VEGETATIONAL CHANGES AND CLIMATIC INFERENCES IN NW OF MEXICO DURING THE LATE HOLOCENE

Lakes sediments are an important source of information for documenting environmental change through time. Changes in plant communities can be inferred by analyzing the pollen content in lacustrine sedimentary sequences. Palynological records provide information about vegetation, which in turn can be translated in terms of climate and other change drivers such as human impact. Lake Santa María del Oro is a crater lake (760 m asl, 60 m of depth) that lies close the boundary between the arid climates of northern Mexico (>23°N) and the humid to sub-humid climates of central Mexico. Thus, it represents a geographic location that is very sensitive to climate changes, providing an exceptional opportunity to study the environmental history of the area. A 9-m sedimentary sequence was retrieved from the central part of the lake, allowing the reconstruction of paleoecological changes during the late Holocene.

Pollen analysis was carried out on modern sediments from traps installed for one year within the crater, as well as from the fossil sedimentary sequence. DCA analysis was used to infer associations between pollen taxa and environmental variables. The amount and rate of ecological change was calculated based on the first four axes of the DCA using Euclidean between adjacent fossil samples. Three periods with high ecological turnover were identified: from 3,000 to 2250 years BC, from 250 years BC to 750 AD, and from 1750 AD to present. Also, high rates of change were observed at 900 years BC, 750 years BC and 1880 years AD. Modern and fossil pollen spectra were compared using the modern analog technique. This analysis suggests that from 3500 years BC to 1000 AD vegetation communities around the lake did not resemble what can be found today. Apparently, the modern plant communities established in the area around 1000 AD.