a fraction
comparable to the tiny masses of P1 and P2 might
have survived such stochastic removal processes.
What about corotation resonances other than
the 4:1 and 6:1? For an eccentric Charon, the 3:1
corotation resonance is nearly overlapped by its
Hill sphere at apocenter and was likely not a
stable niche. Corotation resonances also occur
when (m
þ p)n , pn
C
for p
9 1, but these fall at
distances a
cr
0 (m/p þ 1)
2
=
3
and are shifted
inward. Thus, those that fall in the vicinity of
P1 and P2 have amplitudes that are dependent
on a higher power of Charon
_s eccentricity
(i.e., Èe
C
3p
, e
C
5p
) and are weaker. Although
transient forced eccentricities may interfere with
the stability of adjacent p
0 1 resonances, it re-
mains an intriguing possibility that smaller, yet
undetected moons may orbit Pluto near the 5:1.
Because the corotation resonances we in-
voke no longer exist, direct diagnostic evi-
dence of this mechanism is elusive. However,
a circumstantial case can be made by consider-
ing the alternatives. Although there are capture
mechanisms (4, 5) to create well-separated sec-
ondaries such as some Kuiper belt binaries,
they do not select a common orbital plane or
direction. In addition, the subsequent hardening
of these configurations tends to produce large
eccentricities that could not be damped by tidal
forces given the small masses of P1 and P2 (2).
Alternatively, if a protosatellite disk were to ex-
tend to sufficient distance to allow the accretion
of P1 and P2 in situ, there is no obvious reason
why they should be found in near-resonant
orbits, because tidal torques are also too weak
to migrate them into such configurations. A final
unanswered question is how the moons were
initially trapped in corotation resonances. One
possibility is that a small amount of vapor and/or
small particles extended past the location of the
6:1 resonance (È3.3 a
C
) and their free eccen-
tricities damped by collisional viscosity. This
could have initially populated many resonance
sites, but most would be later cleared as eccen-
tricities were excited by resonant migration.
Indeed, material comprising P1 and P2 may
have begun as ring arcs, except that by lying
external to Pluto
_s Roche limit, single moons
were able to accumulate.
References and Notes
1. H. A. Weaver et al., Nature 439, 943 (2006).
2. S. A. Stern et al., Nature 439, 946 (2006).
3. M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young,
S. A. Stern, Astron. J. 132, 290 (2006).
4. P. Goldreich, Y. Lithwick, R. Sari, Nature 420, 643
(2002).
5. S. J. Weidenschilling, Icarus 160, 212 (2002).
6. W. B. McKinnon, Astrophys. J. 344, L41 (1989).
7. R. M. Canup, Science 307, 546 (2005).
8. R. M. Canup, Ann. Rev. Astron. Astrophys. 42, 441 (2004).
9. A. R. Dobrovolskis, S. J. Peale, A. W. Harris, in Pluto and
Charon, S. A. Stern, D. J. Tholen, Eds. (Univ. Arizona
Press, Tucson, AZ, 1997), pp. 193219.
10. P. Goldreich, S. Tremaine, N. Borderies, Astron. J. 92, 490
(1986).
11. F. Namouni, C. Porco, Nature 417, 45 (2002).
12. C. D. Murray, S. F. Dermott, Solar System Dynamics
(Cambridge Univ. Press, Cambridge, 2001).
13. S. Dermott, R. Malhotra, C. D. Murray, Icarus 76, 295
(1988).
14. D. Brouwer, Astron. J. 68, 152 (1963).
15. P. Goldreich, S. Tremaine, Astrophys. J. 243, 1062
(1981).
16. Methods are available as supporting material on Science
Online.
17. C. B. Olkin, L. H. Wasserman, O. G. Franz, Icarus 164,
254 (2003).
18. The libration period is 2p/w
lib
0 T
orb
(3e
C
m
f
m
(a)m
C
)
½
/(m
þ 1).
With e
C
0 0.2, the libration periods for the 4:1 and 6:1 are
È11 and È112 times the local orbit periods, T
orb
,
respectively.
19. Because the tidal expansion rate (Eq. 7) decreases
strongly as a
C 11
=
2
, the critical e
C
value decreases with
orbital radius as well, i.e., e
crit
º (R
P
/a
cr
)
5
=
m
, so that
the adiabatic constraint on e
C
eases as Charons orbit
expands.
20. P. Goldreich, S. Soter, Icarus 5, 375 (1966).
21. M. H. Lee, S. J. Peale, 2006. Preprint available at
http://arxiv.org/abs/astro-ph/0603214.
22. This work was supported by NASAs Planetary Geology and
Geophysics Program (W.R.W.) and NSFs Planetary Astrono-
my Program (R.M.C.). We thank S. A. Stern for communi-
cating discovery results of Plutos new moons just before
publication and the referees for thoughtful comments.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1127293/DC1
SOM Text
13 January 2006; accepted 23 June 2006
Published online 6 July 2006;
10.1126/science.1127293
Include this information when citing this paper.
