Rodent midden pollen record from the Atacama Desert: from modern signal calibration to tracing past climatic variability

DE PORRAS, M.E; ZAMORA-ALLENDES, A.; MALDONADO, A.

Tunuyan

Conferencia; 4th Southern Deserts Conference; 2014

Past climate dynamics of the Atacama Desert has been studied because of two main reasons (1) the strong impact that past climate variability would have had on the hydrologic resources of this region where water is a crucial factor for plant and animal survival as well as for human occupation and (2) its potential links with the continental and inter-hemispheric scale atmospheric circulation. Fossil rodent midden have not only been the main but a valuable source of palaeoecological and palaeoclimatic evidence in the Atacama desert where ?standard? depositional environments are scarce. The vegetation dynamics in the Atacama Desert during the last 50,000 yrs has been inferred from the joint analysis of pollen and plant macrofossils from fossil rodent middens. Then, past climatic changes were inferred based on the distribution of plant communities in the past.Up to the present the fossil pollen rodent midden record has been interpreted using surface pollen assemblages. However, whether these samples are adequate modern analogous or not is not fully clear, given the different pollen deposition processes that could have affected both kinds of samples.The present paper presents a modern vegetation-pollen from modern rodent middens-climate calibration set from the Atacama Desert (18-25°S) developed to be applied to the fossil record in order to trace palaeoclimate dynamics since the Pleistocene-Holocene. The modern rodent middens were collected along five west-east altitudinal transects (18°, 21°, 22°, 24°, 25°S) following the vegetation and climatic gradients at the regional scale. Furthermore, three or more modern rodent middens were collected at each point so as to analyze the modern pollen robustness to reflect vegetation.Modern rodent midden pollen reflects the vegetation belts along the climatic gradients, particularly annual (mostly summer) precipitation and mean annual temperature, in the Atacama Desert. Along these gradients (from west to east), pollen assemblages represent the Prepuna belt (Chenopodiaceae, 60%; Brassicaceae, 20%; Malvaceae, 10%; Ambrosia-type, 15%), the Puna belt (Senecio-type, 40%; Baccharis-type, 20%; Fabaceae, 30%; Portulacaceae, 20%) and the high Andean steppe belt (Poaceae, 80%; Senecio-type, 10%; Apiaceae, 10%). Inter and intra variability of the vegetation belts are reflected in the rodent middens pollen record at the regional scale and along each transect. The joint analysis of several rodent midden pollen samples at each point reveals a high consistence of the pollen signal in this archives. Important differences between samples appear related to an over-representation of Cactaceae family due to the dietary preferences of the rodent, which are perfectly identifiable.These results prove the potential of pollen in rodent middens as a palaeoecological archive and provide a robust modern calibration set to be applied to the fossil record from the Atacama Desert in order to trace its past climatic dynamics and shed light to some unclear issues and discrepancies that are still a matter of discussion.FONDECYT #3130511, 1130279, 1100916