Reconstruction of palaeoprecipitation based on pollen transfer functions – the record of the last 16 ka from Laguna Potrok Aike, southern Patagonia

Abstract

Based on modern pollen samples from different vegetation units in southern Patagonia, showing a close relation to yearly amounts of precipitation and mean annual temperatures, different pollen–climate transfer functions are developed and tested. Comparing the performance of MAT (Modern Analogue Techniques), WA (Weighted Average), as well as WAPLS (Weighted Average Partial Least Square) statistical techniques, it is possible to determine the statistically most robust model (WAPLS for precipitation). This transfer function is then used to estimate palaeoprecipitation amounts based on Laguna Potrok Aike pollen results for the last 16,000 years. Generally, the results of the precipitation model indicate less precipitation during the Lateglacial and alternating wet and dry periods during the Holocene. The Holocene started with higher amounts of precipitation until about 8 ka cal. BP, followed by a period with lower amounts between 8 and 2.5 ka cal. BP, while the Late Holocene shows a general increase in precipitation. Comparisons with former shoreline reconstructions and carbonate concentrations in the sediments of Laguna Potrok Aike not always show similarities due to the complex environmental factors recorded by these proxies. Moreover, changes in the moisture availability due to the interplay of precipitation and temperature, cannot be reconstructed directly. Nevertheless, the general long-term trend of palaeoprecipitation is in accordance with the absolute moisture content in the air, which is determined mainly by temperature: during cold periods with less absolute moisture, the model shows less precipitation. Moreover, the model also points to a relation with the position and strength of the Southern Hemisphere Westerlies.

Introduction

The present climate of the southern South American continent (Patagonia) is mainly influenced by the Southern Hemisphere west winds (SHW) and the topography of the landmass (Mayr et al., 2007a, 2007b). The eastern part is much semi-arid because of the rain shadow effect of the Andes and is covered by drought-resistant Patagonian steppe vegetation. In contrast, the Andes and the south-western Chilean lowlands where the SHW arrive at the continent first are very wet due to orographic rainfall. The areas along the Chilean coast are covered by Nothofagus forests while in the westernmost, extremely wet fjord coast moorlands occur. Above the forest in the highest southern Andes grass dominated altoandine vegetation predominates. This strong ecological and climatic gradient has been investigated for the late Quaternary since nearly 70 years by numerous studies (e.g. Auer, 1946; Markgraf, 1993; Heusser, 1995; Haberzettl et al., 2005; Lamy et al., 2010; Moreno et al., 2010), using different archives (terrestrial, lacustrine and marine) in order to understand their environmental settings in the past. Located close to the Antarctic continent, the Quaternary history of southernmost South America is connected to and triggered by the ice balance and climatic development of its neighbouring continent. Furthermore, Southern Patagonia is an important source region for dust particles found in Antarctic ice cores (Delmonte et al., 2008). To understand the transport mechanisms of these particles, reconstruction of the southern hemisphere wind belts throughout the Quaternary is crucial. Moreover, the outlined environmental setting of southern South America allows testing different hypothesis for the position and strength of the SHW by using proxy data from different archives.

Here we present the first pollen-based quantitative precipitation reconstruction for the drier eastern part of southern Patagonia using samples from the ICDP deep drilling site Laguna Potrok Aike at about 52° southern latitude. This reconstruction is based on the last 16 ka pollen data analysed with an average temporal resolution of 67 years (Wille et al., 2007). The main questions are (I) whether the statistical model is robust enough to be used for the reconstruction of late Pleistocene climate changes, (II) how the results compare to other important proxies from sediment cores of Laguna Potrok Aike, as well as (III) how these results relate to regional palaeoclimate studies. The palaeoprecipitation signal could further be used to detect and quantitatively describe dry and wet intervals during Quaternary times and to correlate them to deflation events in Patagonia, which were detected in Antarctic ice cores as prominent dust accumulation peaks. This could then be interpreted as a direct link between terrestrial and glacial archives on both southern hemispheric continents.