Five Younger Dryas black mats in Mexico and their stratigraphic and paleoenvironmental context

The Younger Dryas interval (YD) was a period of widespread, abrupt climate change that occurred between 12,900 and 11,700 cal yr BP (10,900–10,000 14 C BP). Many sites in the Northern Hemisphere preserve a sedimentary record across the onset of the YD interval, including sites investigated in sedimentary basins located in central Mexico (Chapala, Cuitzeo, Acambay), the Basin of Mexico (Tocuila), and northern Mexico (El Cedral). Deposits consist of lacustrine or marginal lake sediments that were deposited during the Pleistocene and the Holocene. At the Tocuila and Acambay sites, Pleistocene fossil vertebrate assemblages, mainly mammoths (Mammuthus columbi), are found in association with a distinctive organic layer, sometimes called the black mat that formed during the YD. At the Chapala, Cuitzeo, Acambay, and Tocuila sites the black mats contain a suite of distinctive microscopic and mineralogical signatures and are accompanied by a sharp change in the depositional environments as supported by diatom and pollen studies reported here. The signatures include magnetic, Fe-rich microspherules, silica melted droplets with aerodynamic shapes (tektites), large amounts of charcoal, and sometimes nanodiamonds (Cuitzeo), all of which were deposited at the onset of the YD. The geochemistry of the microspherules indicates that they are not anthropogenic, authigenic or of cosmic or volcanic origin, and instead, were produced by melting and quenching of terrestrial sediments. Here, we present the stratigraphy at five field sites, the analyses of magnetic microspherules, including major element composition and scanning electron microscopy images. All of these materials are associated with charcoal and soot, which are distinctive stratigraphic markers for the YD layer at several sites in Mexico. © 2017 Springer Science+Business Media B.V

Estimation of the tectonic slip-rate from Quaternary lacustrine facies within the intraplate Albacete province (SE of Spain)

The Quaternary lacustrine basin of Cordovilla (CB) represents one of the most active tectonic areas of the Prebetic Zone (Albacete, SE of Spain). The Quaternary sedimentary deposits of this basin are mainly endoreic lacustrine carbonate and alluvial deposits, developed in a semi-arid climate (Pleistocene-present). The basin is a NW–SE-elongated graben bounded by a major right-lateral oblique-fault, the Pozohondo Fault. This fault trendsNW–SE, with an approximate trace of 55 km, and is composed of various segments which are identified by fault scarps. In order to establish the slip-rate of the most active segment of the Pozohondo Fault, called the Cordovilla segment, we carried out a detailed study of the affected Quaternary lacustrine deposits. We found that the lacustrine facies could be related to episodic moderate paleoearthquakes. The slip-rate is calculated to be 0.05 and 0.09 mm/yr, using radiometric dating for the vertical offsets of the lacustrine facies. A trenching study at the northern part of the Cordovilla segment revealed two events caused by paleoearthquakes, with the most recent expressed as an oblique-fault off-setting a poorly-developed soil. The magnitude of the last event was greater than 6, using various empirical relationships for the fault displacement and the surface-length rupture. We estimate episodic activity across the Cordovilla segment, to be characterized by moderate-sized paleoearthquakes (M6), which is in agreement with the tectonic context of an intraplate zone of the Iberian plate

The Younger Dryas black mat from Ojo de Agua, a geoarchaeologicalsite in Northeastern Zacatecas, Mexico

New explorations in the desert of northeastern Zacatecas, in central-northern Mexico, revealed dozens ofarchaeological and geoarchaeological sites. One of them, Ojo de Agua, contains the remains of a Pleis-tocene spring-fed hydrographic system located at the southeastern end of a large elongated endorheicbasin. The locality yielded a particularly dark, highly organic stratigraphic layer commonly known in theAmericas as Black Mat (BM), exposed on the natural profiles of a creek, but not associated with culturalmaterials. Several radiocarbon assessments confirmed the formation of the Ojo de Agua Black Mat duringthe Younger Dryas chronozone, with ten calibrated results clustering between 12,700e12,100 cal BP. Thismulti-proxy study confirmed the peculiarity of the deposit and found similarities and differences withother contexts of Younger Dryas age. The Ojo de Agua Black Mat (stratum C2) is far richer in charcoalspecks than the related strata, but lacks phytoliths, diatoms or ostracods. No further biological remainswere found in it, except for intrusive capillary roots. Clearly water-lain in a shallow pond, the stratumqualifies as a clayey silt with an acidic-to-neutral pH. Rich in heavy metals and with high contents oftitanium, the Ojo de Agua Black Mat yielded significant indicators of intense wildfires during the YoungerDryas, but produced no carbon spherules or nanodiamonds supposedly linked to the impact theory

Paleoindian Sites from the Basin of Mexico: Evidence from stratigraphy, tephrochronology and dating

