Comparison of Holocene temperature reconstructions based on GISP2 multiple-gas-isotope measurements
Abstract. Nitrogen and argon stable-isotope data extracted from ancient air in ice cores provides the possibility to reconstruct Greenland past temperatures when inverting firn-densification and heat-diffusion models (firn-models) to fit the gas-isotope data (δ15N, δ40Ar, δ15Nexcess). This study uses the Döring and Leuenberger (2018) fitting-algorithm coupled on two state of the art firn-models to fit multiple Holocene gas-isotope data measured on the GISP2 ice core. We present for the first time the resulting temperature estimates when fitting δ15N, δ40Ar and δ15Nexcess as single targets with misfits generally in the low permeg level. Whereas the comparison between the reconstructions using δ15N and δ40Ar shows a high agreement, the use of δ15Nexcess for reconstructing temperature is problematic due to higher statistical and systematic data uncertainty influencing especially multi-decadal to multi-centennial signals and results in an unrealistic temperature estimate that differs significantly from the two other reconstructions. We find evidence for systematic too high δ40Ar data in the early- and late-Holocene potentially caused by post coring gas-loss or an insufficient correction of this mechanism. Next, we compare the performance of the Goujon et al. (2003) firn-model and the Schwander et al. (1997) firn-model for Holocene temperature reconstructions. Besides small differences of the reconstructed temperature anomalies – potentially caused by slightly different implementation of firn physics and parameters in the two models – the reconstructed temperature anomalies are highly comparable. We were able to quantify the contribution of the firn-model difference to the uncertainty budget of our reconstruction. Furthermore the fractions of uncertainty on the reconstructed temperatures arising from the non-perfect reproducibility of the fitting algorithm and from the remaining final misfits (low permeg level) were quantified. Together with the published measurement uncertainty of the gas-isotope data and the analysis of the impact of accumulation-rate uncertainty on the reconstruction we were able to calculate the mean uncertainty (2σ) for the nitrogen and the argon based temperature estimates with 2σT = 0.80…0.88 K for T(δ15N), and 2σT = 0.87…1.81 K for T(δ40Ar), respectively. Finally, we compare our reconstructed temperatures to two recent reconstructions based on the same gas-isotope data as used here, but following different reconstruction strategies: first the study of Buizert et al. (2018), which uses a combination of δ18Oice-calibration and δ15N-fitting, and second the study of Kobashi et al. (2017), where δ15Nexcess was fitted in order to conduct the temperature reconstruction. We find generally higher agreement between our T(δ15N) estimate and the Buizert et al. (2018) temperature – in terms of variability and correlation in three investigated periodic-time bands (multi-decadal, multi-centennial and multi-millennial) – as if our T(δ15N) reconstruction is compared to the Kobashi et al. (2017) temperature. However, all three reconstruction strategies lead to distinct temperature realizations.
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