A Neoproterozoic Snowball Earth

Abstract

Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with
estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions
of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial
volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have
resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to
the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate
rocks observed globally.

Full text

DOI: 10.1126/science.281.5381.1342
, 1342 (1998); 281Science
et al.Paul F. Hoffman,
A Neoproterozoic Snowball Earth
www.sciencemag.org (this information is current as of May 23, 2007 ):
The following resources related to this article are available online at
http://www.sciencemag.org/cgi/content/full/281/5381/1342
version of this article at:
including high-resolution figures, can be found in the onlineUpdated information and services,
http://www.sciencemag.org/cgi/content/full/281/5381/1342#otherarticles
, 19 of which can be accessed for free: cites 39 articlesThis article
468 article(s) on the ISI Web of Science. cited byThis article has been
http://www.sciencemag.org/cgi/content/full/281/5381/1342#otherarticles
96 articles hosted by HighWire Press; see: cited byThis article has been
http://www.sciencemag.org/cgi/collection/geochem_phys
Geochemistry, Geophysics
: subject collectionsThis article appears in the following
http://www.sciencemag.org/about/permissions.dtl
in whole or in part can be found at: this article
permission to reproduce of this article or about obtaining reprintsInformation about obtaining
registered trademark of AAAS.
c 1998 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a
CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005.
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the
on May 23, 2007 www.sciencemag.orgDownloaded from

References and Notes
1. J. F. Nye, R. Soc. London Proc. Ser. A 219, 477 (1953);
R. LeB Hooke, Rev. Geophys. Space Phys. 19, 664
(1981); C. J. van der Veen and I. M. Whillans, J.
Glaciol. 36, 324 (1990).
2. Examples include velocity variations occurring over
months to weeks [R. LeB Hooke, P. Calla, et al., J.
Glaciol. 35, 235 (1989)] and days to hours [A. Iken
and R. A. Bindschadler, ibid. 32, 101 (1986)].
3. S. M. Hodge, ibid. 13, 349 (1974); B. Kamb et al., J.
Geophys. Res. 99, 15231 (1994); J. Harbor et al.,
Geology 25, 739 (1997).
4. A dense array of radio-echo sounding measurements
were processed with three-dimensional migration tech-
niques. Comparisons of these measurements with bore-
hole observations suggest that the radar is accurate to
within about 8.5 m (B. C. Welch, W. T. Pfeffer, J. T.
Harper, N. F. Humphrey, J. Glaciol., in press).
5. J. T. Harper and N. F. Humphrey, Geology 23, 901
(1995).
6. M. F. Meier, U.S. Geol. Surv. Prof. Pap. 351 (1960);
W. S. B. Paterson and J. C. Savage, J. Geophys. Res. 68,
4537 (1963); C. F. Raymond, J. Glaciol. 10, 55 (1971);
R. LeB. Hooke, P. Holmlund, N. R Iverson, ibid. 33,72
(1987) were all forced to smooth inclinometry data
because of high levels of noise.
7. The instrument was constructed by Slope Indicator
Canada, Ltd. (Vancouver, BC). Measurement errors
associated with a prototype of this instrument are
discussed by E. W. Blake and G. K. C. Clarke [J.
Glaciol. 38, 113 (1992)]. However, analysis of ac-
tual data from the instrument used suggests that
instrument errors are slightly improved from man-
ufacturer specifications [ J. T. Harper, thesis, Uni-
versity of Wyoming (1997); S. V. Huzurbazar, un-
published material]. Additionally, the uniformity of
the borehole walls enabled a high degree of repeat-
ability for the measurements.
8. We follow the method of C. F Raymond, J. Glaciol.
10, 39 (1971).
9. We use a cubic spline function with an iterative
scheme designed to minimize the curvature of the
function between data points [I. C. Briggs, Geophysics
1974, 39 (1974)]. This interpolation was tested ex-
tensively with synthetic data.
10. J. T. Harper, N. F. Humphrey, W. T. Pfeffer, B. C.
Welch, U.S. Army Cold Reg. Res. Eng. Lab. Spec. Rep.
96-27 (1996), p. 41.
11. This measurement was made within the same
reach and time of year as the deformation exper-
iments, but during a subsequent year. Sliding and
surface velocities were determined by continuous
filming of the base of a borehole with concurrent
surveying of velocity at the surface.
12. Funded by grants from NSF (OPP-9122966 to N.F.H.
and OPP-9122916 to W.T.P.). Additional funding for
computer visualization was provided by NSF’s
EPSCoR (Experimental Program to Stimulate Com-
petitive Research) program (EPS9550477), through
the University of Wyoming’s Spatial Data and Visu-
alization Center project. D. Bahr, B. Welch, and B.
