A high-performance, safer and semi-automated approach for the delta18O analysis of diatom silica and new methods for removing exchangeable oxygen.

The determination of the oxygen isotope composition of diatom silica in sediment cores is important for paleoclimate reconstruction, especially in non-carbonate sediments, where no other bioindicators such as ostracods and foraminifera are available. Since most currently available analytical techniques are time-consuming and labour-intensive, we have developed a new, safer, faster and semi-automated online approach for measuring oxygen isotopes in biogenic silica. Improvements include software that controls the measurement procedures and a video camera that remotely records the reaction of the samples under BrF(5) with a CO(2) laser. Maximum safety is guaranteed as the laser-fluorination unit is arranged under a fume hood in a separate room from the operator. A new routine has been developed for removing the exchangeable hydrous components within biogenic silica using ramp degassing. The sample plate is heated up to 1100 degrees C and cooled down to 400 degrees C in approximately 7 h under a flow of He gas (the inert Gas Flow Dehydration method--iGFD) before isotope analysis. Two quartz and two biogenic silica samples (approximately 1.5 mg) of known isotope composition were tested. The isotopic compositions were reproducible within an acceptable error; quartz samples gave a mean standard deviation of <0.15 per thousand (1sigma) and for biogenic silica <0.25 per thousand (1sigma) for samples down to approximately 0.3 mg. The semi-automated fluorination line is the fastest method available at present and enables a throughput of 74 samples/week.

Methane in ocean waters: Concentration and carbon isotope variability at East Pacific Rise and in the Arabian Sea.

Methane concentrations and stable carbon isotope ratios of water samples from the East Pacific Rise (EPR) at 21°S and the Arabian Sea (24°N, 65°E) have been determined. EPR surface water is in equilibrium (ca. 50 nl/L and -50‰<δ(13)CH4<-46‰) with atmospheric methane. Deep "background" water has the signature of the remaining fraction of atmospheric methane partially oxidized in the water column by bacteria. Bottom near, hydrothermally influenced vent methane (>100nl/L and -30‰<δ(13)CH4<-22‰) is detectable only close to the seep site. There is no input of hydrothermal methane into the atmosphere. EPR water is considered to be rather a sink than a source of atmospheric methane. Surface waters of the Arabian Sea are enriched in methane relative to the atmosphere (source for atmospheric methane). Carbon isotope ratios point to a bacterial origin of methane (δ(13)CH4<-55‰) that is generated in the surface waters. Concentration changes and variations of carbon isotope ratios also suggest that methane seeping from the sea floor sediments of the Arabian Sea is oxidized by bacterial activity and does not reach the atmosphere.