We present new data on the stratigraphy, dating and tephrochonology at the most important Paleoindian sites in the Basin of Mexico. These include: a) Peñon Woman III, with the oldest directly radiocarbon dated human remains (10,755 ± 75 BP); b) Tlapacoya, with two human crania dated to just over 10 ka BP; c) Tocuila, an important mammoth site with incorporation of fossils and suggested bone tools within the Upper Toluca Pumice (UTP) lahar (volcanic mudflow). The Tocuila site also includes potential evidence for a layer associated with the Younger Dryas meteorite airburst, with charcoal, iron microspherules, micro-tektites (melted glass) and volcanic ash, dated to 10,800 ± 50 BP and d) the Santa Isabel Iztapan mammoths I and II with lithics of Scottsbluff, Lerma and Angostura types and obsidian prismatic blades but lacking the characteristic fluted Clovis type points normally associated with mammoth kills and butchering and dated after the Pumice with Andesite (PWA) layer between 14,500 BP and 10,900 BP, before the Younger Dryas interval. These results show that these lithic traditions in Central Mexico are older than in the Great Plains of USA. Several tephra markers are recognised in the sites that help to constrain the stratigraphy and dating of the archaeological sequences. However tephra reworking in marginal lake sites is present and has been carefully considered, especially for the PWA tephra

Estimation of the epicentral area of the 1912 Acambay earthquake (M 6.9, Mexico) determined from the earthquake archaeological effects (EAE) and the ESI07 macroseismic scale

Earthquake Environmental Effects describe the geological features of an earthquake, including the surface faulting, ground cracks, soil liquefaction and landslides, among others. Historical large earthquakes could be classified according to the EEE macroseismic geological scale (ESI07), with the advantage that it is possible to compare different earthquakes which have occurred in different places and estimate the parameterization of their seismogenic sources. Hence, we have determined the ESI07 scale by the EEE description of the geological effects observed during the 1912 Acambay earthquake, which took place in the central basin of Mexico. Additionally, we have estimated the epicentral area of this earthquake, the total rupture length and the potential seismicity of the Acambay-Tixmadejé fault zone. Furthermore, we have estimated the earthquake archaeological effects (EAEs) of the oriented damage. To do this, we have modelled the S-wave travelling by assuming different epicentre locations at several active fault segments and thus, we have compared with the oriented damage mapped by Urbina and Camacho (1913) at different villages. Therefore, we propose a model of rupture of 1912 Acambay earthquake, releasing a M ≥ 7 earthquake and with the epicentre located at Tixmadejé. This rupture model implies earthquake directivity towards East and the S-wave travelling towards South. The total rupture length was estimated 33 km and the maximum coseismic offset 52 cm.Instituto Geológico y Minero de España, EspañaInstituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Méxic

Calibration of the diffusion constant (k0) for dating coseismic fault scarps by using the diffusion equation: application to the Alborax earthquake, Albacete, SW Spain)

The diffusion equation was formulated to estimate the age of fault scarps. This technique is rather simple, taking into account the degradation of the unconsolidated scarp with the time. The Tobarra-Cordovilla segment consists in a surface rupturing associated to undated paleoearthquakes. This segment configures a graben basin with normal faults affecting Quaternary deposits. Preliminary study of the paleoearthquake indicates a magnitude greater than 5.5. As a primary approach, we have applied the Diffusion Equation in order to delimitate the age interval of occurrence. However, the diffusion constant for such equation (K0) is only calibrated for arid environmental. Therefore, we have introduced a new equation to obtain the K0-value according the fault throw, the erodability and the apparent density of the unconsolidated faulted sediment.Ministerio de Educación y CienciaDepto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasFALSEpu

Extraordinary biomass-burning episode and impact winter triggered by the younger dryas cosmic impact ∼12,800 years ago. 1. Ice cores and glaciers

The Younger Dryas boundary (YDB) cosmic-impact hypothesis is based on considerable evidence that Earth collided with fragments of a disintegrating ≥100-km-diameter comet, the remnants of which persist within the inner solar system ∼12,800 y later. Evidence suggests that the YDB cosmic impact triggered an “impact winter” and the subsequent Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural shifts and population declines. The cosmic impact deposited anomalously high concentrations of platinum over much of the Northern Hemisphere, as recorded at 26 YDB sites at the YD onset, including the Greenland Ice Sheet Project 2 ice core, in which platinum deposition spans ∼21 y (∼12,836–12,815 cal BP). The YD onset also exhibits increased dust concentrations, synchronous with the onset of a remarkably high peak in ammonium, a biomass-burning aerosol. In four ice-core sequences from Greenland, Antarctica, and Russia, similar anomalous peaks in other combustion aerosols occur, including nitrate, oxalate, acetate, and formate, reflecting one of the largest biomass-burning episodes in more than 120,000 y. In support of widespread wildfires, the perturbations in CO2 records from Taylor Glacier, Antarctica, suggest that biomass burning at the YD onset may have consumed ∼10 million km2, or ∼9% of Earth’s terrestrial biomass. The ice record is consistent with YDB impact theory that extensive impact-related biomass burning triggered the abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous YD climate episode