6. K. L. Smith Jr., R. S. Kaufmann, R. J. Baldwin, Limnol.
Oceanogr. 39, 1101 (1994).
7. R. J. Baldwin, R. C. Glatts, K. L. Smith Jr., Deep-Sea
Res. Part II Top. Stud. Oceanogr. 45, 643 (1998).
8. K. L. Smith Jr., Limnol. Oceanogr. 32, 201 (1987).
9. K.L.SmithJr.,R.S.Kaufmann,Science 284, 1174
(1999).
10. K. L. Smith Jr., R. J. Baldwin, D. M. Karl, A. Boetius,
Deep-Sea Res. Part I Oceanogr. Res. Pap. 49, 971
(2002).
11.D.Roemmich,J.A.McGowan,Science 267, 1324
(1995).
12. D. Roemmich, J. A. McGowan, Science 268, 352 (1995).
13. J. A. McGowan, D. R. Cayan, L. M. Dorman, Science
281, 210 (1998).
14. C. E. Reimers, R. A. Jahnke, D. C. McCorkle, Global
Biogeochem. Cycles 6, 199 (1992).
15. E. M. Druffel, B. H. Robison, Science 284, 1139 (1999).
16. M. W. Silver, S. L. Coale, C. H. Pilskaln, D. R. Steinberg,
Limnol. Oceanogr. 43, 498 (1998).
17. W. M. Hamner, B. H. Robison, Deep-Sea Res. 39,
1299 (1992).
18. E. G. Barham, Science 205, 1129 (1979).
19. P. J. Davoll, M. J. Youngbluth, Deep-Sea Res. 37, 285
(1990).
20.J.C.Hunt,D.J.Lindsay,Plankt. Biol. Ecol. 46,75
(1999).
21. C. P. Galt, Fish. Bull. 77, 514 (1979).
22. S. E. Beaulieu, K. L. Smith Jr., Deep-Sea Res. Part II
Top. Stud. Oceanogr. 45, 781 (1998).
23. Small larvacean species are often very abundant in
near-surface waters. Most have bodies less than 10 mm
long, with house diameters commonly twice as large.
Their houses may be produced at a rate of six or more
each day, depending on the density of food particles.
Discarded small houses are important components of
organic aggregate flux in the ocean’s upper layers, but
they rarely reach the deep sea floor (34–36).
24. Materials and methods are available as supporting
material on Science Online.
25. Sediment traps catch what they were designed to catch,
namely, small, slowly sinking particles. Although sedi-
ment traps may occasionally collect sinker fragments,
physical contact, particularly with traps that have
interior baffles, is certain to exclude, disrupt, or disperse
this material (16). The easily recognized rectangular
mesh structures of larvacean filters have not been
reported in analyses of sediment trap contents.
26. R. Fenaux, Q. Bone, D. Deibel, in The Biology of Pelagic
Tunicates, Q. Bone, Ed. (Oxford Univ. Press, New York,
1998), chap. 15.
27. Bathochordaeus sp. is found chiefly at depths from
100to300m;Mesochordaeus erythrocephalus
occurs principally between 300 and 500 m (17, 28).
28. R. R. Hopcroft, B. H. Robison, J. Plankton Res. 21,
1923 (1999).
29. J. H. Martin, G. A. Knauer, D. M. Karl, W. W. Broenkow,
Deep-Sea Res. 34, 267 (1987).
30. C. H. Pilskaln, J. B. Paduan, F. P. Chavez, R. Y. Anderson,
W. M. Berelson, J. Mar. Res. 54, 1149 (1996).
31. This value considerably exceeds the amount of flux es-
timated by Silver, Coale, Pilskaln, and Steinberg (16),
for Bathochordaeus inthesameregion.Althoughour
measurements of the abundance and turnover of
houses and sinkers agree, our measurements of the
carbon content of sinkers are substantially greater, prin-
cipally because of incomplete sampling in the earlier
study.
32. F. P. Chavez et al., Prog. Oceanogr. 54, 205 (2002).
33. A. B. Burd, G. A. Jackson, R. S. Lampitt, M. Follows,
Eos 83, 573 (2002).
34. A. L. Alldredge, Science 177, 885 (1972).
35. A. L. Alldredge, Limnol. Oceanogr. 21, 14 (1976).
36. D. Deibel, Mar. Biol. 93, 429 (1986).
37. We thank the pilots of the ROVs Ventana and
Tiburon, for their skills and patience in the difficult
task of collecting these specimens, and the officers
and crews of the research vessels Point Lobos and
Western Flyer. Supported by the David and Lucile
Packard Foundation.