Raup all made significant contributions to portions of
the work presented here.
16 April 1998; accepted 28 July 1998
A Neoproterozoic Snowball
Earth
Paul F. Hoffman,* Alan J. Kaufman, Galen P. Halverson,
Daniel P. Schrag
Negative carbon isotope anomalies in carbonate rocks bracketing Neoprotero-
zoic glacial deposits in Namibia, combined with estimates of thermal subsi-
dence history, suggest that biological productivity in the surface ocean col-
lapsed for millions of years. This collapse can be explained by a global glaciation
(that is, a snowball Earth), which ended abruptly when subaerial volcanic
outgassing raised atmospheric carbon dioxide to about 350 times the modern
level. The rapid termination would have resulted in a warming of the snowball
Earth to extreme greenhouse conditions. The transfer of atmospheric carbon
dioxide to the ocean would result in the rapid precipitation of calcium carbonate
in warm surface waters, producing the cap carbonate rocks observed globally.
During the 200 million years (My) preceding
the appearance of macroscopic metazoans,
;750 to 550 million years ago (Ma) (1), the
fragmentation of a long-lived supercontinent
(2) was accompanied by intermittent, but wide-
spread, glaciation (3–5). Many of the glacial
deposits contain carbonate debris or are directly
overlain by carbonate rocks (6, 7), including
inorganic sea-floor precipitates, which are nor-
mally limited to warm-water settings (8). Post-
glacial carbonate rocks (cap carbonates) occur
even in terrigenous-dominated sections (6, 7).
Certain glacial units contain large sedimentary
iron formations (9), which reappear after a
1-billion-year hiatus in the stratigraphic record.
The glacial intervals are spanned by decreases
of as much as 14 per mil in the d
13
C value of
the surface ocean (10, 11). These isotopic ex-
cursions are enormous in comparison with any
excursions in the preceding 1.2 billion years
(12) or in the Phanerozoic eon (13).
Paleomagnetic evidence suggests that the
ice line reached sea level close to the equator
during at least two Neoproterozoic glacial epi-
sodes (14). The origin of these extreme glacia-
tions has been controversial (1, 15, 16). Kirsch-
vink (17) proposed a snowball Earth, created
by a runaway albedo feedback, in which the
world ocean was virtually covered by sea ice
but continental ice cover was thin and patchy
because of the virtual elimination of the hydro-
logic cycle. Kirschvink applied this hypothesis
to explain the low-paleolatitude glacial deposits
as well as the occurrence of banded iron for-
mations, suggesting that an ocean sealed by sea
ice would quickly become anoxic and rich in
dissolved ferrous iron (17). Here, we present
new data on the amplitude, timing, and duration
of inorganic d
13
C variations in Neoproterozoic
rocks of northern Namibia and the relation
between these variations and glaciation.
We show that the snowball Earth hypothe-
sis best explains the geological and geo-
chemical observations, including the d
13
C
excursions and the existence of carbonates
immediately following glaciations.
We studied the Otavi Group (Fig. 1), a
carbonate platform covering the southern prom-
ontory of the Congo Craton in northern Nami-
bia (15, 18, 19). In the late Neoproterozoic, the
Congo Craton was a Bahama-type sea-level
platform that was about the size of the conter-
minous United States. Paleomagnetic data from
the eastern part of the craton (20) imply that the
Otavi Group was at ;12°S paleolatitude at
743 6 30 Ma and at ;39°S at 547 6 4 Ma. The
Otavi Group contains two discrete glacial units
(Chuos and Ghaub formations) of Sturtian
(;760 to 700 Ma) age (15, 19). Both units are
underlain by thick carbonate successions with
high d
13
C values, and both units are overlain by
distinctive cap carbonates, recording negative
d
13
C excursions (10, 11).
The younger of the two glacial units (the
Ghaub Formation) is represented by unstratified
diamictons, debris flows, and, at the top, varve-
like detrital couplets crowded with ice-rafted
dropstones (15). Both the onset and the termi-
nation of glaciogenic sedimentation were
abrupt. The glacial deposits are composed pre-
dominantly of dolomite and limestone debris
derived from the underlying Ombaatjie plat-
form (Fig. 1). Clast and matrix lithologic com-
positions covary; thus, we interpreted the ma-
trix as being detrital in origin and not as a
seawater proxy. Glacial deposits on the plat-
form are thin and highly discontinuous (not due
to subsequent erosion). Alternately grounded
and floating sea ice caused large horizontal
plates to be detached from the directly under-
lying bedrock. The subglacial erosion surface
has remarkably little relief on the platform
(;50 m relative to underlying strata over a
distance of 150 km), suggesting that any fall in
relative sea level was limited or short-lived.