Ice Record of d
13
C for
Atmospheric CH
4
Across the
Younger DryasPreboreal Transition
Hinrich Schaefer,
1,2
* Michael J. Whiticar,
1
Edward J. Brook,
2
Vasilii V. Petrenko,
3
Dominic F. Ferretti,
4,5
Jeffrey P. Severinghaus
3
We report atmospheric methane carbon isotope ratios (d
13
CH
4
) from the Western Greenland ice
margin spanning the Younger DryastoPreboreal (YD-PB) transition. Over the recorded È800 years,
d
13
CH
4
was around 46 per mil (°); that is, È1° higher than in the modern atmosphere and
È5.5° higher than would be expected from budgets without
13
C-rich anthropogenic emissions. This
requires higher natural
13
C-rich emissions or stronger sink fractionation than conventionally assumed.
Constant d
13
CH
4
during the rise in methane concentration at the YD-PB transition is consistent with
additional emissions from tropical wetlands, or aerobic plant CH
4
production, or with a multisource
scenario. A marine clathrate source is unlikely.
I
ce core records reveal prominent changes in
atmospheric methane concentration
ECH
4
^
associated with abrupt climate change (1)
but the causes, including source and sink changes,
remain controversial (1, 2). Modern contributions
from individual sources or sinks have been con-
strained by the
13
C/
12
C ratio of atmospheric
methane (d
13
CH
4
) (3, 4). New analytical tech-
niques extend this approach to air samples from
gas occlusions in polar ice. Using ice samples
from the Pakitsoq outcrop (Western Greenland)
(5), we measured d
13
CH
4
in air dating between
11,360 and 12,220 years before the present (yr
B.P.) (6). The record covers the transition be-
tween the Younger Dryas (YD) and Preboreal
Holocene (PB), when temperature (7) and
ECH
4
^
(1) increased rapidly at the termination of the
last ice age (Fig. 1).
The suitability of Pakitsoq ice for paleostudies
has been demonstrated by the agreement of
ECH
4
^
and other geochemical tracers with records from
the Greenland Ice Sheet Project 2 (GISP2) ice
core (5). Samples were collected during three
field campaigns (2001 to 2003) by means of oil-
free chainsaws and shipped frozen to the Univer-
sity of Victoria. The main data set was measured
after wet extraction by gas chromatography
isotope ratio mass spectrometry (GC-IRMS) (8).
ECH
4
^ measurements were duplicated at Wash-
ington State University. Six samples from three
time periods were analyzed for d
13
CH
4
at the
National Institute of Water and Atmospheric Re-
search (NIWA) using È100-liter air samples
extracted in the field (8) (Fig. 1). All samples
were dated with a gas age scale derived by com-
parison of geochemical records from Pakitsoq
and GISP2 (8). Results are consistent throughout
the three field seasons and form a composite data
set (Fig. 1).
1
School of Earth and Ocean Sciences, University of Victoria,
Post Office Box 3055, V8W 3P6, Canada.
2
Department of
Geosciences, Oregon State University, 104 Wilkinson Hall,
Corvallis, OR 97331, USA.
3
Scripps Institution of Oceano-
graphy, University of California, San Diego, Mail Code 0244,
La Jolla, CA 92093, USA.
4
Institute of Arctic and Alpine
Research, University of Colorado, Boulder, CO 80309, USA.
5
National Institute of Water and Atmospheric Research
Limited, Post Office Box 14901, Wellington, New Zealand.
*To whom correspondence should be addressed. E-mail:
schaefeh@geo.oregonstate.edu
REPORTS
www.sciencemag.org
SCIENCE
VOL 313
25 AUGUST 2006
1109 Our d
13
CH
4
data reveal several interesting
features, two of which we discuss here. First, the
YD-PB methane is
13
C-enriched by È1 per mil
(°) relative to modern atmospheric d
13
CH
4
(47.1°) (9) and È5.5° higher than expected
from previously proposed natural CH
4
budget
scenarios (table S1) (3, 10). Second, there is no
significant change in d
13
CH
4
across the YD-
PB transition. In the Pakitsoq record,
ECH
4
^
rises from 490 to 750 parts per billion by
volume (ppbv) at the transition, which is
consistent with GISP2 data (1, 5) (Fig. 1A).
During the YD, d
13
CH
4
has a mean of 46.0
T
0.5° (1s) (Fig. 1C). Slight variations fall
within the envelope of uncertainty. The PB
mean d
13
CH
4
is 45.7
T 1.2°. Surprisingly,
there is no significant difference in d
13
CH
4
between the two climatic intervals, nor is there
an isotope shift during the È250-ppbv
ECH
4
^
increase.
Pre-anthropogenic d
13
CH
4
was expected to
be depleted in
13
C (relative to modern atmo-
spheric CH
4
) because of the absence of fossil
fuel combustion, slash-and-burn agriculture, and
landfills, all of which emit
13
C-enriched CH
4
(3).
Such
13
C depletions are observed in ice from
100 to 300 yr B.P. (1113), whereas 400 to
2000 yr B.P. values from the late preindustrial
Holocene (LPIH) are unexpectedly
13
C-enriched,
similar to our YD-PB d
13
CH
4
(12).
An initial explanation for our high d
13
CH
4
was enrichment during postocclusion microbi-
al oxidation of CH
4
; that is, a storage artifact.
However, this is ruled out because the amount
of oxidation required for the observed isotopic
shift would be between 15 and 48% (8). This
would be readily detected as discrepancies
between the Pakitsoq and GISP2
ECH
4
^ re