Supporting Online Material
www.sciencemag.org/cgi/content/full/308/5728/1609/
DC1
Materials and Methods
SOM Text
Figs. S1 to S4
References and Notes
23 December 2004; accepted 15 April 2005
10.1126/science.1109104
Rapid Acidification of the Ocean
During the Paleocene-Eocene
Thermal Maximum
James C. Zachos,
1
*
Ursula Ro
¨
hl,
2
Stephen A. Schellenberg,
3
Appy Sluijs,
4
David A. Hodell,
6
Daniel C. Kelly,
7
Ellen Thomas,
8,9
Micah Nicolo,
10
Isabella Raffi,
11
Lucas J. Lourens,
5
Heather McCarren,
1
Dick Kroon
12
The Paleocene-Eocene thermal maximum (PETM) has been attributed to the
rapid release of È2000 10
9
metric tons of carbon in the form of methane. In
theory, oxidation and ocean absorption of this carbon should have lowered
deep-sea pH, thereby triggering a rapid (G10,000-year) shoaling of the calcite
compensation depth (CCD), followed by gradual recovery. Here we present
geochemical data from five new South Atlantic deep-sea sections that
constrain the timing and extent of massive sea-floor carbonate dissolution
coincident with the PETM. The sections, from between 2.7 and 4.8 kilometers
water depth, are marked by a prominent clay layer, the character of which
indicates that the CCD shoaled rapidly (G10,000 years) by more than 2
kilometers and recovered gradually (9100,000 years). These findings indicate
that a large mass of carbon (d2000 10
9
metric tons of carbon) dissolved in
the ocean at the Paleocene-Eocene boundary and that permanent seques-
tration of this carbon occurred through silicate weathering feedback.
During the Paleocene-Eocene thermal maxi-
mum (PETM), sea surface temperature (SST)
rose by 5-C in the tropics and as much as 9-C
at high latitudes (1–3), whereas bottom-water
temperatures increased by 4- to 5-C(4). The
initial SST rise was rapid, on the order of È10
3
years, although the full extent of warming was
not reached until some È30,000 years (30 ky)
Fig. 3. Comparative plot of active
houses of giant larvaceans (blue
line) and discarded sinkers (red
line) versus depth, in square meters
of area swept. The data are derived
from a 10-year time series of
quantitative video transects at
depth intervals between 100 and
1000 m (n 0 679 transects). With
an average sinking rate of 800 m
day
j1
, the difference between
the integrated areas beneath the
curves indicates that these ani-
mals produce a new house each
day (24).
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later (5). The most compelling evidence for
greenhouse forcing is a coeval global carbon
isotope excursion (CIE) of roughly –3.0 per
mil (°)indeep-seacores(4). The pattern of
the CIE—an initial rapid decrease (È20 ky)
followed by a more gradual recovery (130 to
190 ky) (1, 6–8)—indicates the input of a large
mass of isotopically depleted carbon into the
ocean and atmosphere. Quantitatively, meth-
ane hydrates, with a mean d
13
CofG–60°,
appear to be the most plausible source of this
carbon (9). For example, only È1200 10
9
metric tons of carbon (GtC) of biogenic
methane would be required to produce a CIE
of 2.5° (10, 11). Thermogenic methane has
been implicated as well (12), although the
mass required to produce the CIE would be
roughly double that of the biogenic methane.
Regardless of its source, the released
methane was rapidly oxidized to CO
2
.Subse-
quent oceanic dissolution of this CO
2
would
alter ocean carbon chemistry, principally by
lowering the pH and carbonate ion content
ECO
3
2–
^ of seawater. These changes would be
partially neutralized by a transient rise in the
level of the lysocline and calcite compen-
sation depth (CCD) (13), resulting in the
widespread dissolution of sea-floor carbonate.
Eventually, the CO
2
would be sequestered and
ocean carbonate chemistry would be restored,
primarily through chemical weathering of
silicate rocks (10). The extent and duration
of lysocline/CCD shoaling and subsequent
recovery would depend largely on the source,
mass, and rate of carbon input. For example,
modeling of a 1200-GtC input over 10 ky
produces a lysocline shoaling of 300 m (less
in the Pacific) with a recovery time of È40 ky
(10). Such changes in ECO
3
2–
^ should produce
distinct patterns in pelagic carbonate sedimen-
tation and lithology, characterized by an abrupt
transition from carbonate-rich sediment to clay,
followed by a gradual recovery to carbonate.
