DOI: 10.25921/xhs5-8y36
title: NOAA/WDS Paleoclimatology - Pakitsoq Greenland Younger Dryas-Preboreal 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: 14C measurements in large-volume ice samples from 
Pakitsoq, West Greenland. This data set contains measurements 
of 14C of methane made on ancient air extracted from large-volume 
glacial ice samples spanning the Younger Dryas - Preboreal climatic 
transition about 11,600 years ago. The data set also contains 
supporting measurements of 14C of carbon monoxide and 14C 
of carbon dioxide. 

The measurements were made to test the hypothesis that catastrophic 
destabilization of marine methane clathrates caused the methane 
increase at the end of the Younger Dryas. The results suggested 
little to no change in 14CH4 over the time of the methane increase, 
arguing against major clathrate involvement.

The ice samples were collected between Jul 1 and Aug 15, 2005. 

INSTRUMENT AND METHOD DESCRIPTION
The field and analytical system for determinations of 14CH4 
in large-volume glacial ice samples 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. 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. CO2 derived from CO was diluted with 14-C 
free CO2 to increase the carbon mass to allow for a 14C measurement.
For sample 14CO2 measurements, CO2 was extracted cryogenically 
from ~0.5 L of air.

DATA COLLECTION AND PROCESSING
The 14CH4 data are corrected the procedural blank. Where indicated, 
14CH4 data are also corrected for cosmogenic 14C production in ice. 
For 14CO, corrections are applied for ambient air inclusion 
and processing blank.  For 14CO2, processing blank corrections 
are applied where indicated.