Leaf wax n-alkane distributions record ecological changes during the Younger Dryas at Trzechowskie paleolake (northern Poland) without temporal delay
Authors: Aichner Bernhard , Ott Florian , Słowiński Michał , Noryśkiewicz Agnieszka M. , Brauer Achim , Sachse Dirk 
Published: November 2018

Abstract. While of higher plant origin, a specific source assignment of
sedimentary leaf wax n-alkanes remains difficult. In addition, it is
unknown how fast a changing catchment vegetation would be reflected in
sedimentary leaf wax archives. In particular, for a quantitative
interpretation of n-alkane C and H isotope ratios in terms of
paleohydrological and paleoecological changes, a better understanding of
transfer times and dominant sedimentary sources of leaf wax n-alkanes is
required. In this study we tested to what extent compositional changes in leaf wax
n-alkanes can be linked to known vegetation changes by comparison with
high-resolution palynological data from the same archive. We analyzed leaf
wax n-alkane concentrations and distributions in decadal resolution from a
sedimentary record from Trzechowskie paleolake (TRZ, northern Poland),
covering the Late Glacial to early Holocene (13 360–9940 yr BP). As
an additional source indicator of targeted n-alkanes, compound-specific carbon
isotopic data have been generated in lower time resolution. The results
indicated rapid responses of n-alkane distribution patterns coinciding with
major climatic and paleoecological transitions. We found a shift towards
higher average chain length (ACL) values at the Allerød–Younger Dryas (YD) transition between 12 680 and 12 600 yr BP, coevaled with a decreasing
contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher
abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae,
Artemisia), shrubs, and dwarf shrubs (Juniperus and Salix). The termination of the YD was
characterized by a successive increase in n-alkane concentrations coinciding
with a sharp decrease in ACL values between 11 580 and 11 490 yr BP,
reflecting the expansion of woodland vegetation at the YD–Holocene
transition. A gradual reversal to longer chain lengths after 11 200 yr BP,
together with decreasing n-alkane concentrations, most likely reflects the
early Holocene vegetation succession with a decline of Betula. These results show that n-alkane distributions reflect vegetation changes
and that a fast (i.e., subdecadal) signal transfer occurred. However, our
data also indicate that a standard interpretation of directional changes
in biomarker ratios remains difficult. Instead, responses such as changes in
ACL need to be discussed in the context of other proxy data. In addition, we
find that organic geochemical data integrate different ecological
information compared to pollen, since some gymnosperm genera, such as
Pinus, produce only a very low amount of n-alkanes and for this reason their
contribution may be largely absent from biomarker records. Our results
demonstrate that a combination of palynological and n-alkane data can be used
to infer the major sedimentary leaf wax sources and constrain leaf wax
transport times from the plant source to the sedimentary sink and thus pave
the way towards quantitative interpretation of compound-specific hydrogen
isotope ratios for paleohydrological reconstructions.