We present elemental, lipid biomarker and, in the supplement, compound-specific isotope (
Climate-controlled changes in hydrology are reflected in the development of
the terrestrial biome, which determines the amount and the quality of
organic matter (OM) supplied to environmental archives (Meyers and
Lallier-Vergès, 1999). Currently, reconstructions of biome dynamics are
largely based on pollen data, focusing on the development of the vegetation
(Birks and Birks, 2006). The dynamics of the soil organic carbon pool,
however, are poorly constrained, mainly because palynologists cannot easily
discriminate between direct supply of pollen and intermediate storage of
pollen in soils. Similarly, paleoclimatologists using established organic
geochemical proxies such as the organic carbon to nitrogen ratio (C
We focus on Lake Ohrid in the Western Balkans (Fig. 1), which provides an outstanding archive of continental environmental change dating back at least 1.2 million years (Wagner et al., 2014). In 2013, almost 600 m of sediment core was recovered during a drilling campaign for the International Continental Drilling Program (ICDP). Initial analyses of core catcher material and logging data reveal a continuous limnic sequence, showing pronounced cyclicity that clearly relates to eccentricity and obliquity cycles (Wagner et al., 2014; Baumgarten et al., 2015). This exceptional continental sediment record thus appears very promising for high-resolution palaeoenvironmental reconstructions of orbitally controlled Northern Hemisphere climate fluctuations. Earlier studies of pilot cores suggest a close relationship between the North Atlantic climate of the past 130 000 years and the sedimentation of inorganic as well as organic matter in the lake (Vogel et al., 2010; Holtvoeth et al., 2010). Whichever proxy is used to reconstruct changes in biogeochemical fluxes towards a sedimentary archive, the sources of each component have to be known in order to correctly interpret proxy variability. For distal marine sediment archives, mixing associated with long-distance transport results in averaged end-member characteristics for marine and terrestrially sourced matter that are reasonably well defined. Terrestrial ecosystems, by contrast, show far greater complexity and diversity, with a great variety of local factors and processes that modify specific end-members. The local vegetation type, geology, geomorphology or hydrology may all reveal peculiarities that potentially interfere with the regional or global climate signal that paleoclimatologists are keen to extract from terrestrial archives.
Locations of sediment core Lz1120 and sampling sites for modern materials in the Ohrid Basin. Lake Prespa contributes 28 % of the inflow of meteoric water through karst systems. Inset: location of Lake Ohrid in the Balkans region of southeastern Europe.
As part of a longer-term study of the Ohrid sedimentary record, we aim to improve the end-member definition of OM sources, and test new geochemical proxies across the 8.2 ka climate event. We provide lipid biomarker distributions and elemental compositions of modern materials from the Lake Ohrid Basin, including terrestrial plant litter, soils, macrophytes and lake water particles, all of which may influence the geochemical signature of the OM deposited in the lake.
A great advantage of the Lake Ohrid climate archive is its small,
mountainous catchment (Fig. 1). Up to 55 % of water inflow derives from
karst springs fed by seepage from the higher altitude Lake Prespa. Although
these waters do not deliver any sediment they do supply small amounts of
nutrients to Lake Ohrid, e.g. 7 % of the total phosphorous load
(Matzinger et al., 2006), as well as calcium ions allowing carbonate
precipitation. As there are no major rivers entering the Ohrid Basin, the
supply of allochthonous sediment to the lake depends almost entirely on
surface runoff from the surrounding mountain ranges in the form of small
streams and gullies. The Triassic limestones forming the eastern,
southeastern and northwestern slopes of the Ohrid Basin are highly permeable
so that much of the precipitation is taken up by karstic systems before
reaching the lake. Thus, the quantity and composition of terrestrial OM
supplied to the lake will reflect hydrologically controlled changes in
vegetation and soil stocks of the immediate surroundings, while productivity
in the lake reflects runoff-controlled nutrient supply. The steep
morphology and the geological conditions in the catchment force a rapid
response of the vegetation cover to precipitation decrease and are
responsible for relatively low soil stability. Primary productivity in Lake
Ohrid is low due to the low nutrient levels (e.g. total phosphorous: 0.15
While the use of biological markers as proxies for environmental change is well established, we present the first data set of this kind for the Ohrid Basin to improve the interpretation of sedimentary OM compositional changes. We incorporate observations made during the study of sediments from pilot cores taken prior to the 2013 ICDP drilling. Our catchment data then allow us to identify the main OM sources to the Lake Ohrid sediments.
