Geochemical characteristics of pore waters from sediment cores of the Wagner Basin, Gulf of California

摘要

The Wagner Basin (WB) is a shallow basin (depth < 225 m) belonging to the northernmost section of the Gulf of California rift system. Hydrothermal activity and high heat fluxes prevail in some regions of the WB. For this contribution, we report the first dataset of chemical (major and some trace elements) and isotopic compositions (delta O-18, delta D, Sr-87/Sr-86, delta C-13) from pore water sampled at the bottom of the WB, in areas affected by hydrothermal activity. The goals of the study are to determine the origin of the fluids emanating from the anomalous heat flow zones and to characterize the physical and chemical processes controlling their composition. The 18 pore water samples are classified into two groups: low temperature (LT) and high temperature (HT) samples, according to the sampling temperature (from 16.4 to 25.6 degrees C, and 32.5-99.6 degrees C, respectively). LT samples have chemical and isotopic (delta O-18 and delta D) compositions similar to those of present-day seawater. On the opposite, HT cores are typically more enriched in Cl (26,100-37,074 mg L-1) and other elements (Br, Na, K, Ca, B and Sr) than those of present-day seawater (Cl = 20,284 mg L-1). HT samples are also strongly depleted in deuterium isotopes (up to 30.48 parts per thousand). This characteristic could be related to the mixing between ancient evaporated seawater and Colorado river waters. Conceptually, the origin of a saline paleo-aquifer/reservoir can be related with the gradual marine flooding of shallow lagoons and depressions at the time Gulf of California was rifting (6-8 Ma) or during the Last Glacial Maximum (20-26 Ky). Additionally, it is not ruled out that some of the deuterium depletion observed in HT samples may be related to secondary processes (e.g., clays exchange, organic matter). Radiogenic Sr-87/Sr-86 signatures (0.70929-0.70997) of the HT samples likely reflect the leaching of radiogenic continental sediments from the Colorado River (filling the WB) and authigenic minerals (e.g., calcite or barite) precipitated from seawater. Solute geothermometry indicates that HT pore fluids underwent water-rock interactions at temperature of at least 220 degrees C. Finally high delta C-13 values (up to +10.5 parts per thousand) in DIC from HT samples indicates partial equilibration of methane with DIC, or partial reduction of DIC.