83 research outputs found
Molecular diffusion of stable water isotopes in polar firn as a proxy for past temperatures
Polar precipitation archived in ice caps contains information on past
temperature conditions. Such information can be retrieved by measuring the
water isotopic signals of and in
ice cores. These signals have been attenuated during densification due to
molecular diffusion in the firn column, where the magnitude of the diffusion is
isotopologoue specific and temperature dependent. By utilizing the differential
diffusion signal, dual isotope measurements of and
enable multiple temperature reconstruction techniques. This
study assesses how well six different methods can be used to reconstruct past
surface temperatures from the diffusion-based temperature proxies. Two of the
methods are based on the single diffusion lengths of
and , three of the methods employ the differential diffusion
signal, while the last uses the ratio between the single diffusion lengths. All
techniques are tested on synthetic data in order to evaluate their accuracy and
precision. We perform a benchmark test to thirteen high resolution Holocene
data sets from Greenland and Antarctica, which represent a broad range of mean
annual surface temperatures and accumulation rates. Based on the benchmark
test, we comment on the accuracy and precision of the methods. Both the
benchmark test and the synthetic data test demonstrate that the most precise
reconstructions are obtained when using the single isotope diffusion lengths,
with precisions of approximately 1.0\,^\mathrm{o}\mathrm{C}. In the benchmark
test, the single isotope diffusion lengths are also found to reconstruct
consistent temperatures with a root-mean-square-deviation of
0.7\,^\mathrm{o}\mathrm{C}
The effect of a Holocene climatic optimum on the evolution of the Greenland ice sheet during the last 10 kyr
Publisher's version (útgefin grein)The Holocene climatic optimum was a period 8–5 kyr ago when annual mean surface temperatures in Greenland were 2–3°C warmer than present-day values. However, this warming left little imprint on commonly used temperature proxies often used to derive the climate forcing for simulations of the past evolution of the Greenland ice sheet. In this study, we investigate the evolution of the Greenland ice sheet through the Holocene when forced by different proxy-derived temperature histories from ice core records, focusing on the effect of sustained higher surface temperatures during the early Holocene. We find that the ice sheet retreats to a minimum volume of ~0.15–1.2 m sea-level equivalent smaller than present in the early or mid-Holocene when forcing an ice-sheet model with temperature reconstructions that contain a climatic optimum, and that the ice sheet has continued to recover from this minimum up to present day. Reconstructions without a warm climatic optimum in the early Holocene result in smaller ice losses continuing throughout the last 10 kyr. For all the simulated ice-sheet histories, the ice sheet is approaching a steady state at the end of the 20th century.This work is supported by the Danish National Research
Foundation under the Centre for Ice and Climate,
University of Copenhagen and Villum Investigator Project
IceFlow. Brice Noël and Michiel van den Broeke (IMAU,
Utrecht University) are thanked for providing the
RACMO2.3 Greenland SMB, precipitation and temperature
data. B. Vinther is thanked for providing the Holocene accumulation
reconstruction for the GRIP site. We are grateful for
computing resources provided by the Danish Center for
Climate Computing, a facility build with support of the
Danish e-Infrastructure Corporation and the Niels Bohr
Institute. Development of PISM is supported by NASA
grants NNX13AM16G and NNX13AK27G. We thank the
anonymous reviewers and Ralf Greve for their helpful suggestions
which substantially improved the paper.Peer Reviewe
Continuous CH4 measurements with the NIPR CFA system
第6回極域科学シンポジウム分野横断セッション:[IA] 急変する北極気候システム及びその全球的な影響の総合的解明―GRENE北極気候変動研究事業研究成果報告2015―11月19日(木) 国立極地研究所1階交流アトリウ
δ17Oおよび17O-excess の測定手法の確立および南極ドームふじルート上の表面積雪の分析結果
第6回極域科学シンポジウム[OM] 極域気水圏11月16日(月) 国立極地研究所1階交流アトリウ
High Resolution Continuous Flow Analysis System developed at the Ice Core Research Center National Institute of Polar Research
第6回極域科学シンポジウム[OM] 極域気水圏11月16日(月) 統計数理研究所 セミナー室2(D304
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Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores
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 cm<sup>2</sup> 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
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