We also determined the carbon and hydrogen stable isotope composition for
Sampling was carried out along the southern and southeastern shores of Lake
Ohrid (Fig. 1). We did not sample each component of the ecosystem, but
instead focused on the major OM sources. Leaf litter and underlying
topsoils, i.e. O
At two littoral sites, near the DG site and
Lake water samples were collected from two sites (DEEP and Co1202; Fig. 1)
by deployment of Niskin bottles (20 L). The water samples were filtered
immediately after collection in the laboratory of the Hydrobiological
Institute in Ohrid using an electrical pump and pre-combusted glass fibre
filters with a nominal pore size of 0.7
Eleven sediment samples were taken from core Lz1120 situated close to the
southeastern shores of Lake Ohrid (Fig. 1). The detailed site description
and age model are provided by Wagner et al. (2009). Accordingly, our samples
cover the time span from 8.65 to 8.05 ka (8647–8049
Total carbon (TC) and total nitrogen (TN) contents were measured in
duplicate (values were < 10 % of the mean) using a CE Instruments
NC 2500 elemental analyser. Total organic carbon (TOC) was determined after
acid vapour (HCl) digestion of the carbonate fraction (Yamamuro and Kayanne,
1995). Carbonate contents (assuming all carbonate was as CaCO
For extraction of the lipid fraction, about 0.7 g of plant matter and 2 g of
soil and sediment were homogenised and sonicated (3
Water filtrates were extracted using a Dionex Accelerated Solvent Extraction
system (ASE), with DCM and methanol (
Aliquots of the transmethylated and derivatised extracts of samples
collected in 2012/2013 were injected onto a Trace 2000 Series gas
chromatograph (GC) fitted with an on-column injector and a fused
high-temperature silica column (60 m
Statistical analyses (multi-dimensional scaling, MDS; analysis of
similarity, ANOSIM; similarity percentages, SIMPER) were conducted on lipid
biomarkers, their concentrations being normalised to percentage of total
identified lipids (%
Leaf litter samples from the near-shore low-altitude forests (sites SN and
TP) have high TOC contents of 43
The macrophyte samples can be separated into two types of plant that (a) do
precipitate carbonate, i.e.
Composition of the total lipid extracts (TLEs) and amounts of
compound classes, sub-categories and individual lipids of soils, sediments,
leaf litter, macrophytes and water filtrates from the Lake Ohrid Basin.
Values are given as percentages of the total lipids (%
Carbonate contents of the Lz1120 sediments range from 20 to 56 % and TOC
from 1.5 to 2.3 % (Fig. 2). The carbonate record of Lake Ohrid appears to
be controlled by temperature and terrestrial runoff, with precipitation and
production of largely endogenic carbonate occurring mostly during the summer
(Wagner et al., 2008; Vogel et al., 2010) and relying on the supply of
calcium ions and nutrients from the catchment. Accordingly, minima in
sedimentary carbonate represent periods of drier and cooler climate. Between
8.8 and 7.9 ka, the Lz1120 carbonate record features two pronounced minima
at around 8.3 and 8.15 ka that appear to correspond to phases of the 8.2 ka
event (Fig. 2) as documented in sediment records from the North Atlantic
(MD99-2251) and Greenland ice cores (GISP 2; Ellison et al., 2006). TOC
decreases from 2.3 to a minimum of 1.5 % at ca. 8.3 ka and remains below 2 % thereafter. Carbonate and organic carbon contents appear to co-vary
apart from in the three youngest samples between 8.05 and 7.9 ka and the
sample separating the two carbonate minima at 8.23 ka. In the remaining
samples, TOC and carbonate correlate closely (
Records of carbonate (CaCO
Leaf litter under both types of vegetation, low-altitude oak-dominated
forest (sites SN, TP) and high-altitude beech forest (GN), contain very
similar total amounts of lipids: ca. 458
There are compositional differences in the major lipid compound classes,
between leaf litter and corresponding topsoils and between high-altitude and
low-altitude sites (see Fig. 3 and Table 2). Lipids of the near-shore
oak-dominated leaf litter (sites SN, TP) are dominated by saturated FAs that
account for 37
Chain-length distributions of
The proportions of the major compound classes in the topsoils differ from
those observed in the overlying leaf litter. While the relative amounts of
FAs remain similar (35
The higher amounts of
The relative amount of steroids also decreases from leaf litter to soil. At