Comparatively thick sections (,180 m) of dia-
mictons and debris flows occur on the conti-
nental slope, suggesting that the ice grounding
line remained close to the platform edge (Fig.
1). These observations are consistent with an
abrupt development and a subsequent dissipa-
tion of grounded sea ice on a tropical or sub-
P. F. Hoffman, G. P. Halverson, D. P. Schrag, Depart-
ment of Earth and Planetary Sciences, Harvard Uni-
versity, Cambridge, MA 02138, USA. A. J. Kaufman,
Department of Geology, University of Maryland, Col-
lege Park, MD 20742, USA.
*To whom correspondence should be addressed. E-
mail: hoffman@eps.harvard.edu
R EPORTS
28 AUGUST 1998 VOL 281 SCIENCE www.sciencemag.org1342
on May 23, 2007 www.sciencemag.orgDownloaded from

tropical platform, consistent with a snowball
glaciation.
We measured inorganic d
13
C values of car-
bonate rocks that spanned the glacial interval
from several sections (Fig. 2) (10, 11). In gen-
eral, d
13
C values are rather insensitive to di-
agenesis because aqueous fluids contain little
carbon in comparison with carbonate rocks
(21). This inference is supported by the overall
agreement of the pattern of isotopic variations
from multiple sections. The d
13
C data on the
platform (summarized in a composite section in
Fig. 3) show that (i) preglacial values are 5 to 9
per mil through .200 m of section just below
the subglacial surface; (ii) values fall abruptly
to as low as 25 per mil in the final regressive
platformal parasequences and slope apron di-
rectly beneath the subglacial uncomformity;
and (iii) immediate postglacial values are about
23 per mil (;2 per mil higher than minimum
preglacial values), decline through ;40mof
section to a nadir of 26 per mil, and then rise to
0 per mil at about 480 m above the base of the
cap carbonate. Lesser subglacial d
13
C down-
turns are known elsewhere on the Otavi plat-
form (22) and on other continents (10, 11). The
overall negative d
13
C excursion occupies ;500
m of the platformal carbonate section. Much of
the lateral variance in d
13
C curves between
sections (Fig. 2) can be accounted for in terms
of subglacial erosional truncation and slope
progradation.
Constraints on the duration of the isotopic
excursion from a model of thermally driven
subsidence of the platform (15) allow a maxi-
mum subsidence rate of 14 m/My (equivalent to
a maximum carbonate accumulation rate, with
sediment loading, of ;50 m/My). The d
13
C
excursion begins and ends in sediments depos-
ited near nonglacial sea level and occupies a
total thickness on the platform of ;500m(;50
m of which can be accounted for isostatically as
a consequence of subglacial erosion). The re-
mainder of the thickness (;450 m) required
time-dependent thermal subsidence for its ac-
commodation. Thus, the minimum time re-
quired to accommodate the d
13
C excursion (be-
low 0 per mil) was 9 My [450 m/(50 m/My)].
Stratigraphic mapping shows that no tectonic
activity occurred at the time of the d
13
C excur-
sion that would affect the subsidence calcula-
tion. We cannot estimate the time span of the
deposition of the cap carbonate because we do
not know the water depth or the potential gla-
cioeustatic and ice-loading effects at the onset
of deposition.
If we interpret the d
13
C excursion in terms
of carbon burial fluxes, then the proportion of
organic carbon to total carbon burial changed
from almost 0.5 before the glacial deposits to
virtually zero immediately after. Carbonates,
precipitated from an ocean in which most bio-
logical productivity had ceased for a time peri-
od greatly exceeding the carbon residence time
(.10
5
years), would approach a value of 25to
27 per mil, which is the isotopic composition
of carbon entering the ocean (23, 24). The
isotopic pattern, therefore, is consistent with the
hypothesis of a snowball Earth, in which oce-
anic photosynthesis would be severely reduced
for millions of years because the ice cover
would block out sunlight. Meltwater pools and
bare ground, exposed through gravitational
thinning and ablation of ice sheets without
much rejuvenative snowfall, might provide
refugia for a variety of bacteria and simple
eukaryotes.
