On certain occasions Nature provides us with privileged sources of
information. Such is the case of the diatomite beds of an extinct
paleolake in Mexico, which was subjected to intense volcanic activity. By
studying the laminations observed in mines presently dug out from the
lake's bed, we have been able to relate statistical properties of the
volcanic events and variations in the diatom populations to the lake's
development. A scaling analysis of computed tomography X-ray attenuation
measurements along blocks chiseled out of the mine walls, exhibits a
transition in the correlations of the sediment density fluctuations
dominated by strong volcanic perturbations. We argue that this transition
provides a regime for the maintenance of the dominant diatom species under
suboptimal conditions. This situation leads to marginally stable
conditions, under which environmental changes can produce a sharp shift in
the dominance between two of the coexisting diatom species. The interplay
between the information obtained from biological microscopy observations
and the statistical anlaysis of geological data suggests a general
scenario for the lake's evolution.