58 research outputs found
Evaluation of bulk carbonate delta C-13 data from Triassic hemipelagites and the initial composition of carbonate mud
No full textBulk carbonate samples of hemipelagic limestone–marl alternations from the
Middle and Upper Triassic of Italy are analysed for their isotopic
compositions. Middle Triassic samples are representative of the Livinallongo
Formation of the Dolomites, while Upper Triassic hemipelagites were sampled
in the Pignola 2 section, within the Calcari con Selce Formation of the
Southern Apennines in Southern Italy. Triassic hemipelagites occur either as
nodular limestones with chert nodules or as plane-bedded limestone–marl
alternations which are locally silicified. In the Middle Triassic Livinallongo
Formation, diagenetic alteration primarily affected the stable isotopic
composition of sediment surrounding carbonate nodules, whereas the latter
show almost pristine compositions. Diagenesis lowered the carbon and oxygen
isotope values of bulk carbonate and introduced a strong correlation between
d13C and d18O values. In the Middle Triassic successions of the Dolomites,
bulk carbonate of nodular limestone facies is most commonly unaltered,
whereas carbonate of the plane-bedded facies is uniformly affected by
diagenetic alteration. In contrast to carbonate nodules, plane-bedded facies
often show compaction features. Although both types of pelagic carbonate
rocks show very similar petrographic characteristics, scanning electron
microscopy studies reveal that nodular limestone consists of micrite (< 5 lm
in diameter), whereas samples of the plane-bedded facies are composed of
calcite crystals ca 10 lm in size showing pitted, polished surfaces. These
observations suggest that nodular and plane-bedded facies underwent
different diagenetic pathways determined by the prevailing mineralogy of
the precursor sediment, i.e. probably high-Mg calcite in the nodular facies and
aragonite in the case of the plane-bedded facies. Similar to Middle Triassic
nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are
also characterized by uncorrelated d13C and d18O values and exhibit small, less
than 5 lm size, crystals. The alternation of calcitic and aragonitic precursors in
the Middle Triassic of the Dolomites is thought to mirror rapid changes in the
type of carbonate production of adjacent platforms. Bioturbation and
dissolution of metastable carbonate grains played a key role during early
lithification of nodular limestone beds, whereby early stabilization recorded
the carbon isotopic composition of sea water. The bulk carbonate d13C values
of Middle and Upper Triassic hemipelagites from Italy agree with those of
Tethyan low-Mg calcite shells of articulate brachiopods, confirming that
Triassic hemipelagites retained the primary carbon isotopic composition of the
bottom sea water. A trend of increasing d13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in
successions from tropical palaeolatitudes elsewhere. The carbon isotopic
composition of Middle and Upper Triassic nodular hemipelagic limestones
can thus be used for chemostratigraphic correlation and palaeoenvironmental
studies
Possible influence of the δ13C variations on the Carnian/Norian conodonts diffusion.
No full textQuantitative analyses of the absolute abundances of the species belonging to the five most widespread Upper Carnian/Lower Norian conodont genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella) from the Pizzo Mondello section (Monti Sicani, Western Sicily, Italy), GSSP candidate for the Norian stage (Balini et al., 2008), show potential ecological competition between these genera. Cross checks of the quantitative curves evidence the presence of three major assemblage changes, named T events: at T1 Carnepigondolella is replaced by its descendant Epigondolella in an evolutionary step; at T2 Epigondolella is substituted by the mass occurrence of Metapolygnathus and at T3 Metapolygnathus is succeeded by advanced Epigondolellae and by Norigondolella. A comparison between these quantitative curves and coeval δ18O and δ13C isotopic curves, obtained from the carbonate bulk of the section, shows a correspondence between higher13C/12C ratios and event T2 (and partially event T3), but not with T1. Thus, while the shifts in the C isotopic composition of seawater do not affect conodont evolution, they have an influence on the diffusion of the studied genera, which react differently to the δ13C variations. Epigondolella and Carnepigondolella proliferate in fact when seawater δ13C is lower than 2.5‰, Metapolygnathus is instead limited to environmental conditions related to δ13C values higher than 2.5‰ (Mazza et al., 2009). The rapid expansion of photosynthetically active organisms in the Upper Carnian may explain these perturbations in the carbon cycle and have influenced the trophic chain of the different conodont animal genera, producing thus mutual local migrations
Corrosion morphology and cave wall alteration in an Alpine sulfuric acid cave (Kraushohle, Austria).
No full textWhereas most karstic caves worldwide are formed by carbonic acid, a small but significant number of sub-surface cavities are the product of sulfuric acid speleogenesis (SAS). In the Eastern Alps, no cave has so far been attributed to this type. In this multidisciplinary study we demonstrate that Kraushöhle in northern Styria was indeed formed by SAS. The cave pattern shows individual chambers, 3D-mazes and blind galleries, as well as typical SAS morphologies such as cupolas, gypsum replacement pockets, corrosion notches and convection niches. “Ceiling pendants drip holes” are described here for the first time and these corrosion features are fully consistent with the SAS model. Other features of Kraushöhle include thick gypsum deposits with strongly depleted δ34S values and other minerals - mostly sulfates - indicating highly acidic conditions.
We also studied acid-rock interaction processes giving rise to widespread corrosion and concomitant replacement by gypsum. Petrographic and geochemical analyses reveal the presence of a distinctive alteration layer of highly increased porosity at the interface between the host limestone and the secondary gypsum. Dissolution and replacement of the limestone was fast enough to prevent the development of C and O isotopic alteration halos but resulted in selective leaching of elements. This stable isotope signal is thus different from the pronounced isotope gradient commonly observed in CO2-dominated hypogenic caves. Petrographic observations reveal that the limestone-gypsum replacement was a nearly constant volume process
Aragonite dissolution, sedimentation rates and carbon isotopes in deep-water hemipelagites (Livinallongo Formation, Middle Triassic, northern Italy)
No full textHemipelagic nodular limestones are a widespread facies in the Triassic of the Tethys, often considered as being deposited at
rather constant sedimentation rates. The aim of this paper is to investigate the sedimentation rate variability in a case-study from
the Middle Triassic of the Dolomites, northern Italy, and to suggest possible causes. The nodular cherty limestones of the
Livinallongo Fm. and correlated units were studied in four stratigraphic sections and compared to classical successions from the
literature. Correlations between sections are based on ammonoid biostratigraphy and tephra stratigraphy. Correlation highlighted
conspicuous changes in sedimentation rates through time and between sections, associated with sedimentological
evidence of deep-water aragonite dissolution. Deep-water dissolution is believed to have resulted in small hiatuses and in a bias
of ammonoid assemblages towards taxa with more resistant shells. Carbonate petrography and geochemistry provide evidence
of differential diagenesis of the Livinallongo Fm. Carbonate nodules were lithified at the water–sediment interface, and their C
isotope composition is regarded as a proxy of the d13C of Middle Triassic bottom seawater. The onset of nodular cherty
limestones, occurring contemporaneously at the basin scale, and the coeval carbonate platform drowning events are tentatively
explained by the inflow and local upwelling of cool deep water from Panthalassa into the western Tethys
Climate variability in the SE Alps of Italy over the past 17000 years reconstructed from a stalagmite record.
No full textThe European Alps is a region very sensitive to Dryas (YD) marked the end of the Palaeolithic painclimatic
change (Beniston & Jungo 2002), and it is terso A better insight into the climate evolution of the
important to extend the existing climate records as far SE European Alps from the Last Glacial Maximum
back in rime as possible to detect any anthropogenic would greatly improve our understanding of Alpine
climate signals. At present, there is a large set of history. Rere we present an exceptional c. 17 kyr
palaeoclimate and climate data from the CentraI and speleothemr ecord from the southeasternm argin of the
Western Alps, but a fragmentary record from the ltalian Alps.
Eastern Alps (cf. Davis et al. 2003 and references
therein). A stalagmite record from northem ltaly
(McDermott et al. 1999) suggested that the CentraI. .
Alps had a similar Rolocene climatic evolution as the Samplmg and analytlcal procedures
JUTa Mountains and the French Alps. The eastern sec- G h. l tI ' tor of the ltalta..ne oAglrpasp ltes at the boundary between the lca se mg
CentraI and Southern European climate regions as The 27-cm-high, candle-shaped, stalagmite SVI was
defined by pollen data (Davis et al. 2003) and, there- sampled in Grotta Savi, a cave located at 441 m a.s.l.
fore, may show different climate evolution with north of Trieste (4S03ios"N, 13°S3'10"E) (Fig. l).
respect to the CentraI and Western Alps. The Eastern The outer surface of the stalagmite was whitish,
ltalian Alps bave great pre-history importance, which translucent and wet, a typical feature of active
is documented by abundant archaeological evidence stalagmites from the southem watershed of the Alps.
for both indigenous and imported cultural 'packages' At the time of removal, SVI was fed by a constant
(Mithen 2003). In the Dalmeri Shelter site, at 1240m drip (c. 10drops/min) from a stalactite growing about
a.s.l. in NE ltaly, a unique finding of painted stones 30m above the stalagmite. The Savi cave is cut in
dated at c. 13cal.kyr BP, which show lberian fissuredlimestoneoverlainbythin«SOcm)grassland
naturalistic influence (Dalmeri et al. 2004), seems to soil cover, a situation which optimizes the potential for
support Mithen's (2003) hypothesis that the Younger Dryas (YD) marked the end of the Palaeolithic painters. A better insight into the climate evolution of the SE European Alps from the Last Glacial Maximum would greatly improve our understanding of Alpine history. Here we present an exceptional c. 17 kyr speleothem record from the southeastern margin of the Italian Alps
Aragonite dissolution, sedimentation rates and carbon isotopes in deep-water hemipelagites (Livinallongo Formation, Middle Triassic, northern Italy) - Reply
No full textReply to the discussion of Preto et al., 2005, [Maurer, F., Brack, P., in press. Aragonite dissolution, sedimentation rates and carbon isotopes in deep-water hemipelagites (Livinallongo Formation, Middle Triassic,
northern Italy) — Comment. Sedimentary Geology doi:10.1016/j.sedgeo.2006.09.003
Holocene climate variability in Sicily from a discontinuous stalagmite record and the Mesolithic to Neolithic transition
No full textFabric and stable isotopic composition of a Holocene stalagmite (CR1) from a cave in northern Sicily record changes in paleorainfall in the early Holocene. High δ13C stable isotope values in the calcite deposited from ca. 8500 to ca. 7500 yr ago are interpreted as reflecting periods of high rainfall. The wet phase was interrupted by two periods of multi-century duration characterized by relatively cool and dry winters centered at ca. 8200 and ca. 7500 yr ago, highlighted by low δ13C and δ18O values. A high variability of δ13C values is recorded from ca. 7500 to ca. 6500 yr ago and indicates that the transition from a pluvial early Holocene to the present-day climate conditions was punctuated by decadal-scale periods of relatively dry winters. In northern Sicily, the traditional elements of the Neolithic appear at ca. 7700 yr ago. It is possible that changes in rainfall influenced the passage from hunter-gathering to farming and sheep-herding economies
Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon-oxygen isotopes
No full textHypogene dissolution-precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly-investigated research topic. Studies exploring the complexity of silica dissolution-precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km-long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon-oxygen stable isotope analyses and U-Th-Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro-crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50-100 degrees C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep-rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165-210 degrees C. Similar temperature estimates (110-200 degrees C) were obtained from the delta Si-30 and delta O-18 isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution-precipitation phases were obtained from monazite U-Th-Pb dating. However, our results, interpreted in the regional context of the Sao Francisco Craton, suggest that the Cambrian tectono-thermal events could have been amongst the possible drivers for this hypogene process in the basin
Northeastern Caribbean Rainfall Variability Linked to Solar and Volcanic Forcing
No full textWe present a 500-year precipitation-sensitive record based on co-varying speleothem delta 18O values and Mg/Ca ratios from Larga cave in Puerto Rico. This multi-proxy record shows that the evolution of rainfall in the northeastern Caribbean was characterized by alternating centennial dry and wet phases corresponding to reduced versus enhanced convective activity. These phases occurred synchronous with relatively cool and warm tropical Atlantic sea-surface temperatures (SSTs), respectively. While the observed pattern suggests a close link of northeastern Caribbean rainfall to the Atlantic Multidecadal Variability, a regional comparison reveals intermittent regional heterogeneity especially on decadal timescales, which may be related to a superimposing influence of the Pacific and Atlantic basins. Furthermore, the speleothem-based hydroclimate reconstruction indicates a significant volcanic impact during the past two centuries, and further reveals a potential solar signal in the preceding three centuries. We posit that the forcing likely shifted from solar to volcanic during the eighteenth century in being an important source of multidecadal to centennial Caribbean rainfall variability. The link between convective rainfall and natural forcing may be explained through a modulation of SST variations in the tropical Atlantic and Pacific oceans.Climate change is expected to affect rainfall throughout the Caribbean and Central America, where over 200 million people are heavily dependent upon rain as their main source of freshwater. We have looked at how the amount of rainfall in the Caribbean has changed through time, and possible reasons for these changes, to inform predictions of future rainfall patterns for the region. Cave mineral deposits, also known as speleothems, can be used to assess the amount of regional rainfall in the past by looking at their slight changes over time in the mineral's chemical composition. We created a record of Caribbean rainfall spanning the last five centuries using a speleothem from Puerto Rico. Our results indicate that rainfall changes since the sixteenth century were strongly influenced by changes in temperatures of the surface waters of the Atlantic Ocean. We further found that changes in the sun's brightness and volcanic eruptions may alter Atlantic Ocean sea-surface temperature, which in turn affect the overall changes in Caribbean rainfall patterns.Speleothem-based Puerto Rican rainfall reconstruction shows prominent multidecadal-to-centennial variability during the past five centuries Puerto Rican rainfall fluctuations are linked to solar variations before the eighteenth century and volcanic forcing thereafter Caribbean Sea-surface temperature anomalies as part of the Atlantic Multidecadal Variability emerge as robust precursor of rainfall amount
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