Earth and Planetary Science Letters 365 (2013) 190–197

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Earth and Planetary Science Letters

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High-precision 14C measurements demonstrate production of in situcosmogenic 14CH4 and rapid loss of in situ cosmogenic 14COin shallow Greenland firn

Vasilii V. Petrenko a,b,c,n, Jeffrey P. Severinghaus c, Andrew M. Smith d, Katja Riedel e, Daniel Baggenstos c,Christina Harth c, Anais Orsi c, Quan Hua d, Peter Franz e,1, Yui Takeshita c, Gordon W. Brailsford e,Ray F. Weiss c, Christo Buizert f,2, Andrew Dickson c, Hinrich Schaefer e

a Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USAb Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USAc Scripps Institution of Oceanography (SIO), University of California, San Diego, La Jolla, CA 92093, USAd Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, NSW 2232, Australiae National Institute of Water and Atmospheric Research Ltd. (NIWA), P.O. Box 14901, Kilbirnie, 301 Evans Bay Parade, Wellington, New Zealandf Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, DK-2100-Copenhagen, Denmark

a r t i c l e i n f o

Article history:

Received 8 August 2012

Received in revised form

11 January 2013

Accepted 25 January 2013

Editor: J. Lynch-Stieglitzin situ cosmogenic—are present in a combined form, as well as by a very limited understanding of the

14 14

Keywords:

radiocarbon

ice core

cosmogenic

firn

methane

carbon monoxide

1X/$ - see front matter & 2013 Published by

x.doi.org/10.1016/j.epsl.2013.01.032

esponding author at: Department of Earth a

tchison Hall, University of Rochester, Rochest

585 276 6094.

ail address: vpetrenk@z.rochester.edu (V.V. P

ow at: Mighty River Power, P.O. Box 245, Rot

ow at: College of Earth, Ocean and Atmosp

ity, Corvallis, OR 97331, USA.

a b s t r a c t

Measurements of radiocarbon (14C) in carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO)

from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric

CH4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of 14C

signals in ice is complicated by the fact that the two major 14C components—trapped atmospheric and

in situ component. This study measured CH4 and CO content in glacial firn with unprecedented

precision to advance understanding of the in situ 14C component. 14CH4 and 14CO were melt-extracted

on site at Summit, Greenland from three very large (�1000 kg each) replicate samples of firn that

spanned a depth range of 3.6–5.6 m. Non-cosmogenic 14C contributions were carefully characterized

through simulated extractions and a suite of supporting measurements. In situ cosmogenic 14CO was

quantified to better than 70.6 molecules g�1 ice, improving on the precision of the best prior ice 14CO

measurements by an order of magnitude. The 14CO measurements indicate that most (499%) of the

in situ cosmogenic 14C is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid14C loss, our measurements successfully quantified 14CH4 in the retained fraction of cosmogenic 14C

(to 70.01 molecules g�1 ice or better), and demonstrate for the first time that a significant amount of14CH4 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results

of an earlier study that used measurements of 14CH4 in glacial ice to show that wetlands were the likely

main driver of the large and rapid atmospheric CH4 increase approximately 11.6 kyr ago.

& 2013 Published by Elsevier B.V.

1. Introduction

1.1. The potential and complications of 14C records in polar ice

The radiocarbon (14C) content of accumulating polar ice is deter-mined by two main processes. The first is trapping of 14C-bearingatmospheric trace gases such as carbon dioxide (CO2), methane (CH4)

Elsevier B.V.

nd Environmental Sciences,

er, NY 14627, USA.

etrenko).

orua 3010, New Zealand.

heric Sciences, Oregon State

and carbon monoxide (CO) into bubbles during the transformation ofcompacted snow (firn) into ice. The second process is production of14C in relatively shallow ice from oxygen-16 (16O) directly in the ice

lattice by cosmic rays (e.g., Lal et al., 1987). Each of these two

components of 14C in ice, when considered separately, contains a

wealth of paleoenvironmental information.The 14C activity of CH4 (expressed in percent modern carbon,

pMC (Stuiver and Polach, 1977)) in trapped ancient air contains

information about the fossil component of the past atmospheric

CH4 budget (Petrenko et al., 2009). This allows the assessment

of CH4 emissions to the atmosphere from potentially unstable

marine CH4 clathrates (e.g., Archer et al., 2009; Westbrook et al.,

2009) as well as from the large old organic carbon reservoir in

thawing permafrost (e.g., Walter et al., 2007) at times of past

Table 1Estimates of surface 14C production rates, absorption mean free paths and

e-folding depths in ice for the neutron, negative muon capture and fast muon

mechanisms. r is taken as 0.92 g cm�3. Cosmic ray flux scaling is as in Lifton et al.

(2005). We note that uncertainties in both production rates and absorption mean

free path remain relatively large and poorly quantified. Nesterenok and Naidenov

(2012), for example, estimated 21–25 atoms g�1 yr�1 for ice at sea level and high

latitude, and L of �130 g cm�2 for the neutron mechanism.

