Eur. Phys. J. Special Topics 143, 217–222 (2007) c  EDP Sciences, Springer-Verlag 2007 DOI: 10.1140/epjst/e2007-00090-2 T HE EUROPEAN P HYSICAL JOURNAL SPECIAL TOPICS Power spectrum crossover in sediments of a paleolake disturbed by volcanism G. Mart´ ınez-Mekler 1 , E. Ugalde 2 , and G. Vilaclara 3 1 Instituto de Ciencias F´ ısicas, Universidad Nacional Aut´ onoma de M´ exico, Apdo. Postal 48-3, 62251 Cuernavaca, Morelos, M´ exico e-mail: mekler@fis.unam.mx 2 Instituto de F´ ısica, Universidad Aut´ onoma de San Luis Potos´ ı, ´ Alvaro Obreg´ on 64, 7800 San Luis Potos´ ı, SLP, M´ exico 3 Limnolog´ ıa Tropical, Divisi´ on de Investigaci´ on y Posgrado, FES-Iztacala, Universidad Nacional Aut´ onoma de M´ exico, Apdo. Postal 314, Tlanepantla, Edo. M´ exico, M´ exico Abstract. We study density fluctuations from sediments of a paleolake in central Mexico that was subjected to volcanic perturbations by means of computed to- mography (CT) measurements on blocks chiselled out of mines at the lake’s bed. The mine walls show laminations corresponding to the alternation of low density diatom sediments and high density volcanic ash depositions. We have previously shown that there is a range of scales where these fluctuations present a self-similar behavior [1]. Here we relate density correlation calculations to the power spectrum of the fluctuations. We show that a scaling region in the power spectrum coincides with the scaling region in the correlations produced by relaxation from intense volcanic perturbations to steady state fluctuations. There appears to be a kink- like crossover in the power spectrum from mid range scaling to a shorter range scale invariance. This, together with the density probability distribution of the fluctuations, draws attention to the dominant role of rare events. We believe that our analysis may be useful for the understanding of other phenomena with similar power spectrum properties, in which a scale invariance in the unperturbed system is altered by external perturbations that induce an additional scaling behavior. 1 Paleolake sediments Mexico has numerous diatomite beds from paleolakes in regions subjected to intense volcanic activity [2]. Laminae are common in these beds (see Fig. 1.(a)) when diatomite blocks are exposed to X-Rays, a rich and complex structure becomes manifest which may be related to short-period environmental changes (see tomography plate in Fig. 1.(b)). For the case of di- atomite sediments, X-ray attenuation measured with computed tomography (CT) is particularly suited for the fine scale determination of this structure. We have performed such measurements for blocks dug out of the walls of the Santa Barbara and El Lucero mines which lie at the bed of an extinct Pliocene-Pleistocene lake in the state of Tlaxcala, Mexico (N19 ◦ 24’, W98 ◦ 18). Fig. 2 shows the CT X-ray attenuation series ρ(n) expressed in Hounsfield units (based on a relative scale, 0 for water and -1000 for air) of concatenated blocks with data taken every 0.25 mm, along transects perpendicular to the lamination pattern, measuring a total length of 1.4 meters, corresponding to the range 0 <n< 5607. Since we have verified that these X-ray attenuation values are proportional to sediment density values [3], we shall hereafter interchange both terms indiscriminately. Particle induced X-ray emission (PIXE) measurements which can determine chemical composition at the spatial resolution of the CT, as well as our microscope