GN, their relative abundance nearly halves compared to the leaf litter (24
to 13 %
A notable difference in the total lipid composition between leaf litter and
underlying soils is the significantly higher proportion of
Minor compound classes present in leaf litter and soils include hydroxy
acids,
Compared to leaf litter and topsoils, TLEs of grasses show a far less
complex composition, with the alkyl compounds being dominated by only a few
individual biomarkers (Fig. 3b). Apart from representing single-species
samples, a further reason for this may be that the grasses were not sampled
as litter from the ground but as upright, although dry, late-season plant
matter. This means that degradation was at the earliest stage and
contribution of lipids from degrading organisms likely minimal, which is
also indicated by the absence of branched FAs. The grasses show
significantly higher amounts of PUFAs (29
Chain length distributions of
The
Correlations of compound class-specific relative abundances of
Comparison of the relative abundances of individual FAs and OHs in the two
samples from site TP provides clues as towards the effect of white rot on
chain-length distribution (Fig. 4b, c). Fungal biomass contribution and
breakdown of plant material appears to increase the proportion of
short-chain C
Chain lengths of
The
As for TOC, the subsoil samples, DG-SS and DG-SS 2, contain the lowest
amounts of lipids: 1.3 and 2.3
The lipid composition of the weathered sample contrasts with the
un-weathered subsoil sample and the topsoils. Almost half of the lipids (48 %) are
MDS plot showing separation of sample types in Euclidean space. For details see methodology Sect. 2.4. Low-carbon sediment samples are labelled with their age (ka). DG-SS and DG-SS II refer to the weathered and un-weathered subsoil samples (Terra Rossa) from site DG.
The dominant
The differences between the un-weathered and the weathered/degraded Terra
Rossa sample may be indicative of the compositional changes soil lipids may
undergo when the soil dries out frequently and becomes aerated.
Submerged macrophytes contain between 0.2 and 1.1 mg g
The macrophyte lipids are dominated by unsaturated and saturated fatty acids
that, together, account from 64 %
Saturated
The total lipid composition of the lake particulate organic matter (POM) was
not determined, as extracts had previously been separated into non-polar and
polar fractions. Here we report the results from analyses of the polar
components that include fatty acids, alcohols and steroids of the water
filtrates taken at Co1202 and the DEEP site. At both sites, these were
dominated by short-chain FAs (C
Total lipid contents of sediments from core Lz1120 range from 7.6 to 79
With regard to the major lipid compound classes, there are clear
compositional differences between the samples with low and high carbonate
contents, i.e. between the samples dated to 8.29, 8.17 and 8.11 ka,
representing cool, dry episodes associated with the 8.2 ka event, and those
from the rest of the investigated time slice. In particular, the proportion
of
The concentrations of OH-FAs range from 1 to 6.7 %
In contrast to macrophytes, leaf litter and soils, the early Holocene
sediment samples contain almost no unsaturated fatty acids. The only
detectable MUFA is C
Other compounds account for 17 % of the total lipids in both high- and
low-carbonate samples. While most of these compounds are minor components
(< 2 %
The principle behind paleoenvironmental reconstructions using
organic-geochemical proxies is that OM fluxes from aquatic and terrestrial
sources within the catchment of a given sedimentary archive change in
response to changes in regional or global climate. The main purpose of this
study is to characterise the organic geochemical composition of potential
sources of OM buried in sediments of Lake Ohrid in order to improve the
interpretation of organic geochemical proxy data in reconstructions of
hydrology-controlled environmental change in the Ohrid Basin. Our initial
data set provides significant insight towards an improved interpretation of
changes in elemental (TOC
TOC
Straightforward interpretation of the Lz1120 TOC
Nearly all of the biomarkers preserved in the sediments can be ascribed to terrestrial OM, or have a non-specific source. The exception is dinosterol (from dinoflagellates), but this is a very minor component of the total lipid extracts (see Table 2, Supplement Sect. S2) and we consider it not to be a robust quantitative proxy for lake-derived OM.
