Testate amoeba assemblages of southern Patagonian peat bogs record drastic 20th century shifts: unprecedented climate change or increased UV-B radiation?

English Abstract:

The ombrotrophic peat bogs of southern Patagonia are located within the southern westerly wind (SWW) belt, the position of which is the dominant mechanism of climate variability in this region. We have sampled cores from peat bogs to reconstruct water table depths during the last 2000 years and aim to relate past fluctuations in water table depth to shifts in regional climate in general and the position and intensity of the SWW in particular.

Cores were extracted from peat bogs located along a longitudinal transect across Tierra del Fuego. We reconstructed local water table depths based upon quantitative analyses of testate amoeba (TA) assemblages. These unicellular organisms live at the surface of humid ecosystems and create shells (‘tests’) that are generally well preserved in peat and their relative presence is closely linked to the local position of the water table. We therefore constructed a transfer function that allows for the quantitative reconstruction of past water table depths from historical assemblages. This transfer function has predictive ability, but relatively high prediction errors given the wide range of sampled water tables in excess of 100 cm. Statistical analyses of TA assemblages were performed to explore spatio-temporal patterns and relationships with climate and environmental variables.

Although water table depth remained the main environmental variable in explaining fossil TA assemblages, sensitivity to environmental factors varied between sites. During the last 2000 years, bogs recorded slightly wetter conditions between ~500-1000 AD, and drier conditions during the LIA. However, the sharpest change in both amoeba assemblages and reconstructed water table depths, suggesting important drying, was found during the 20^th century. Given other evidence from previous publications and considering the regional climatic context, this shift, replicated at several sites, may have been forced by a decrease in precipitation. Nevertheless, we cannot exclude the influence of increasing UV-B radiation resulting from the local degradation of the ozone layer since the late 1970s. This may have affected and possibly still affects living TA assemblages in the southern Patagonian region.