Caldeira and Kasting (25) estimated that, at
19º S
14º E
14º E
15º E
20º S
DATUM
RIDGE
DATUM
758.5
100
200
300 m
3.5 Ma
Mulden clastics
Elandshoek slope
Elandshoek platform
Maieberg cap carbonate
Ghaub glacial deposits
Ombaatjie platform
Gruis ramp
Rasthof cap carbonate
Chuos glacial deposits
Nosib clastics
Basement
Onlap
Truncation
PLATFORM
HUAB
TSUMEB SUBGROUPSUBGROUPABENAB
SUBGROUPOMBOMBO
MULDEN GROUP
Te(p)
Tm
Ab
Ag
Ar
O5
O3
Ac
Tg
0 10 20 30km
N
M
Te(s)
Te(p)
Tm
Tg
Ab
Ag
Ar
Ac
O5
O4
O3
O2
O1
N
B
O4
O2
O1
B
B
Ar
Ab
B
Ag
Ab
M
Te(s)
Te(s)
M
M
Tm
Te(p)
B
O4
Ar
Tg
Ac
ABCDG H
EF
Ar
Tg
Ag
Ombombo
ramp
STUDY
AREA
CONGO
CRATON
Rockeys
Fault
TERTIARY
CRETACEOUS
MULDEN GROUP
OTAVI GROUP
NOSIB GROUP
BASEMENT
measured sections
0 30 km
HUAB RIDGE
OTAVI GROUP
Ombombo Abenab Tsumeb
4
2
3
1
SLOPE
3
7
5
6
8
9
H
G
B
A
F
E
D
C
Fig. 1. Stratigraphic
cross sections of the
Otavi carbonate slope
(A–B and C–D) and
platform (E–F and
G–H) in northwest
Namibia, showing the
measured sections in
Fig. 2 (indicated by
circled numbers).
R EPORTS
www.sciencemag.org SCIENCE VOL 281 28 AUGUST 1998 1343
on May 23, 2007 www.sciencemag.orgDownloaded from

present, it would take ;0.12 bar of atmospheric
CO
2
from volcanic input to overcome a snow-
ball albedo and cause meltback. This estimate
implies that a snowball glaciation would last
;4 My at the modern rates of CO
2
release from
subaerial volcanism [;5.4 3 10
12
mol/year
(26)] with no air-sea gas exchange. Partial gas
exchange through cracks in sea ice would in-
crease this estimate. In the Neoproterozoic, the
duration of a snowball glaciation would be
longer because of lower solar luminosity as
well as reduced pelagic deposition of carbonate,
-20
OMBAATJIE FORMATION
MAIEBERG CAP CARBONATE
ELANDSHOEK FORMATION
SLOPE
PLATFORM
S
PLATFORM
N
200km
-5 0 5
SLOPE PLATFORM
-5 0 5
-5 0 5
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5
-5 0 5
-5 0 5
-5 0 5
-5 0 5
-5 0 5
-5 0 5
-5 0 5
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5 10
-5 0 5 10
-40
-60
20
40
60
80
100
120
140
160
180
200
220
240
260
280
GHAUB GLACIATION
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ
13
C(‰)
δ C(‰)
13
δ
13
C(‰)
glacial deposits
microbialaminite
grainstone
stromatolite
ribbon rock
rhythmite/
rhythmite breccia
siliciclastic
sequence
boundary
flooding
surface
LITHOFACIES
water
depth
19º S
14º E
14º E
15º E
20º S
9
8
7
6
5
4
3
2
3
1
4
7
8
9
5
6
Rockeys
measured sections
0 30 km
Cretaceous
Mulden Group
Otavi Group:
platform
/slope
Nosib Group
Basement
Fault
3
4
5
6
7
9
2
1
Fig. 2. Measured sec-
tions of carbonates
bracketing the Ghaub
glaciation in a south-
north profile across
the Otavi platform
and slope. The extend-
ed negative d
13
C ex-
cursion is centered on
the Maieberg cap car-
bonate on the plat-
form, and its con-
densed stratigraphic
equivalent is on the
continental slope. The
d
13
C values decline
from positive pregla-
cial values that are
stratigraphically be-
neath Ghaub glacial
deposits or the bare
glaciated surface. The
crossover from posi-
tive to negative val-
ues occurs in a 25-m-
thick parasequence,
which can be corre-
lated regionally and
which is variably
truncated by the sub-
glacial surface.
R EPORTS
28 AUGUST 1998 VOL 281 SCIENCE www.sciencemag.org1344
on May 23, 2007 www.sciencemag.orgDownloaded from

which would lower the release rates of volcanic
CO
2
at convergent margins (27). A minimum
value of 4 My and a maximum estimate of 30
My (25) are broadly consistent with the 9 My
duration of the isotopic excursion in the Otavi
Group.
During a snowball glaciation, the Ca/Mg
ratio of seawater would have increased because
of hydrothermal activity at mid-ocean ridges
and low-temperature alteration of basalt (28).
Without the input of alkalinity from rivers,
carbonate would dissolve in the deep sea, driv-
en by the input of CO
2
from mid-ocean ridge
volcanism, although this flux of CO
2
[;0.83 3
10
12
mol/year (24)] in the modern ocean is
smaller than the CO
2
input to the atmosphere
from subaerial volcanism. Air-sea gas exchange
through cracks in the sea ice would intensify the
carbonate dissolution. Hydrothermal activity
without continental weathering would also de-
crease the Sr isotopic composition of seawater,
although this effect might be small considering
the buffering effect of carbonate dissolution
during the glaciation as well as the lower Sr/Ca
partitioning in inorganically precipitated car-
bonates (29) and, therefore, the higher Sr con-
centrations and the longer Sr residence time in
Proterozoic oceans.