Moreover, the clay layer should increase in
thickness with increasing water depth.
Clay or low-carbonate layers coincident
with the PETM were previously identified in
several deep-sea cores and land-based marine
sections (14–16). However, these sections,
which are either geographically isolated or not
completely recovered, or both, are inadequate
for constraining CCD variations and for testing
the methane hypothesis. Ocean Drilling Program
(ODP) Leg 208 was designed to recover an
array of pelagic cores spanning the Paleocene-
Eocene (P-E) boundary over a broad depth
range. The primary drilling target was the
Walvis Ridge, in the southeastern Atlantic
(fig. S1), where the Deep Sea Drilling Project
(DSDP) Leg 74 rotary cored portions of the
P-E boundary sequence near the base and
summit of the ridge (sites 527 and 525) (17).
By using advanced piston coring in multiple
offset holes at five sites (1262, 1263, 1265,
1266, and 1267), Leg 208 successfully recov-
ered stratigraphically complete and undisturbed
upper Paleocene–to–lower Eocene successions
at four of five sites between 2.7 and 4.8 km
water depth (18). At each site, the P-E bound-
ary sequence was characterized by an abrupt
transition from carbonate-rich ooze to a dark
Fig. 1. Digital core
photos and weight %
CaCO
3
content plotted
versus meters of com-
posite depth (MCD)
across the P-E bound-
ary interval at ODP
sites 1262 (hole A),
1263 (hole C/D),
1265 (hole A), 1266
(hole C), and 1267
(hole B) on Walvis
Ridge (fig. S1) (18).
Records are plotted
from left to right in
order of increasing wa-
ter depth. The core
photos for each site
represent composites
of the following sec-
tions: 1262A-13H-5
and -6; 1263C-14H-1
and core catcher (CC);
1263D-4H-1 and -2;
1265A-29H-6 and -7;
1266C-17H-2, -3, and
-4; 1267B-23H-1, -2,
and -3.
2717 m water depth
1263C/D
3060 m water depth
1265A
3798 m water depth
1266C
4355 m water depth
1267B
4755 m water depth
1262A
Depth (mcd)
334
335
336
305
306
307
230
231
232
315
316
139
140
100
CaCO
3
(wt%)
500
100
CaCO
3
(wt%)
500
100
CaCO
3
(wt%)
500
100
CaCO
3
(wt%)
500
100
CaCO
3
(wt%)
500
1
Earth Sciences Department, Earth and Marine Sci-
ences Building, University of California, Santa Cruz,
Santa Cruz, CA 95064, USA.
2
Deutsche Forschungs-
gemeinschaft (DFG) Research Center for Ocean
Margins, University of Bremen, Leobener Strasse,
28359 Bremen, Germany.
3
Department of Geological
Sciences, San Diego State University, 5500 Campanile
Drive, San Diego CA 92182–1020, USA.
4
Laboratory of
Palaeobotany and Palynology, Department of Palaeo-
ecology;
5
Faculty of Geosciences, Department of
Earth Sciences; Utrecht University, Budapestlaan 4,
3584 CD Utrecht, Netherlands.
6
Department of
Geological Sciences, University of Florida, 241 Wil-
liamson Hall, Post Office Box 112120, Gainesville, FL
32611, USA.
7
Department of Geology and Geophysics,
University of Wisconsin, Madison, 1215 West Dayton
Street, Madison, WI 53706, USA.
8
Wesleyan Univer-
sity, 265 Church Street, Middletown, CT 06459–0139,
USA.
9
Department of Geology and Geophysics, Yale
University, New Haven, CT 06520–8109, USA.
10
De-
partment of Earth Science, Rice University, 6100 Main
Street, MS-126, Houston, TX 77005–1892, USA.
11
Dipartimento di Scienze della Terra, Universitario
G. D’Annunzio, Campus Universitario, Via dei Vestini
31, 66013 Chieti Scalo, Italy.
12
Faculty of Earth and
Life Sciences, Vrije Universiteit, De Boelelaan 1085,
HV 1081 Amsterdam, Netherlands.
*To whom correspondence should be addressed.
