Person: Martín García, Rebeca
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First Name
Rebeca
Last Name
Martín García
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Mineralogía y Petrología
Area
Petrología y Geoquímica
Identifiers
4 results
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Item Loss of primary texture and geochemical signatures in speleothems due to diagenesis: Evidences from Castañar Cave, Spain(Sedimentary geology, 2009) Martín García, Rebeca; Alonso Zarza, Ana María; Martín Pérez, AndreaGeochemical signals from speleothems are commonly used in the investigation of palaeoenvironments. In most cases, however, little attention is paid to whether or not these signals are primary or altered by diagenesis. The speleothems of the Castañar Cave (Cáceres, Spain), which are initially formed of calcite or aragonite, have undergone a variety of meteoric diagenetic processes such as micritization and neomorphism (inversion), that collectively modify their primary features (textures, mineralogy, geochemical signals). The mean δ13C and δ18O values of the aragonites in the cave are −8.66 and −4.64 respectively, whereas the primary calcites have mean δ13C and δ18O values of −9.99 and −5.77, respectively. Following the diagenetic process of micritization, the aragonite isotopic signals averaged −7.63 δ13C and −4.74 δ18O and the calcite micrite signals −9.53 δ13C and −5.21 δ18O. Where inversion took place, some secondary calcites after the aragonite show preserved aragonite, whereas others do not. The secondary calcites without aragonite relics show isotopic values slightly higher than those of the primary calcite due to the inheritance of the aragonite signal. Where aragonite relics are preserved, the isotopic signatures are very similar to those of the aragonite micrite. In addition, the stable isotopic values and Sr and Mg contents of the speleothems became also modified by micritization and/or inversion. These diagenetic processes were driven by the changes in composition of the cave waters over time and space, but also, in the case of aragonite, by its initial unstable mineralogy. The present results highlight how important diagenesis is in caves and how the initial features of cave minerals may be lost. These changes alter the geochemical signals shown by speleothems, which may have an impact on the interpretation of the results obtained in palaeoenvironmental studies.Item Geoquímica isotópica (δ18O yδ13C) de los espeleotemas de aragonito, calcita y dolomita de la Cueva de Castañar de Ibor (Cáceres)(Geotemas, 2008) Martín Pérez, Andrea; Martín García, Rebeca; Alonso Zarza, Ana MaríaEn la cueva de Castañar de Ibor (Cáceres), aparecen una gran variedad de espeleotemas de aragonito, calcita, dolomita, huntita y magnesita. Muchos de estos minerales son de origen primario, pero también se han observado transformaciones diagenéticas recientes como disolución, micritización, inversión aragonito–calcita y dolomitización. En este contexto se ha realizado un estudio preliminar de los isótopos estables de algunos espeleotemas centrándonos en los que son de calcita, aragonito y dolomita, para ver si su composición isotópica puede aportar datos sobre sus condiciones de formación y transformación. Se observa un enriquecimiento en isótopos pesados en las muestras de aragonito respecto a la calcita, lo cual es indicativo de la formación del aragonito a partir de soluciones más enriquecidas en isótopos pesados que la calcita. La dolomita presenta también valores más altos en δ18O, sin embargo, se produce un enriquecimiento en isótopos ligeros del δ13C lo cual podría ser explicado por una posible participación de actividad orgánica en su formación. [ABSTRACT] In Castañar de Ibor cave (Cáceres), aragonite, calcite, dolomite, huntite and magnesite speleothems are found. These minerals can be primary in origin, but also suffer recent diagenetic processes such as dissolution, micritization, aragonite-calcite inversion or dolomitization. In this context, a preliminary study of stable isotope has been done focusing on calcite, aragonite and dolomite, in order to check if their isotopic geochemistry can help to understand their formation and transformation conditions. The aragonite is a little more enriched in the heavy isotopes than the calcite. This indicates that aragonite precipitated from waters enriched in δ18O and δ13C. Dolomite shows higher values in δ18O as well, but there is a lighter value for 13C, what may indicate organic activity.Item Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess(Sedimentary Geology, 2009) Alonso Zarza, Ana María; Zhao, Z.; Song, C. H.; Li, J. J.; Zhang, J.; Martín Pérez, Andrea; Martín García, Rebeca; Wang, X. X.; Zhang, Y.; Zhang, M. H.The Tianshui Basin in central China contains a thick sedimentary sequence (~1400 m) of continental deposits, Aragonian to Villafranchian (Miocene-Pliocene) in age. Intense Himalayan movements around the Paleogene/Miocene boundary triggered the uplift of mountain ranges around the Tianshui Basin, providing the deposition site for continental sediments. The sedimentary infill of the basin consists of four stratigraphic units (I to IV). This paper focuses on Unit II. Most of the accommodation space was occupied by Unit I, so during the sedimentation of Unit II, the morphology of the basin was relatively flat, promoting the development of wide distal fan/mudflat areas and wide shallow lakes. Deposits include: red mudstones, pedogenic and groundwater calcretes, reworked calcrete deposits, sheet-floods, fluvial channels, rippled sandstones/siltstones, ooidal/peloidal packstones, palustrine limestones, bioturbated marls and intraclastic limestones/marls. The characteristics and organization of the deposits indicate the gradual transition from alluvial to lake environments. Within the distal fan/mudflat, the deposition of reworked calcrete clasts as one of the most striking facies of the basin may be attributed to substantial recycling of calcrete levels and red clays. At the lake margins, the presence of ooids and palustrine limestones suggests the possibility of ramplike margins within different energy settings. The origin of some of the deposits of the basin's QA-I section is under discussion, and their consideration as eolian has been recently proposed. However, we believe this possibility is precluded by the characteristics of the deposits. Our proposal has important implications for revising the interpretation of Miocene paleoclimatic conditions in central Asia.Item Micritización de espeleotemas en ambiente meteórico vadoso (Cueva de Castañar de Ibor, Cáceres)(Geogaceta, 2007) Martín García, Rebeca; Alonso Zarza, Ana María; Martín Pérez, AndreaThe aragonite and calcite speleothems of the Castañar de Ibor Cave show white and matt coatings that formed by micritization and dissolution of the initial crystals. These processes are also recognised in the inner part of the speleothems. Either in the inner or outher parts the result is the lost of brightness and transparency of the speleothems. Petrographically these coatings are seen as lines of micrite liked to dissolution processes that probably occurred because of the circulation of undersaturated fluids within the cave. At the moment no organic structures have been recognised in relation to the micritizated areas, indicating very probably that the process was inorganically driven.