Water stable isotope record of ice core NEEM (early Holocene) measured discretely

A new technique for on-line high resolution isotopic analysis of liquid water, tailored for ice core studies is presented. We built an interface between a Wavelength Scanned Cavity Ring Down Spectrometer (WS-CRDS) purchased from Picarro Inc. and a Continuous Flow Analysis (CFA) system. The system offers the possibility to perform simultaneuous water isotopic analysis of d18O and dD on a continuous stream of liquid water as generated from a continuously melted ice rod. Injection of sub µl amounts of liquid water is achieved by pumping sample through a fused silica capillary and instantaneously vaporizing it with 100% efficiency in a~home made oven at a temperature of 170 °C. A calibration procedure allows for proper reporting of the data on the VSMOW-SLAP scale. We apply the necessary corrections based on the assessed performance of the system regarding instrumental drifts and dependance on the water concentration in the optical cavity. The melt rates are monitored in order to assign a depth scale to the measured isotopic profiles. Application of spectral methods yields the combined uncertainty of the system at below 0.1 per mil and 0.5 per mil for d18O and dD, respectively. This performance is comparable to that achieved with mass spectrometry. Dispersion of the sample in the transfer lines limits the temporal resolution of the technique. In this work we investigate and assess these dispersion effects. By using an optimal filtering method we show how the measured profiles can be corrected for the smoothing effects resulting from the sample dispersion. Considering the significant advantages the technique offers, i.e. simultaneuous measurement of d18O and dD, potentially in combination with chemical components that are traditionally measured on CFA systems, notable reduction on analysis time and power consumption, we consider it as an alternative to traditional isotope ratio mass spectrometry with the possibility to be deployed for field ice core studies. We present data acquired in the field during the 2010 season as part of the NEEM deep ice core drilling project in North Greenland.本文介绍了一种专为冰芯研究设计的液态水在线高分辨率同位素分析新技术。我们搭建了一套接口装置，将采购自Picarro公司的波长扫描腔衰荡光谱仪（Wavelength Scanned Cavity Ring Down Spectrometer，WS-CRDS）与连续流分析（Continuous Flow Analysis，CFA）系统进行集成。该系统可对连续熔融冰柱产生的液态水流开展δ¹⁸O与δD的同步水同位素分析。通过将样品注入熔融石英毛细管，并在自制的170℃烘箱中以100%效率瞬间汽化，可实现亚微升级液态水样品的进样。本方法采用校准程序，可确保数据在VSMOW-SLAP标准尺度（Vienna Standard Mean Ocean Water - Standard Light Antarctic Precipitation）下进行准确报告。我们基于系统评估的性能，针对仪器漂移以及光学腔室中水浓度的依赖性进行了必要校正。通过监测熔融速率，可为测得的同位素剖面赋予深度标尺。通过光谱方法分析，该系统对于δ¹⁸O和δD的综合不确定度分别低于0.1‰和0.5‰，该性能可与传统同位素比值质谱法的结果相媲美。样品在传输管路中的弥散效应限制了该技术的时间分辨率，本研究对上述弥散效应进行了探究与评估。通过采用最优滤波方法，我们证明了可针对样品弥散带来的平滑效应，对测得的同位素剖面进行校正。鉴于该技术具备多项显著优势：可同步测定δ¹⁸O与δD，可与传统CFA系统常规测量的化学成分相结合，且能显著缩短分析时长、降低功耗，因此该技术可作为传统同位素比值质谱法的替代方案，有望应用于野外冰芯研究。本文呈现了2010年野外科考季采集的数据，该数据隶属于北格陵兰地区的东北格陵兰埃姆间冰期冰芯钻探项目（North Greenland Eemian Ice Drilling，NEEM）。