Experimental simulation of melt migration in stromatic metatexite migmatites

Melting experiments on a gneissic protolith using the diamond trap technique produced an accumulation of crystals of biotite in the boundary of the diamond trap. Several hypotheses can be inferred to explain this feature, but textural evidence and calculations of the settling and drag velocities of the crystals suggest a model of extraction of melt due to a pressure gradient inside the capsule as the most likely origin of this biotite layer. This biotite layer and the trapped melt resemble the melanosome and leucosome, respectively, of stromatic metatexite migmatites. The melt extraction model developed is similar to the “filter pressing” model for migmatite formation. The diamond trap technique is a suitable method to develop migmatites experimentally.     

A first-order seismotectonic regionalization of Mexico for seismic hazard and risk estimation

The purpose of this work is to define a seismic regionalization of Mexico for seismic hazard and risk analyses. This seismic regionalization is based on seismic, geologic, and tectonic characteristics. To this end, a seismic catalog was compiled using the more reliable sources available. The catalog was made homogeneous in magnitude in order to avoid the differences in the way this parameter is reported by various agencies. Instead of using a linear regression to converts from m _b and M _d to M _s or M _w , using only events for which estimates of both magnitudes are available (i.e., paired data), we used the frequency-magnitude relations relying on the a and b values of the Gutenberg-Richter relation. The seismic regions are divided into three main categories: seismicity associated with the subduction process along the Pacific coast of Mexico, in-slab events within the down-going COC and RIV plates, and crustal seismicity associated to various geologic and tectonic regions. In total, 18 seismic regions were identified and delimited. For each, the a and b values of the Gutenberg-Richter relation were determined using a maximum likelihood estimation. The a and b parameters were repeatedly estimated as a function of time for each region, in order to confirm their reliability and stability. The recurrence times predicted by the resulting Gutenberg-Richter relations obtained are compared with the observed recurrence times of the larger events in each region of both historical and instrumental earthquakes.     

UV habitable zones around M stars

During the last decade there was a change in paradigm, which led to consider that terrestrial-type planets within liquid-water habitable zones (LW-HZ) around M stars can also be suitable places for the emergence and evolution of life. Since many dMe stars emit large amount of UV radiation during flares, in this work we analyze the UV constrains for living systems on Earth-like planets around dM stars. We apply our model of UV habitable zone (UV-HZ; Buccino, A.P., Lemarchand, G.A., Mauas, P.J.D., 2006. Icarus 183, 491–503) to the three planetary systems around dM stars (HIP 74995, HIP 109388 and HIP 113020) observed by IUE and to two M-flare stars (AD Leo and EV Lac). In particular, HIP 74995 hosts a terrestrial planet in the LW-HZ, which is the exoplanet that most resembles our own Earth. We show, in general, that during the quiescent state there would not be enough UV radiation within the LW-HZ to trigger the biogenic processes and that this energy could be provided by flares of moderate intensity, while strong flares do not necessarily rule-out the possibility of life-bearing planets.     

Vertical profiles of aeolian sand mass flux

Vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel. Considering lower sampling efficiency of the sand trap in the near-bed region, this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap. The experiments were conducted with two test sands and five different stream velocities.In the upper part of the vertical profile, the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near-bed region. The measured longitudinal profiles are similar to the measured vertical profiles. Linking both profiles and the modes of sand transport, it is possible that saltating sand grains give rise to the well-known exponential decay distribution of mass flux, and that creeping and reptating grains force a deviation from it. A simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data.     

High-pressure,low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain)Métamorphisme de haute pression et basse température dans les unités Alpujarrides du Sud-Est de la cordillère Bétique (Espagne)

We present the first occurrences of high-pressure, low-temperature ferro-magnesiocarpholite-bearing mineral assemblages associated to quartz segregations in the Alpujarride units of southeastern Betics (Sierra de Almagro, Sierra de los Pinos and Sierra Cabrera). Thermobarometric results show that the carpholite-bearing rocks underwent the same P–T conditions in the three outcrops, i.e. 8–10 kbar, 350–400 °C. Metamorphic and structural data allow us to conclude that these rocks belong to the same Alpujarride unit. In the Sierra de Almagro, tectonic units with carpholite-bearing rocks overlie low-pressure, low-temperature Alpujarride units, then forming a stack with an inverted tectono-metamorphic sequence, as observed in the central and western part of the Alpujarride complex. The preservation of carpholite-bearing assemblages in these rocks implies that no significant temperature increase occurred during the exhumation history. To cite this article: G. Booth-Rea et al., C. R. Geoscience 334 (2002) 857–865.     

