Abstract
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.
Highlights
In the past decades organic geochemical proxies, such as concentrations and ratios of n-alkanes derived from aquatic and terrestrial organisms, have increasingly been used as paleoecological indicators
The following age–anchor points have been used (Table 1): (a) five 14C AMS radiocarbon dates calibrated with the Intcal13 data set (Reimer et al, 2013); (b) two tephra layers (Askja-S: 1164 cm, 228±226 yr BP, age from Ott et al, 2016; Laacher See Tephra (LST): 1253.75 cm, 880 ± 40 yr BP, age from Brauer et al, 1999b); (c) varve chronology within the laminated section anchored by the LST (Wulf et al, 2013); and (d) two biostratigraphic changes at the Younger Dryas cold period (YD) onset characterized by a decrease in Pinus together with an abrupt increase in non-arboreal pollen (NAP) and Juniperus
The YD cold interval was characterized by relatively low concentrations of n-alkanes and pronounced changes in their relative abundances, which resulted in significant changes in n-alkane-based proxies
Summary
In the past decades organic geochemical proxies, such as concentrations and ratios of n-alkanes derived from aquatic and terrestrial organisms, have increasingly been used as paleoecological indicators. Changes in nalkane ratios in sedimentary records have frequently been interpreted to reflect changes in vegetation in lakes and their catchments (e.g., Schwark et al, 2002; Hanisch et al, 2003; Liu and Huang, 2005; Zhang et al, 2006; Nichols et al, 2006; Hockun et al, 2016) While most of these studies delivered plausible indications that the application of these proxies reliably reflect changes in local ecological conditions, a compilation of published plant leaf wax n-alkane data revealed that there is little agreement between specific n-alkanes and potential groups of source organisms, at least if considering a global data set (Bush and McInerney, 2013). These might differ from the transition times of plant pollen depending on the scale and type of the respective catchment (Feakins, 2013)
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