Leaf wax <i>n</i>-alkane distributions record ecological changes during the
Younger Dryas at Trzechowskie paleolake (Northern Poland)
without temporal delay
Abstract. While of higher plant origin, a specific plant source assignment of sedimentary leaf wax n-alkanes remains difficult. Recent compilations of global plant data sets have demonstrated an overlapping and non-systematic production of different chain-length homologues among different classes of terrestrial vegetation. Further, n-alkane distributions can change within the same species due to environmental changes. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. However, 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 aim to identify the major leaf wax contributors to a Central European lacustrine system. Therefore, we tested to what extent leaf wax n-alkane compositional changes (expressed through compound concentration ratios, such as nC27 vs. nC31, average chain length ACL, etc.) can be linked to known vegetation changes, specifically during the Younger Dryas cold period (YD), by comparison with high-resolution palynological data from the same archive. We analysed 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 (13,360–9,940 yrs BP). As additional source indicator of targeted n-alkanes, compound specific carbon isotopic data have been generated in lower time resolution. The results showed rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We find a shift towards higher ACL values at the Allerød/YD transition between 12,680 and 12,600 yrs BP, coeval 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), as well as shrubs and dwarf shrubs Juniperus and Salix. The termination of the YD was characterized by a successive increase of n-alkane concentrations coinciding with a sharp decrease of ACL values between 11,580–11,490 years BP, reflecting the expansion of woodland vegetation at the YD/Holocene transition. Centennial reversals to longer chain lengths during the Allerød could possibly be linked to Greenland Interstadial 1b (GI-1b). A similar pattern during the early Holocene has more likely been triggered by rapid ecological responses in course of warming, rather than to reflect a local impact of a Preboreal Oscillation or 11.4 yr event. Another gradual increase in ACL values after 11,200 yrs 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 results also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes of ACL need to be discussed in context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm species, such as Pinus, produce only very low amount of n-alkanes and thus 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.
