Modeling speleothem δC variability in a central Sierra Nevada cave using C and Sr/Sr     

Jessica L. Oster  Isabel P. Montañez  Warren D. Sharp 《Geochimica et cosmochimica acta》2010,74(18):5228-4950

Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid-rock interaction in the host limestone, degassing of CO₂ and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water-rock interaction and soil processes in controlling stalagmite δ¹³C. This approach uses the dead carbon proportion (dcp) estimated from coupled ¹⁴C and ²³⁰Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water-rock interaction and soil processes in determining stalagmite δ¹³C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased δ¹³C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals.We use geochemical and isotope mixing models to estimate the host-carbonate contribution throughout the δ¹³C time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite. The degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators, with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals. Modeled δ¹³C values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite δ¹³C values, even during relatively wet intervals. These results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem δ¹³C records, particularly those from caves that formed in seasonal semi-arid to arid environments.  相似文献   

The Holocene record of Alberca de Tacámbaro,a tropical lake in western Mexico: evidence of orbital and millennial-scale climatic variability     

Beatriz Ortega-Guerrero  Margarita Caballero  Isabel Israde-AlcáNtara 《第四纪科学杂志》2021,36(4):649-663

Laminated sediments in a core from Alberca de Tacámbaro, western central Mexico, provide a high-resolution record of palaeoclimatic variability for most of the Holocene. Using compositional and textural characteristics we identified 14 facies classified in biogenic, clastic and volcaniclastic groups, in which diatoms and other algal remains are the most common components. In addition to facies analysis, high-resolution geochemical data from continuous core scanning XRF and the analysis of discrete samples for organic and inorganic carbon content, provide the basis for palaeoclimatic reconstruction. The age model indicates that the record spans from 9.4 cal ka bp toad 1760. Three main phases in the sequence are identified, controlled by millennial-scale variability. During the period between 9.4 and 5.7 cal kabp , the lake is dominated by siliceous organisms. Drier conditions between 8.2 and 6.3 cal kabp are linked to decreases in summer insolation and solar irradiance. By 5.7 cal kabp , the change in sedimentation is related to the increase in El Niño Southern Oscillation frequency. Evidence of the occurrence of dry conditions during the archaeological Late Classic period (ad 600–900) and historical droughts are also documented in the Alberca de Tacámbaro record. Unprecedented changes in the landscape are recorded afterad 1520.  相似文献   

Erratum to: Eight-hundred years of summer temperature variations in the southeast of the Iberian Peninsula reconstructed from tree rings     

Isabel Dorado Liñán  Eduardo Zorita  Jesús Fidel González-Rouco  Ingo Heinrich  Filipe Campelo  Elena Muntán  Laia Andreu-Hayles  Emilia Gutiérrez 《Climate Dynamics》2015,44(7-8):2329-2329

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Effects of ultraviolet radiation on the abundance,diversity and activity of bacterioneuston and bacterioplankton: insights from microcosm studies     

Ana L. Santos  Isabel Henriques  Newton C. M. Gomes  Adelaide Almeida  António Correia  Angela Cunha 《Aquatic Sciences - Research Across Boundaries》2011,73(1):63-77

The effects of ultraviolet-B (0.4 W m⁻²) radiation on the abundance, diversity and heterotrophic metabolism of bacterioneuston and bacterioplankton communities from Ria de Aveiro (Portugal) were assessed and compared to those of freshwater communities from Lake Vela (Portugal) in microcosm experiments. Exposure to 9 h of artificial ultraviolet radiation (UVR) led to 24–33% reduction in bacterial abundance and up to a 70% decrease in bacterial diversity. Maximum extracellular enzyme activity and monomer incorporation rates were reduced by 16–90% and 80–100%, respectively. Recovery of bacterial activity during post-UV dark incubations ranged from 10 to 100% for extracellular enzyme activity and 40% for monomer incorporation rates. In general, the heterotrophic activity of bacterioneuston was more inhibited by UVR than that of bacterioplankton. However, DGGE profiles revealed greater UVR-induced reductions in the diversity of bacterioplankton compared to bacterioneuston. The similarity between bacterioneuston and bacterioplankton communities in samples collected at early morning was lower than at noon (pre-exposed communities) and increased upon experimental irradiation, possibly indicating selection for UV-resistant bacteria. The observation that UV exposure resulted in enhanced reduction of bacterioneuston activity, but a lower reduction in bacterial diversity accompanied by enhanced dark recovery potential compared to bacterioplankton, indicates re-directioning of bacterioneuston metabolism towards stress defence/recovery strategies rather than the sustained heterotrophic metabolism. Our results indicate that UVR can significantly decrease the abundance, diversity and activity of bacteria inhabiting the surface and sub-surface layers of freshwater and estuarine systems with potentially important impacts on the biogeochemical cycles in these environments.  相似文献   