The amount of lipids relative to the total OM of individual sources is likely to vary with environmental conditions. Thus, without knowledge on how representative individual biomarkers are for the size of the OM pool they derive from under specific hydrological conditions, assessments of relative contributions remain speculative unless they are backed up by other proxy data (e.g. elemental data, petrology, bulk isotopes).
As evident from the pie diagrams in Fig. 3, topsoils contain significantly
higher amounts of
The chain-length distributions in Fig. 3 also reveal another important
difference between leaf litter and soils. In the near-shore oak-dominated
setting, the proportion of mid-chain FAs, C
Records from elemental (CaCO
MDS shows separation of sample types, namely soils, leaf litter, grasses,
macrophytes, lake POM and sediments, which 1-way ANOSIM shows to be
significantly different. The “weakest” differences are between subsoil and
grass (
Figure 6 compares the elemental records across the 8.2 ka event to biomarker
proxies that are based on our geochemical fingerprinting of modern OM
sources. The carbonate record shows two pronounced minima in response to
changes in temperature and hydrology (Wagner et al., 2008) that likely
reflect the Northern Hemisphere climate development controlled by two major
phases of freshwater release into the North Atlantic (Lajeunesse and
St-Onge, 2008; Roy et al., 2011). In the following, we refer to the timing
of these minima as Phase 1 and Phase 2 according to their chronological
succession. While the carbonate record is closely linked to climate forcing,
the proxies that incorporate an organic component, i.e. TOC, TOC
Between 8.7 and 8.35 ka, TOC and TOC A slowly decreasing lake level, exposing the flood plains to the south of
site Lz1120 and reducing the flux of lipid-rich terrestrial OM to the lake. A change in the soil moisture budget leading to increased degradation of
lipids. A slow decline in terrestrial productivity.
Due to its tectonic origin, the Ohrid Basin is characterised by a pronounced
terraced morphology, particularly at its northern and southern ends where
today's agricultural plains were once marsh land and/or lake floor. As a
result, lake level change can introduce a threshold-type control of biome
characteristics and, thus, cause abrupt changes in material fluxes within
the catchment of site Lz1120.
A major change occurred shortly after 8.35 ka, with the onset of Phase 1,
when values for carbonate, TOC and TOC
Degraded soils as the main source of sedimentary lipids and little
contribution from topsoils or of terrestrial vegetation suggests that the
latter were both diminished, leaving the deeper soil layers vulnerable to
erosion. A depleted topsoil pool and, possibly, a vegetation cover that had
not yet fully recovered from a period of at least seasonally increased
aridity would also explain why the samples from 8.23 ka (separating Phases 1
and 2) and from 8.05 ka show TOC values that appear low relative to their
carbonate contents, with these two proxies otherwise correlating closely
(
Finally, the composition of
Since Lake Ohrid appears to have been an oligotrophic ecosystem for most of its history, it is not surprising that soil OM is the major source of the sedimentary lipids. Our lipid-based proxies suggest that soil OM supply was relatively enhanced during the 8.2 ka event in response to a drier and cooler climate and the associated recession of terrestrial vegetation. This contradicts the conclusions of Lacey et al. (2014) who assume a reduction in primary productivity and an even more profound reduction in the contribution of soil-derived OM for the period between 8.5 and 8.0 ka, based on their interpretation of proxy data from bulk organic carbon isotope measurements and Rock-Eval pyrolysis of sediments from the western part of Lake Ohrid. The authors postulate lower soil-OM contributions on the basis of heavier bulk carbon isotopes and higher values of the oxygen index (OI) from Rock-Eval pyrolysis. However, soil OM is frequently characterised by the same pattern, i.e. bulk carbon isotope ratios that are heavier by 1–3 ‰ compared to the original vegetation (Lichtfouse et al., 1995; Ehleringer et al., 2000 and references therein) and have higher OI values (Disnar et al., 2003). This results from initial OM degradation in the soils rather than in the water column as assumed by Lacey et al. (2014). We therefore suggest that the data reported by Lacey et al. (2014) actually support our conclusion of relatively increased amounts of soil-derived OM in the Lake Ohrid sediments between about 8.5 and 8.0 ka.
Lake Ohrid is an outstanding sedimentary archive of SE European continental climate change, recording environmental changes in the Ohrid Basin continuously for at least 1.2 million years. OM buried in Lake Ohrid sediments can deliver information on the response of the ecosystem to climatically controlled changes in hydrology. In order to optimise the reconstruction of environmental changes we carried out the first organic geochemical survey of potential sources of sedimentary OM in the Ohrid Basin.
We determined the organic geochemical fingerprints of leaf litter, topsoil,
un-weathered and weathered subsoil, macrophytes and filtrates of suspended
and slow-sinking particles from the Ohrid water column. Comparison with the
TLE composition of Early Holocene sediments from site Lz1120 in the
southeast of Lake Ohrid reveals that little of the sedimentary lipids appear
to derive from aquatic OM sources such as macrophytes or phytoplankton.
Labile compounds including MUFAs and PUFAs, accounting for a third of the
polar lipids in water filtrates and up to half of the total lipids in
macrophytes, are not preserved in the sediments, while the dominant
saturated fatty acid in these samples, the
We tested a set of new proxies based on our improved end-member
characterisations for a section of sediment core Lz1120 that includes the
prominent 8.2 ka event, a 400–500-year episode of cold and dry climate
in the North Atlantic realm affecting much of the Northern Hemisphere,
including SE Europe. Both the carbonate record and the biomarker proxies
highlight two distinct phases of the 8.2 ka event affecting the ecosystems
of the Ohrid Basin. Changing proportions of suberin-derived mid-chain
compounds relative to cuticular long-chain compounds as expressed in ratios
(m-FA/l-FA) or average chain lengths (ACL
Overall, it appears that soil-derived lipids have the greatest potential to
be preserved in the sedimentary archive of the Ohrid Basin. However, the
extent as to which terrigenous and aquatic biomarkers are representative of
the terrestrial and aquatic OM pools is still a matter of uncertainty.
Multi-proxy approaches combining lipid analyses with, for example, bulk
carbon and hydrogen stable isotope data, pyrolysis GC-MS analyses or
Rock-Eval analyses of both OM sources and sediments may help resolve this
issue. Further improvements of biomarker-based reconstructions of the
terrestrial environment may be achieved by completing the geochemical
characterisation of elements of the Ohrid Basin's habitats that are
currently missing, including e.g. marshland soils and soils developing on
the ultrabasic rocks along the western shores. However, our first survey of
major OM sources in the Ohrid Basin demonstrates that organic-geochemical
fingerprinting and the development of proxies adjusted to the local OM
inventory can lead to an improved understanding of terrestrial biome
dynamics and OM fluxes towards the lake sediments, in particular, when
short-distance OM transport in a heterogeneous and variable catchment
significantly increase complexity. When applied to the sedimentary record of
a well-known climate fluctuation, the 8.2 ka event, the adjusted proxies
demonstrably improve the interpretation of bulk-proxy data such as TOC and
TOC
These are abbreviated by their carbon number, for example:
C
C
Monounsaturated fatty acids (MUFAs) are abbreviated using IUPAC recommended numerical symbols, for example:
C
C
C
Polyunsaturated acids (PUFAs) are abbreviated in the same way, but where double bond positions are unsure, no designated position is given, for example:
C
Branched fatty acids are abbreviated as follows:
iso-C
anteiso-C
iso-C
iso-C
anteiso-C
Hydroxy fatty acids, either
C
C
Diacids are denoted
C
C
C
Branched alcohols are abbreviated as follows:
iso-C
anteiso-C
Long-chain diols are abbreviated as follows:
C
C
Long-chain hydroxy-ketones are abbreviated as follows:
1, 15(
Isoprenoids
stigmasterol – 24
stigmastanol – 24-ethyl-5
cholesterol – cholest-5-en-3
cholestanol – 5
coprostanol – 5
epicoprostanol – 5
epicholestanol – 5
brassicasterol – 24
campesterol – 24
ergosterol ergosta – 5,7,22-trien-3
dinosterol – 4
dinostanol – 4
lanosterol – 5
taraxasterol – 5
tetrahymanol– gammaceran-3
We are grateful to D. Kurti (Pogradec) for logistical support during macrophyte sampling, O. Cara (Durrës) and B. Muceku (Tirana) for support during soil sampling and treatment and B. Wagner and A. Franke (Cologne) for providing the sediment samples. Finally, we thank A. Thompson, S. Blackbird (Liverpool) and P. Donohoe (Newcastle) for support in the laboratories. The study was funded by the Leverhulme Trust (grant F/00 025/AU). Edited by: B. Wagner