Frequency and duration of historical droughts from the 16th to the 19th centuries in the Mexican Maya lands, Yucatan Peninsula

Using unprecedented catalogues of past severe drought data for the Yucatan Peninsula between 1502 and 1900 coming from historical written documentation, we identified five conspicuous time lapses with no droughts between 1577–1647, 1662–1724, 1728–1764, 1774–1799 and 1855–1880, as well as time epochs with most frequent droughts between 1800 and 1850. Moreover, the most prominent periodicity of the historical drought time series was that of ∼40 years. Using the Palmer Drought Severity Index for the Yucatan Peninsula for the period 1921–1987 we found prominent negative phases between ∼1942–1946 and 1949–1952, 1923–1924, 1928–1929, 1935–1936, 1962–1963, 1971–1972 and 1986–1987. Two prominent periodicities clearly appear at ∼5 and 10 years. Most modern and historical severe droughts lasted 1 year, and share a quasi-decadal frequency. Also, in the first 66 years of the twentieth century the frequency of occurrence of severe drought has been lower compared with the nineteenth century. Some of the major effects and impacts of the most severe droughts in the Yucatan region are examined. We also studied the relation between historical and modern droughts and several large scale climate phenomena represented by the Atlantic Multidecadal Oscillation (AMO) and the Southern Oscillation Index (SOI). Our results indicate that historical droughts and the cold phase of the AMO coincide, while the influence of the SOI is less clear. The strongest coherence between historical droughts and AMO occurred at periodicities of ∼40 years. For modern droughts the coherence of a drought indicator (the Palmer Drought Severity Index) is similar with AMO and SOI, although it seems more sustained with the AMO. They are strongest at ∼10 years and very clearly with the AMO cold phase. Concerning the solar activity proxies and historical droughts, the coherence with a record of beryllium isotope Be¹⁰, which is a good proxy of cosmic rays, is higher than with Total Solar Irradiance. We notice that the strongest coherence between historical droughts and Be¹⁰ occurs at periods ∼60–64 years. When studying modern droughts and solar activity, frequencies of ∼8 years appear, and the coherences are similar for both sunspots and cosmic rays. Comparing natural terrestrial and solar phenomena, we found that the most sustained and strongest modulation of historical drought occurrence is at ∼60–64 years and is between the historical drought series and the solar proxy Be¹⁰. For modern droughts we notice that the coherence is similar among AMO, SOI and the solar indices. We can conclude that the sea surface temperatures (AMO) and solar activity leave their signal in terms of severe droughts in the Maya lands, however in the long term, the influence of the SOI on this type of phenomenon is less clear.     

Response of Acartia populations to environmental variability and effects of invasive congenerics in the estuary of Bilbao, Bay of Biscay

The effect of environmental factors (river discharge, water temperature and dissolved oxygen saturation) on the abundance and distribution of Acartia populations and the interactions between their congenerics was evaluated by means of transfer function (TF) models in the estuary of Bilbao during the period 1998–2005. The recorded species were Acartia clausi, Acartia tonsa, Acartia margalefi and Acartia discaudata. Acartia clausi dominated in the entire euhaline region of this estuary until 2003 when it was displaced from the inner part by A. tonsa. This invasive species (A. tonsa) was found for the first time in 2001 and colonized successfully the inner (salinity 30) and intermediate (salinity 33) waters of this estuary since 2003. The TF models revealed an immediate and negative effect of A. tonsa on A. clausi in the intermediate salinity (33) waters, where these species showed the highest spatial overlap. The results indicate that environmental changes from 2003 influenced the abundance of Acartia species, being unfavourable for A. clausi. The decrease of A. clausi abundance in low salinity waters was related to a significant decrease of dissolved oxygen saturation levels, whereas the increase of temperature was linked to a significant increase of A. tonsa. Acartia margalefi and A. discaudata were scarce over the entire period, but they were found to be valuable indicators of hydrological changes, which were associated to climate factors. These two latter species increased in abundance and expanded their seasonal distribution, and in the case of A. margalefi also its spatial distribution, in 2002, coinciding with the period in the time-series when autumn–winter rainfall and summer temperatures were lowest, and dissolved oxygen saturation levels were highest.     

Paleoshoreline reconstruction: A proposed method to approach submerged prehistoric landscapes of Espiritu Santo Island,Baja California Sur,Mexico

When the first Americans inhabited the area now known as Isla Espiritu Santo, around 12.5 and 6 ka, the sea level continuously rose. This resulted in the loss of the coastal territory and the retreat of the human population further inland. Part of the archaeological evidence of this period currently lies over the seafloor and under the seabed. Therefore, reconstructing paleoshorelines is a necessary first step toward submerged precontact archaeology and a broad understanding of the spatial context in which those human populations interacted and how the landscapes changed. Isla Espiritu Santo is of prehistoric importance as it contains numerous Paleocoastal tradition sites as early as ~12,400 Cal B.P. This research aims to reconstruct the changing shoreline positions as sea levels rose around Isla Espiritu Santo between 12.5 and 6.5 ka. We apply numerical modeling to map digital elevation models at 1 ka intervals by estimating the changes in the morphology of the seafloor according to the deposition rates and global sea-level curve. The results show the evolution of coastal morphology and paleoshoreline's position. Three primary geoforms are proposed: (1) coastal plains, (2) tombolo, and (3) a coastal lagoon–island barrier system. This also offers insights into the physical aspect of submerged prehistoric landscapes and the possible resource exploitation options of early societies. In this research, the reconstruction of submerged landscapes seeks to contribute to the long-term goal of locating submerged precontact sites.