Sources of n-alkanes
Potential and constraints of n-alkane-based proxies
Sedimentary leaf wax n-alkane sources can be broadly distinguished based on
n-alkane chain length. Principally, mid-chain alkanes (nC23 and
nC25) are mostly attributed to aquatic sources or Sphagnum species, while
long-chain compounds can be derived from terrestrial plants or emergent
macrophytes (Eglinton and Hamilton, 1967; Baas et al.,
2000; Ficken et al., 2000). In addition, it has been postulated that nC31 is indicative of
grasses as trees preferably synthesize nC27 and nC29 alkanes.
Consequently, the ratios of these compounds, such as nC27 vs. nC31, have
frequently been applied in sedimentary archives to infer ecological changes
in lake catchments such as shifts from grassland to trees (Meyers and
Ishiwatari, 1993; Schwark et al., 2002; Meyers, 2003).
Nevertheless, an increasing number of studies have indicated that alkane patterns
of plants are heterogeneous and compound distributions often overlap. For
example, in some environments so-called “aquatic” mid-chain compounds
(nC23 and nC25) have been shown to be derived partly from
terrestrial sources (Aichner et al., 2010a; Gao et al., 2011), while
long-chain “terrestrial” compounds (nC27 and nC29) were derived
mainly from aquatic sources (Liu et al., 2015). Further, compilations of
global plant data sets clearly indicate that indices such as ACL and long-chain
n-alkane ratios do not reliably reflect classes of terrestrial vegetation
(Bush and McInerney, 2013). In addition, the sedimentary n-alkane pattern
will be dominated by plants producing and delivering the largest amount of
n-alkanes into the sedimentary sink. Based on a large-scale survey of
different tree species, Diefendorf et al. (2011) found that angiosperms
showed a tendency to produce higher concentrations of n-alkane leaf wax
lipids compared to gymnosperms, a result later confirmed by Bush and McInerney (2013).
Carbon isotopes as source indicators
Source allocation of n-alkane homologues can also be inferred from their
carbon isotope composition. For instance, in West African lakes with
catchments dominated by C4 vegetation, only nC31 alkanes showed a
C4 isotopic signal. In contrast, other long-chain n-alkanes carried a
C3 plant signature (Garcin et al., 2014), likely resulting from higher
n-alkane production in the few C3 trees in the lake catchment compared
to the dominating C4 grasses. While no C4 plants were present in
eastern Europe, aquatic plants are also characterized by more positive
δ13C values (Allen and Spence, 1981; Keeley and Sandquist,
1992; Ficken et al., 2000; Mead et al., 2005; Aichner et al., 2010a), and an
aquatic origin of mid- and long-chain n-alkanes can be confirmed or rejected:
the constant δ13C values of ca. -33 ‰ for
nC31 and slightly more variable values for nC29 in our data set from
TRZ paleolake suggest a stable terrestrial C3 vegetation source for
these compounds (Meyers, and Ishiwatari, 1993; Meyer, 2003) during the study
period. The 13C enrichment in nC23, nC25, and nC27 during
the YD could be explained by (a) a change in the sources of these compounds or
(b) limited CO2 availability to potential aquatic sources. The latter
could be induced by low atmospheric pCO2 or by the establishment of
diffusion barriers such as ice cover and/or a thickened boundary layer in
stagnant water. CO2 limitation due to enhanced aquatic productivity,
higher water temperature, salinity, or pH can be excluded (Street-Perrott et
al., 2004; Aichner et al., 2010a, b). More positive δ13C values
could also result from a reduced isotopic fractionation of terrestrial
plants subjected to water stress (Farquhar, 1989), in particular during the
YD, when 8 %–15 % lower humidities have been reconstructed at MFM (Rach et
al., 2014, 2017). On the other hand, all terrestrial plants would be
affected by increased aridity, which should be reflected in all n-alkane
homologues, contrary to our observations.
n-Alkane concentrations and average chain lengths (ACLs)
during the studied interval in comparison to pollen and macrofossil counts
(Wulf et al., 2013; Słowinski et al., 2017; unpublished macrofossil data
from 10 000–12 400 yr BP). NAP: non-arboreal pollen. Grey shaded
interval: YD according to pollen cluster.
