noaa-icecore-12256
Summit, Greenland Firn Cosmogenic 14CH4 Data
Petrenko, V.V.; Severinghaus, J.
Summit, Greenland Firn Cosmogenic 14CH4 Data
2011-10-16
NCDC-Paleoclimatology
ONLINE Files
Pending
https://www.ncdc.noaa.gov/paleo/study/12256
Investigator
V.V.
Petrenko
Investigator
J.
Severinghaus
earth science
paleoclimate
ice cores
depth at sample start,null,null,unspecified unit,null,ice cores,null,null,N,null
earth science
paleoclimate
ice cores
14C,carbon monoxide,null,molecule per gram,null,ice cores,null,accelerator mass spectrometry,N,null
earth science
paleoclimate
ice cores
depth at sample end,null,null,unspecified unit,null,ice cores,null,null,N,null
earth science
paleoclimate
ice cores
14C,methane,null,molecule per gram,null,ice cores,null,accelerator mass spectrometry,N,null
earth science
paleoclimate
ice cores
14C,carbon monoxide,null,molecule per gram,null,ice cores,null,accelerator mass spectrometry,N,modeled
earth science
paleoclimate
ice core
radiogenic isotopes
geoscientificInformation
1950 AD
2005 AD
0 cal yr BP
-55 cal yr BP
Complete
72.57963
72.57963
-38.49253
-38.49253
3200
3200
Continent
North America
Greenland
Summit Firn>LATITUDE 72.57963>LONGITUDE -38.49253
None
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English
DOC/NOAA/NESDIS/NCEI
National Centers for Environmental Information, NESDIS, NOAA, U.S. Department of Commerce
https://www.ncdc.noaa.gov/data-access/paleoclimatology-data
DATA Center Contact
Bruce
Bauer
bruce.a.bauer@noaa.gov
paleo@noaa.gov
303-497-6280
303-497-6513
325 Broadway, E/NE31
Boulder
CO
80305-3328
USA
online
ASCII
Petrenko, V.V., A.M. Smith, E.J. Brook, D. Lowe, K. Riedel,
G. Brailsford, Q. Hua, H. Schaefer, N. Reeh, R.F. Weiss,
D. Etheridge, and J.P. Severinghaus. 2009.
14CH4 Measurements in Greenland Ice: Investigating Last Glacial
Termination CH4 Sources.
Science, Vol. 324, pp. 506-508, 24 April 2009.
10.1126/science.1168909
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.
STUDY NOTES: 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.
GET DATA
https://www1.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/summitfirn14ch4.xls
GET DATA
https://www1.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/summitfirn14ch4.txt
USA/NOAA
DIF
Version 9.8.4
2018-12-11
2018-12-11