Late Glacial and Holocene avulsions of the Rio Pastaza Megafan (Ecuador–Peru): frequency and controlling factors     

Carolina Bernal  Frédéric Christophoul  José Darrozes  Jean-Claude Soula  Patrice Baby  José Burgos 《International Journal of Earth Sciences》2011,100(7):1759-1782

The geomorphological study by means of remote sensing imagery of the Rio Pastaza Megafan (Ecuador and northern Peru) reveals the traces of numerous avulsions. One hundred and eight avulsion sites have been defined. The location of these sites, the available radiocarbon ages as well as historical maps of the seventeenth century, enable us to propose an evolution history of the migration and avulsions of the Rio Pastaza since the Last Glacial Maximum. The first avulsions of the Río Pastaza occurred after the LGM in a zone close to and roughly parallel to the sudandean front, where the developed avulsion gave a distributive pattern to the ancient stream of the Río Pastaza in an area located between the modern Río Morona and Pastaza, where they caused the Rio Pastaza to develop a fan-like distributary pattern. This is interpreted as a response to thrust-related forelimb tilt, progressively shifting eastward the Rio Pastaza and the apex of the megafan. This sequence of events ended with the Great Diversion of the Rio Pastaza towards the modern Rios Corrientes and Tigre. Avulsions occurred in the Tigre-Corrientes Area between 9200 and 8,500 years Cal BP. Afterwards, the Río Pastaza was diverted to its present-day north–south course. This last significant avulsion occurred before AD 1691. In the area located between the modern Río Morona and Pastaza, avulsion frequency—probably overestimated—ranges between 100 and 200 years. In the Ríos Tigre and Corrientes area, avulsion frequency—probably underestimated—ranges from 300 to 400 years. Regional tectonics is likely to have triggered most of the avulsions in the Morona Pastaza area but its influence is restricted to this area. The factors controlling the avulsions in the Tigre-Corrientes area are less clear because the frequently described “hydrologic”-driven avulsion as observed in areas characterized by contrasted hydrologic cycles are inconsistent with the characteristics of the hydrologic cycles of the Rio Pastaza.  相似文献   

The metamorphic aureole of the Nisa-Alburquerque batholith (SW Iberia): implications for deep structure and emplacement mode     

Luís?González?MenéndezEmail author  Antonio?Azor  Alvaro?Rubio?Ordó?ez  Isabel?Sánchez-Almazo 《International Journal of Earth Sciences》2011,100(7):1533-1550

The Nisa-Alburquerque granitic batholith (southern Variscan Belt, Iberian Peninsula) has been studied by petrological, structural and geophysical approaches, obtaining contrasting models for its deep structure and emplacement sequence. In order to test these models and gain knowledge on the thermal increase induced by the intrusion, we have studied its contact aureole, which was developed in similar country rock lithologies (mica schists alternating with metasandstones and feldespatic schists) all along the northern external contact of the batholith. Our results indicate no change in metamorphic grade and some variations in aureole width, which narrows toward the western sectors of the batholith. Cordierite is the only contact metamorphic mineral developed together with a high temperature biotite probably related to the granite thermal input. By considering these new data, together with zircon saturation temperatures within the granite and previous petrological and geophysical studies, we propose a model in which the feeder zones of the granitic magmas were an eastern main one and a western secondary one. We have also made comparisons of the metamorphic grade in the country rocks and the xenoliths within the granite. Most of the xenoliths have the same metamorphic facies as the country rocks (Crd-zone), though some of them contain slightly different assemblages (And + Crd), which could be explained in different ways: (1) differences in the primary schist compositions, (2) increased time-span of xenoliths in contact with the melt and (3) xenolith incorporation at slightly higher depths during final granite ascent.  相似文献   

The Bizielle vein (valle de Gistain): A case of iron oxide transformations at the Pyrenees of Spain     

Irene DeFelipe  Isabel Fanlo  Ester Mateo  Ignacio Subías 《Chemie der Erde / Geochemistry》2014