Comparison with palynological proxies
Mid-chain n-alkanes (nC23, nC25)
Concentrations of mid-chain-length compounds were low throughout the whole
record. For instance, nC23 exceeds 10 µg/g d.w. only during
some episodes in the Allerød. (Fig. 2). Submerged aquatic macrophytes
principally produce low amounts of pollen and mainly use vegetative
strategies for reproduction. Consequently, very low concentrations of
aquatic pollen have been counted in the sediment samples. However,
macrofossil remains of submerged species Potamogeton and Chara have also been found only in low
amounts in some samples from the Allerød and early YD (Fig. 3). This
gives evidence that submerged aquatic species have been of low abundance and
contributed little to sedimentary organic matter in TRZ throughout the
studied time interval, with a somewhat higher proportional input during the
Allerød and early YD. Low δ13C values, which vary between
-33 ‰ and -28 ‰, also indicate a
primary terrestrial origin of mid-chain n-alkanes. Submerged macrophytes use
13C-enriched bicarbonate as a carbon source, especially when carbon is
limited at highly productive stands, leading to δ13C values of
n-alkanes that can reach up to -12‰ (Allen and
Spence, 1981; Keeley and Sandquist, 1992; Aichner et al., 2010a, b). Hence,
low δ13C values for mid-chain compounds as measured in TRZ
either indicate (a) low productivity of submerged macrophytes and/or (b) a relatively high proportional contribution of terrestrial sources to
nC23 and nC25. We conclude that these mid-chain compounds comprise a
mixture of aquatic and terrestrial sources, especially during the YD and
Holocene.
High-resolution palynological data from the Allerød
and transition into the YD in comparison to concentration of n-alkanes,
ACL,
and the C27 / (C27+ C31) n-alkane ratio. The light grey
shaded period indicates the transition from the Allerød to the YD (12 680–12 620 yr BP) as defined by Słowinski et al. (2017), while the dark
grey shaded interval marks the YD. NGRIP ice core stratification according
to Rasmussen et al. (2014). Σherbs: NAP without Poaceae and Cyperaceae.
Long-chain n-alkanes (nC27, nC29,
nC31)
To further elucidate plant sources of n-alkanes in the sediments of TRZ we
compared our organic geochemical data to pollen and macrofossil spectra
(Fig. 3; low-resolution pollen data from Wulf et al., 2013; decadal
pollen–macrofossil data across the Allerød–YD transition from Słowinski
et al., 2017; unpublished low-resolution macrofossil data from 12 400–10 000 yr BP). Based on these data, the YD was characterized by relatively
low amounts of tree and shrub pollen, except for Juniperus, which expands after the
YD onset, and enhanced proportional input from herbaceous plants (Poaceae,
Cyperaceae, Artemisia, Chenopodiaceae). The arboreal communities during the Allerød
and Holocene were dominated by Pinus. Relatively high amounts and percentages of
pollen and macrofossils derived from Betula sp. were mainly observed in samples
from the YD to Holocene transition and also in samples from the early
Allerød (Figs. 3 and 4).