Mechanism

i

Pi0 (14C

atoms

g�1 yr�1),

sea level

Pi0 (14C atoms g�1 yr�1),

calculated for Greenland

Summit (3200 m)

Absorption

mean free

path, Li

e-Folding

depth in

solid ice,

Li/r(g cm�2) (m)

Neutrons 30.7b 407 150a 1.6

Muon

capture

4.75b 9.8 1510b 16.4

Fast muons 0.74b 1.4 4320c 47.1

Production rates and attenuation lengths are based on:a Lal et al. (1987), van de Wal et al. (2007).b Heisinger et al. (2002a).c Heisinger et al. (2002b).

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197 191

global warming. Using the 14CO2 activity of trapped air forabsolute dating has long been a goal of ice core studies (e.g.,Andree et al., 1984; Fireman and Norris, 1982; van de Wal et al.,1990, 2007). Such absolute dating would result in improved agescales for ice cores and add value to these important records ofenvironmental change.

The in situ cosmogenic 14C component contains informationabout the past cosmic ray flux. Because the cosmic ray flux ismodulated by the solar magnetic field (Masarik and Beer, 1999), itshistory can contain information about past changes in solar activity(Suess, 1965) and possibly even solar irradiance (Steinhilber et al.,2009). In situ cosmogenic 14C content in accumulating ice inprinciple has some important advantages over other tracers thathave been used to study the past cosmic ray flux and solar activity.Unlike 10Be content in ice and 14C activity of atmospheric CO2 (e.g.,Beer et al., 1988; Knudsen et al., 2009), in situ 14C content in polarice is free from uncertainties associated with variations in thegeomagnetic field (e.g., Lal et al., 2005). In situ 14C content is alsonot subject to uncertainties arising from transport and deposition(which affect 10Be content) (Field et al., 2006), or uncertaintiesfrom the carbon cycle (which affect 14C activity of atmosphericCO2). Of the different 14C-bearing species in glacial ice, 14COappears most promising as a tracer of past cosmic ray flux becausethe in situ 14CO component is expected to be much larger than thetrapped air 14CO component (e.g., van Roijen et al., 1995).

Unfortunately, the paleoatmospheric and in situ cosmogenic14C signals are present in a combined form. In order to extractmeaningful information from 14C measurements in glacial ice, athorough understanding is required of how in situ 14C is producedat all depths, how well it is retained in the firn (in situ 14C in thefirn can escape to the atmosphere via the interconnectedpore space if it leaks out of the ice grains), and how it partitionsbetween the 14CO2, 14CO and 14CH4 phases. A complete under-standing of these processes is currently lacking.

1.2. Brief introduction to polar firn

This section provides a very brief introduction to polar firn forthose readers not closely familiar with firn studies. Firn is thecompacted snow layer that comprises typically the top 40–110 mof an ice sheet in the accumulation zone. The firn is composed ofthe solid ice matrix (ice grains) that can be thought of as sponge-like in its structure, as well as the air that fills the open space(porosity). In an ice sheet at steady-state, a snow layer depositedin a particular year moves downward with time through the firncolumn. As the snow layers move down, they become progres-sively more dense, owing to the reduction of the porosity volume(Herron and Langway, 1980). In the upper firn, the porosity isalmost entirely interconnected, with only a very small fraction ofair enclosed in microbubbles (e.g., Siegenthaler et al., 2005). Gasescan move freely through this part of the firn and exchange withthe atmosphere, primarily by molecular diffusion (e.g., Schwanderet al., 1988). In the deeper firn, progressively more air becomestrapped in bubbles, some impermeable ice layers begin to form,and gas diffusion mostly stops (e.g., Schwander et al., 1993). Thetransition from firn to ice occurs at the depth at which there is nolonger a significant amount of interconnected porosity and all airis enclosed in bubbles.

1.3. In situ cosmogenic 14C production, retention and partitioning in

glacial ice

14C is produced both in accumulating (e.g., Jull et al., 1994) andablating (e.g., van der Kemp et al., 2002) glacial ice by energeticneutrons (Lal et al., 1990), negative muon capture (van der Kempet al., 2002), and likely also by interactions with fast muons

(Nesterenok and Naidenov, 2010). The production rate for eachmechanism decreases exponentially with depth in ice and firn,following:

PiðzÞ ¼ P0i e�rz=Li ð1Þ

where P0 is the surface production rate, z the depth in cm, L theabsorption mean free path in g cm�2, and r the density of ice ing cm�3. i denotes the specific production mechanism (neutrons,muon capture, or fast muons). The total in situ 14C production rateis the sum of production rates from the three individual mechan-isms. Table 1 shows the surface production rates, absorptionmean free paths and the effective e-folding depths in solid icefor the three mechanisms. After production, the majority of thehot 14C atoms form 14CO or 14CO2 (e.g., Lal et al., 2000; van de Walet al., 2007).

