosol preserved between 5.9 and 6.2 m depth must have developed
earlier although probably still during MIS 3, because it developed into loess
deposited during MIS 4 based on OSL and IR50 ages of 68.1 and 58.1 kyr,
respectively. Our radiocarbon age from 6.8 m depth is only
28.7–32.2 cal kyr BP and very likely underestimates the real
sedimentation age of the loess at this depth. The F14C of the sample is
only 0.038 and the smallest amounts of contamination (in the lab or from
other compounds) can readily explain the discrepancy between
the luminescence and radiocarbon ages. Another possibility might be incorporation of
root-derived n-alkanes by roots from plants growing after deposition, as
suggested, for example, by Gocke et al. (2014). But a post-sedimentary
production of roots can most probably be excluded, as shown by Häggi et
al. (2014), Zech et al. (2017) and Haas et al. (2017).
In general, our radiocarbon results are in reasonable agreement with the ages
of Gaar and Preusser (2017) and corroborate that radiocarbon analyses of
n-alkanes are a promising new tool for dating LPS back to ∼ 30 ka (Häggi et al., 2014; Haas et al.,
2017; Zech et al., 2017). Moreover, the stratigraphic integrity and
synsedimentary nature of the long-chain n-alkanes could be confirmed for
the uppermost 6 m.
The OSL, IR50 (Gaar and Preusser, 2017) and our radiocarbon ages show that
the major part of the sequence was developed between 35 and 30 kyr and that
older deposits are probably influenced by erosion. The paleosol might show a
hiatus. Our interpretation is thus mainly valid for the time between 35 and
30 ka, when the rapid loess accumulation occurred.
Paleovegetation
The uppermost two samples from depths of 0.2 and 0.4 m have a relatively low
ACL compared to most other samples from the profile. This indicates more
input of n-alkanes from deciduous trees and shrubs; however, concentrations
are also lower, and lower OEP values show enhanced degradation (Fig. 4). The
plot also illustrates that changes in the alkane ratio (the same is true for
the ACL) are partly an artefact of degradation. However, the sample from
0.4 m depth plots furthest below the grass endmember, which is numerically
expressed as lowest %grass (Fig. 3). This possibly reflects the remnant
leaf wax signal from the natural potential vegetation at Möhlin during
the Holocene, i.e., mainly deciduous trees, whereas the site is used today as
grassland.
Endmember plot after Schäfer et al. (2016a). Our samples from
Möhlin are plotted in blue.
While the n-alkanes between 0.6 and 1.6 m are strongly degraded and too low
in concentration to robustly infer any paleoenvironmental conditions during
MIS 2, high concentrations and good preservation allow this for the rest of
the sequence. The positive trend in the ACL from depths of ∼ 6 to 2 m
indicates an increase of grass-derived n-alkane input during late MIS 3. However, the observed trend in ACL is probably an artefact of
degradation. This is illustrated in the endmember again (Fig. 4), in which
all these samples plot very close to the grass endmember. Accordingly,
%grass (Fig. 3) does not show much of a trend and no major vegetation
changes seem to be documented in the n-alkane patterns for the lower part of
the profile between 1.8 and 6.8 m depth, i.e., during MIS 4 and MIS 3. This
example shows that it is imperative to not over-interpret ACL and to account
for degradation, even when preservation is generally good.
Our results suggest that the n-alkanes preserved at Möhlin during MIS 3
and possibly MIS 4 were mainly produced by grasses and herbs and that
deciduous trees and shrubs played a minor role, if any. Based on n-alkanes
only, however, one cannot rule out that conifer trees grew at the research
site. n-Alkane concentrations in conifer needles are mostly about an order
of magnitude lower than in deciduous trees or grass, with the exception of
Juniperus. Hence n-alkane proxy records are quite insensitive for
detecting conifer vegetation (Diefendorf et al., 2011; Tarasov et al.,
2013; Schäfer et al., 2016b). Open conifer forest during parts of MIS 3
have been reported from peat sequences at Gossau (Schlüchter et al.,
1987; Preusser et al., 2003) and Niederwenigen (Drescher-Schneider et
al., 2007), both located a few dozen kilometers to the east of Möhlin.
Similar vegetation may have prevailed at Möhlin, with the long-chain
n-alkanes having only recorded thinly recorded grasses, i.e., the
understory of the open conifer forest.