Once atmospheric CO
2
reached the criti-
cal concentration [;120,000 parts per mil-
lion (ppm) (25)], a transformation from ice-
house to greenhouse conditions would occur
quickly, as the albedo and water vapor feed-
backs would enhance the warming with the
opening of low-latitude oceans. This abrupt
climate change would make Pleistocene gla-
cial terminations seem slow in comparison.
On meltback, gas exchange between the sur-
face ocean and the high-CO
2
atmosphere
would first drive carbonate dissolution and
then drive precipitation as cold deep waters
with high concentrations of calcium and dis-
solved inorganic carbon mixed with warm
tropical surface waters. Additional sources of
alkalinity would come from intense continen-
tal weathering that was driven by warm tem-
peratures, high levels of CO
2
, and a strong
hydrologic cycle. Reducing atmospheric CO
2
pressure from 0.12 to 0.001 bar [that is, from
terminal snowball conditions to normal Neo-
proterozoic values (25)] would provide
;2.5 3 10
20
g of carbon, sufficient to pro-
duce ;8 3 10
5
km
3
of carbonate, which is
enough to cover the entire present-day con-
tinental crust with a layer ;5 m thick. The
space that was created by thermal subsi-
dence during a prolonged glacial period
could be rapidly filled by the cap carbonate
sequence, which is consistent with textural
evidence in the Maieberg and other Neo-
proterozoic cap carbonates suggesting rap-
id deposition (6, 7, 30). Precipitation would
be strongly localized on warm shallow-
water platforms, where CaCO
3
solubility is
minimized, which is in agreement with the
regional variation in cap carbonate thick-
ness (6, 7) and the observed increase in
thickness of the negative d
13
C excursion
from the slope to the platform (Fig. 2). If
the observed millimeter-scale laminations
in cap carbonates are diurnal (the dominant
cycle in the tropics), accumulation rates
were ;40 cm/year (31).
The d
13
C values in the Otavi Group are
consistent with the snowball hypothesis. The
initial decrease in d
13
C values before the glaci-
ation on the tropical platform implies a decrease
in productivity relative to carbonate deposition,
perhaps because of colder conditions (220 to 0
m; Fig. 3). During the glaciation, if there was no
air-sea gas exchange, both the ocean and the
atmosphere would have similar d
13
C values,
equivalent to the hydrothermal or the volcanic
input (25to27 per mil). At the termination,
isotopic fractionation associated with the hydra-
tion of CO
2
would raise the d
13
C of dissolved
inorganic carbon in the surface ocean, which is
dominated by the large atmospheric reservoir.
As the amount of CO
2
in the atmosphere sub-
sided, the continued uptake of carbon with
higher d
13
C values would drive atmospheric
d
13
C down through Rayleigh distillation, while
the ocean would read 25 per mil because of
mass balance and mixing with the deep ocean.
Thus, the d
13
C values of the cap carbonate start
out somewhat higher than 25 per mil but
quickly decrease to the low values of the glacial
atmosphere (0 to 40 m; Fig. 3). The reestablish-
ment of the biological pump drove values back
up toward preglacial levels over a stratigraphic
thickness determined by sedimentation rate,
which is much higher on the platform than on
the continental slope (Fig. 2).
A review of the alternative hypotheses
that attempt to explain various aspects of
Neoproterozoic isotopic excursions and
glacial events reveals contradictions be-
tween each of the hypotheses and our data
from Namibia (15). A popular model as-
serts that the isotopic anomalies were driv-
en by alternating periods of ocean stagna-
tion and overturn, corresponding to positive
and negative surface-water d
13
C values, re-
spectively (10, 11, 32). The model predicts
that the duration of the negative excursion
should be limited by the residence time of
carbon in the ocean [,10
5
years) (6, 7,
23)], which is inconsistent with our esti-
mate of the duration of the excursion in the
Otavi Group.