To-date, there has been no agreement on how well cosmogenic14C is retained in accumulating ice. While some studies find eitherno in situ cosmogenic 14C, or a relatively small amount (e.g.,Andree et al., 1984; de Jong et al., 2004; Smith et al., 2000; Wilsonand Donahue, 1990), other studies suggest that either all or alarge fraction of 14C is retained (e.g., Lal et al., 2000, 2001). Thereis also no consensus on the partitioning of in situ 14C between thedifferent species. In prior studies that have characterized the 14COfraction of total cosmogenic 14C in ice, estimates of this fractionvary greatly, sometimes spanning almost the full range from0.0 to 1.0 within the same study (e.g., Lal et al., 2001; van de Walet al., 2007).

1.4. Goals of this study

The first goal of this study was to unambiguously test thehypothesis that cosmic rays produce a significant amount of14CH4 in glacial ice, as suggested by recent work on 14C ofpaleoatmospheric CH4 (Petrenko et al., 2009). The cosmogenic14CH4 component inferred by Petrenko et al. (2009) introduced anelement of uncertainty to the interpretation of the 14CH4 activityresults and highlighted the need to better understand thisprocess. Prior to the Petrenko et al. (2009) results, in situ 14CH4

had not been considered in natural ice, but its likely existence wassuggested by laboratory studies in which water or ice werebombarded by protons to produce hot 11C atoms or by energetic14Cþ or 14COþ beams (Evans, 1970; Rossler et al., 1984). Thesestudies found that a small fraction of the hot C atoms formed CH4

as well as other simple organics. The second goal of the study was

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197192

to provide a more definitive answer to the question of how wellin situ cosmogenic 14C is retained in the ice grains in shallow firn.

Table 2

Measured 14CH4 activity (normalized to measured d13CH4), measured [CH4], and14CH4 activity after all corrections for non in situ cosmogenic 14C. 14CH4 activity

value for the standard gas is an average of 3 replicate extractions. 14CH4 activity

and [CH4] in Summit ambient air are also shown for comparison (see SOM).

Uncertainties for measured 14C are 1 standard error.

Sample

name

Measured 14CH4

activity

Measured

[CH4]

Corrected 14CH4

activity

ANSTO

sample code

(pMC) (nmol mol�1) (pMC)

Standard

gas

0.770.5 483.270.2 OZN382, 383,

390

Blank 1 1.670.2 491.570.2 OZN389

Blank 2 1.270.1 476.070.4 OZN385

Blank 3 1.270.3 480.170.6 OZN384

Sample 1 9.270.4 496.370.3 4.370.5 OZN387

Sample 2 10.770.4 516.070.3 5.670.8 OZN386

Sample 3 11.470.3 508.570.6 6.270.6 OZN388

Ambient

air

130.075.0 1854.0712.7

2. Methods

2.1. Field sampling

The sampling site (N 72.579631, W 38.492531) was located�2 km from the main research station at Summit, Greenland. Thestation is at �3200 m above sea level and has an annualaccumulation rate of 22.8 cm a�1 ice equivalent. A large-volumesystem (Petrenko et al., 2008a) was used for field melt-extractionsof 14C from the firn. Firn blocks from 3.6 to 5.6 m below snowsurface were cut with clean electric chainsaws and furthercleaned by removing �0.5 cm from all surfaces with electropol-ished stainless steel scrapers. After loading into the melting tank,the firn was subjected to a repeating evacuate–flush sequence toensure complete removal of ambient air from the firn openporosity. During this sequence the tank was evacuated to 1 Torrabove the expected vapor pressure over ice at the given firntemperature (this usually meant 3 Torr). Ultra-high-purity (UHP)nitrogen or argon was then introduced to a total pressure of100 Torr. The evacuate–flush sequence was done two times, and afinal evacuation step followed, with continued pumping on thetank (using a high-vacuum pump) for 10 min after the pressurereached the expected water vapor pressure þ1 Torr value.�20 L STP of a standard gas, containing 483 nmol mol-1 of

14C-depleted CH4 and 3100 nmol mol�1 of 14C-depleted CO inUHP air was then introduced to the tank. Such a carrier gas wasneeded because shallow firn contains almost no enclosed air.The standard gas also contained artificially high levels of Kr(�105 mmol mol�1) and Xe (�55 mmol mol�1), added to serveas tracers of gas dissolution and ambient air inclusion.

