Iron and Trace Metals in Microbial Mats and Underlying Sediments: Results From Guerrero Negro Saltern, Baja California Sur, Mexico

Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 ± 10 ??mol g^?1; n = 22) as well as the low pyrite (0.89?C7.9 ??mol g^?1) and AVS (0.19?C21 ??mol g^?1) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12?C50%), high degrees of sulfidization (14?C100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EF_Me = (Me/Al)_sample/(Me/Al)_background] of Co, Pb, and Cd were high in the mat (EF_Me = 2.2 ± 0.4, 2.8 ± 1.6 and 34.5 ± 9.8, respectively) and even higher in the underlying sediment (EF_Me = 4.7 ± 1.5, 14.5 ± 6.2 and 89 ± 27, respectively), but Fe was slightly impoverished (average EF_Fe of 0.49 ± 0.13 and 0.50 ± 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 × 10²?C27 × 10²) and sediment (0.81 × 10²?C6.6 × 10²) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 ± 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., >100) and low reactive Fe.     

Tidally flooded back-barrier dunefield, Guerrero Negro area, Baja California, Mexico

A 45 km long barrier island exists west of the town of Guerrero Negro, Mexico, along the western coast of the Baja California peninsula, about 720 km south of San Diego, California. This barrier has developed in a mesotidal, arid-climate regime characterized by steady, strong, onshore winds from the NW. The barrier island W of Guerrero Negro has prograded seaward about 1·6 km in the last 1800 years while an aeolian dunefield fed by sand blown from beaches has advanced inland up to 13 km. Landward progradation of the dune system from the barrier has occurred during relative rise in sea-level; thus, aeolian sediments exist at or below the water table over a wide area. The progradation of dunes across marshes, tidal flats, and tidal channels, as well as the repeated submergence of interdune areas by tidal waters, has created a complex suite of mixed aeolian and subaqueous sediments in the back barrier. The complexity of the suites of aeolian sedimentary structures, together with the inclusion of subaqueously formed structures such as current and oscillation ripples, would make recognition of the aeolian origin of much of the sediments difficult in ancient rocks. In addition to the scientific importance of recognizing the aeolian deposits, the sedimentation model represented by the Guerrero Negro barrier has applications in petroleum exploration and development. Currently, most preservational models for barrier islands attach little volumetric importance to aeolian deposits. This modern example suggests that volumetrically significant aeolian deposits can be preserved behind a barrier, particularly in an arid-climate regime. If preserved and charged with oil, the resulting productive sandstone could have an extremely irregular landward edge comprised in part of onshore-prograded aeolian dune sandstone with excellent reservoir characteristics. As with current barrier models, the reservoir would be sealed landward and above by lagoonal mudstone and silt, evaporites, or evaporitic, sandy sabkha deposits. High organic productivity occurs in lagoons immediately adjacent to the dunefields of Guerrero Negro, suggesting that organic-rich source rock may exist near aeolian sandstone in ancient settings similar to Guerrero Negro.     

Biological influences on modern sulfates: Textures and composition of gypsum deposits from Guerrero Negro,Baja California Sur,Mexico

Gypsum (CaSO₄·2H₂O) deposits from a range of sedimentary environments at Guerrero Negro, Baja California Sur, Mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent. Gypsum deposits were classified according to their sedimentary environment, textures, crystal habit, brine composition and other geochemical factors. The environments studied included subaqueous sediments in anchialine pools and in solar salterns, as well as subsurface sediments of mudflats and saltpans. Gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines. Subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles. Water column precipitates had euhedral prismatic habits and extensive penetrative twinning. Gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds. Gypsum precipitating within benthic biofilms, and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits. This evidence included irregular, high relief surface textures, accessory minerals (S°, Ca-carbonate, Sr/Ca-sulfate and Mg-hydroxide) and distinctive crystal habits such as equant forms and crystals having distorted prism faces.     