E-mail: jzachos@emerald.uscs.edu
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red Bclay layer,[ whichthengradedbackinto
ooze (Fig. 1 and table S1). Carbonate content
was G1 weight percent (wt %) in the clay lay-
ers, and 980 and 90 wt % in the underlying and
overlying oozes, respectively; the only excep-
tion was site 1265, where the basal portion of
the clay layer was not recovered. The thickness
of the clay layers increased with depth, from 5
cm at the shallowest site (1263) E2717 m; pa-
leodepth È1500 m (19)^ to 35 cm in the deepest
site (1262) E4755 m; paleodepth È 3600 m
(19)^ (Fig. 1). The benthic foraminiferal ex-
tinction horizon, which is characterized by the
disappearance of long-lived Paleocene species
and a rapid drop in diversity, occurred at the
base of the clay layer in each site (18).
Bulk sediment carbon isotope (d
13
C) records
were constructed at 1- to 5-cm resolution for
each boundary section (table S2) (20). Each
record is marked by a decrease in d
13
Catthe
base of the clay layer, followed by gradual re-
covery. Minimum carbon isotope values within
the clay layer are not uniform, but increase
from the shallowest to the deepest site (mini-
mums of –0.9 and 0.0° at sites 1263 and
1262, respectively), a feature we attribute to
truncation by dissolution and the presence of
residual pre-excursion calcite (21). Also, the
base of the CIE differs across sites, occurring
in two steps at site 1263 and in a single step
at the deeper sites. As a result, the excursion
layer, from the onset of the CIE to the point of
full recovery (i.e., stability), decreases in thick-
ness from 2.1 m at site 1263 to 1.0 m at 1262.
In this spatially tight array of sites, the
production and export of carbonate and the
accumulation of clay should be similar at any
given time, leaving dissolution as the major
process that drives differences in carbonate
accumulation between sites. We can there-
fore infer from the weight % carbonate and
carbon isotope data that rapid shoaling of the
lysocline/CCD occurred, followed first by a
more gradual descent or recovery of the CCD
and then by the recovery of the lysocline. The
duration of the lysocline/CCD descent from
the shallowest to the deepest sites was
estimated by first correlating several key
inflection points in the carbon isotope records
(Fig. 2, tie points A to G), as well as in the
Fe concentration and bulk magnetic suscep-
tibility (MS) records (fig. S2). The tie points,
particularly E and F, were then verified with
biostratigraphic data (table S3) (20). We then
correlated the site 1263 carbon isotope record
to that of south Atlantic ODP site 690 (22),
which has an orbitally derived age model (8),
and ordinated the weight % carbonate and
isotope data for each site within that age
model (Fig. 3 and table S4). An alternate age
model based on
3
He exists for site 690 (23),
but the two models are roughly similar for the
initial 100 ky of the PETM; thus, the choice
of model makes little difference in our in-
terpretation of events up to that point. The
greatest uncertainty in the site-to-site correla-
tions and age estimates is in the basal portion
of the clay layer, where the carbon isotope
and other records are compromised by dis-
solution. The correlations (Fig. 1, tie points D
to G) are most reliable in the recovery in-
terval where the weight % carbonate is higher
and the ocean d
13
C is rapidly shifting.
Given these age constraints, the CCD is
inferred to have shoaled more than 2 km
within a few thousand years (Fig. 3). Recov-
ery was gradual, with the CCD descending to
the shallowest site (1263) within È10 to 15
ky of the CIE onset and to the deepest site
(1262) within È60 ky. By þ110 ky, carbon-
ate content had fully recovered. This pattern
of change, particularly the recovery, has im-
portant implications. According to theory, the
initial uptake of CO
2
and buffering should
occur mainly via deep-sea calcite dissolution,
but eventually, chemical weathering of sili-
cate rocks takes over accelerating the flux
of dissolved ions (including HCO
3
–
)tothe
ocean, thereby increasing ECO
3
2–
^ and the rate
of calcite accumulation (24). The distribution
of carbonate between þ60 and þ100 ky
indicates that the CCD had descended, but
the lysocline was still shallow and the deep
sea was largely undersaturated. The percent-
age of CaCO
3
continued to increase, and by
þ110 kyr, it had reached 90% over the entire
MBSF (690) and MCD (Leg 208)
A
B
C
D
E
F
H
A-
164
166
168
170
172
332
333
334
335
336
313
314
315
316
303
304
305
306
307
228
229
230
231
232
137
138
139
140
N4
N4
N3
N3
N2
N2
N1
N1
1263C/D
δ
13
C (vPDB)
-1 0 1 2 3 -1 0 1 2 3 -1 0 1 2 3
-1 0 1 2 3-1 0 1 2 3
-1 0 1 2 3
1262A
δ
13
C (vPDB)
1266C
δ
13
C (vPDB)
δ
13
C (vPDB)
690B
δ
13
C (vPDB)
1265A
δ
13
C (vPDB)
1267B
G
Fig. 2. Bulk sediment carbon isotope records for holes 1262A, 1263C/D,
1265A, 1266C, and 1267B plotted versus MCD. Also plotted are
nannofossil horizons (N1 to N4, arrows in red) for holes 1262B and
1263C/D (20). Data for ODP site 690 (22) are plotted to the far left
versus meters below the sea floor (MBSF). Lines of correlation are based
on inflections in the carbon isotope (A to G above the P-E boundary, –A
below), Fe/Ca, and magnetic susceptibility (MS) records (20). vPDB, Vienna
PeeDee Belemnite.