Paleoclimatology
The δ2Hwax values from the uppermost two samples
(ranging from -167 to -205 ‰; Fig. 3) are perfectly in the range observed in topsoil samples from central
Europe (Schäfer et al., unpublished data) and in agreement with
what can be expected based on theoretical considerations. Assuming a
constant metabolic fractionation of -160 ‰, δ2Hleafwater for the uppermost two samples ranges from
-8.7 to -53.4 ‰. This is close but
tends to be a little bit higher than today's δ2Hprecip in
Möhlin, which has a value of ∼-40 ‰ in
summer (Bowen et al., 2005; Bowen, 2008). As spring and
possibly also winter precipitation, which is more depleted than summer
precipitation, may have been used by the plants as well, some
evapotranspirative enrichment certainly occurred.
The δ2Hwax values below 1.8 m depth are relatively
constant around -180 ‰, i.e., similar to the values from the
uppermost two samples. Most conspicuously, a sudden drop occurs just below
5 m depth to values as low as -215 ‰ (Fig. 3).
δ2Hwax then increases again in the lowermost meter of
the Möhlin sequence. The observed δ2Hwax pattern
strongly resembles the pattern from the LPS Bobingen, Germany, 200 km
northeast of Möhlin (R. Zech et al., 2015). Here, n-alkanes are
∼-200 ‰ in sediments dated to latest MIS 3 (∼ 30 ka)
and MIS 2, and values drop to < -220 ‰ below, before they
increase again in the lowest part of the profile dated to ∼ 45 ka.
Overall, we are therefore confident that the δ2Hwax
record from Möhlin is a local signal but also carries valuable regional
information about paleoclimate changes. Interpretation in terms of changes in
δ2Hprecip and evapotranspirative enrichment is,
however, very challenging because disentangling these two major controls is
not yet possible. To the best of our knowledge, there are no independent
continuous records of δ2Hprecip in Europe during the
last glacial. Luetscher et al. (2015) presented a δ18O record
from a northern Alpine speleothem, but it overlaps only slightly with ours
and shows very little variation from 30 to 15 ka (∼ 1 ‰
δ18O, which is equivalent to ∼ 8 ‰ δ2H).
The only long continuous isotope records spanning the last 40 to 60 kyr
currently come from an LPS in Crvenka, Serbia (R. Zech et al., 2013), which
shows not much more than 10 ‰ variability, and from marine cores
offshore Portugal (Abreu et al., 2003) and in the Mediterranean Sea
(Frigola et al., 2008). The marine δ18O records (measured on
foraminifera) show a minor trend towards more enriched values (corresponding
to a δ2H trend < 10 ‰) from ∼ 60 to 20 ka.
Afterwards, during the last glacial termination and the Holocene, values
become much more negative (∼ 20 ‰ δ2H). A comparison
of these records suggests the following hypotheses: the sudden enrichment in
δ2Hwax after 32 kyr does not reflect a sudden change
in the source, i.e., the North Atlantic. It might therefore document a sudden
onset of more arid conditions and enhanced evapotranspiration.
The interpretation of the uppermost two samples is trickier: on one hand, the
elevated δ2Hwax of nC29 in the uppermost two
samples in Möhlin might document even more arid conditions than during
MIS 3, because they do not show the more negative source values and thus may
have experienced even more evapotranspirative enrichment, possibly caused by
elevated temperatures and similar precipitation compared to MIS 3. On the
other hand, δ2Hwax of nC31 is much more negative
than nC29. The reason for this might be different sources of
nC29 and nC31. It makes a difference whether the alkanes are
produced by grasses or deciduous trees: grasses produce their n-alkanes
mainly in the intercalary meristem and are thus much less influenced by
evaporative enrichment than deciduous trees and shrubs (Kahmen et al.,
2013), which produce their n-alkanes mostly at leaf flush (Tipple et al.,
2013) and are thus much more influenced by relative humidity. Nevertheless,
grasses produce also n-alkanes in their leafs (Gao et al., 2012) and also
show evaporative enrichment to some degree. In older publications, it has
been stated that nC29 is mainly produced by deciduous trees
(Cranwell, 1973; Zech et al., 2010); the publication of Schäfer et
al. (2016a) shows that both nC29 and nC31 are
more or less equally produced by grasses and shrubs. However, relative to
nC31, Schäfer et al. (2016b) state that
deciduous trees produce more nC29. Thus
the elevated amount of deciduous trees, recorded in the uppermost two
samples, might show evaporative enrichment in nC29. Also different
biosynthetic fractionation of different plants might be a possible influence
on the different values (e.g., Gao et al., 2014). As δ2Hwax values of shrubs, trees and grasses are quite similar
(Sachse et al., 2012), we think that evapotranspirative enrichment plays a
more important role.
