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.