To simulate a snowball Earth, coupled
energy-balance models require that atmo-
spheric CO
2
levels be lowered dramatically
(;10
24
bar), even with lower-than-present
solar luminosity (33). Fragmentation of the
Rodinia supercontinent may have contributed
to the CO
2
drawdown (1, 2) by creating many
new continental margins, which are major
repositories for organic carbon in the modern
ocean (34), consistent with the high d
13
C
values observed before the glaciation. This is
also consistent with the observation that Stur-
tian (;760 to 700 Ma) and Varangian (;620
to 550 Ma) glaciations accompanied the
opening of the Pacific and Iapetus oceans,
respectively (5), and might explain why the
only known older examples of similar carbon
isotope excursions and low-latitude glacia-
tions (35) accompanied the fragmentation of
a late Archean megacontinent. We speculate
that higher solar luminosity, less efficient
burial of organic carbon due to bioturba-
tion, and limits on primary productivity due
to lower levels of nutrient iron and phos-
phorus (36) in the more oxic Phanerozoic
ocean (37 ) prevented Phanerozoic snow-
ball Earth conditions.
Postglacial cap carbonates are predictable
consequences of the recovery from a snowball
Earth. Accordingly, the succession of late Neo-
proterozoic glaciations characterized by cap
carbonates and large d
13
C excursions (10, 11)
should represent multiple episodes of runaway
ice albedo. These episodes (cryochrons) should
be useful for global correlation (3–5). A snow-
ball Earth followed by extreme greenhouse
conditions represents a strong source of selec-
tive pressure on the evolution of life in the
Neoproterozoic. Although the absence of skel-
etal organisms makes any extinction difficult to
evaluate, there is some evidence for a substan-
tial turnover among acritarchs (38). Many pro-
karyotic organisms, which dominated the Neo-
proterozoic biosphere, are able to survive ex-
-5 0 5
-10
-20
-30
-40
-50
10
-5 0 5
10
20
40
10
30
m
δ
13
C (‰)
0
0
OMBAATJIE FM MAIEBERG CAP
GHAUB GLACIATION
6
7
Fig. 3. Composite section across the Ghaub
glacial surface on the platform, showing high-
resolution d
13
C data. An abrupt downturn in
d
13
C occurs at the base of the penultimate
preglacial parasequence, and there is a postgla-
cial descent to a nadir of 26 per mil, ;40 m
above the glacial surface and ;20 m above the
maximum flooding interval. In the snowball
Earth model, the glacial surface would repre-
sent ;10 My, but the 400-m-thick cap carbon-
ate would only represent thousands of years.
R EPORTS
www.sciencemag.org SCIENCE VOL 281 28 AUGUST 1998 1345
on May 23, 2007 www.sciencemag.orgDownloaded from

treme and prolonged environmental stress (39)
and were likely unaffected. Many eukaryotic
phyla (including red, green, and chromophytic
algae) evolved before the late Neoproterozoic
glaciations and also must have survived the
environmental stress (40). However, a succes-
sion of snowball glaciations must have imposed
an intense environmental filter, resulting in a
series of genetic “bottleneck and flush” cycles
(41), possibly leading to an initial metazoan
radiation before the terminal glaciation (42) and
an Ediacaran radiation in its aftermath (11).
References and Notes
1. A. H. Knoll, Sci. Am. 265, 64 (October 1991); iiii
and M. R. Walter, Nature 356, 673 (1992); A. H. Knoll,
in Origin and Evolution of the Metazoa, J. H. Lipps and
P. W. Signor, Eds. (Plenum, New York, 1992), pp. 53–84.
2. G. C. Bond, P. A. Nickeson, M. A. Kominz, Earth
Planet. Sci. Lett. 70, 325 (1984); P. F. Hoffman,
Science 252, 1409 (1991); I. W. D. Dalziel, Annu.
Rev. Earth Planet. Sci. 20, 501 (1992); C. M. Powell,
Z. X. Li, M. W. McElhinny, J. G. Meert, G. K. Park, Geology
21, 889 (1993); A. B. Weil, R. Van der Voo, C. Mac
Niocall, J. G. Meert, Earth Planet. Sci. Lett. 154,13
(1998).
3. W. B. Harland, Geol. Rundsch. 54, 45 (1964).
4. M. J. Hambrey and W. B. Harland, Earth’s Pre-Pleis-
tocene Glacial Record (Cambridge Univ. Press, Cam-
bridge, 1981).
5. G. M. Young, Geology 23, 153 (1995).
6. J. D. Roberts, J. Geol. 84, 47 (1974); J. D. Aitken, Bull.
Geol. Surv. Can. 404, 1 (1991); I. J. Fairchild, in
Sedimentology Review 1, V. P. Wright, Ed. (Blackwell,
Oxford, 1993), pp. 1–16.
7. M. J. Kennedy, J. Sediment. Res. 66, 1050 (1996).
8. Although Phanerozoic cool-water skeletal carbonates
are not uncommon [N. P. James and J. A. D. Clarke,
Cool-Water Carbonates (Society for Sedimentary Ge-
ology, Tulsa, OK, 1997)], the inverse solubility of
CaCO
3
with temperature mitigates against such an
origin for inorganic carbonates with abundant arago-
nitic sea-floor cements, which are typical of Neopro-
terozoic postglacial cap carbonates [T. M. Peryt et al.,
Sedimentology 37, 279 (1990); (7)].
