LOESSFEST'09 | Aug. 31st – Sept. 3rd, 2009 |Novi Sad-Serbia
600 KYR of Quaternary Landscape History in SE
Europe
What do Molecular Fossils (n-Alkanes) Tell us?
Buggle, B.1, Zech, M.1,2, Gerasimenko, N.3, Markovic, S.B.4, Hambach, U.2,
Glaser, B.1, Zoeller, L.2
1Department of Soil Physics, University of Bayreuth, Universitätsstr. 30, D-95440 Bayreuth, Germany
2Chair of Geomorphology, University of Bayreuth, D-95440 Bayreuth, Germany
3Earth Science and Geomorphology Department, Тarasa Shevchenko National University of Kyiv, Ukraine
4Chair of Physical Geography, Faculty of Sciences, University of Novi Sad, Serbia
Introduction
In the middle and lower Danube Basin, loess paleosol sequences (LPSS) of several decameter thickness are widely distributed. They provide valuable long-term archives for the Quaternary landscape development of the region and possibly for SE- and Central Europe, in general. Paleopedologic features indicate a trend of increasing aridity or cooling for the mid- and late Pleistocene interglacials with older pedocomplexes being often interpreted as fossil forest soils and younger pedocomplexes and the recent soil being described as (fossil) steppe soils. In lack of macrofossils, biomarker analyses are promising to gain new insight into the landscape history of the region. Here, we investigate two key sections in the Danube Basin for their lipid biomarker composition. To our knowledge the following results represent the first long-term (~600.000 years) alkane record in loess paleosol sequences.
Regional setting / Methods
As key sections of these areas, we studied the loess-paleosol sequences Mircea Voda (44° 19´ 15´´ N, 28° 11´ 21´´ E, Romania) and the stacked LPSS Batajnica/ Stari Slankamen (44° 55´ 29´´ N, 20° 19´ 11´´ E and 45° 7´ 58´´ N, 20°18´ 44´´ E, Serbia) for their n-alkane biomarker composition. On selected samples (one per paleosol horizon and loess layer, respectively) additionally pollen analyses have been conducted in order to compare the findings of both methods. The potential natural vegetation (PNV) of the Romanian site is a feather-grass steppe (Frey&Lösch, 1998) or according to Horvath et al. (1974) a light steppe forest ( Aceri tatarici- Quercion). The PNV of the Serbian site is a Aceri tatarici-Quercion steppe forest.
n-Alkane biomarkers – fundamentals
N-alkanes are important compounds of epicuticular plant waxes. Characteristic distribution pattern of n-alkane homologues are used for chemotaxonomic differentiation of certain plant species or plant types. Due to their relative recalcitrance, n-alkanes have the potential to serve as markers (so-called biomarkers) for different palaeovegetation types recorded in sediment archives. Classically the ratios of the homologues n-C27 vs. n-C31 and n-C29 vs. n-C31 are applied as proxies for tree and shrub vs. grass vegetation in paleoenvironmental studies. However such a classical straightforward interpretation of n-alkanes in the Danube Basin region is difficult, since species of the potential interglacial vegetation only show small differences in the n-alkane patterns of the grasses and trees. Therefore, observed small scale variations in n-alkane ratios may also reflect 1) changes in the species composition within the wood or grass subgroup (e.g. trees with different C27, C29, C31 patterns) and 2) adaptation of the cuticular waxes to increasing (decreasing) water- and heat stress, causing an increase (decrease) of the chain length.
Results
The distribution patterns of the fossil n-alkanes show a bimodal pattern with a microbial derived subpopulation in the short-, medium chain length range (< C24) having a low predominance of odd over even C-numbered homologues (OEP) and abundance maxima (Cmax) at C17 or C18. However, the presence of a distinct longchain n-alkane subgroup with pronounced OEP and Cmax mostly at C31 give evidence for the preservation of plant-derived n-alkanes also in the oldest samples under study (MIS 17). The paleoecological interpretation of the biomarker record is based on values corrected for degradation effects according to the procedure of Buggle et al. (see corresponding abstract).
Cold stages vs. warm stages
A classical interpretation of the biomarker record would
indicate more woody vegetation during cold stages than
during warm stages. Yet, the variations are only relatively
small and reflect rather reduced water - and / or
heat stress of the vegetation. Nevertheless, the results for
both sites are in line with the presence of woody vegetation
during cold stages, being in contrary to what is
suggested from Greek Pollen records as well as from the
pollen data from the studied sections. However, the biomarker
findings are in agreement with charcoal evidences
for trees in Hungarian LGM loesses, tree pollen
findings in late LGM lake sediments in Hungary and
climate models for the LGM showing higher P-E i.e. less
water stress in this region than under interglacial conditions.
Also bioclimatic studies support that this region
was well within the potential area of tree growth. A possible
model for the cold stadial landscape could be grassland
with (sparsely) presence of trees or shrubs or pure
grassland with intermittent spread of woody vegetation
during more favorable periods (e.g. D/O events).
Fossil steppe soils vs. fossil forest soils i.e MIS 5,
MIS 7, MIS 9 vs. MIS 13-15, MIS 17
For the studied sections the biomarker record does not
indicate a higher abundance of trees for the fossil “forestsoils”
as compared to the fossil and recent “steppe soils”.
The biomarker distribution rather points to less trees and
/ or more intense heat -, drought stress of the vegetation.
Also the pollen results do not indicate higher tree abundance
for the fossil so-called “forest-soil”. On a first look
this seems contrarious to the stronger soil development
of the fossil “forest soils”. However, an increase mainly
of winter and spring precipitation, favouring soil formation
(Cambisols, Luvisols), and a concomitant increase in
summer dryness, limiting tree growth, may resolve the
apparent contradiction. Such climatic features would be
characteristic of Mediterranean-like climate conditions,
as supported by rubification of the respective paleosols.
General Conclusion
The application of biomarker approach in LPSS studies can shed new light on classical paradigms such as treeless cold stadial vegetation or forests on fossil so-called “forest soils” (e.g. fossil Luvisols, Cambisols). Discrepancies between pollen and n-alkane records could be due to following reasons: The n-alkane record shows on site vegetation patterns and reflect also ecophysiological adaptation of plants to water-, and heat stress. Pollen records in open landscapes reflect more regional vegetation patterns. They are influenced by far-distance transport, differences in pollen production, dispersal and selective preservation.