From the aliphatic compounds in TRZ, the nC27 alkane shows the largest
variability in concentration, which ranges from < 10 µg/g d.w. during the YD up to > 100 µg/g d.w. during the
Allerød. This seems to reflect the trend of a contribution of arboreal
pollen, specifically from Betula, to the lake sediments (Figs. 3 and 7). Leaf waxes
of the gymnosperm tree species Pinus sylvestris have been shown to often contain high
relative abundances of nC27 alkanes, but absolute concentrations are in
most cases very low (Maffei et al., 2004; Ali et al., 2005; Dove and Mayes,
2005; Diefendorf et al., 2011; Bush and McInerney, 2013). It is therefore
likely that Pinus does not contribute significant amounts of n-alkanes to the
sedimentary archive of TRZ. On the other hand, Betula sp. as the second most
abundant tree genera in the lake catchment could have been a major
contributor to the sedimentary nC27 pool (Schwark et al., 2002;
Diefendorf et al., 2011) and likely biosynthesizes higher absolute amounts
compared to Pinus. Comparing palynological abundances and nC27 concentration
data supports this idea. The high concentrations of nC27 alkanes during
the early Allerød, between ca. 13 250 and 13 100 yr BP, coincide with an
interval characterized by a dominance of Betula over Pinus (Wulf et al., 2013).
Further, the increasing concentrations of nC27 between ca. 11 700 and
11 500 yr BP were in phase with Betula expansion at the YD–Holocene transition
(Fig. 3). Finally, the expansion of Pinus after the onset of the Holocene was not
reflected in increasing nC27 concentrations; rather, a decrease in
n-alkane concentrations was observed. This decrease could be explained by a
synchronous decrease in absolute and relative Betula abundances. Based on the
covariation of nC27 concentrations and palynological data we consider
Betula sp. as a dominant contributor to this compound, while Pinus sylvestris appears to be a less
relevant source.
Heat map table illustrating Pearson correlation
coefficients between concentrations of pollen and n-alkanes at the
Allerød–YD transition (1230–1271 cm). Data have been z-transformed
before correlation. Bold black numbers: p<0.01. Black numbers: p<0.05. Positive correlations in green and negative correlations
in red. AP: arboreal pollen. NAP: terrestrial non-arboreal pollen.
Another gymnosperm shrub species, Juniperus communis, was expanding during the early YD,
peaking between ca. 12 300 and 12 500 yr BP, before gradually declining
(Fig. 3; Wulf et al., 2013; Słowinski et al., 2017). Reported n-alkane
concentrations for this species ranged from very low (Maffei et al., 2004)
to high (Mayes et al., 1994). In the study of Diefendorf et al. (2011),
Juniperus osteosperma was the only analyzed gymnosperm that showed intermediate concentrations of
n-alkanes. All these studies reported that Juniperus sp. biosynthesizes relatively
long chain lengths, in the range C31–C35, a pattern which has also
been observed in plant samples from Lake Steisslingen (southern Germany;
Schwark et al., 2002). This suggests that Juniperus sp. could be a significant
contributor to nC31 also in the TRZ sediments, especially during the
early YD. During phases of high nC31 abundance in the Allerød and
Holocene other contributors must have been predominant.
Herbaceous plants could theoretically contribute to all measured
n-alkanes, but have often been associated with a dominance of longer
n-alkane chain lengths (e.g., Maffei, 1994, 1996a, b; Zhang et al., 2004;
Rommerskirchen et al., 2006). In TRZ, grasses and other herb pollen showed
varying concentrations, with a tendency of higher amounts during the YD.
Their proportional contribution significantly increased or decreased at the
YD onset or termination, which was enhanced by the strong decline of tree
pollen during the cold interval. We assume that these types of plants could indeed
be strong contributors to the longer chain lengths, i.e., nC29 and
nC31, specifically during the YD when concentrations of those compounds
approach values of the dominating nC27 alkane. A proportional
contribution from trees cannot be fully excluded, especially during the
abovementioned phase of Juniperus expansion (early YD) as well as during parts of
the early Holocene and Allerød when Betula was dominating.
Considering emergent aquatic and telmatic species, macrofossils were counted
for species such as Typha latifolium and Sparganium as well as multiple species of ferns (Fig. 4).
Those species might have contributed to the pool of long-chain n-alkanes
(Ficken et al., 2000), at least during the Allerød and possibly also
during the Holocene. After the YD onset, those species widely disappeared
from the littoral due to their low tolerance for cool summer temperatures
(Fig. 4; Słowinski, et al., 2017).