9. H. Martin, The Precambrian Geology of South West
Africa and Namaqualand (Precambrian Research Unit,
University of Cape Town, Cape Town, South Africa,
1965); J. V. N. Dorr Jr., Econ. Geol. 68, 1005 (1973);
G. M. Young, Precambrian Res. 3, 137 (1976).
10. A. H. Knoll, J. M. Hayes, A. J. Kaufman, K. Swett, I. B.
Lambert, Nature 321, 832 (1986); A. J. Kaufman, J. M.
Hayes, A. H. Knoll, G. J. B. Germs, Precambrian Res.
49, 301 (1991); A. J. Kaufman and A. H. Knoll, ibid.
73, 27 (1995). The carbon isotopic compositions of
carbonates were determined according to procedures
described in these references and in (11) and were
reported as d
13
C values relative to Vienna Pee
Dee belemnite in units per mil defined as [(R
sample
/
R
standard
) 2 1] 3 10
3
, where R 5
13
C/
12
C.
11. A. J. Kaufman, A. H. Knoll, G. M. Narbonne, Proc. Natl.
Acad. Sci. U.S.A. 94, 6600 (1997).
12. R. Buick, D. J. Des Marais, A. H. Knoll, Chem. Geol.
123, 153 (1995); L. C. Kah and J. K. Bartley, Geol. Soc.
Am. Abstr. Programs 29, 115 (1997); M. D. Brasier
and J. F. Lindsay, Geology 26, 555 (1998).
13. W. T. Holser, in Patterns of Change in Earth Evolution,
H. D. Holland and A. F. Trendall, Eds. (Springer-Verlag,
Berlin, 1984), pp. 123–143.
14. P. W. Schmidt and G. E. Williams, Earth Planet. Sci.
Lett. 134, 107 (1995); L. E. Sohl, Geol. Soc. Am. Abstr.
Programs 29, 195 (1997); J. K. Park, Can. J. Earth Sci.
34, 34 (1997).
15. P. F. Hoffman, A. J. Kaufman, G. P. Halverson, GSA
Today 8(5), 1 (1998).
16. M. R. Rampino, J. Geol. 102, 439 (1994); G. E. Wil-
liams, Geol. Mag. 112, 441 (1975); Earth Sci. Rev. 34,
1 (1993).
17. J. L. Kirschvink [in The Proterozoic Biosphere,J.W.
Schopf and C. Klein, Eds. (Cambridge Univ. Press, New
York, 1992), pp. 51–52] extended an idea originally
proposed by W. B. Harland (3).
18. R. M. Miller, in African Basins,vol.3ofSedimentary
Basins of the World, R. C. Selley, Ed. (Elsevier, Am-
sterdam, 1997), pp. 237–268.
19. K. H. Hoffmann and A. R. Prave, Communs. Geol.
Surv. Namibia 11, 47 (1996).
20. J. G. Meert, R. Van der Voo, S. Ayub, Precambrian Res.
74, 225 (1995); J. G. Meert and R. Van der Voo, J.
Geol. 104, 131 (1996).
21. Discussions of diagenesis in Neoproterozoic carbonates
and descriptions of methods for screening samples be-
fore isotopic analysis are contained in (10) and (11).
22. M. J. Kennedy, A. R. Prave, K. H. Hoffmann, Geol. Soc.
Am. Abstr. Programs 29, 196 (1997).
23. L. R. Kump, Geology 19, 299 (1991).
24. D. J. Des Marais and J. G. Moore, Earth Planet. Sci.
Lett. 69, 43 (1984).
25. K. Caldeira and J. F. Kasting, Nature 359, 226 (1992).
The time required to achieve meltback would be less
if the ice surface became darkened by volcanic ash.
However, the effect of ash accumulation would be
countered by hoarfrost formation that originated
from volcanic outgassed H
2
O. Caldeira and Kasting
reported the possibility of irreversible icehouse con-
ditions due to the formation of CO
2
clouds, as did
Kirschvink (17), but it now appears that cooling ac-
tually may be limited by CO
2
clouds [F. Forget and
R. T. Pierrehumbert, Science 278, 1273 (1997)].
26. S. N. Williams, S. J. Schaefer, V. Calvache, M. D. Lopez,
Geochim. Cosmochim. Acta 36, 1765 (1992).
27. K. Caldeira, Geology 19, 204 (1991).
28. H. Elderfield and A. Schultz, Annu. Rev. Earth Planet.
Sci. 24, 191 (1996); R. E. McDuff and J. M. Gieskes,
Earth Planet. Sci. Lett. 33, 1 (1976).
