Constraints for precise and accurate fluid inclusion stable isotope analysis using water-vapour saturated CRDS techniques

TL;DR

This study refines a laser spectroscopy method for analyzing water isotopes in speleothem fluid inclusions, establishing constraints for precision and accuracy crucial for paleoclimate research.

Contribution

It introduces optimized constraints and validation procedures for water-vapour saturated CRDS techniques in fluid inclusion isotope analysis.

Findings

Precision improves with water amount above 1 μl.

Measurement accuracy better than 0.08 permil for δ18O.

Reproducibility is within 0.5 permil for δ18O.

Abstract

Hydrogen ($\delta$2H) and oxygen ($\delta$18O) isotopes of water extracted from speleothem fluid inclusions are important proxies used for paleoclimate reconstruction. In our study we use a cavity ring-down laser spectroscopy system for analysis and modified the approach of Affolter et al. (2014) for sample extraction. The method is based on crushing of small sub-gram speleothem samples in a heated and continuously water-vapour purged extraction line. The following points were identified: Injection of reference water shows a precision (1$\sigma$) of 0.4-0.5 permil for $\delta$18O values and 1.1-1.9 permil for $\delta$2H values for water amounts of 0.1-0.5 $\mu$l, which improves with increasing water amount to 0.1-0.3 permil and 0.2-0.7 permil, respectively, above 1 $\mu$l. The accuracy of measurements of water injections and water-filled glass capillaries crushed in the system is better than 0.08 permil for $\delta$18O and 0.3 permil for $\delta$2H values. The reproducibility (1$\sigma$) based on replicate analysis of speleothem fluid inclusion samples with water amounts > 0.2 $\mu$l is 0.5 permil for $\delta$18O and 1.2 permil for $\delta$2H values, respectively. Isotopic differences between the water vapour background of the extraction system and the fluid inclusions have no significant impact on the measured fluid inclusion isotope values if they are within 10 permil for $\delta$18O and 50 permil for $\delta$2H values of the background. Tests of potential adsorption effects with inclusion free spar calcite confirm that the isotope values are unaffected by adsorption for water contents of about 1 $\mu$l (fluid inclusion) water per g of carbonate or above. Fluid inclusion analysis on three different modern to late Holocene speleothems from caves in northwest Germany resulted ...