Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores

Abstract

Abstract. Water isotopes in ice cores are used as a climate proxy for local temperature and regional atmospheric circulation as well as evaporative conditions in moisture source regions. Traditional measurements of water isotopes have been achieved using magnetic sector isotope ratio mass spectrometry (IRMS). However, a number of recent studies have shown that laser absorption spectrometry (LAS) performs as well or better than IRMS. The new LAS technology has been combined with continuous-flow analysis (CFA) to improve data density and sample throughput in numerous prior ice coring projects. Here, we present a comparable semiautomated LAS-CFA system for measuring high-resolution water isotopes of ice cores. We outline new methods for partitioning both system precision and mixing length into liquid and vapor components -useful measures for defining and improving the overall performance of the system. Critically, these methods take into account the uncertainty of depth registration that is not present in IRMS nor fully accounted for in other CFA studies. These analyses are achieved using samples from a South Pole firn core, a Greenland ice core, and the West Antarctic Ice Sheet (WAIS) Divide ice core. The measurement system utilizes a 16-position carousel contained in a freezer to consecutively deliver ∼ 1 m × 1.3 cm 2 ice sticks to a temperature-controlled melt head, where the ice is converted to a continuous liquid stream and eventually vaporized using a concentric nebulizer for isotopic analysis. An integrated delivery system for water isotope standards is used for calibration to the Vienna Standard Mean Ocean Water (VSMOW) scale, and depth registration is achieved using a precise overhead laser distance device with an uncertainty of ±0.2 mm. As an added check on the system, we perform inter-lab LAS comparisons using WAIS Divide ice samples, a corroboratory step not taken in prior CFA studies. The overall results are important for substantiating data obtained from LAS-CFA systems, including optimizing liquid and vapor mixing lengths, determining melt rates for ice cores with different accumulation and thinning histories, and removing system-wide mixing effects that are convolved with the natural diffusional signal that results primarily from water molecule diffusion in the firn column.

Molecular diffusion of stable water isotopes in polar firn as a proxy for past temperatures

“…In the case of high resolution measurements carried out with CFA measurement systems, there exist a number of ways to characterize the sampling diffusion length. Typically the step or impulse response of the CFA system can be measured yielding a Gaussian filter specific for the CFA system [23], [41], [19], [32]. The Gaussian filter can be characterized by a diffusion length σ 2 cfa that can be directly used to perform a sampling correction.…”

“…Real measurements with CFA systems indicate that most likely due to surface effects in the experimental apparatus that lead to sample memory, the transfer functions of such systems depart from the ideal model of Gaussian dispersion showing a slightly skewed behavior. For some systems, this behavior resembles more that of a slightly skewed Log-Normal distribution [23], [41], [19] or a more skewed distribution that in the case of [32] requires the product of two Log-Normal distributions to be accurately modeled. The result of this behavior to the power spectral density is still a matter of further study as high resolution datasets obtained with CFA systems are relatively recent.…”

“…With the advent of present commercial high-accuracy, high-precision Infra-Red spectrometers [11], [9], simultaneous measurements of δ 18 O and δD have become easier to obtain. Coupling of these instruments to Continuous Flow Analysis systems [23], [41], [19], [32] can also result in measurements of ultra-high resolution, a necessary condition for accurate temperature reconstructions based on water isotope diffusion.…”

Automated calibration of laser spectrometer measurements of δ18O and δ2H values in water vapour using a Dew Point Generator

“…Several researchers have developed customised calibration systems and protocols, mainly based on the Picarro and LGR designs, to provide improved performance for long-term automated monitoring of water vapour isotope compositions and flash evaporation systems for ice-core analysis. 1,[6][7][8][9] Wang et al 10 used a LI-610 Dew Point Generator (DPG) from LI-COR (Lincoln, NE, USA) in manual mode to calibrate an LGR DLT-100 water vapour isotope analyser (WVIA). The DPG produces a water-saturated air stream with a known dew point (± 0.2°C) from a liquid water reservoir.…”

Calculating uncertainty for the RICE ice core continuous flow analysis water isotope record

“…Recent advances in laser absorption spectrometry have allowed continuous flow analysis (CFA) to become common in ice core studies and are an essential measurement technique for obtaining highresolution climate records (e.g. Kaufmann et al, 2008;Gkinis et al, 2011;Kurita et al, 2012;Emanuelsson et al, 2015;Jones et al, 2017). However, the simultaneous operation of seven measurement systems and the continuous nature of CFA pose challenges for calibration and uncertainty estimation.…”