RESUMEN ABSTRACTPaleolimnological studies require a broad understanding of the geometry and depth of the sedimentary fill of lake basins prior to coring campaigns. Seismic methods are routinely employed for the indirect characterization of lake-bottom sediments. However, the use of seismic methods might occasionally be limited due to surface conditions or poor seismic contrasts between the stratigraphic units, which lead to data quality that is too poor to provide sufficient information for the selection of drill sites. Sources of cultural noise are one of the main reasons why seismic methods fail in study areas near or within large cities, such as Mexico City. Under certain conditions, electrical and electromagnetic methods might be a suitable substitute or complement to seismic exploration, as they respond to different physical properties of the subsurface. To evaluate the applicability of such methods, here we present two recent case studies from the desiccated lakes Chalco and Xochimilco both located in urban areas within the Basin of Mexico where electrical resistivity tomography (ERT), transient electromagnetic (TEM) and magnetotelluric (MT) soundings were conducted for the characterization of lake basement and lacustrine sediments. In both examples, the results of the geophysical exploration contributed valuable stratigraphic information for the following drilling and core recovery, which highlights the potential of electrical and electromagnetic methods in paleolimnological studies.

Testate amoebae are unicellular organisms characterized by a shell-like test. Due to their potential use as bioindicators (and paleoindicators), these organisms have been increasingly studied in the last decade, particularly in temperate latitudes. This study's objective was two-fold: to identify the testate amoeba communities sampled from 29 water bodies in Mexico and to determine if their presence and distribution also made them suitable bioindicators for tropical latitudes. A total of 40 taxa were recorded within 12 genera, and six significant variables -oxygen, pH, depth, temperature, conductivity, and total alkalinity -that explained testate amoeba distribution within and among the water bodies were identified through a canonical correspondence analysis. The Q-mode clusters rendered five assemblages, each named after their respective dominant species: 1) Centropyxis aculeata strain "aculeata" assemblage, 2) Difflugia oblonga strain "bryophila" assemblage, 3) diverse assemblage, 4) Cucurbitella tricuspis assemblage, and 5) Difflugia protaeiformis strain "acuminata" assemblage. We found that Cucurbitella tricuspis and the Difflugia protaeiformis strain "acuminata" have similar ecological preferences to those reported previously for temperate lakes, with the former identified as an indicator of eutrophic environments and the latter as an indicator of low oxygen levels. On the other hand, Centropyxis aculeata strain "aculeata" and Arcella vulgaris seem to indicate adverse conditions, but the source of this environmental stress apparently differs from that reported in temperate latitudes. Although this stress source could not be identified in all cases, our study nonetheless demonstrates that testate amoebae in the water bodies of central Mexico could reveal the presence of environmental stress.

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Con el propósito de documentar los cambios paleoambientales, se lleva a cabo una investigación multidisciplinaria en la Laguna Seca de San Felipe, Baja California. Los resultados preliminares incluyen análisis sedimentológicos de diatomeas y propiedades magnéticas de un núcleo de 9.5 m de longitud. Fechamientos de 14C sugieren que la secuencia abarca los últimos 70 000 años. Los resultados de propiedades magnéticas (χ, χfd%, MRIS, MRA, parámetros de histéresis y cocientes S), contenido de materia orgánica y tamaño de partículas indican condiciones contrastantes entre el último glacial del Pleistoceno y el glacial tardío-Holoceno. Se infieren condiciones secas entre 70 000 y 45 000 años A.P. El pleniglacial, entre 34 000 y 19 000 años A.P., está caracterizado por condiciones húmedas. A partir de 12 000 años A.P. la tasa de sedimentación y los parámetros magnéticos se incrementan en un factor de 5, cambio interpretado como el aumento en el escurrimiento de aguas superficiales. Entre 7000 y 6000 años A.P. se registra un periodo de aridez, a partir del cual se recuperan las condiciones húmedas. El establecimiento de las actuales condiciones áridas se infiere alrededor de 4000 años A.P.

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