E. Selmo

Aragonite-Calcite Relationships in Speleothems (Grotte De Clamouse, France): Environment, Fabrics, and Carbonate Geochemistry

ABSTRACT In Grotte de Clamouse (France), aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation. Prolonged degassing and evaporation yield needle aragonite, which is more enriched in 18O and 13C than aragonite ray crystals, which form near isotopic equilibrium. At present, aragonite ray crystals form at the tops of stalagmites at very low discharge (0.00035 ml/ min), and when fluid Mg/Ca ratio is > 1.1. Temperature and evaporation do not seem to have a significant role in their formation. The presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology. Fossil aragonite was in part replaced by calcite in a time frame 13C signal and U content), and preserved aragonite relicts (up to 16 weight %). The isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters. The real extent of aragonite-to-calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor.

Concentrations and δ13C values of atmospheric CO2 from oceanic atmosphere through time : Polluted and non-polluted areas

CO2 is one of the primary agents of global climate changes. The increase of atmospheric CO2 concentration is essentially related to human-induced emissions and, particularly, to the burning of fossil fuel whose δ13C values are quite negative. Consequently, an increase of the CO2 concentration in the atmosphere should be paralleled by a decrease of its δ13C. Continuous and/or spot measurements of CO2 concentrations were repeatedly carried out during the last decade and in the same period of the year along hemispheric courses from Italy to Antarctica on a vessel of the Italian National Research Program in Antarctica. During these expeditions, discrete air samples were also collected in 4-l Pyrex flasks in order to carry out precise carbon isotope analyses on atmospheric CO2 from different areas, including theoretically “clean” open ocean areas, with the main purpose of comparing these open ocean results with the results obtained by the National Oceanic and Atmospheric Administration/World Meteorological Organization (NOAA/WMO) at land-based stations. According to the data obtained for these two variables, a relatively large atmospheric pollution is apparent in the Mediterranean area where the CO2 concentration has reached the value of 384 ppmv while quite negative δ13C values have been measured only occasionally. In this area, southerly winds probably help to reduce the effect of atmospheric pollution even though, despite a large variability of CO2 concentrations, these values are consistently higher than those measured in open ocean areas by a few ppmv to about 10 ppmv. A marked, though non-continuous, pollution is apparent in the area of the Bab-el-Mandeb strait where δ13C values considerably more negative than in the Central and Southern Red Sea were measured. The concentration of atmospheric CO2 over the Central Indian Ocean increased from about 361 ppmv at the end of 1996 to about 373 ppmv at the end of 2003 (mean growth rate of about 1.7 ppmv yr−1 in excellent agreement with the NOAA/WMO data from land-based stations). Simultaneously, the mean δ13C value of atmospheric CO2 over the Central Indian Ocean (Equator) decreased from −7.92‰ at the end of 1998 to −8.22‰ at the end of 2003; the mean decrease rate being of about −0.06‰ yr−1. This rate as well as that calculated at 12°S (−0.05‰ yr−1) are not far from the rates that may be calculated according to the data from the nearest NOAA sites (Crozet and Mahe islands); the rates calculated South of Australia and between Tasmania and N.Z. are almost identical to those calculated according to the data from Cape Grim NOAA site (Tasmania).

Atmospheric CO2 concentrations and δ13C measurements along a hemispheric course (1998/99, Italy to Antarctica)

Continuous measurements of atmospheric CO2 concentrations were carried out along three ship-routes from Italy to Antarctica in 1994/95, 1996/97 and 1998/99. The growth rate of CO2 for four latitudinal belts with arbitrarily chosen boundaries (from 45‡N to 80‡S) is reported for the three expeditions. The northernmost belt shows the largest growth rate between 1996 and 1998 (close to 6 ppm/yr) while southern belts show an almost constant growth rate of 1.7 ppm/yr. Anthropogenic activities clearly affect both CO2 concentrations and growth rates. N 13 C measurements were carried out on CO2 from 23 flask air samples collected during the 1998/99 expedition. Apart from negative values observed in highly polluted areas, a negative trend is observed across the Antarctic Convergence Area with no changes in the atmospheric concentration of CO2. The hypothesis of a biochemical origin of these light carbon isotope values is suggested. fl 2001 Elsevier Science B.V. All rights reserved.