The firn (�260 kg per melt-extraction) was melted, and thegas was then recirculated through the tank for 20 min via abubbler manifold at the bottom, equilibrating the gases betweenthe water and the headspace. In-situ cosmogenic 14CO and 14CH4

are produced inside the ice grains and would therefore initially bepresent in the water after the firn is melted. The recirculation stepallows to transfer most of the cosmogenic 14CO and 14CH4 into thecarrier gas in the headspace (where it can be extracted from themelting tank by the transfer pumps), and partitions the 14CO and14CH4 between the water and headspace in a well-characterizedmanner governed by the solubility of each gas. A detaileddiscussion of this solubility equilibrium characterization is pro-vided in the Supplementary Online Material (SOM). Air extractedfrom the headspace during 4 such melt-extractions was combinedinto a single canister to produce an individual sample; overall,3 complete replicate samples were collected.

The standard gas used in firn melt-extractions was also usedfor simulated extractions to characterize the procedural 14C blank.Prior to introduction of the standard gas to the melting tank in asimulated extraction, the melt water was purged with UHP air toremove cosmogenic 14CH4 and 14CO remaining in the water aftera firn extraction (99.97% removal estimated for 14CH4 and 99.99%removal for 14CO). In total, 10 such simulated extractions werecarried out, producing 3 procedural blank samples (four extrac-tions each combined into blanks 1 and 2; two extractions intoblank 3).

We note that this field method does not allow for reliablein situ 14CO2 determination because of issues inherent in a melt-extraction, such as CO2 dissolution and partitioning betweendifferent species in the melt water, as well as extraneous CO2

production in the water from organics and carbonates releasedfrom the ice.

2.2. Laboratory analyses

In the laboratory, samples, procedural blanks and the standardgas were first analyzed for CH4 molar fraction ([CH4]; by GC-FID),d(O2/N2) (using a Thermo Delta V IRMS), d(Xe/Ar), d(Kr/Ar),d(Xe/N2) and d(Kr/N2) (Finnigan MAT 252 IRMS) at SIO. d13CH4

(needed for 14C normalization) was measured at NIWA using anIsoPrime IRMS with a customized inlet system (Ferretti et al.,2005). d13CO was measured in the standard gas at NIWA followingBrenninkmeijer (1993). The air was then processed through asystem that oxidizes either CH4 or CO to CO2, and captures thisCO2 for further handling, as described in Petrenko et al. (2008b)and Petrenko et al. (2009). This CO2 was converted to graphite overultra-high-purity iron powder as described in Petrenko et al.(2008b) and subsequently measured for 14C by AMS on the10 MV ANTARES accelerator at ANSTO (Fink et al., 2004). The finalsample sizes were �16 mg C for all 14CH4 samples and �17 mg Cfor all 14CO samples.

3. Results and discussion

3.1. Procedural corrections and calculations of in situ 14CH4 and14CO content

Tables 2 and 3 present the [CH4], 14CH4 activity, [CO] and 14COactivity measured in the samples, blanks and the standard gas.Before the true in situ cosmogenic 14CH4 and 14CO content in thefirn can be determined, we need to account for the effects of any14C in our samples that is not due to in-situ cosmogenic produc-tion in ice grains. This includes:

1)

14C contained in the standard gas (the carrier gas). This 14Cis completely accounted for by our full-process proceduralblanks, as they use the same standard gas.

2)

14C added from ambient air trapped in micro-bubbles in thesampled firn (�1% of total sample air by volume). This isquantified with the help of supporting measurements of Krand Xe in the samples (see SOM).

3)

14C introduced via ‘‘extraneous carbon’’ addition during sam-ple processing. This includes carbon release from parts of theapparatus at any stage of sample processing, as well as carbonadded as a result of ambient air leaks into the apparatus. This14C addition is mostly well-characterized by our full-processprocedural blanks. An additional component resulting fromCH4 or CO outgassing from the melting tank during the firn

Table 4Maximum depth span for each sample, determined in situ cosmogenic content in the 14CH4 and 14CO phases, and the expected in situ cosmogenic 14CO content based on

production rates and assuming full 14CO retention in ice grains.

Sample name Top depth Bottom depth In situ cosmogenic 14CH4 content In situ cosmogenic 14CO content Expected in situ cosmogenic 14CO content

(m) (m) (molecules/g ice) (molecules/g ice) (molecules/g ice)

Sample 1 3.84 5.60 0.05670.007 2.6370.49 482

Sample 2 3.84 5.62 0.07170.011 488

Sample 3 3.67 5.54 0.07670.009 2.3670.58 480

Table 3

Measured 14CO activity (normalized for d13C as discussed in SOM), measured [CO], and 14CO activity after all corrections for non in situ cosmogenic 14C.14CO activity and [CO] in Summit ambient air are also shown for comparison (see SOM). Uncertainties for measured 14C are 1 standard error. 14CO sample

2 was lost during processing (see SOM).

