ALBERT

Description: This article presents the first results of chlorine residual salt analysis (Cl-RSA), a new technique that allows investigation of the isotopic composition of chlorine in oil-field waters from core analysis. As water evaporates, the residual salts precipitate in the pores of a core. Unlike the routinely used strontium isotope residual salt analysis (Sr-RSA) technique, which determines the composition of a trace element (strontium), the Cl-RSA technique allows the determination of the isotopic composition and origin of one of the most abundant anions in natural waters. The results show that combining the δ37Cl of two successive leachates of a core sample leads to an isotopic composition that is representative of pore water. The reproducibility of the method applied to sandstone samples is considered to be ±0.05‰. The Cl-RSA technique was tested on two wells from the Elgin area (North Sea, United Kingdom). The δ37Cl results in formation waters range from −1.03 to −0.57‰, with an overall increase with depth. These results have been compared to the 87Sr/86Sr ratios of pore waters determined in the same wells using the Sr-RSA technique. The similarity of form between the δ37Cl and 87Sr/86Sr profiles with depth confirms that residual salts permit a reliable investigation of the chlorine isotopic composition from core analysis. As with the Sr-RSA technique, Cl-RSA could be used to test the isotopic homogeneity of a sample and to assess the compartmentalization of oil fields and reservoirs. This technique could provide key information regarding the origin of the salinity and its evolution during the reservoirs' filling, as well as help in the monitoring during field production. Véronique Woulé Ebongué is a Ph.D. student in the Stable Isotope Laboratory of the Institut de Physique du Globe de Paris (France). Her field of research is the application of the isotopic geochemistry of chlorine to fluid circulation in sedimentary basin and oil fields. Nathalie Jendrzejewski, Frédéric Walgenwitz, and Françoise Pineau are her Ph.D. advisors.Nathalie Jendrzejewski is a senior research fellow in the Institut de Physique du Globe de Paris (IPGP). After a Ph.D. in geochemistry (1994), she joined the IPGP Laboratoire de Geochimie des Isotopes Stables in 1996. Her research focuses on the geochemistry of chlorine using its stable isotopes. Her main application areas include volcano survey, magma degassing, fluid circulation, and pollution characterization. Frédérick Walgenwitz obtained his Ph.D. in 1976 from the University of Besançon (France), and he joined Elf Exploration and Production in 1977. He is now working with Total in Pau (France) and is an expert in diagenesis and inorganic geochemistry applied to the study of reservoirs and oil-field waters. Françoise Pineau was has been director of research in Centre National de la Recherche Scientifique at the Laboratoire de Géochimie des Isotopes Stables (Université of Paris 7 and the Institut de Physique du Globe de Paris) since 1969. Her main interests focus on the geodynamical cycles of C, H, N, and O and the origin of the isotopic variability of materials like carbonatites, peridotites, basalts, and deep continental crust (granulite facies) within the Earth's mantle. Marc Javoy is professor at University Denis Diderot and at the Institut de Physique du Globe de Paris, where he developed the Stable Isotope Laboratory. He was chairman of the Laboratoire de Physico-Chimie des Fluides Géologiques of the Centre National de la Recherche Scientifique–Institut de Physique du Globe de Paris–University of Paris 7. His present main areas of research are the geodynamical cycles of volatile elements and the problems of the Earth's formation.

Description: The combination of concentrations and δ13C signatures of Particulate Organic Carbon (POC) and sterols provides a powerful approach to study ecological and environmental changes both in the modern and ancient ocean, but its application has so far been restricted to the surface area. We applied this tool to study the biogeochemical changes in the modern ocean water column during the BONUS-GoodHope survey (Feb–Mar 2008) from Cape Basin to the northern part of the Weddell Gyre. Cholesterol and brassicasterol were chosen as ideal biomarkers of the heterotrophic and autotrophic carbon pools, respectively, because of their ubiquitous and relatively refractory nature. We document depth distributions of concentrations (relative to bulk POC) and δ13C signatures of cholesterol and brassicasterol from the Cape Basin to the northern Weddell Gyre combined with CO2 aq. surface concentration variation. While relationships between surface water CO2 aq. and δ13C of bulk POC and biomarkers have been previously established for surface waters, our data show that these remain valid in deeper waters, suggesting that δ13C signatures of certain biomarkers could be developed as proxies for surface water CO2 aq. Our data suggest a key role of zooplankton fecal aggregates in carbon export for this part of the Southern Ocean. We observed a general increase in sterol δ13C signatures with depth, which is likely related to a combination of particle size effects, selective feeding on larger cells by zooplankton, and growth rate related effects Additionally, in the southern part of the transect south of the Polar Front (PF), the release of sea-ice algae is hypothesized to influence the isotopic signature of sterols in the open ocean. Overall, combined use of δ13C and concentrations measurements of both bulk organic C and specific sterol markers throughout the water column shows the promising potential of analyzing δ13C signatures of individual marine sterols to explore the recent history of plankton and the fate of organic matter in the SO.

Description: The combination of concentrations and δ13C signatures of Particulate Organic Carbon (POC) and sterols provides a powerful approach to study ecological and environmental changes in both the modern and ancient ocean. We applied this tool to study the biogeochemical changes in the modern ocean water column during the BONUS-GoodHope survey (February–March 2008) from Cape Basin to the northern part of the Weddell Gyre. Cholesterol and brassicasterol were chosen as ideal biomarkers of the heterotrophic and autotrophic carbon pools, respectively, because of their ubiquitous and relatively refractory nature. We document depth distributions of concentrations (relative to bulk POC) and δ13C signatures of cholesterol and brassicasterol combined with CO2 aq. surface concentration variation. While the relationship between CO2 aq. and δ13C of bulk POC and biomarkers have been reported by others for the surface water, our data show that this persists in mesopelagic and deep waters, suggesting that δ13C signatures of certain biomarkers in the water column could be applied as proxies for surface water CO2 aq. We observed a general increase in sterol δ13C signatures with depth, which is likely related to a combination of particle size effects, selective feeding on larger cells by zooplankton, and growth rate related effects. Our data suggest a key role of zooplankton fecal aggregates in carbon export for this part of the Southern Ocean (SO). Additionally, in the southern part of the transect south of the Polar Front (PF), the release of sea-ice algae during the ice demise in the Seasonal Ice Zone (SIZ) is hypothesized to influence the isotopic signature of sterols in the open ocean. Overall, the combined use of δ13C values and concentrations measurements of both bulk organic C and specific sterols throughout the water column offers the promising potential to explore the recent history of plankton and the fate of organic matter in the SO.