Effects of hurricane Odile on the infrastructure of Baja California Sur,Mexico

In September 2014, the state of Baja California Sur (BCS), Mexico, was struck by hurricane Odile. The hurricane caused extensive damage to the infrastructure of the state of BCS, where the energy and water supply were interrupted for several days. The direct economic impact was estimated to be more than 1654 million USD. Considering this damage, few days after the cyclonic event, a technical visit to BCS was carried out. The objective of this study is to present the relevant information on the performance of the infrastructure observed after the event, and to make recommendations in order to mitigate the effects of future cyclonic storms in Mexico, based on the observations during the technical visit. The main causes of damage were the use of materials for non-structural elements with a weak performance under high wind speeds, failures in the design and construction of connections of structural and non-structural elements, accumulated damage, lack of maintenance and impact of wind-born debris, as well as the absence of a building code in the region. Background information about meteorological and climatological aspects of the cyclones that have affected the southern region of BCS, the characteristics of hurricane Odile, the analysis of structural damages and the conclusions, are also presented.     

Salinization in coastal aquifers of arid zones: an example from Santo Domingo,Baja California Sur,Mexico

Groundwater quality in the Santo Domingo Irrigation District area in Baja California Sur, Mexico, indicates the presence of various salinization processes, (1) the geological matter of marine origin comprising the aquifer material suffers diagenetic effects due to its interaction with groundwater of low salinity, (2) the effects of intensive agriculture practices produce effluents that infiltrate to the saturated zone, and (3) the extraction of groundwater causes modifications in the natural flow system induces lateral flow of seawater from the coast line. However, groundwater management has been carried out with the belief that the latter is the main source of salinization. This has resulted in a policy of installing wells increasingly far from the coast, which is not solving the problem. Irrigation-return and seawater that remains in the geological units have been identified as major sources of salinization. Controls should be imposed when installing wells in contact with clayey units that form the base of the aquifer. Extracted groundwater consists of a mixture of (1) groundwater of relatively low salinity that circulates in the aquifer and (2) an extreme member with salinity different to seawater contained mainly in formations that have low permeability, which limits the aquifer underneath. The geochemistry of carbonates and cation-exchange reactions (both direct and reverse) control the concentration of Ca, Mg, Na, and HCO₃, as well as pH values. The concentrations of dissolved trace elements (F, Li, Ba, Sr) suggest that the extreme saline member is different from the average seawater composition. A distinction between the salinization caused by farming practices and that blamed on seawater is defined by the use of NO₃.     

Occurrence of landslides during the approach of tropical cyclone Juliette (2001) to Baja California Sur,Mexico

The approach of tropical cyclone (TC) Juliette, in 2001, to the Baja California Peninsula triggered at least 419 landslides. Most of the landslides were shallow slips and debris slides of limited areal extent, which were converted rapidly into debris flows to be exported quickly out of the mountain areas towards the lowlands. Main factors affecting landslide occurrence were total storm rainfall and intensity, aspect, geology and vegetation association. Two processes can be distinguished as initiating slope failure. Accumulation of rainfall from exposed bedrock slopes, generating excess overland flow, was the main process linked to failures in concave topography. A combination of wind and excess overland flow in the more convex or planar upper slopes was a secondary process related to heterogeneity of vegetation associations in the oak-dry tropical forests ecotone, as uprooted trees dislodged large regolith and bedrock blocks, priming hillslopes for further runoff concentration. An estimative threshold curve for triggering landslides in this region is sketched. From the analysis of historical information, storms like Juliette approach the southern peninsula on average once every 100 years. Denudation estimates are in the higher end of the spectrum for a tectonically passive margin. These estimates should be considered when taking decisions regarding management of water resources in this area through damming of streams. The results emphasize the need for a more detailed representation of the spatial distribution of the rainfall and winds for this mountainous region affected by TCs.     