Sample name Measured14CO activity

Measured [CO] Corrected14CO activity

ANSTO sample code

(pMC) (nmol mol�1) (pMC)

Standard gas 6.470.2 3184764 OZN478

Blank 1 11.470.4 31917139 OZN477

Blank 2 11.670.2 33057144 OZN473

Blank 3 10.770.2 33577146 OZN471

Sample 1 43.970.5 33957148 32.175.6 OZN474

Sample 2 37927165 OZN475

Sample 3 43.070.6 35537155 30.877.4 OZN476

Ambient air 430.37177.2 125

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197 193

melting step is quantified based on [CH4] and [CO] elevation inthe samples as compared to the procedural blanks.

4)

For 14CO, this also includes in-situ cosmogenic production inair sample canisters during storage and transport (Lowe et al.,2002). This is also completely characterized by our proceduralblanks, as they experience exactly the same storage andtransport conditions as the samples.

Results after all the above corrections are shown in Tables 2and 3, and the corrections themselves are described in detail in theSOM. Cumulative contributions to sample 14C from sources otherthan in situ cosmogenic production in firn on average account for28% of the measured signal for 14CO and 49% for 14CH4. Unlessotherwise specified, all uncertainties in Tables 2–4 are 1s; uncer-tainties for calculated quantities are determined using standarderror propagation techniques.

It can be shown that the in-situ cosmogenic 14C content in firnor ice for the gas species X can be calculated as:

14CX ¼pMC

100�ð1þðd13C=1000ÞÞ2

0:9752

!� 1:176� 10�12

� X½ � � A

�1

1000�

1

22:4� NA ð2Þ

where 14C content (14CX) is expressed as the number of14C-bearing molecules of gas X per gram of ice, pMC is the 14Cactivity of the gas after all above corrections, d13C is the d13C of thegas after corrections for addition of extraneous carbon as well ascarbon from air trapped in firn micro-bubbles, 0.975 is a factor arisingfrom 14C activity normalization to d13C of �25% (part of definition ofpMC (Stuiver and Polach, 1977)), (1.17670.010)�10�12 is the 14C/(13Cþ12C) ratio corresponding to the absolute international standardactivity (AISA) (Stuiver and Polach, 1977; Stuiver, 1980), [X] is the gasmolar fraction in extracted air after all corrections (see SOM fordetails), A is the effective air content of the ice in cc STP g�1 (see SOMfor details), 22.4 is the number of STP liters of gas per mole, and NA isthe Avogadro constant. It can be shown that for this experiment, [X] iseffectively the [CH4] or [CO] in our standard gas, and d13C is the

d13CH4 or d13CO in the standard gas (values provided in SOM).Table 4 presents the in situ cosmogenic 14CH4 and 14CO content inthe samples calculated using Eq. (2) after applying all the abovecorrections.

3.2. Estimated uncertainties of final results

As can be seen from the uncertainties in Table 4 (final 1suncertainties estimated by error propagation), 14CH4 content wasquantified to about 0.01 molecules g�1 ice or better, and 14COcontent was quantified to 0.6 molecules g�1 or better. 14CO contentvalues agree well between the two successful samples (1 and 3) giventhe estimated 1s uncertainties. 14CH4 content values agree amongmost samples, but the difference between samples 1 and 3 is slightlygreater (0.020 molecules g�1) than allowed by the combined esti-mated 1s uncertainties (0.016 molecules g�1). It may be possible thatsamples 2 and 3 have been affected by unaccounted-for extraneousCH4 introduced with the flush gas used during the melt-extractionprocedure. In the case of sample 1, the firn was flushed with UHP N2

that was verified to be CH4-free. For samples 2 and 3, the firn wasflushed with UHP Ar, which was not measured for [CH4]. It is thuspossible that some modern CH4 was present in the Ar and con-tributed to the slightly higher observed [CH4] and 14CH4 activity insamples 2 and 3.

3.3. Comparison with and discussion of prior in-situ cosmogenic 14C

measurement methods

Our method represents a significant improvement over prior14C analyses in ice for several reasons. First, our ice sample sizes aremuch larger. This allows us to measure 14CH4 in ice samples(previously impossible) and detect even very small (down to0.01 molecules g�1, based on the uncertainties) amounts of 14CH4.For 14CO, the most precise prior measurements in ice are those of vande Wal et al. (2007). Their method included careful procedural blankdetermination using gas-free ice and produced low 14C blanks and atypical sample uncertainty of 77 14CO molecules g�1. Our studyyielded an order of magnitude lower uncertainties of o0.6 14CO

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197194

molecules g�1, and allowed to characterize 14CO content accuratelyeven in the case when less than 0.5% of all cosmogenic 14C appearedto be retained (see below). We note that we were not able to getprecise [CO] determinations on the samples in this study because ofan instrument malfunction (see SOM); normal [CO] determinationsshould result in further improvement in 14CO content uncertaintiesby another order of magnitude.

In contrast to prior studies, our extractions of in-situ cosmogenic14C from ice are performed on-site within a few hours of ice recovery.This is important, because in situ cosmogenic production of 14C in icecontinues after the ice is drilled and adds further uncertainty to theresults. Post-coring 14C production at high-elevation ice drilling sites(e.g., de Jong et al., 2004) and even at lower-altitude laboratorystorage sites (e.g., Lal et al., 2000) can add a substantial new 14Ccomponent.

