Primary carbonates are a common feature of many modern and ancient lacustrine deposits. Carbonates from hydrologically open lakes show little or no correlation between δ13C and δ18O. In short-residence-time open lakes, carbonate oxygen isotopic composition is relatively invariant and typically is closely related to the bulk isotopic composition of inflow waters to the lake. Suites of carbonates which display covarying 13C and 18O compositions precipitate from waterbodies having relatively long residence times. Where the correlation between carbon and oxygen isotopic variations is high (r⪖ 0.7 ), the carbonates have normally precipitated from a closed lake. In addition, because of large changes in water balance, the δ18O of closed-lake carbonates usually varies over a range of several %o. Therefore, the combination of degree of covariance and spread of δ18O-values can be used to discriminate between carbonates produced in hydrologically open and closed basins. Within individual basins, covariant trends may have remarkable long-term persistence despite major environmental changes, indicating considerable stability in basin hydrology. Each closed lake has a unique isotopic identity defined by its covariant trend, which is a function of the basin's geographical and climatic setting, its hydrology, and the history of the waterbody. Any major interruption or realignment of this trend reflects a fundamental change in basin hydrology. Isotopic trends based upon the carbon and oxygen isotopic composition of primary lacustrine carbonates have several applications in palaeolimnology. The oxygen isotopic composition of open-lake carbonates may, with caution, be used as a proxy indicator of the composition of regional rainfall. Covariant trends can be used to trace the hydrological history of a basin, the evolution of individual water masses, and to correlate carbonate-bearing sediments from different parts of a basin.

A review of the palaeohydrological interpretation of carbon and oxygen isotopic ratios in primary lacustrine carbonates

Environmental isotopes in lake studies

Proxy data reflecting the oxygen isotope composition of meteoric precipitation (δ18Oppt) are widely used in reconstructions of continental paleoclimate and paleohydrology. However, actual geographic variation in modern water compositions is difficult to estimate from often sparse data. A first step toward understanding the geologic pattern of change in δ18Oppt is to describe the modern distribution in terms of principal geographic parameters. To this end, we empirically model relationships between 18O in modern precipitation and latitude and altitude. We then identify geographic areas where large-scale vapor transport patterns give rise to significant deviations from model δ18Oppt compositions based on latitude and altitude. Model value and residual grids are combined to derive a high-resolution global map of δ18Oppt that can serve as a spatial reference against which proxy data for paleoprecipitation can be compared. Reiteration of the procedure outlined here, for paleo-δ18Oppt data, may illuminate past changes in the climatic and physiographic parameters controlling the distribution of δ18O regimes.

Spatial distribution of δ18O in meteoric precipitation

Prevailing opinion assigns the Tibetan Plateau a crucial role in shaping Asian climate, primarily by heating of the atmosphere over Tibet during spring and summer Accordingly, the growth of the plateau in geologic time should have written a signature on Asian paleoclimate Recent work on Asian climate, however, challenges some of these views The high Tibetan Plateau may affect the South Asian monsoon less by heating the overlying atmosphere than by simply acting as an obstacle to southward flow of cool, dry air The East Asian “monsoon” seems to share little in common with most monsoons, and its dynamics may be affected most by Tibet's lying in the path of the subtropical jet stream Although the growing plateau surely altered Asian climate during Cenozoic time, the emerging view of its role in present-day climate opens new challenges for interpreting observations of both paleoclimate and modern climate

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Orographic controls on climate and paleoclimate of Asia: thermal and mechanical roles for the Tibetan Plateau.

Climatic and environmental controls on speleothem oxygen-isotope values

The altitude effect on the isotopic composition of tropical rains

Comportement isotopique au cours de l'evaporation de deux bassins sahariens

/pdf/deuterium-et-oxygene-18-dans-les-eaux-du-lac-tchad-u2buaqujar.pdf

Deutérium et oxygène-18 dans les eaux du lac Tchad

Isotopes du milieu et circulations dans les aquiferes du sous-sol Vénitien

Depuis plusieurs annees, l’Office de la Recherche Scientifique et Technique Outre-Mer, a entrepris une operation concertee multidisciplinaire visant a une definition des mecanismes d’alteration, erosion, transport et sedimentation, dans la cuvette tchadienne Actuellement, les phenomenes impliques dans cette etude peuvent etre rapportes au cycle hydrologique des tributaires du lac Tchad, a la dynamique de ce dernier ainsi qu’a ses rapports avec les nappes environnantes

/pdf/donnees-isotopiques-preliminaires-sur-les-rapports-du-lac-jcjrbj84iu.pdf

Jean-Charles Fontes

Papers

The altitude effect on the isotopic composition of tropical rains

Comportement isotopique au cours de l'evaporation de deux bassins sahariens

Deutérium et oxygène-18 dans les eaux du lac Tchad

Isotopes du milieu et circulations dans les aquiferes du sous-sol Vénitien

Données isotopiques préliminaires sur les rapports du lac Tchad avec les nappes. de la bordure nord-est