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transect, a state that implies that the lysocline
descended below the deepest site (93.6 km)
as well as its pre-excursion level. This phe-
nomenon is consistent with theory (10) and
likely represents a transitional period during
which the excess ions supplied to the ocean
by the weathering of silicate rocks greatly
increased deep-sea CO
3
2–
concentration and
thus carbonate accumulation. The site 690
record is marked by a similar pronounced
interval of high carbonate content (23, 25),
demonstrating that CO
3
2–
oversaturation was
not a local phenomenon.
This scenario for acidification of the deep
sea and initial neutralization by calcite disso-
lution is not unlike that simulated by models
in response to the anthropogenic rise in CO
2
(26–28). Because dissolution layers are also
present in P-E boundary sections in the Pa-
cific and Tethys Oceans and at depths G1km
(29–33), it appears that for a brief period of
time, much of the ocean beneath the ther-
mocline was highly undersaturated with re-
spect to calcite. The mass of CO
2
required to
shoal the CCD to G1kmwaterdepthwould
be substantial. In a series of simulations with
an ocean/sediment carbon-cycle model de-
signed to evaluate the ocean-buffering capac-
ity in response to a range of anthropogenic
CO
2
fluxes, 4500 GtC was required to ter-
minate carbonate accumulation over the en-
tire ocean (26).
For the PETM, the release of 94500 GtC
would be more consistent with the magnitude
of global temperature rise (2, 3, 9). Such a
large mass of carbon, however, would require
a reevaluation of the source of carbon and
its isotopic composition. With bacterially
produced methane at –60°, the total input
from hydrates is limited by the d
13
C excur-
sion to G2000 GtC (10). To increase the mass
of carbon added while adhering to the isotope
constraints requires the input of isotopically
heavier carbon, such as thermogenic CH
4
/CO
2
(È–30 to –20°) or oxidation of organic car-
bon (standing or stored) (–20°)(34). In this
regard, recent documentation of an unusual
concentration of upper Paleocene fluid/gas seep
conduits associated with volcanic intrusions
in the North Atlantic (12) merits additional
attention. An alternative explanation, that the
magnitude of the marine CIE has been greatly
underestimated because of dissolution or damp-
ing by pH affects, seems unlikely given the
constraints provided by continental isotope
records (35). Finally, proximity to where car-
bon (CO
2
or CH
4
) enters the deep sea via
circulation will dictate where neutralization
by carbonate dissolution is most intense (36).
For example, severe dissolution in the Atlantic
may indicate direct input of methane into
bottom waters entering this basin.
Excessive carbonate undersaturation of the
deep ocean would likely impede calcification
by marine organisms and therefore is a poten-
tial contributing factor to the mass extinction
of benthic foraminifera at the P-E boundary.
Although most plankton species survived, car-
bonate ion changes in the surface ocean might
have contributed to the brief appearance of
weakly calcified planktonic foraminifera (6)
and the dominance of heavily calcified forms
of calcareous algae (37). What, if any, impli-
cations might this have for the future? If com-
bustion of the entire fossil fuel reservoir (È4500
GtC) is assumed, the impacts on deep-sea pH
and biota will likely be similar to those in the
PETM. However, because the anthropogenic
carbon input will occur within just 300 years,
which is less than the mixing time of the
ocean (38), the impacts on surface ocean pH
and biota will probably be more severe.