The Bizielle vein has some unique features among the Pyrenean alpine veins that allow us to address the question of the nature of iron oxides transformations under low temperature hydrothermal conditions, which is well known to prevailed over wide areas of western Europe between early Triassic to early Cretaceous times. Isotopic studies indicate a deep-seated origin of the ore-forming fluids and suggest that the metals were leached from the Variscan basement (mainly from granites). Isotopic geothermometry and regional evidences point to a 250 °C and reducing fluid, being SH₂ the predominant S specie. Under such conditions, the proposed in situ deposition of hematite is a consequence of Fe carbonates dissolution and oxidation involving dissolution/precipitation processes in the sense of Putnis. Non-redox model is a quite plausible origin for subsequent hematite to magnetite conversion.  相似文献   

Contaminated soils and sediments associated with Zn ore metallurgy near the São Francisco River,Minas Gerais (Brazil)     

Isabel Ribeiro da Costa  Rita Fonseca  Catarina Pinho  Alexandre Araújo  Luísa Catarina Martins  Nuno Dias  Ana Isabel Janeiro  Guilherme Freitas 《Environmental Earth Sciences》2018,77(5):202

Draining through industrial areas of the Minas Gerais mining state (Brazil), some tributaries of the São Francisco River constitute a potential environmental hazard for this great river and threaten the quality of the regional soils for agriculture and other activities. Extensive geochemistry and mineralogy of sediments, soils and alluvial plains from six selected areas within the Consciência drainage basin close to an important Zn-extraction plant, have been carried out. In this report, detailed mineralogy of those samples and supporting geochemical data are discussed, taking into account their specific climactic and environmental context. Petrographic and electron microprobe characterization of the sand-grained fraction of these materials was complemented by XRD on their finer fraction: the main contaminant minerals are willemite (one of the Zn ores used in the industrial plant) and jarosite, though their contents are quite variable in the studied areas and also with depth; minor amounts of Zn-, Pb-, Cd-, and Mn-bearing mineral phases are also frequent, usually as inclusions in willemite or in polycrystalline clasts, or adsorbed on the finer materials, such as clay minerals and associated Fe-hydroxides. Mineralogical contamination is responsible for high metal contents in the soils and sediments of the areas closer to the plant (e.g. Zn ? 2000 mg kg^?1 and Cd ? 20 mg kg^?1, which are the Intervention Values for Industrial Areas) and the greatest contamination risks are related to the more labile phases that circulate throughout the alluvial plains, the shallow sediments and the stream bed. Monitoring the mineral/chemical contamination and its extent also constitutes a useful basis for future proposals to remediate and recover this industrial area in order to decrease medium- and long-term negative impacts of metal contamination on the local and downstream environments.  相似文献   

Trojan resonant dynamics,stability, and chaotic diffusion,for parameters relevant to exoplanetary systems     

Rocío Isabel Páez  Christos Efthymiopoulos 《Celestial Mechanics and Dynamical Astronomy》2015,121(2):139-170

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Stratigraphy and architecture of the Upper Triassic Ischigualasto Formation,Ischigualasto Provincial Park,San Juan,Argentina     

Brian S. Currie  Carina E. Colombi  Neil J. Tabor  Todd C. Shipman  Isabel P. Montañez 《Journal of South American Earth Sciences》2009,27(1):74-87

The Ischigualasto Formation in northwestern Argentina contains abundant fluvial channel sandstones, overbank mudstones, and paleosols that were deposited in a northwest-trending continental-rift basin during Late Triassic time. In the study area the formation progressively thins from ～700 m in the west to ～400 m in the east, over a distance of 7 km. This thinning is accompanied by a relative decrease in the abundance of fluvial channel sandstones and an increase in mud-rich overbank deposits and paleosols. While preserved channel deposits in the formation are highly variable in terms of their size and stratigraphic distribution, four general channel forms can be recognized based on their overall cross sectional geometry and internal sedimentary structures. Of these, the dominant channel-body types are interpreted as the deposits of sandy multi-channel fluvial systems. The internal stratigraphic architecture of the Ischigualasto Formation indicates that during deposition, the central part of the basin was the location of a long-lived, north flowing, fluvial channel belt that received relatively continuous channel and proximal overbank deposition. To the east, however, channel-related deposition was more infrequent, resulting in enhanced pedogenic modification of alluvial deposits. The overall thickness and facies trends observed in the Ischigualasto Formation most likely correspond to variations in fault-related accommodation development within the basin during the time of deposition.  相似文献