Statistical correlation across the Allerød–YD transition
To analyze possible correlations between occurrences of alkanes with
vegetation more in detail, we compared n-alkane concentrations throughout the
Allerød and transition into the YD (ca. 13 350–12 400 yr BP) with
high-resolution pollen data from the same interval (Figs. 4 and 5). For the
latter, data published in Słowinski et al. (2017; 1230–1262 cm) have
been complemented by further unpublished data extending further into the
Allerød (1262–1271 cm). Macrofossils have been excluded from this
survey (a) due to lower sampling resolution (1 vs. 0.5 cm for organic
geochemical proxies) compared to pollen and organic geochemical data and (b) as macrofossil samples were not taken from the same core.
We found that AP counts as well as some herbaceous plants and ferns showed
a significant positive correlation with concentrations of C25, C27,
and C29 n-alkanes. In contrast, nC31 concentrations showed a
significant correlation with NAP counts, but also with Salix. Significant
correlations are also found between n-alkane concentrations and algae
(Botryococcus, Pediastrum) since the latter have been found in relatively high concentrations during
the Allerød. Since Botryococcus does not produce n-alkanes (Lichtfouse et al., 1994),
this could well be considered as an autocorrelation.
It has to be noted that concentrations of n-alkane homologues strongly
correlated with one other (Table S3 in the Supplement). This autocorrelation is
due to the fact that basically all n-alkanes showed the same rough trend with
high concentrations during the Allerød and low concentrations during the
YD. Autocorrelations could also be found within pollen data; e.g., Pinus counts
significantly correlated with Betula (R=0.79; p<0.01;
Table S4 in the Supplement). Hence, significant correlation does not necessarily indicate
origin of n-alkanes but could be due to intercorrelations between
concentrations of source organisms, which is specifically relevant for
negative correlations. Principally, the correlation coefficients reflect the
major trends of pollen counts and n-alkane concentrations throughout the
Allerød–YD transition. Significant positive correlations indicated that
the pollen counts follow the rough trend of n-alkanes, i.e., high
concentrations during the Allerød, low concentrations during the YD. In
contrast, significant negative correlations suggested opposite trends, as
relevant for Juniperus and some herbs (Artemisia, Chenopodiaceae, Helianthemum)
We conclude that compounds can be attributed to arboreal and non-arboreal
sources. Comparison with pollen data suggest that the abundances of
Betula and Juniperus in the catchment have had a strong impact on n-alkane concentrations
and the composition of n-alkanes in TRZ sediments. Decreasing concentrations of
mid-chain compounds, which are often interpreted as of aquatic origin, are
here rather a mixture of aquatic and terrestrial sources, with high
proportional input of the latter during certain time periods (see above).
Leads and lags between n-alkanes and pollen
Potential pre-aging of leaf waxes, i.e., residence time in soils before being
deposited in sediments, has been a recent matter of debate. Some studies
have shown that significant lag times occur before compounds are deposited
in sediments, mainly due to long residence time in soils and transport via
rivers as well as mixing processes (Eglinton et al., 1997; Uchikawa et al.,
2008; Kusch et al., 2010). This is specifically the case for marine
sediments but has recently also been shown for lacustrine systems by Douglas
et al. (2014). Those authors quantified pre-aging of plant waxes from
several hundreds to thousands of years in a warm and humid environment,
in which transport times are expected to be low due to high rainfall and
runoff. In contrast, Lane et al. (2016) inferred very short lag times for
another tropical lake characterized by a small catchment area.