Rapid development of gravelly high-density turbidity currents in marine Gilbert-type fan deltas, Loreto Basin, Baja California Sur, Mexico

The Pliocene Loreto basin is an asymmetrical half graben located on the eastern margin of Baja California Sur, Mexico, which formed by rapid subsidence along the dextral-normal Loreto fault. The southern Loreto basin contains numerous, well exposed coarse-grained Gilbert-type fan deltas that were derived from the footwall of the Loreto fault. Detailed sedimentological study of individual foreset beds provides information about down-slope flow transformations of cohesionless sediment gravity flows in shallow water. Deposits of Gilbert-delta foresets consist of ungraded, normal-graded, inverse- to normal-graded, and bipartite conglomerate and sandstone. Lateral transitions in sorting, grading style and internal structure are commonly observed within individual beds, both across and down slope, suggesting heterogeneity within flows and a close relationship between high-density turbidity currents and gravel traction carpets. A conceptual model for flow transformation and deposition of high-density turbidity currents on Gilbert-delta foreset slopes is developed for Pliocene strata in the Loreto basin. In this model, ungraded cohesionless debris flows evolved rapidly down-slope into normal-graded gravelly turbidity currents. With continued down-slope transport, the gravel fraction collapses and becomes concentrated into a basal traction carpet undergoing laminar shear, and is over-ridden by a sandy turbulent suspension. The short distances (10–20 m) over which lateral transitions within single beds are observed indicate very rapid flow transformations (10–20 s) and rapid deposition of gravel traction carpets by frictional freezing on and near the base of the foreset slope.     

Relative inactivity during the last 140,000 years of a portion of the La Paz fault,southern Baja California Sur,Mexico

Uranium-series dating of corals overlying the undeformed Punta Coyote gravels indicates that the underlying La Paz fault zone has been relatively inactive in this part of the Baja California peninsula during the last 140,000 years, and possibly for a significantly longer period. However, Holocene seismic activities along extensions of the fault zone north of Cabo San Lucas suggest potential seismic hazards for the city of La Paz (population 200,000), which lies about 6 km from the fault.     

Geoarchaeology and geochronology of pluvial Lake Chapala,Baja California,Mexico

Stratigraphic exposures in natural profiles, archaeological excavation units, backhoe trenches, and an uncased water well from the Laguna Seca Chapala basin in the Central Desert of Baja California (29°N, 115°W) record lake level and climate changes and provide a context for prehistoric occupation predating 9070 yr B.P. and extending through the Holocene. Lithofacies analysis points to the presence of a large (ca. 66 km²) lake prior to 9070 yr B.P., which desiccated by 7.45 ka yr B.P., promoting rapid dune growth. New dating and redefinition of stratigraphic units in the basin refutes earlier models of lacustrine history and prehistoric occupation including a proposed series of Pleistocene lake levels with associated cultural occupations. The geologic record from the Laguna Seca Chapala basin compares well with other paleoenvironmental records in southwestern North America, supporting interpretations of wet and cool conditions in Baja California during the late Pleistocene and early Holocene. © 2003 Wiley Periodicals, Inc.     

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.     

永定河沉积物中磷的存在形态及其指示意义

采集官厅水库的永定河入库口附近沉积物,采集深度77cm,以1cm做为一个层位单元进行分割,测定其中11个层位的总磷,同时对这些层位沉积物中二钙结合态磷、八钙结合态磷、铝结合态磷、铁结合态磷进行连续分级提取,有机磷通过灼烧法测定,残留态磷由总磷与上述五种形态磷差减确定。结果显示:沉积物中总磷含量在580.3~1276.2μg/g,主要成分为无机磷,平均占总磷的比例为88%;残留态磷为沉积物中磷的主要形态,含量占总磷50%以上。在沉积物深度75~20cm的沉积年份中,永定河向官厅水库输送的残留态磷量不断增加,且增速逐渐加快,主要为水土侵蚀的产物。从沉积深度20cm以来的沉积年份里,总磷的输入量不断减少,但以二钙结合态磷为主的生物有效磷输入量随总磷的含量降低而持续增加,主要来自工业和生活污水,反映了官厅水库上游永定河流域内工业和生活污水排放所占的比重日益增加,成为官厅水库富营养化污染的主要来源,以致官厅水库富营养化的趋势增强。     

Tidal flat macrofaunal communities and their associated environments in estuaries of southern California and northern Baja California, Mexico