Our method combines accurate and thorough characterizationof extraneous carbon added during sample handling with completeextraction of cosmogenic 14C from the ice grains. Dry extraction of14C from ice (involving grating, milling or crushing ice) used inmany past studies may not effectively liberate all in situ cosmo-genic 14C because it leaves behind relatively large fragments of theice matrix (e.g., Smith et al., 2000; van de Wal et al., 2007). Melt-extraction is expected to recover all in situ 14C, provided that this14C is transferred from the water to the headspace. However, priorstudies that involved melt-extractions only conducted partial blanktests that may not have accurately characterized procedural 14Caddition. Further, in these studies, acid was added to the extractionvessel to achieve a pH o3 and drive off all dissolved CO2 (e.g., Lalet al., 1997, 2001). This acidification may have resulted in addi-tional 14C release from carbonates or organics in the ice.

Ice cores contain sufficient amounts of carbonate dust to signifi-cantly alter the measured CO2 concentration; this is one of the mainreasons why ice core melt-extraction techniques are generallyconsidered unreliable for CO2 (e.g., Tschumi and Stauffer, 2000).Carbonate dust becomes highly soluble at low pH and is thusexpected to contribute CO2 especially if acid is added to the meltwater. While most carbonate rocks are geologically old and should bedevoid of 14C, we can still identify two possible mechanisms by whichcarbonate dust deposited in ice cores can contain significant amountsof 14C. The first mechanism involves carbonate dust derived fromseasonal lake beds by wind erosion. Dissolved carbonate in such lakeswould be expected to be largely in equilibrium with 14C of atmo-spheric CO2. The second mechanism has to do with the fact thateroding carbonate rocks near the surface also experience cosmogenic14C production. Surface carbonates from the Provo shoreline in Utahhave been shown to contain very high cosmogenic 14CO concentra-tions of �0.4�106 molecules/g (Handwerger et al., 1999). Thus,contributions from carbonate dust to both 14CO and 14CO2 appearpossible with a melt-acidify technique.

In addition to extraneous 14C contributions from carbonate dust,significant contributions from trace organics in the ice are alsopossible with a melt-acidify approach. Production of both CO2

(Tschumi and Stauffer, 2000) and CO (Haan and Raynaud, 1998)from trace organics even in-situ in frozen ice appears likely forGreenland ice cores, and may proceed much faster under theconditions of an acidified melt-extraction. Organic compounds inice layers deposited before �1750 AD are derived primarily fromsources such as biomass burning and biospheric emissions (e.g.,Kawamura et al., 2001; Legrand and DeAngelis, 1996), and wouldbe expected to have a modern 14C signature.

3.4. In-situ cosmogenic 14CH4 production

Our results (Table 4) indicate that a small but significant amountof in situ cosmogenic 14CH4 is clearly present (0.06870.010 14CH4

molecules per gram ice, average and 1s of 3 replicate samples). This

represents the first definitive identification of such a signal in naturalice. The results also support the hypothesis that elevated 14CH4

activity values observed in an earlier study conducted in ablationice from the West Greenland ice margin (Petrenko et al., 2009) areattributable to in situ 14CH4 production. That earlier study usedmeasurements of 14CH4 activity in ancient air to constrain thecontribution of fossil (14C-free) CH4 to the large and rapid atmo-spheric CH4 increase associated with an abrupt climate warmingaround 11,600 years ago. The Petrenko et al. (2009) results indicatedthat increased emissions from wetlands in response to the warmingwere the main driver of the CH4 increase, but the conclusions wereweakened by the speculative correction for cosmogenic 14CH4. Thenew results presented here increase confidence in the conclusions ofthe Petrenko et al. (2009) paleoatmospheric 14CH4 study.

3.5. Retention of in-situ cosmogenic 14C in firn

Table 4 compares measured and expected 14CO content in thesamples. Expected total 14C content in the sampled shallow firnwas calculated using a recently developed model for in situcosmogenic 14C production in ice (Buizert et al., 2012), in combina-tion with a depth–density profile from a Summit shallow coredrilled in the summer of 2007 (Mary Albert, personal communica-tion, 2012). For accumulating ice, Lal et al. (1987) described the 14Ccontent of ice (in atoms g�1) at depth z (z expressed as solid iceequivalent depth) from in-situ production by mechanism i as:

14Ci ¼P0

i

ðra=LiÞ�lðe�lz=a�e�rz=Li Þ ð3Þ

where a is the accumulation rate in cm a�1 solid ice equivalent, l isthe 14C decay constant in a�1, and r is the density of solid ice (weuse 0.92 g cm�3). The Buizert et al. (2012) model adds solarmodulation of spallogenic 14C production using the observationalrecord of sun spot numbers. The expected 14C values shown inTable 4 are based on individually-determined, mass-weightedaverages taken over the depth span of each sample. For ourrelatively shallow samples, the model predicts that 95% of in situ14C is from the neutron mechanism.