In TRZ we do not observe significant offsets between pollen and n-alkane
signals neither at the YD onset nor during the termination (Figs. 3, 4, and 7). Especially the data at the YD onset show the most pronounced change
in
integrative n-alkane proxies between 12 680 and 12 620 kyr BP. This is
precisely the time interval defined by Słowinski et al. (2017) as
the transition phase between the Allerød and the YD (Fig. 4) based on
decadally resolved pollen data. We conclude that leaf waxes experience very
short transit times (i.e., subdecadal) before being deposited to the
sediments in this small catchment system. We suggest that the major
transport pathway of leaf waxes into the sedimentary record in this
temperate system is due to direct litterfall into the lake during autumn,
when trees shed their leaves.
Potential of n-alkane ratios as paleoecological proxies in TRZ
Downcore changes in proxies based on proportional contribution of specific
n-alkanes (e.g., Paq, ACL, n-alkane ratios such as
nC27/nC31) have frequently been applied to decipher
paleoecological changes. Nevertheless, due to the abovementioned
constraints, the applicability of alkane ratios as source indicators has
been questioned, acknowledging that a uniform interpretation of proxies such
as ACL is not possible (Rao et al., 2011; Bush and McInerney, 2013;
Hoffmann et al., 2013). Our data illustrate that vegetation changes
co-occurred with compositional changes in sedimentary n-alkanes and we argue
that if constraints from other proxy data can be made, specific additional
information can be derived from such proxies.
Heat map table showing Pearson correlation coefficients
between pollen percentages and n-alkane ratios. Data have been z-transformed
before correlation. Bold black numbers: p<0.01. Black numbers: p<0.05. Positive correlations in green and negative correlations
in red. Intercorrelation between relative abundances of pollen and
n-alkane ratios are in Tables S3 and S5 in the Supplement. AP: arboreal
pollen. Terr. NAP: terrestrial non-arboreal pollen. Σ other herbs: NAP
without Poaceae and Cyperaceae.
To test their applicability in our study area we correlated standard
n-alkane ratios with pollen percentages during the interval 13 350–12 400 yr BP (Allerød–YD transition) (Fig. 6),
when the strongest changes
occurred. Positive correlations were found between n-alkane ratios and
arboreal pollen from Pinus and Betula. The same ratios showed negative correlations to
non-arboreal pollen (NAP) and arboreal pollen (AP) from Juniperus and Salix. The highest
positive PCC was given by the ratio nC27/(nC27+nC31)
versus percentage of ΣAP excluding Juniperus and Salix (R=0.81, p<0.001;
Fig. 6). The same ratio also delivered the most negative
coefficients for correlations versus relative amounts of NAP, grasses,
herbs, Juniperus, and Salix. Slightly lower correlations were found for the ratio
(nC27+nC29)/(nC27+nC29+nC31), but principally
all the analyzed ratios were good indicators to express changes in
vegetation. Interestingly, the ratio nC29/(nC29+nC31)
strongly correlated with percentages of Betula (R=0.80, p<0.001;
Fig. 6), which is considered as a major nC27 producer. Given the
discussion of sources of the compounds above, this could indicate that this
proxy is useful to assess the proportional contribution of Betula to long-chain
n-alkanes C29 and C31.
Similar to pollen vs. n-alkane concentrations, autocorrelations need to be
considered when interpreting n-alkane-based proxies. For instance, if the aquatic
contribution to the sediment is low as in TRZ, then the Paq must not be
interpreted as a measure for aquatic influx but instead as an alternative
expression for ACL, which explains the similarity of both curves (Fig. 2).
Data comparison to MFM (Brauer et al., 1999b; Rach et
al., 2014), Lake Kråkenes (Bakke et al., 2009), and NGRIP (Rasmussen et
al., 2014).
Although an exact assignment to sources was difficult, significant changes
in n-alkane ratios mirrored shifts of pollen percentages throughout the
YD onset. Therefore, we consider alkane ratios to be a good indicator for
major ecological changes in the lake catchment. We suggest that a shift
towards longer chain lengths at the YD onset (reflected by higher ACL and by
long-chain alkane ratios) was indicative of a shift of a more
tree-dominated catchment (Pinus–Betula communities, Wulf et al., 2013) to a higher
abundance of herbaceous plants, complemented by shrubs Juniperus and Salix.