References and Notes
1. J. P. Kennett, L. D. Stott, Nature 353, 225 (1991).
2. J. C. Zachos et al., Science 302, 1551 (2003).
3. A. K. Tripati, H. Elderfield, Geochem. Geophys.
Geosyst. 5, 2003GC000631 (2004).
4. E. Thomas, N. J. Shackleton, in Correlation of the
Early Paleogene in Northwest Europe,R.W.O.B.
Knox, R. M. Corfield, R. E. Dunay, Eds. (Geological
Society, London, 1996), vol. 101, pp. 401–441.
5. D. J. Thomas, J. C. Zachos, T. J. Bralower, E. Thomas,
S. Bohaty, Geology 30, 1067 (2002).
6. D. C. Kelly, T. J. Bralower, J. C. Zachos, I. P. Silva, E.
Thomas, Geology 24, 423 (1996).
7. T. J. Bralower, D. J. Thomas, E. Thomas, J. C. Zachos,
Geology 26, 671 (1998).
8. U. Roehl, T. J. Bralower, R. D. Norris, G. Wefer,
Geology 28, 927 (2000).
9. G. R. Dickens, J. R. Oneil, D. K. Rea, R. M. Owen,
Paleoceanography 10, 965 (1995).
10. G. R. Dickens, M. M. Castillo, J. C. G. Walker, Geology
25, 259 (1997).
11. G. R. Dickens, Bull. Soc. Geol. Fr. 171, 37 (2000).
12. H. Svensen et al., Nature 429, 524 (2004).
13. The lysocline, also referred to as the calcite satura-
tion horizon, represents the depth in the ocean where
the carbonate ion concentration falls below the sat-
uration level (currently È4 km in the South Atlantic).
Carbonate accumulation can occur below this level if
the flux of carbonate to the sea floor exceeds the rate
at which it dissolves. The depth at which dissolution
is greater than the flux and where carbonate does not
accumulate defines the CCD.
14. T. J. Bralower et al., Geology 25, 963 (1997).
15. E. Thomas, in Late Paleocene–Early Eocene Biotic and
Climatic Events in the Marine and Terrestrial Records,
M.-P. Aubry, S. Lucas, W. A. Berggren, Eds. (Columbia
Univ. Press, New York, 1998), pp. 214–243.
16. D. J. Thomas, T. J. Bralower, J. C. Zachos, Paleocean-
ography 14, 561 (1999).
54.75
54.80
54.85
54.90
54.95
55.00
55.05
CaCO
3
(wt%)
Age (Ma)
Age (ky ±PEB)
1266C
1265A
1263C/D
1262A
δ
13
C (vPDB)
II
III
IV
1500 m
3600 m
2600 m
250
200
150
100
50
-50
0
20 40 60 80 100
-1.0 0.0 1.0 2.0 3.0
0
I
1267B
Fig. 3. Bulk sediment d
13
C and weight % carbonate content (g
CaCO3
/g
Total
100) plotted versus
age for ODP sites 1262, 1263, 1265, 1266, and 1267. Age (ky) relative to the P-E boundary is
plotted on the left axis and absolute age (Ma) along the right. Age models (table S4) are based on
correlation to site 690 (8) using the carbon isotope stratigraphy as verified with the nannofossil
events in Fig. 2 and with the Fe and MS cycles in fig. S2. Transferring the 1263 age model to
deeper sites with carbon isotopes could only be achieved where sufficient carbonate was present.
Ages within the clay layers for sites 1266, 1267, and 1262 were derived through linear inter-
polation from tie points E and A. Paleodepths (È55 Ma) are provided for sites 1263 (1500 m),
1266 (2600 m), and 1262 (3600 m). Key events in the evolution of south Atlantic carbonate
chemistry were (i) the rapid drop in content to G1% for all sites with the exception of site 1265,
where the lowermost Eocene is absent (marked I); (ii) the return of the CCD to site 1263 roughly
5 ky after the excursion (marked II); (iii) the return of the CCD to site 1262 at 60 ky (marked III);
and (iv) the lysocline descending to a point below the deepest site at 110 ky after the excursion
(marked IV). PEB, Paleocene-Eocene boundary.
10 JUNE 2005 VOL 308 SCIENCE www.sciencemag.org
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17. T. C. J. Moore et al., Eds., Leg 74 (U.S. Government
Printing Office, Washington, DC, 1984), vol. 74.