To calculate the expected 14CO content, we had to make anassumption about the fraction of cosmogenic 14C that forms 14COin ice. As mentioned above, estimates of this fraction vary greatlyin prior studies (full range from 0.0 to 1.0), making it impossibleto estimate this fraction with high confidence. For our purposes,we used an estimate of 0.31 for the 14CO fraction, based on vander Kemp et al. (2002). The van der Kemp et al. (2002) study waschosen because it included careful procedural blank characteriza-tion and was conducted in ablating ice, where, unlike in firn, 14Closs from ice grains is not a concern. We stress that theuncertainty of this 0.31 estimate is relatively large and difficultto characterize because of potential complications with incom-plete cosmogenic 14C release associated with the dry extractiontechnique used in that study, as well as the fact that the estimateis based on data from only a single study site.

The actual measured 14CO content in the Summit firn samplesrepresents less than 0.5% of the estimated expected amount. Ourcalculations may overestimate the expected amount due touncertainties in 14C production rate and absorption mean freepath. For example, if we use recent estimates from Nesterenokand Naidenov (2012) (see Table 1 caption) the expected valuesare lower by as much as 30%. It is also possible that ourcalculations further overestimate the expected amount becauseof the uncertainty in the 14CO fraction. However, studies inablation ice clearly show that a large fraction of cosmogenic 14Cforms 14CO (van der Kemp et al., 2002; van Roijen et al., 1994) inboth the shallowest ice (production dominated by neutrons) and

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197 195

deeper ice (production dominated by muons). The uncertaintiesin our calculations of the expected 14CO content therefore do notaffect our conclusion that almost all in situ cosmogenic 14CO islost quite rapidly from shallow firn.

If we assume that in situ 14CO2 escapes the firn in the sameway, and that all in situ 14C above 5.6 m depth is lost, while all 14Cbelow 5.6 m is completely retained, we can attempt to place anabsolute upper limit on the fraction of in situ produced 14C thatcould potentially be retained in Summit ice. This fraction is 0.4 ifwe consider ice at 300 m depth, where 14C production by allmechanisms is negligible. Of course, the assumption that all 14C isfully retained below 5.6 m depth is almost certainly incorrect andthe actual retained fraction is likely much smaller. We furthernote that at least some of the in-situ 14C that escapes from the icegrains into the open firn porosity would be transported down-wards in the firn and ultimately trapped in the air bubbles.However, quantitative estimates of this process require majormodifications to existing firn gas transport models as well as adetailed understanding of in-situ 14C retention at all depth levelsin the firn, and are beyond the scope of this paper.

Our findings regarding in situ 14C retention in firn agree withmost prior studies that did not use the melt-acidify approach forthe air extraction (see Table S15 in SOM for a summary of resultsfrom prior work). These studies primarily used some form of dryextraction, but also include some studies that used the wetextraction (Fireman and Norris, 1982) and sublimation (Wilsonand Donahue, 1990; Wilson, 1995) techniques. Data from thesestudies indicate that in situ cosmogenic 14C retention in firn iseither insignificant or minimal (Andree et al., 1984, 1986; de Jonget al., 2004; Fireman and Norris, 1982; Levchenko et al., 1996,1997; Smith et al., 2000; van de Wal et al., 1994; van der Kempet al., 2000; Wilson and Donahue, 1990, 1992; Wilson, 1995). Ourconclusion disagrees with all the studies using the melt-acidifyapproach, which included studies of 14CO in upper firn (Jull et al.,1994; Lal et al., 1997, 2001). Our results thus support thehypothesis that the melt-acidify technique for ice core 14Canalyses is problematic.

As our samples targeted only a single depth level at a single site,the data do not allow for a thorough exploration of the mechanismsof 14CO loss from the ice grains. We speculate, however, that the mostlikely mechanism is the relatively rapid recrystallization in the firnresulting from water vapor transport between ice grains. This well-known process is responsible for a dramatic decrease with depth inthe amplitude of the seasonal cycle in H2O stable isotopes (e.g., Steen-Larsen et al., 2011). For example, results from the NGRIP ice core sitein north-central Greenland (climatically similar to Summit) show thatat 4.5 m depth, the seasonal d18OH2O cycle is already attenuated by�30%, implying a very large water vapor mass flux on at least thelength scale of �20 cm (½ annual layer thickness for this depth inNGRIP) (Johnsen et al., 2000). Such a length scale is 2 ordersof magnitude larger than the diameter of a typical firn grain(Johnsen et al., 2000).

3.6. Limitations of this study

We note that while there is very high confidence in the large-sample firn 14C results presented here, the utility of these results ininforming deeper firn and ice core studies at this stage is limited. Itmay be possible that in situ 14C retention is more efficient at deeper,less ventilated levels in Summit firn where muogenic 14C productionstill occurs. Retention of in situ 14C may also be higher at other firnsites with higher accumulation rates, shallow convective zones ormelt layers. We further note that our results with regard to 14Cretention are not applicable to ablation-zone ice, where all in-situ 14Cis expected to be retained.

4. Conclusions and future outlook

Our measurements in Summit shallow firn have provided thefirst definitive proof of in situ cosmogenic production of 14CH4 innatural ice. This cosmogenic component of 14CH4 in ice will need tobe carefully quantified and corrected for in future studies of pastatmospheric CH4 budgets that utilize 14C. Our technique, evenunder the conditions of a critical instrument malfunction duringsample analyses, succeeded in improving the measurement preci-sion for 14CO content in ice by an order of magnitude. Ourmeasurements have clearly indicated that almost all in situ cos-mogenic 14CO is being rapidly lost from the ice grains in shallowfirn. The 14CO measurements allow us to place an absolute upperlimit of 0.4 on the fraction of total in-situ cosmogenic 14C that isretained in Summit firn, but lower values are more likely.

We now briefly discuss the potential of further 14C measure-ments in ice. Ablation-zone ice sites with surface exposures ofancient ice are needed for reconstructions of paleoatmospheric14CH4 activity, because of the large sample requirement. Suchsites exist both in Greenland (e.g., Pakitsoq (Petrenko et al., 2006))and Antarctica (e.g., Taylor Glacier (Aciego et al., 2007)). OurSummit firn 14CH4 measurements suggest that an in situ cosmo-genic signal will be combined with paleoatmospheric 14CH4 inablating ice. To reconstruct the absolute values of past atmo-spheric 14CH4 activity, a reliable correction is needed for thecosmogenic component. Such a correction could be possible if (1)the in situ cosmogenic 14CH4/14CO ratio is constant, and (2) 14COis dominated by the in situ component. Our results are consistentwith the hypothesis of constant 14CH4/14CO ratio withinestimated uncertainties (0.02170.005 for Sample 1 and 0.03270.009 for Sample 3). However, these results are only representa-tive of neutron-dominated 14C production in an environmentwhere most in situ 14C is being rapidly lost, and it is unclearwhether they are directly applicable to ablation-zone ice. Furtherstudies in ablation zone ice are needed to investigate in situ 14CH4

production and the in situ 14CH4/14CO ratio. With regard to point(2), predicted in situ cosmogenic 14C values at Taylor Glacier(Buizert et al., 2012) suggest that cosmogenic 14CO will exceedthe trapped paleoatmospheric component by a factor of 100or more.

If dynamic recrystallization is indeed the process responsiblefor the observed 14CO loss, then cosmogenic 14CO2 would beexpected to be lost from the ice grains as well. It may then indeedbe possible to use trapped atmospheric 14CO2 for absolute datingin both ice accumulation and ice ablation zones. For accumulatingice, further work is needed to better understand in situ 14Cproduction and retention in the entire firn column as well asbelow the firn zone. Even in the case of a non-negligible in situ14CO2 component, corrections may be possible if 14CO measure-ments are available and the 14CO/14CO2 partitioning is wellunderstood (van Roijen et al., 1995).

As mentioned above, 14CO may be promising as a tracer of thepast cosmic ray flux. Some 14C production by muons is stillexpected below the firn zone, where all in situ 14C would bequantitatively retained. The trapped atmospheric 14CO content isrelatively small because of low atmospheric [CO]. Preliminarycalculations (not shown) using production rates as in Table 1 andassuming instant release of all in situ 14C from ice grains into theopen porosity in the firn column suggest that Summit ice belowthe firn zone should still contain �10 times more in situ cosmo-genic 14CO than trapped atmospheric 14CO. If this is the case, 14COat accumulation sites could still be useful as a tracer of pastcosmic ray flux. Much further work is needed to better character-ize 14C production, retention and partitioning in the entirefirn column before such an application of 14CO in ice is possible,however.

V.V. Petrenko et al. / Earth and Planetary Science Letters 365 (2013) 190–197196

Acknowledgments

This work was supported by NSF OPP award 0806450 (Sever-inghaus), NOAA Climate and Global Change Postdoctoral Fellow-ships (Petrenko, Buizert) and New Zealand Ministry for Science andInnovation contract C01X0703 (Brailsford, Franz, Riedel, Schaefer).We thank James White for hosting Petrenko at INSTAAR, MaryAlbert and Zoe Courville for sharing their unpublished Summit firndensity data, CH2MHILL and 109th Air National Guard for excellentfield logistical support, and Ross Beaudette for help with fieldpreparations and testing. We thank Guy Emanuele and RalphKeeling for extracting CO2 from sample air aliquots.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2013.01.032.

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