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 semi-automated 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 cm2 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.
Highlights
The measurement of water isotopes in ice cores provides records of past hydrologic cycle variability (Dansgaard, 1964)
We present a semi-automated water isotope cavity ring-down laser spectroscopy (CRDS)-continuous-flow analysis (CFA) system developed at the Institute of Arctic and Alpine Research (INSTAAR) Stable Isotope Lab (SIL)
We find that the majority of the mixing in the CRDS-CFA system occurs in the liquid phase, while the remainder occurs in the vapor phase of the system
Summary
The measurement of water isotopes in ice cores provides records of past hydrologic cycle variability (Dansgaard, 1964). The parameters δD and δ18O are proxies for both local temperature and regional atmospheric circulation, while the second-order parameter deuterium excess has been used to obtain information about source water evaporative conditions (temperature, humidity, and wind speed), as well as changes in the location of moisture source regions (Jouzel and Merlivat, 1984; Jouzel et al, 1997; Johnsen et al, 2001; Kavanaugh and Cuffey, 2003; Steig et al, 2013) These parameters have routinely been analyzed since the origins of ice core science, first in a precipitation experiment in Copen-. Jones et al.: Improved methodologies for continuous-flow analysis hagen in 1952, on the visible layers of icebergs in the North Atlantic, and later in the Camp Century ice core (described in Dansgaard, 2005) Since these early experiments, a large collection of ice core water isotope records have been recovered from the Greenland Ice Sheet, the Antarctic Ice Sheet, and many high-latitude and/or high-altitude ice caps. CFA has widely been used for chemical measurements in ice cores (e.g., Röthlisberger, 2000; Osterberg et al, 2006; Bigler et al, 2011; and Rhodes et al, 2013)
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