Radiocarbon age inversions and progression: Source and causes in Late Holocene sediments from Lake Chapala, western Mexico

The results of eight radiocarbon datings of Lake Chapala sediments (site T46) are presented, the age inversions (AI) observed and their age progression discussed. As deduced from some AIs and the 210Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5-25 cm. The linear sedimentation rates (LSRs) calculated from 14C ages do not match the LSR calculated from unsupported 210Pb activity for the upper sediments. This demonstrates the usefulness of dating sediments with complementary radiometric techniques such as short-lived isotope counting (SLIC), i. e., 210Pb and 137Cs. This approach leads to the following conclusions: (1) The incorporation of detrital particles with ancient carbon into the sedimentary column of the lake occurred by a combination of: (a) the presence of outcrops of hydrothermal petroleum with ages >40 ka (ka = thousands of years) in the lake, and (b) mass transport due to the presence of two elongated gyre circulation patterns integrated by cyclonic circulation (counterclockwise) in the north portion of the lake and anticyclonic circulation in the southern part. (2) Consequently, the 14C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the 210Pb method. (3) A bioturbation mechanism is not necessary to explain the 14C AI in the top 70 cm and from 110 to 150 cm depth of the sediments. (4) According to the biological proxies data for the last 600 years B. P., the paleoclimate at Lake Chapala has changed from sub-humid to dry environmental conditions, and eutrophication has increased over the past 100 years due to local input from ongoing agricultural activities. � 2010 Springer-Verlag

Rock-magnetic and paleomagnetic study on a 27-m-long core from Lake Chapala, western Mexico: Paleoenvironmental implications for the last 10\u202fka

We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions

A reexamination of human-induced environmental change within the Lake Pátzcuaro Basin, Michoacán, Mexico

This paper presents 2,000 years of settlement and land use within the Lake Pátzcuaro Basin, Mexico. Three findings challenge the conclusions of previous research. We show (i) that initial land degradation was caused by settlement, not by agriculture; (ii) that population density inversely correlates with erosion; and (iii) that land degradation was associated with European Conquest but not from the introduction of the Euro-agro suite. Instead, demographic collapse caused by European-introduced disease prevented human-generated landscapes from being maintained, resulting in widespread degradation. These findings support the use of indigenous landscape technology for modern conservation if past failings can be resolved

The younger dryas black mat from ojo de agua, a geoarchaeological site in Northeastern Zacatecas, Mexico

New explorations in the desert of northeastern Zacatecas, in central-northern Mexico, revealed dozens of archaeological and geoarchaeological sites. One of them, Ojo de Agua, contains the remains of a Pleistocene spring-fed hydrographic system located at the southeastern end of a large elongated endorheic basin. The locality yielded a particularly dark, highly organic stratigraphic layer commonly known in the Americas as Black Mat (BM), exposed on the natural profiles of a creek, but not associated with cultural materials. Several radiocarbon assessments confirmed the formation of the Ojo de Agua Black Mat during the Younger Dryas chronozone, with ten calibrated results clustering between 12,700–12,100 cal BP. This multi-proxy study confirmed the peculiarity of the deposit and found similarities and differences with other contexts of Younger Dryas age. The Ojo de Agua Black Mat (stratum C2) is far richer in charcoal specks than the related strata, but lacks phytoliths, diatoms or ostracods. No further biological remains were found in it, except for intrusive capillary roots. Clearly water-lain in a shallow pond, the stratum qualifies as a clayey silt with an acidic-to-neutral pH. Rich in heavy metals and with high contents of titanium, the Ojo de Agua Black Mat yielded significant indicators of intense wildfires during the Younger Dryas, but produced no carbon spherules or nanodiamonds supposedly linked to the impact theory

New constraints on tectonism and magmatism from the eastern sector of the Trans-Mexican Volcanic Belt (Chignahuapan Horst, Puebla, México)

The Quaternary Acoculco (<2.7 my) and Los Humeros (<46 ky) calderas, located in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB), are separated by a regional structure known as the Chignahuapan Horst (ChH). The tectonic and magmatic evolution of the ChH comprises four stages: 1) Upper Cretaceous-Paleocene Laramide orogeny, related to the deformation of sedimentary successions; 2) a sedimentary hiatus followed by the initial stage of the TMVB (Middle Miocene); 3) a Miocene NE-SW oriented extension responsible for the NW-SE normal faults that controlled the volcanic processes and the emplacement of calc-alkaline magmas that formed the Aquixtla sequence; 4) a magmatic hiatus prolonged until 1.6 ± 0.15 my; 5) Pliocene-Quaternary normal faults controlled the emplacement of tholeiitic-transitional magmas and the monogenetic volcanism distribution that formed the Apizaco-Chignahuapan sequence. Coevally to the Miocene NE-SW and Pliocene-Quaternary NW-SE tectonic extensional phases, NE-SW and NW-SE transfer faults were developed. NE-SW normal faults and NW-SE strike-slip faults correspond to fault systems optimally oriented with the current stress field, thus, these faults can be considered as potentially active structures controlling magma emplacement and the distribution of volcanic structures at the surface

Paleoseismology of the southwestern Morelia-Acambay fault system, central Mexico

El sistema de fallas Morelia–Acambay (MAFS) consiste en una serie de fallas normales de dirección E–W y NE–SW que cortan la parte central del Cinturón Volcánico Transmexicano. El sistema de fallas se asocia a la formación de las depresiones lacustres de Chapala, Zacapu, Cuitzeo, Maravatio y Acambay. Las fallas E–W de MAFS aparecieron hace 7–9 millones de años durante Mioceno temprano. Las fallas NNW–SSE son más viejas y se han reactivado en el tiempo, desplazando y controlando a las depresiones lacustres E–W. Se estimaron las magnitudes sísmicas de las estructuras E–W de la región de Morelia–Cuitzeo asumiendo una ruptura cosismica. Cartografía, geología estructural y paleosismología sugieren que las estructuras de MAFS en la región de Morelia–Cuitzeo han estado activas durante el Holoceno, controlando los terremotos históricos que afectan a paleosuelos con cerámica de las culturas Pirinda–Purepecha. Estos terremotos históricos también están registrados en las fallas E–W del campo geotérmico de Los Azufres. En la región de Pátzcuaro, las estructuras E–W de MAFS también se ligan a sismos fuertes ocurridos durante épocas prehistóricas e históricas. Por ejemplo, la secuencia lacustre de Jarácuaro, en el sector meridional del lago Pátzcuaro ha registrado por lo menos tres sismos importantes (Período de PostClassic, 1845 y 1858). El sismo de 1858 (Magnitud estimada de ~7.3) generó un tsunami de 2 m de altura descrito en los archivos históricos. Un sismo similar en la actualidad devastaría esta zona del estado de Michoacán, la cual está ocupada por las poblaciones de más alta densidad. De hecho, el 17 de octubre de 2007 mientras que este articulo era escrito, ocurrieron tres sismos en la ciudad de Morelia que fueron ligados a la falla normal derecha de la Central o de La Paloma. Este hecho corroboraba la sismicidad potencial de las fallas E–W y NE–SW del TMBV. doi: https://doi.org/10.22201/igeof.00167169p.2009.48.3.2

Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis

We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka