Zeolitization of Oligo-Miocene volcaniclastic rocks from Logudoro (northern Sardinia, Italy)

The present study reports the results of research on volcaniclastic products from Logudoro (northern Sardinia), a reconstructed stratigraphic succession some hundreds of meters thick, comprising two different ignimbritic units separated by an epiclastic layer (generally ˜10 m thick). Clinoptilolite is the most widely distributed authigenic phase in epiclastite and unwelded units, always associated with opal-CT and sometimes with smectite. An adularia-like feldspar, cristobalite and epigenetic quartz are typical phases of welded units. Within the zeolitized units (unwelded ignimbrite and epiclastic units) clinoptilolite and opal-CT constitute the cement deriving from the transformation of the precursor rhyolitic glass, in agreement with a single zeolitization process that developed after the deposition of the entire succession. Silica increases in zeolitized rocks with respect to the precursor material, leading to hypothesize that the secondary mineralization process was favored by mineralized hydrothermal fluids preferentially circulating through the fault system of the area. Quartz veins in welded subunit and K-feldspar (adularia-like) preferentially located near the faults are in agreement with this hypothesis. The interaction of these fluids with the glassy fraction favored its dissolution and the consequent crystallization of clinoptilolite. Furthermore, the pH increase and the silica supersaturation enhanced the contemporary or subsequent precipitation of opal-CT. Received: 5 June 2000 / Accepted: 14 August 2000     

On the interpretation of magnetic anomalies for strong remanent magnetizations

In this paper some aspects concerning the interpretation of magnetic anomalies are treated, particularly when the remanent magnetization intensity is strong. In this case, since total and induced magnetization vectors can have very different directions, a correct anomaly interpretation strictly depends on the knowledge of their declinations and inclinations.Thus, a specific procedure is described to determine such parameters from well-known semi-empirical techniques and vectorial relations.Furthermore, the classical definition of apparent susceptibility is shown to be inadequate to this problem and a more general formulation is suggested, which is not only related to the true susceptibility and to the Koenisberger ratio, but also to the declinations and inclinations of the induced and remanent magnetization vectors.The two apparent susceptibilities are then compared for some synthetic magnetic anomalies and significant differences are found.     

Continuous in situ measurements of volcanic gases with a diode-laser-based spectrometer: CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O concentration and soil degassing at Vulcano (Aeolian islands: Italy)

We report on a continuous-measurement campaign carried out in Vulcano (Aeolian islands, Sicily), devoted to the simultaneous monitoring of CO₂ and H₂O concentrations. The measurements were performed with an absorption spectrometer based on a semiconductor laser source emitting around a 2-μm wavelength. The emitted radiation was selectively absorbed by two molecular ro-vibrational transitions specific of the investigated species. Data for CO₂ and H₂O concentrations, and CO₂ soil diffusive flux using an accumulation chamber configuration, were collected at several interesting sampling points on the island (Porto Levante beach- PLB, Fossa Grande Crater – FOG- and Valley of Palizzi, PAL). CO₂/H₂O values, measured on the ground, are very similar (around 0.019 (± 0.006)) and comparable to the previous discrete detected values of 0.213 (Fumarole F5-La Fossa crater rim) and 0.012 (Fumarole VFS – Baia Levante beach) obtaid during the 1977–1993 heating phase of the crater fumaroles.     

Warmer early instrumental measurements versus colder reconstructed temperatures: shooting at a moving target

Comparison of tree-ring-based warm-season temperature reconstructions and their instrumental target data reveals substantial divergence between (warmer) early instrumental measurements and (colder) proxy estimates. Here we detail this systematic misfit for the Northern Hemisphere before 1900 and the European Alps before 1850. Five hypotheses related to both proxy and target uncertainties are presented towards explaining this phenomenon. These include: (1) tree-ring detrending methods, (2) biological persistence in the proxy time-series, (3) uncertainties and instabilities in the growth response to given climatic parameters, (4) reduced instrumental station availability back in time, and (5) instrumental data homogeneity. We suggest that uncertainties in the choice of instrumental targets at the hemispheric scale, and instrumental data inhomogeneities at the Alpine and possibly also the hemispheric-scale are the most important factors in explaining this offset. Assessment of homogeneity at larger scales remains challenging. Attention is drawn to possible warm biases in early thermometer shelters and the relevance of proxy/target discrepancies for understanding and quantifying the amplitude of both recent anthropogenic and past natural forced climate fluctuations.