DOI: 10.25921/k2ya-8863
title: NOAA/WDS Paleoclimatology - Summit, Greenland Firn Cosmogenic 14CH4 Data

description: The cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas–Preboreal CH4 rise.
description: This project successfully examined the question of whether or not 
cosmic ray-produced 14C forms radiomethane (14CH4) in polar firn. 
Firn samples from ~4.5 m depth were taken at Summit, Greenland. 
Around 1000 kg of firn per sample was melted in the presence of 
a 14C-free carrier gas to extract gases from the firn matrix. 
The findings indicate a small but clearly present cosmogenic 
14CH4 component. A larger cosmogenic 14C – carbon monoxide
(14CO) component was also found. The results further indicated 
that almost all cosmogenic 14C is being lost rapidly from the 
firn matrix at this depth level. These results confirm a 
surprising earlier finding (Petrenko et al., 2009) that 
cosmogenic 14CH4 may be present in glacial ice. Work using 
paleoatmospheric 14CH4 to study the fossil source contribution 
to the methane budget therefore needs to take the cosmogenic 
component into account. The results are currently 
in preparation for publication. 

The firn samples were collected between Jul 29 and Aug 18, 2009.

INSTRUMENT AND METHOD DESCRIPTION
The field and analytical system for determinations of in-situ 
cosmogenic 14CO and 14CH4 in glacial ice and firn have been 
described in (Petrenko et al., 2008a; Petrenko et al., 2008b; 
Petrenko et al., 2009). This system involves the melt-extraction 
of occluded air from very large volumes of glacial ice or at 
the sampling / ice coring site. Briefly, the present field system 
consists of a large chemically polished aluminum vacuum melting 
tank (~670 L internal volume) and a series vacuum and transfer 
pumps. The ice is loaded into the tank, and the headspace is 
evacuated and flushed 3x with either ultra-high purity (UHP) air, 
nitrogen or argon. The ice is then melted, releasing the ancient 
air into the headspace. This air is then recirculated through 
the tank via a bubbler manifold at the bottom, equilibrating 
all the gases between the water and the headspace. The air 
is then extracted from the tank by clean diaphragm transfer 
pumps and stored in electropolished stainless steel canisters 
for further laboratory handling and analyses. In the laboratory, 
the air is first processed through a system that converts either 
CH4 or CO to CO2, and captures this CO2 for further handling. 
In the case of CH4 processing, H2O, CO2, N2O and other 
condensibles are first removed by a series of traps at liquid 
nitrogen temperature. CO is then quantitatively oxidized to CO2 
by the Sofonocat reagent and subsequently removed by further 
cryotraps. CH4 is then combusted to CO2 by passing the air 
through a 800°C furnace containing platinized quartz wool. 
The CH4-derived CO2 is then captured. This CO2 is then converted 
to graphite over ultra-high-purity iron powder and subsequently 
measured for 14C by AMS. The combined procedural 14CH4 blank 
for all steps of sampling handling was determined to be 
0.75 ± 0.38 pMC, on the basis of 72 processed blank 
and standard samples (Petrenko et al., 2008b). For CO analyses, 
the sample handling is very similar except that the air bypasses 
Sofnocat, and the furnace temperature is reduced from 800 to 150 °C. 
This allows for complete combustion of CO while CH4 passes 
through unaffected.

DATA COLLECTION AND PROCESSING
The 14CH4 data are corrected for ambient air added to the samples 
from air bubbles in the firn as well as for the procedural blank. 
For 14CO, the ambient air correction is insignificant, but the 
samples are corrected for the procedural blank.