Cinta Osácar

Morphological and speleothemic development in Brujas Cave (Southern Andean Range, Argentine): palaeoenvironmental significance

Abstract Brujas Cave, in the Southern Andean Range, is a well-known endokarstic site in Argentina. However, the origin and evolution of this cave system are poorly known. Based on morphological cave features as well as characteristics of cave deposits, we propose a meteogene drawdown cave genesis, including a change from phreatic to vadose conditions related to the high rate of fluvial downcutting in the area. During the vadose period, various cave-related deposits, including authogenic calcite and gypsum speleothems, allogenic volcanic ash and external tufas were deposited. Gypsum crusts are the oldest cave deposits identified (90.2–64.3 ky BP). Their origin, deduced from isotopic characteristics ( ∂ 34 S=9.6‰), is related to the oxidation of pyrite contained in the Jurassic limestone bedrock as well as the dissolution of overlying Jurassic–Triassic evaporite formations. Gypsum crust deposition is associated with evaporation of water flowing and seeping into the cave during arid environmental conditions. Calcite deposits precipitated from flowing water under equilibrium conditions represent the main speleothem growth period (67.6–34 ky BP in age). Their stable isotope values ( ∂ 13 C=−3‰ to −5‰ and ∂ 18 O=−9‰ to −11‰) may indicate slightly humid and warm conditions related to the regional Minchin lacustrine phase and global oxygen isotope stage 3. Following this stage, a seismic event is evidenced by accumulations of broken stalactites. Seepage calcite speleothems covering cave walls were deposited under disequilibrium conditions by evaporation, probably during Holocene time. Finally, another more recent gypsum deposition period represented by gypsum balls has been differentiated. Micromorphological as well as isotopic ( ∂ 34 S=5.6‰) data indicate that these gypsum forms are related to cyclic processes (solution–deposition) from water seeping into the cave under arid conditions. In addition, intense volcanic activity in the area during Holocene time is deduced from allogenic volcanic ash and lapilli located mainly at the entrance cave. At present, limited hydrological activity is observed in the cave and small tufa deposits at karstic discharge points are evidenced. We conclude that the geomorphological evolution of Brujas Cave was controlled by climatic changes (wet and dry stages) under semiarid environmental conditions in a very active tectonic and volcanic setting during Late Quaternary time.

Benthic bacterial diversity from freshwater tufas of the Iberian Range (Spain)

Aiming to characterize the bacterial diversity of modern tufa systems of the Iberian Range (Spain), we surveyed the 16S rRNA gene sequence diversity from 24 sites within three rivers (Añamaza, Mesa and Piedra). These tufas record substantial calcareous growth under different physicochemical conditions and are part of an important, regional landscape-building system. The bacterial community structure and composition, richness and diversity were quantified from denaturing gradient gel electrophoresis fingerprints. Retrieved DNA sequences could be assigned to 10 bacterial phyla and included a variety of phototrophic and heterotrophic groups. Cyanobacteria, mainly filamentous taxa, constituted 43% of all the retrieved sequences, followed by Firmicutes (11%), Gammaproteobacteria (10%), Alphaproteobacteria (7%), Acidobacteria (6%), Bacteroidetes (5%), Betaproteobacteria (4%), Planctomycetes (4%), Actinobacteria (3%) and Deltaproteobacteria (2%). Diatom and Xanthophyceae chloroplast sequences were also detected. Physicochemical variables measured at each site were modelled with multivariate statistics. Principal component analyses yielded the highest variance for salinity-related variables (conductivity; Na(+) , Cl(-) and SO4(2-) concentrations), which correlated negatively and significantly with diversity indices. However, the highest variance explained by individual principal components was relatively low (< 34%). Overall, we show that these young fluvial tufas are inhabited by a large variety of bacteria in diverse and widespread communities.

Succession and stable isotope composition of gastropods in Holocene semi-arid alluvial sequences (Bardenas Reales, Ebro Basin, NE Spain): Palaeoenvironmental implications

Three alluvial units, ranging from the middle to late Holocene in age, were distinguished in Bardenas Reales Natural Park (Ebro Basin, NE Spain) based on morphosedimentary analysis and radiocarbon data. In this sensitive Mediterranean alluvial system, the succession and isotopic composition of gastropods greatly contribute to establishing the prevailing palaeoenvironmental conditions. The oldest unit (4763±87 to 2848±55 cal. yr BP) represents fluvial channels and swamped areas, includes the greatest proportion of aquatic gastropods (Ancylus fluviatilis and Lymnaea truncatula) and marsh species (Vallonia pulchella) and presents the most negative values of δ18O (mean −2.21‰ PDB). The intermediate unit (1403±60 to 836±65 cal. yr BP) shows a high rate of alluvial activity related to irregular torrential rainfall, with sedimentary features of aridity, and this unit includes only dry open ground gastropods (Xeroplexa sp., Chondrinidae indet. and Granopupa granum) and presents the least negative values of δ18O (mean −1.17‰ PDB). The youngest unit (191±97 to 127±82 cal. yr BP) exhibits a recurrence of flooding events, mostly includes marsh (Truncatellina callicratis and Lauria cylindracea) and aquatic (Lymnae truncatula) species and shows a very heterogeneous isotopic signal, with −1.83‰ PDB as the mean value of δ18O. As a consequence, a palaeoenvironmental pattern comprised of an early wetter/colder phase followed by a warmer/drier period and, finally, an increase in both cold conditions and hydrological variability, is deduced for the middle to late Holocene in the Ebro Basin. Gastropod shell δ13C values, related to vegetation, can also be linked to palaeoenvironmental changes. The succession and stable isotope composition of gastropods in alluvial sequences are revealed as very sensitive indicators of rapid Holocene climatic changes in fragile semi-arid landscape systems.