IMPLICATIONS OF CLIMATE CHANGE AND TEPHRA ACCUMULATION ON THE LACUSTRINE ENVIRONMENT IN A VOLCANIC CALDERA IN MICHOACÁN, MEXICO

La Alberca, a volcanic caldera in the active Michoacan-Guanajuato volcanic field of central Mexico, is a site of ancient occupation. The oldest known burial (~5000 yrs BP) in the state of Michoacan was discovered immediately beneath prehispanic cliff paintings on the caldera wall. The stratigraphy of fluvial and lacustrine sediments exposed in trenches in the caldera floor indicates variations in the geomorphic environment of the caldera, which could have implications for the type of human use of the site.

La Alberca is a closed system with a large alluvial fan that extends from the northwestern wall and drains into the lowest southern corner of the caldera floor. Stratigraphic descriptions in pits up the alluvial fan gradient indicate that since the nearby Paricutin eruption in 1943-52, coarse sand-sized tephra has washed into the lowest part of the caldera at least every 2 years, but the pools rarely reach 1 meter in depth. Over the last ~3000 years, the periods in between volcanic eruptions are characterized by finer sediment accumulating in frequent, shallow pools. This change in the sedimentation pattern is driven by either the extensive tephra blanketing the watershed after volcanic eruptions or by regional changes in climate. The two deepest trenches contain zones dominated by coarse, dark tephra and cobbles at the base, overlain by a thick sequence dominated by finer-grained lacustrine sediments from 2360-530 years BP. A sharp contact divides this central zone from the upper 2m, which is dominated by multiple coarse tephra layers that represent both direct airfall and re-deposition within the watershed. Paricutin volcano is the source of some or all of the upper tephra layers. Geochemical analyses currently underway will help differentiate these tephras.

The zone of fine-grained, laminated sediments in the central section of the stratigraphy corresponds in time with periods of wetter followed by drier climatic conditions in the surrounding region of central Mexico. This increase in the frequency or persistence of ephemeral pools in the lowest portion of the caldera basin would not have been suitable for permanent occupation. During periods with a greater influx of coarse tephra the permeability of the surface substrate increased, likely decreasing the occurrence of sustained periods of standing water.