29. J. W. Morse and M. L. Bender, Chem. Geol. 82, 265
(1990).
30. Sea-floor crystal fans, pseudomorphic after aragonite
(not ikaite) and reeflike in overall construction, occur
locally in the Maieberg cap carbonate (G. Soffer,
personal communication).
31. Analysis of tidal rhythmites indicates ;400 solar days
in the late Neoproterozoic year [G. E. Williams, J. Phys.
Earth 38, 475 (1990); C. P. Sonett, E. P. Kvale, A. Zak-
harian, M. A. Chan, T. M. Demko, Science 273, 100
(1996)].
32. J. P. Grotzinger and A. H. Knoll, Palaios 10, 578
(1995); A. H. Knoll, R. K. Bambach, D. E. Canfield, J. P.
Grotzinger, Science 273, 452 (1996).
33.
H. G. Marshall, J. C. G. Walker, W. R. Kuhn, J. Geophys. Res.
93, 791 (1988); W. W. Hay, E. J. Barron, S. L. Thompson, J.
Geol. Soc. London 147, 749 (1990); T. J. Crowley and S. K.
Baum, J. Geophys. Res. 98, 16723 (1993).
34. J. I. Hedges and R. G. Keil, Mar. Chem. 49, 81 (1995).
35. V. A. Melezhik and A. E. Fallick, Mineral. Mag. 58A,
593 (1994); D. A. Evans, N. J. Beukes, J. L. Kirschvink,
Nature 386, 262 (1997).
36. P. Van Cappellen and E. D. Ingall, Paleoceanography
9, 677 (1994).
37. D. E. Canfield and A. Teske, Nature 382, 127 (1996);
S. J. Carpenter and K. C. Lohmann, Geochim. Cosmo-
chim. Acta 61, 4831 (1997).
38. G. Vidal and M. Moczydlowska-Vidal, Paleobiology
23, 230 (1997); A. H. Knoll, Proc. Natl. Acad. Sci.
U.S.A. 91, 6743 (1994).
39. W. J. Green and E. I. Friedmann, Eds., Physical and
Biogeochemical Processes in Antarctic Lakes, vol. 59
of Antarctic Research Series (American Geophysical
Union, Washington, DC, 1993); K. H. Nealson, J.
Geophys. Res. 102, 23675 (1997).
40. A. H. Knoll, Science 256, 622 (1992).
41. H. L. Carson, Annu. Rev. Genet. 21, 405 (1987); K. J.
Niklas, The Evolutionary Biology of Plants (Univ. of
Chicago Press, Chicago, 1997).
42. H. J. Hofmann, G. M. Narbonne, J. D. Aitken, Geology
18, 1199 (1990).
43. We thank S. Bowring, J. Edmond, B. Farrell, M. Delaney,
J. Grotzinger, A. Knoll, J. Marshall, M. McElroy, and P.
Myrow for discussions. B. Holtzman, G. Hu, A. Maloof, A.
Prave, G. Soffer, and D. Sumner contributed to field-
work. The manuscript benefited from comments by K.
Caldeira, L. Derry, D. Erwin, L. Kump, and an anonymous
reviewer. This work was supported by NSF grants EAR
95-06769, EAR 95-10339, EAR 96-30928, EAR 96-
14070, and OCE 97-33688; the National Sciences and
Engineering Research Council of Canada; the Canadian
Institute of Advanced Research; Harvard University; the
University of Maryland; and the Geological Survey of
Namibia.
21 April 1998; accepted 21 July 1998
Photofragment Helicity Caused
by Matter-Wave Interference
from Multiple Dissociative
States
T. Peter Rakitzis, S. Alex Kandel, Andrew J. Alexander,
Zee Hwan Kim, Richard N. Zare*
Isolated diatomic molecules of iodine monochloride (ICl) were photodissociated
by a beam of linearly polarized light, and the resulting ground-state Cl atom
photofragments were detected by a method that is sensitive to the handedness
(helicity) of the electronic angular momentum. It was found that this helicity
oscillates between “topspin” and “backspin” as a function of the wavelength of
the dissociating light. The helicity originates solely from the (de Broglie) matter-
wave interference of multiple dissociating pathways of the electronic excited
states of ICl. These measurements can be related to the identity and to the
detailed shapes of the dissociating pathways, thus demonstrating that it is
possible to probe repulsive states by spectroscopic means.
The photodissociation of a diatomic molecule
occurs, in the simplest case, as the breakup of
an excited molecule on a single potential
energy surface (1). The molecule then disso-
ciates under the influence of a force directed
along the bond axis. The photodissociation
R EPORTS
28 AUGUST 1998 VOL 281 SCIENCE www.sciencemag.org1346
on May 23, 2007 www.sciencemag.orgDownloaded from