Corrigendum to ``Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland'' published in Atmos. Chem. Phys., 12, 4259–-4277, 2012

Abstract

It was kindly pointed out to us by M. Battle that Eq. (2) on p. 4263 contains a typo, and should instead bewhere [X] corr ([X] meas ) is the gravity-corrected (measured) mixing ratio of gas species X, M = M X − M air is the molar mass difference between gas X and air, and δ grav (z) is the gravitational fractionation per unit mass difference at depth z. All calculations in the study were done correctly, following Eq. (2) as given here.Furthermore, the present-day age value for NEEM is incorrect in the orignal manuscript, and underestimates age by 6 years. The correct value is 188 +3 −9 yr. In our original, incorrect calculation we used the ice age in years before 2000 CE (b2k), while we should have used the ice age relative to the surface ice age. In the updated age calculation we use the ice age found by annual layer counting of the shallow NEEM 2011 S1 core (Sigl et al., 2013). The NEEM chronology published in Rasmussen et al. (2013) uses the correct, updated age estimate.Published by Copernicus Publications on behalf of the European Geosciences Union.

Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation

“…The sharp increase in the amplitude of high-frequency variability by up to 10-fold makes the base of the lock-in zone (close-off depth) easily recognisable in continuous CH 4 data. We estimate the close-off depth to be 82 m at D4, 73 m at Tunu13 and 95 m at B40, comparable to values from firn air field campaigns at the latter two sites (Tunu13: Butler et al, 1999;B40: Weiler, 2008). The D4 continuous CH 4 data appear to encompass the entire lock-in zone.…”

“…Field measurements of firn air (air pumped from the open porosity in the firn) provide strong evidence for such sealing layers by demonstrating a lack of vertical mixing within the lock-in zone. For example, halocarbon tracers linked to anthropogenic industrial activity are effectively absent in the lock-in zone firn air at many sites (Butler et al, 1999;Severinghaus et al, 2010;Sturrock et al, 2002). To first order, the trapping signal we observe in the ice cores therefore suggests that significant bubble closure in the early-closure layers must occur above the lock-in depth, where vertical diffusion of the relatively young air required to form the regular CH 4 oscillations is not impeded.…”

Revised records of atmospheric trace gases CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and <i>δ</i><sup>13</sup>C-CO<sub>2</sub> over the last 2000 years from Law Dome, Antarctica

“…The age scale of the NEEM CH 4 record published in Rhodes et al (2013) (Fig. 4c) has been revised with the updated ice-age scale published in Sigl et al (2015) and the new estimate of age provided by Buizert et al (2014). Mitchell et al (2013) have synchronised the GISP2 CH 4 record with the WAIS CH 4 record to investigate changes of the interpolar difference in the pre-industrial based on the reasoning that "the multidecadal events observed in both ice core records must have occurred simultaneously since the durations of the events were much larger than the atmospheric mixing time (∼ 1 year)" (Mitchell et al, 2013).…”

Authors reply to DE review

“…= mean liquid conductivity; ALC = annual layer count; VS = volcanic synchronisation; gas age scales do not incorporate lock-in zone measurements. a Buizert et al (2014); b Klein (2014); c NEEM community members (2013); d NGRIP community members (2004); e Butler et al (1999); f Mitchell et al (2013);g MacFarling Meure et al (2006); h Rasmussen et al (2013); i Sigl et al (2015); j Mitchell et al (2011). nals we previously observed in NEEM-2011-S1 ice are not unique to this site. Furthermore, we demonstrate how several site characteristics influence the frequency and magnitude of non-atmospheric signals.…”