Estimation and calibration of the water isotope differential diffusion length in ice core records

Abstract

Abstract. Palaeoclimatic information can be retrieved from the diffusion of the stable water isotope signal during firnification of snow. The diffusion length, a measure for the amount of diffusion a layer has experienced, depends on the firn temperature and the accumulation rate. We show that the estimation of the diffusion length using power spectral densities (PSDs) of the record of a single isotope species can be biased by uncertainties in spectral properties of the isotope signal prior to diffusion. By using a second water isotope and calculating the difference in diffusion lengths between the two isotopes, this problem is circumvented. We study the PSD method applied to two isotopes in detail and additionally present a new forward diffusion method for retrieving the differential diffusion length based on the Pearson correlation between the two isotope signals. The two methods are discussed and extensively tested on synthetic data which are generated in a Monte Carlo manner. We show that calibration of the PSD method with this synthetic data is necessary to be able to objectively determine the differential diffusion length. The correlation-based method proves to be a good alternative for the PSD method as it yields precision equal to or somewhat higher than the PSD method. The use of synthetic data also allows us to estimate the accuracy and precision of the two methods and to choose the best sampling strategy to obtain past temperatures with the required precision. In addition to application to synthetic data the two methods are tested on stable-isotope records from the EPICA (European Project for Ice Coring in Antarctica) ice core drilled in Dronning Maud Land, Antarctica, showing that reliable firn temperatures can be reconstructed with a typical uncertainty of 1.5 and 2 • C for the Holocene period and 2 and 2.5 • C for the last glacial period for the correlation and PSD method, respectively.

Challenges associated with the climatic interpretation of water stable isotope records from a highly resolved firn core from Adélie Land, coastal Antarctica

“…In coastal regions, several studies showed no temporal isotope-temperature relationship at all between water stable isotope records in firn cores covering the last decades and near-surface air temperature measured at the closest station. This is for instance the case in Dronning Maud Land, near the Neumayer station (three firn cores, for which the longest covered period is 1958-2012; Vega et al, 2016), in the Ross Sea sector (one snow pit covering the period 1964Bertler et al, 2011), and in Adélie Land, close to Dumont d'Urville (DDU, one firn core covering the period 1946-2006Goursaud et al, 2017). While several three-dimensional atmospheric modelling studies have suggested a dominant role of large-scale atmospheric circulation in the variability of coastal Antarctic snow δ 18 O (e.g.…”

Forward modeling of δ18O in Andean ice cores

“…Our forward model builds upon prior PSMs developed to simulate Alpine ice core [Brönnimann et al, 2012] and specifically Quelccaya 18 O [Dee et al, 2015]. By restricting our analyses to the recent decade of snowfall, we sidestep the need to explicitly model compaction [Li and Zwally, 2011] and diffusion [Gkinis et al, 2014;van der Wel et al, 2015]; such as would be necessary for simulation of the full Quelccaya proxy archive [Dee et al, 2015]. Rather, as a distinct contribution to proxy system modeling, we explore how the hydrologic cycle elements of ablation and sublimation influence the recording of 18 O in the ice sensor [Evans et al, 2013], where the sensor receives the climate signal that is recorded in the archive.…”

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

“…From the moment of deposition, the difference in diffusion between the δ 18 O and δD signals results in a decrease of the r δ 18 O/δD value. Hence, diffusing the δD signal with a Gaussian kernel of standard deviation equal to 18 ∆σ 2 will maximize the value of r δ 18 O/δD [58] as shown in Fig. 9.…”

“…In order to avoid significant overlap with previously published works e.g. [27], [31], [54], [24], [58] we occasionally point the reader to any of the latter or/and refer to specific sections in the SOM. We exemplify and illustrate the use of various techniques using synthetic data prepared such that they resemble two representative regimes of ice coring sites on the Greenland summit and the East Antarctic Plateau.…”

“…The second study presents a combined temperature and accumulation history of the past 16,000 years based on the power spectral density (PSD hereafter) signals of high resolution δ 18 O measurements of the NorthGRIP ice core. More recently, [58] introduced a slightly different approach for reconstructing the differential diffusion signal and testing it on dual δ 18 O, δD high resolution data from the EDML ice core [46]. By artificially forward-diffusing the δD signal the authors estimate differential diffusion rates by maximizing the correlation between the δ 18 O and δD signal.…”