# Informations et ressources scientifiques sur le développement des zones arides et semi-arides

Accueil du site Doctorat Canada Environmental stable isotope (H, C, O, S) geochemistry of the Death Valley sedimentary basin, California, U.S.A.

## Yang, Wenbo

Titre : Environmental stable isotope (H, C, O, S) geochemistry of the Death Valley sedimentary basin, California, U.S.A.

Auteur : Yang, Wenbo

Université de soutenance : University of Calgary

Grade : Doctor of Philosophy (Ph.D.) 1996.

Résumé
In this study, environmental stable isotope (H, C, O, S) techniques have been used to study the modern hydrological and ecological systems, and a 200 ka record of depositional environment and paleoclimate from a 186 m core in Death Valley. Ninety-five water samples were collected in Death Valley from May 1992 to March 1994. The $\rm\delta\sp ?O$ and $\delta$D values of these waters range from $-$15.2 to +2.8$\perthous$ and from $-$109 to $-21\perthous,$ respectively. Rain, snow, and shallow spring waters derived from melting snow on the mountains fall on the meteoric water line (MWL). Salt pan brines fall on a local evaporation line with a slope of 3.6, indicating strong evaporation. Regional spring waters have lower $\delta$-values and are most likely remnants of Pleistocene meteoric waters. At least three major sulfate sources were identified by isotope data in these waters : (1) atmospheric sulfate in precipitation, (2) dissolved sulfate from the local spring flow system, (3) dissolved marine sulfate from Paleozoic carbonate rocks in the regional groundwater system. The isotopic variations of these waters and sulfate therein reflect changing and/or mixing of sources in the basin. Stable carbon isotope data were obtained from 33 desert shrub samples. The $\rm\delta\sp ?C$ values of these samples reflect control by photosynthetic pathways and further reflect the water-use efficiency of the plants. The $\rm\delta\sp ?C$ variation of the cellulose component from 58 Tamarix aphylla tree ring samples provide useful information about the past 50-years basin aridity in Death Valley. The $\rm\delta\sp34S$ values of two dominant xerophytes, Atriplex hymenehytra and Larrea tridentata, in Death Valley, vary from +7 to +18$\perthous,$ corresponding isotopically to sulfate in the water supplies at a given location. Tree ring data from a phreatophyte tree, Tamarix aphylla, show a distinct time dependence, with $\rm\delta\sp34S$ values increasing from +13.5 to +18$\perthous$ for soluble sulfate and from +12 to +17$\perthous$ for total sulfur. These data are interpreted in terms of sulfur sources, water sources and flow paths, and tree root growth. Fluid inclusion brines in 87 halite samples were collected from Core DV 93-1 for H and O isotope analyses. The stable isotope variation of inflow calculated from the isotope data obtained from the fluid inclusions in halite appear to be controlled by storm patterns rather than regional temperatures in Death Valley during the past 200 ka. The $\rm\delta\sp34S$ and $\rm\delta\sp ?O$ values of 172 soluble sulfate samples from Core DV 93-1 varied from +5 to +22$\perthous$ and +9 to +23$\perthous,$ respectively. The $\rm\delta\sp34S$ and $\rm\delta\sp ?O$ variations provide constraints on water inflow sources and the lake water level status. Generally dry, shallow and ephemeral lake conditions result in precipitation of sulfate with the $\delta$-values reflecting the inflow water source. Wet, perennial lake conditions result in precipitation of sulfate with low $\rm\delta\sp34S$ and high $\rm\delta\sp ?O$ values. Carbon and oxygen isotope data from 50 sediment samples in Core DV 93-1 show a positive correlation, indicating an origin as primary carbonates formed in a closed basin. It appears that the $\rm\delta\sp ?C$ values of the carbonates in the Core DV 93-1 sediments depend mainly on dissolved inorganic carbon (DIC) in lake, atmospheric CO$\sb2,$ and lake water residence time. The $\rm\delta\sp ?O$ values of these carbonates are controlled by inflow water and temperature, and can be used to imply paleoclimate changes in the western United States.

Présentation (BAC)

Page publiée le 10 mars 2009, mise à jour le 13 septembre 2019