However, it is unclear how changes in atmospheric circulation, and thus
source areas, have changed in the past and to which degree a temperature
effect at the site of precipitation may have been relevant. If such
a temperature effect was relevant on glacial–interglacial timescales and if
the isotope record offshore Portugal reasonably reflected changes in the source signal, both effects
would have canceled each other out explain the similar
δ2Hprecip and δ2Hwax values
during the Holocene and the late MIS 3. Drawing robust paleoclimatological
conclusions from δ2Hwax records thus remains very
difficult and independent δ2Hprecip records are needed
to reconstruct changes in paleohumidity and evapotranspirative enrichment.
Last but not least, the OEP might also be influenced by climatic conditions: during MIS 3 it is lowest, when δ2Hwax also has its
lowest values. Higher humidity, expressed in lower δ2Hwax values, might allow higher microbial activity and thus an
enhanced degradation, which leads to a lower OEP.
Conclusions
Our investigations of long-chain n-alkanes from the Möhlin sequence
reveal that they are well preserved and occur in sufficient amounts in the
uppermost samples down to 0.4 m depth, as well as from 1.8 to 5.8 m depth,
to use them for the reconstruction of paleovegetation and paleoclimate and
for radiocarbon dating. From 0.6 to 1.6 m depth, concentrations are very
low, probably related to Holocene pedogenesis and priming.
The n-alkane pattern of the uppermost samples reflects today's grassy
vegetation and possibly some leaf wax remnants of the natural deciduous
forests that prevailed during most of the Holocene. No major vegetation
changes are detected below 1.8 m depth. All samples indicate a dominant
input of grass-derived leaf waxes and negligible contributions from
deciduous trees and shrubs.
Compound-specific δ2H analyses have yielded values for the
uppermost samples that one can expect from today's isotopic composition of
the precipitation, the metabolic fractionation and some evapotranspirative
enrichment. The δ2Hwax values from 1.8 to 5 m depth,
i.e.,
during late MIS 3, are not much different from today's values, which might
document that the source effect (more positive source water in the North
Atlantic during the glacial) and the temperature effect (more negative
precipitation during glacial times) cancel each other out. However, we
cannot exclude that changes in evapotranspirative enrichment or in
atmospheric circulation and thus source areas were also relevant.
Independent records of δ2Hprecip would be necessary to
quantitatively derive robust paleoclimatic information, particularly
relative humidity and evapotranspirative enrichment. A very interesting
feature in the δ2Hwax pattern from Möhlin is a sudden
shift of ∼ 30 ‰ towards more negative
values below 5 m depth. This shift is also observed in another LPS in the
northern Alpine foreland and therefore probably a regional phenomenon. It
might document a major change in paleohydrology, namely a shift from more
humid to more dry conditions at ∼ 32 ka. Nevertheless,
independent δ2Hprecip records are needed for more robust
paleoclimatic reconstructions.
Radiocarbon dating of the n-alkanes have yielded ages in reasonable
agreement with published OSL and IR50 ages, although ages of ∼ 30 kyr
are close to the limit of radiocarbon dating of the n-alkanes. The
synsedimentary nature and stratigraphic integrity of long-chain n-alkanes
are thus corroborated, highlighting the great potential of this new tool for
dating loess–paleosol sequences. The chronology of the Möhlin sequence
shows that loess accumulation occurred during MIS 4 and started again
∼ 34 ka, well before the onset of MIS 2.
Overall, our study shows the great potential of leaf wax analyses in LPS.
More high-resolution records of leaf wax patterns and compound-specific
δ2H, complemented by larger numbers of radiocarbon dating, would be
useful to investigate the regional variability of respective proxy patterns.
Ideally, such studies should be accompanied by other sedimentological,
paleopedological and geochemical methods. Provenance analyses at Möhlin,
for example, might help to investigate the proposed changes in wind direction
and loess sources. Other leaf wax compounds, such as long-chain
n-carboxylic acids or n-alkanols, or other biomarkers in general, could be
used to corroborate and refine the vegetation reconstruction. Most
importantly, the lack of independent 2Hprecip records
currently limits the possibility to robustly reconstruct past changes in
relative humidity and evapotranspirative enrichment. A particularly promising
path for future work is to further develop the biomarker-based
“paleohygrometer”, which is based on a coupled δ2Hwax and δ18Osugar approach. It allows
us to disentangle changes in evapotranspiration and
δ2Hprecip and has successfully been tested in topsoils
(Tuthorn et al., 2015) and applied to organic-rich archives (M. Zech et
al., 2013; Hepp et al., 2015).
The dataset used in this paper can be found on the Pangaea
database (Wüthrich et al., 2017b).
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank Jasmin Aschenbrenner and the Chair of Soil Science, TUM Freising,
for support during labwork, Michael Zech and Johannes Hepp for fruitful
discussions and the SNF (131670 and 150590) for funding. We also acknowledge
the comments of the two reviewers which helped to improve the manuscript.
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