Plant-wax D/H ratios in the southern European Alps record multiple aspects of climate variability

Abstract

We present a Younger Dryas–Holocene record of the hydrogen isotopic composition of sedimentary plant waxes (δD_(wax)) from the southern European Alps (Lake Ghirla, N-Italy) to investigate its sensitivity to climatic forcing variations in this mid-latitude region (45°N). A modern altitudinal transect of δD values of river water and leaf waxes in the Lake Ghirla catchment is used to test present-day climate sensitivity of δD_(wax). While we find that altitudinal effects on δD_(wax) are minor at our study site, temperature, precipitation amount, and evapotranspiration all appear to influence δD_(wax) to varying extents. In the lake-sediment record, δD_(wax) values vary between −134 and −180‰ over the past 13 kyr. The long-term Holocene pattern of δD_(wax) parallels the trend of decreasing temperature and is thus likely forced by the decline of northern hemisphere summer insolation. Shorter-term fluctuations, in contrast, may reflect both temperature and moisture-source changes. During the cool Younger Dryas and Little Ice Age (LIA) periods we observe unexpectedly high δD_(wax) values relative to those before and after. We suggest that a change towards a more D-enriched moisture source is required during these intervals. In fact, a shift from northern N-Atlantic to southern N-Atlantic/western Mediterranean Sea sources would be consistent with a southward migration of the Westerlies with climate cooling. Prominent δD_(wax) fluctuations in the early and middle Holocene are negative and potentially associated with temperature declines. In the late Holocene (<4 kyr BP), excursions are partly positive (as for the LIA) suggesting a stronger influence of moisture-source changes on δD_(wax) variation. In addition to isotopic fractionations of the hydrological cycle, changes in vegetation composition, in the length of the growing season, and in snowfall amount provide additional potential sources of variability, although we cannot yet quantitatively assess these in the paleo-record. We conclude that while our δD_(wax) record from the Alps does contain climatic information, it is a complicated record that would require additional constraints to be robustly interpreted. This also has important implications for other water-isotope-based proxy records of precipitation and hydro-climate from this region, such as cave speleothems.

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