Paleoecological and climatic context
During the Allerød, i.e., between 13 360 and 12 680 yr BP, an average ACL
of ca. 26.9 reflects the vegetation communities
consisting of Pinus and Betula forests (Fig. 7). Shifts towards longer chain lengths
(> 27.4) occurred between ca. 13 220–13 050 and 12 970–12 850 yr BP. Climatic variations during the Allerød were detected
in numerous paleoclimatic records. The most recent chronology from NGRIP ice
cores dated the Greenland interstadial cooling phase 1b (GI-1b) to 13261±149 to 13049±143 yr BP (Rasmussen et al., 2014; Fig. 5).
This episode has been suggested to be equivalent to the “Gerzensee
oscillation”, which has been observed in lake sediment records (von
Grafenstein et al., 1999, 2000). In our record the ACL reversal between ca.
13 220 and 13 050 yr BP could probably be considered as a response of
vegetation in the TRZ catchment to this cold oscillation, but the synchronicity
of events could not be fully confirmed due to larger
uncertainties of the age model in the non-varved section of the record.
Another minor and short-term cooling reversal ca. 40 years before the onset of
the YD, at around 12 720 yr BP, has recently been proposed for MFM (Engels
et al., 2016) but was not clearly recorded by n-alkane ratios in TRZ because
of the insufficient time resolution of our data.
The sharpest and most pronounced decrease in ACL was measured between 12 680
and 12 600 yr BP. This was synchronous with the transition phase of major
vegetation changes as inferred from palynological data between 12 680 and
12 620 yr BP (Słowinski et al., 2017). No pronounced lag time for organic
biomarkers, for instance due to prolonged residence time in soils or
transport from the catchment to the lake, was observed. Given the precise
age control in this varved and tephrochronologically anchored section of the
record, the rapid vegetation change marking the YD onset appeared to be
delayed by ca. 170 years compared to the onset of cooling as inferred from
Greenland ice cores (GS-1 onset 12846±138 yr BP; Rasmussen et al.,
2014), similarly as suggested for more westerly locations such as MFM (Rach
et al., 2014). While the combined absolute dating uncertainty of both records
(NGRIP and MFM) exceeds this delay, the relative age uncertainty in this
section is much lower, evidenced by the common occurrence of the Vedde Ash
isochron in MFM and NGRIP with a relative age difference of only 19 years
(Lane et al., 2013; Rach et al., 2014) when based on their independent age
models. The TRZ record can be linked to NGRIP indirectly through MFM, with
which it shares the LST at 12 880 yr BP (Wulf et al., 2013; Brauer
et al., 1999a, b, 2011), resulting in similarly low relative age uncertainties, at
least in the varved core section.
Within the YD cold interval, the ACL was showing a gradual trend towards
lower values. While some potential contributors to longer chain lengths such
as Poaceae, Cyperaceae, and other herbaceous plants showed relatively constant pollen counts
during the YD, some others such as Juniperus were more abundant during the first half
of the YD (Wulf et al., 2013; Figs. 3 and 7). This is in agreement with the
principal trend of δ18O values and Ca2+ concentrations in
Greenland ice cores, which indicated the coldest and driest conditions in the
earlier phase of the YD, followed by a gradual and slight warming (Rasmussen
et al., 2014). A similar pattern was also detected within proxy data from
other European lakes, for example MFM (varve thickness, elemental
composition, and δD values; Brauer et al., 1999a, b; Rach et al.,
2014), Ammersee (δ18O; von Grafenstein et al., 2003),
paleolake Rehwiese (varve thickness; Neugebauer et al., 2012), and Lake
Kråkenes (elemental composition; Lane et al., 2013; Fig. 7). Recently
this bipartitioning of the YD was also observed in Lake Suigetsu (Schlolaut et al.,
2017). Explanations for this phenomenon have been related to a
restrengthening of the AMOC at ca. 12 300 yr BP. Its slowdown and
the consequent southward shifting of the polar front are considered as major
triggers for the YD in Europe (McManus et al., 2004; Bakke et al., 2009;
Elmore and Wright, 2011). The gradual strengthening of the AMOC pushed the
polar front back to the north, which is the reason that the partially rapid
climatic responses occurred earlier at more southern (ca. 12 240 yr BP at
MFM) compared to more northern locations (ca. 12 140 yr BP at Lake
Kråkenes) (Brauer et al., 2008; Lane et al., 2013). At TRZ, ACL, as an
integrative proxy, showed shifts at ca. 12 300–12 280 and 12 200–12 190 yr BP, but changes were gradual and did not
allow for a distinct identification of
the timing of this climatic change. An explanation for this could be the
continental setting of our study area, which showed weaker climatic
responses compared to locations with stronger Atlantic influence (Słowinski et al., 2017).