Several tidal flats in both Estero de Punta Banda and Bahia de San Quintín, Baja California, and one in Mission Bay, southern California, were sampled for macrofaunal properties (taxonomic composition, density, species richness, and functional groups for animals ≥0.3 mm) and associated environmental variables (sediment properties, salinity, plant belowground biomass, and cover ofZostera marina) in order to establish a benchmark data set for these areas. The grouping of macrofauna into higher taxonomic or functional groups for these comparisons reduced variability and revealed stronger relationships. Each estuary had a fairly distinct macrofaunal assemblage, with that of Estero de Punta Banda being different from Bahia de San Quintín and Mission Bay primarily due to dominance by a capitellid polychaete, lower proportions of surface deposit feeders, and higher proportions of fauna with a planktonic stage. The flats in Mission Bay and Bahia de San Quintín were dominated by peracarid crustaceans, oligochaetes and polychaetes and had higher proportions of direct developers and macrofauna with mobile adult stages than did Estero de Punta Banda. There was an overlap of the environmental characteristics among estuaries, with more variability of sediment and vegetation properties within than among estuaries. Within Bahia de San Quintín, there was an oceanic to back-bay distribution gradient of macrofauna that was similar to that found in estuaries in wetter climates, despite the lack of a salinity gradient in San Quintín. A decoupling of the benthos and the assumed anthropogenic stresses was observed with the degraded site, Mission Bay, being most similar to the relatively pristine Bahia de San Quintín. Selection of reference sites and sampling variables should be made cautiously because effects of disturbance factors on the benthos may be site-dependent, scale-dependent, or negligible.     

Anthropogenic subsidence in the Mexicali Valley,Baja California,Mexico, and slip on the Saltillo fault

Deep fluid extraction in the Cerro Prieto geothermal field (CPGF) has caused subsidence and induced slip on tectonic faults in the Mexicali Valley (Baja California, Mexico). The Mexicali Valley is located in the southern part of the Salton Trough, at the boundary between the Pacific and North American plates. The Valley is characterized by being a zone of continuous tectonic deformation, geothermal activity, and seismicity. Within the Cerro Prieto pull-apart basin, seismicity is concentrated mainly in swarms, while strong earthquakes have occurred in the Imperial and Cerro Prieto transform faults, that are the eastern and western bound of the basin. Since 1973, fluid extraction at the CPGF has influenced deformation in the area, accelerating the subsidence and causing rupture (frequently as vertical slip or creep) on the surface traces of tectonic faults. Both subsidence and fault slip are causing damage to infrastructure like roads, railroad tracks, irrigation channels, and agricultural fields. Currently, accelerated extraction in the eastern part of CPGF has shifted eastwards the area of most pronounced subsidence rate; this accelerated subsidence can be observed at the Saltillo fault, a southern branch of the Imperial fault in the Mexicali Valley. Published leveling data, together with field data from geological surveys, geotechnical instruments, and new InSAR images were used to model the observed deformation in the area in terms of fluid extraction. Since the electricity production in the CPGF is an indispensable part of Baja California economy, extraction is sure to continue and may probably increase, so that the problem of damages caused by subsidence will likely increase in the future.     

Metamorphic petrology of a high-pressure, low-temperature subduction complex in west-central Baja California, Mexico

Abstract The Western Baja terrane (WBt) of west-central Baja California is an uplifted subduction complex that is divided into smaller 'subterranes'on the basis of bounding faults and petrological differences. Each subterrane contains coherent Early Jurassic to Early Cretaceous sedimentary and mafic volcanic rocks (not melange) that have been metamorphosed under blueschist facies conditions. Key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene, sodic amphibole, lawsonite, aragonite, chlorite, titanite and white mica. Pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite. Temperature indicators include the presence of lawsonite, the absence of greenschist facies minerals and results from vitrinite reflectance studies. Conditions at the peak of metamorphism were >8 kbar, 225–325°C for subterrane 1, 7–8 kbar, 170–220°C for subterrane 2, and 5–6 kbar, 175–200°C for subterrane 3; these correspond to cold geothermal gradients (6–9/km). Vein assemblages that include aegerine–jadeite and aegerine, albite, aragonite, lawsonite and sodic amphibole indicate uplift during continued cold conditions, probably during steady-state subduction.     

Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez,Mexico

A detailed field analysis of Neogene and Quaternary faults in Baja California has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula. The deformation, which is related to the opening of the gulf, affects the whole peninsula, but decreases from east to west. Most observed faults, normal and/or dextral, strike NNW-SSE to WNW-ESE; their mechanisms include both strike-slip and dip-slip, as well as intermediate motions. Compressional events have occurred since Late Neogene times, but were probably of minor quantitative importance because reverse faults are rare and small.The principal fault pattern includes dextral NNW-SSE Riedel shears and N-S tension faults induced by dextral strike-slip along two main NW-SE fault zones bordering the peninsula: the Gulf of California to the east, which is the most important, and the Tosco-Abreojos fault to the west. The resulting pattern of deformation shows that the eastern part has been a complex transform-extensional zone since Late Miocene-Early Pliocene times.     

A Fresh Look at the Geobiology & Sedimentary Environments of the Hypersaline Great Salt Lake,USA

正1 Introduction The Great Salt Lake(GSL)is the defining hydrological feature of the Great Basin,North America’s largest desert--and it is the largest waterbody in the western USA.Despite the early(Eardley,1938)recognition of"bioherms,"algal layers,and mats covering hundreds of square km of lake area,these features have not been the     

Mineralogy and Geochemistry of Major,Trace and Rare Earth Elements in Sediments of the Hypersaline Urmia Salt Lake,Iran

Urmia Salt Lake(USL) is a hypersaline lake located at the NW corner of the Iran platform. The lake area is estimated to have been over 5000 km~2 at one point, but has now decreased to 1000 km~2 in the last two decades. It contains 4.6×10~9 tons of halite and other detrital and evaporative minerals such as calcite, aragonite, dolomite, quartz, feldspars, augite and sylvite. This study examined the mineralogy and geochemistry of bed sediments along the mid-east toward NE bank sediments collected from 1.5 meters depth and nearby augite placer. Due to the diverse lithology of the surrounding geology, bed sediments vary from felsic in the mid-east to mafic in the northeast. Weathering of tephrite and adakite rocks of the Islamic Island at the immediate boundary has produced a large volume of augite placer over a 40 km length, parallel to the shoreline. Based on the study result, weathering increases from south to north and the geochemistry of the sediments shows enrichment of Mg O, Ca O, Sr and Ba associated with Sr deployment in all samples. Rare earth elements(REE) patterns normalized to the upper continental crust(UCC) indicated LREEs enrichment compared to HREEs with an elevated anomaly of Eu, possibly due to surface absorbance of Mn and Fe minerals, associated with Sr elevation originating from adakites in the lake basin vicinity.     

Geomorphology, deformation, and chronology of marine terraces along the pacific coast of central Baja California, Mexico

Three emergent marine terraces are prominent between Playa El Marron and Arroyo El Salinito and comprise the most extensive Pleistocene planation surfaces in central Baja California, Mexico. The deposits of the lowest terrace, the Tomatal, are 120,000 ± 20,000 yr old (Sangamonian?) while the absolute ages of the two higher and older terraces, the Andrés and Aeropuerto, are unknown. The Tomatal terrace is particularly well developed and comprises degraded sea cliffs, paleodunes, and lagoonal sequences. Shingle paleobeach ridges also occur locally and reflect shore progradation and tombolo formation. The Tomatal shoreline is nearly horizontal at 7 ± 1 m above present mean sea level, whereas the older Aeropuerto terrace has been tilted so that it decreases in elevation toward the southeast. Nonetheless, coastal tilting is not nearly as great as at many other localities in California and Baja California. This is despite the fact that the entire Baja California peninsula has been assumed to be tectonically unstable during the Pleistocene, primarily because of the forces generated by plate motion.