The succession of n-alkane abundances between ca. 11 540 and 11 200 yr BP
reflects the gradual expansion of vegetation around the YD–Holocene
transition (defined by means of biostratigraphy at 11515±35 yr BP; Table 1) and during the early Holocene. An increase in nC27
concentrations starting around 11 540 yr BP, while nC31 reached the
lowest concentrations within the studied interval, was possibly caused by
expansion of Betula and triggered a sharp decrease in ACL between 11 580 and 11 490 yr BP. This shift occurred ca. 70–80 years earlier compared to the defined
onset of the Holocene in Greenland at 11654±4 yr BP based on
oxygen isotopes in the NGRIP ice core (Rasmussen et al., 2014). Due to
age uncertainties between 35 and 90 years within the respective section of the
TRZ record, ultimate quantification of the lag was not possible in this
case.
A sudden increase in nC29 and nC31 concentrations at ca. 11 430 yr BP
(Fig. 2), i.e., 90 years after the start of increasing nC27 concentration,
was associated with a ca. 180-year reversal of ACL and n-alkane ratios, whose
start was in phase with a cold period in Greenland referred to as the 11.4 kyr
event (Rasmussen et al., 2014). In contrast to the YD, which was
characterized by low concentrations of n-alkanes, the increasing ACL at 11.5 kyr BP was triggered by an increase in concentrations of long-chain
compounds. It is likely that the rapid ecological developments in the lake
catchment in the course of warming, such as expansion of forest vegetation and
development of soils, were the main drivers behind the observed shift and
not a response to a cooling within the catchment.
Cold reversals during the early Holocene are mostly referred to as pre-boreal
oscillations (PBOs) and have been inferred from different proxies in many
European lakes. The exact timing and possible synchronicity of these
oscillations is still a matter of debate and PBOs are generally difficult to
detect in lake sediments (e.g., Björk et al., 1997; Bos et al., 2007).
Based on sedimentary analysis of a lake in southern Sweden, Wohlfahrt et al. (2006) suggested a PBO could be stratigraphically located between tephra
layers Hässeldalen and Askja-S, which have been dated to 11380±216 and 11228±226, respectively (Ott et al., 2016). The fact that
the decrease in ACL in the TRZ sediment core was observed ca. 10 cm below
the Askja-S layer is evidence against a connection to the oscillation, at
least according to the stratigraphic boundaries suggested by Wohlfahrt et al. (2006).
The upper 31 cm of the studied core interval covered the early Holocene from
ca. 10 970 to 9940 yr BP at relatively low resolution due to lower
sedimentation rates and increased sample size. This episode was
characterized by a gradual change in n-alkane ratios, e.g., an increase in ACL,
indicating the gradual establishment of Holocene vegetation in the lake
catchment due to further gradual warming. Specifically after ca. 10 600 yr BP, a decrease in mainly nC27 alkane concentrations probably indicated a
decline of Betula and further expansion of Pinus sylvestris as dominating species (Wulf et al.,
2013).