18. J. C. Zachos et al., Proc. ODP Init. Rep., 208 (2004),
available at http://www-odp.tamu.edu/publications/
208_IR/208ir.htm.
19. Paleodepths of the Leg 208 sites at 55 million years
ago (Ma) were estimated using a standard thermal
subsidence curve and a sediment accumulation
model (18). At 55 Ma, the paleodepths of sites
1263 and 1262 were 1.5 and 3.6 km, respectively.
20. Materials and methods are available as supporting
material on Science Online.
21. The initial phase of dissolution would involve Paleocene
sediments already present on the sea floor. As such, the
base of the clay layer, perhaps as much as a few
centimeters in the deepest site, was deposited before
the carbon isotope excursion. The traces of carbonate
remaining must be a mixture of pre-excursion and
excursion fragments that survived dissolution.
22. S. Bains, R. M. Corfield, R. D. Norris, Science 285, 724
(1999).
23. K. A. Farley, S. F. Eltgroth, Earth Planet. Sci. Lett. 208,
135 (2003).
24. The weathering of silicates on land is generally repre-
sented by the following equation: CaSiO
3
þ 2CO
2
þ
H
2
O Y 2HCO
3
–
þ Ca
2þ
þ SiO
2
. Ensuing precipitation
of calcite from the bicarbonate (and carbonate) ions
supplied by the above reaction is represented by this
equation: HCO
3
–
þ Ca
2þ
Y CaCO
3
þ CO
2
þ H
2
O, so
that there is a net uptake of one unit of CO
2
for each
unit of silicate weathered.
25. D. C. Kelly, Paleoceanography 17, 1071 (2002).
26. D. Archer, H. Kheshgi, E. Maier-Reimer, Geophys. Res.
Lett. 24, 205 (1997).
27. K. Caldeira, M. E. Wicket, Nature 425, 365 (2003).
28. R. A. Feely et al., Science 305, 362 (2004).
29. T. J. Bralower et al., Proc. ODP Init. Rep. 198 (2002).
30. B. Schmitz, V. Pujalte, K. Nunez-Betelu, Palaeogeogr.
Palaeoclimatol. Palaeoecol. 165, 299 (2001).
31. B. Schmitz, R. P. Speijer, M. P. Aubry, Geology 24, 347
(1996).
32. N. Ortiz, Mar. Micropaleontol. 26, 341 (1995).
33. R. Coccioni, R. Di Leo, S. Galeotti, S. Monechi,
Palaeopelagos 4, 87 (1994).
34. A. Kurtz, L. R. Kump, M. A. Arthur, J. C. Zachos, A.
Paytan, Paleoceanography 18, 1090 (2003).
35. G. J. Bowen, D. J. Beerling, P. L. Koch, J. C. Zachos, T.
Quattlebaum, Nature 432, 495 (2004).
36. G. Dickens, Geochem. Geophys. Geosyst. 2, NIL_1 (2001).
37. T. J. Bralower, Paleoceanography 17, 1060 (2002).
38. The primary buffering capacity of the ocean is
provided by the deep ocean and sea-floor sediments.
Because the mixing time of the ocean is 9500 years,
most of the anthropogenic CO
2
will accumulate in
the atmosphere and surface ocean before it can be
conveyed to the deep sea to be neutralized (27).
39. We thank the ODP Leg 208 Science Crew for their
contributions and C. John and S. Bohaty for technical
assistance. The ODP supplied samples. Supported by
NSF grant EAR-0120727 to J.C.Z. and E.T. and by
DFG grant no. Ro 1113/3 to U.R.
Supporting Online Material
www.sciencemag.org/cgi/content/full/308/5728/1611/
DC1
Materials and Methods
Figs. S1 and S2
Tables S1 to S4
References
21 December 2004; accepted 18 April 2005
10.1126/science.1109004
Photoinduced Plasticity in
Cross-Linked Polymers
Timothy F. Scott,
1
Andrew D. Schneider,
1
Wayne D. Cook,
2
Christopher N. Bowman
1
*
Chemically cross-linked polymers are inherently limited by stresses that are
introduced by post-gelation volume changes during polymerization. It is also
difficult to change a cross-linked polymer’s shape without a corresponding loss
of material properties or substantial stress development. We demonstrate a
cross-linked polymer that, upon exposure to light, exhibits stress and/or strain
relaxation without any concomitant change in material properties. This result
is achieved by introducing radicals via photocleavage of residual photoinitia-
tor in the polymer matrix, which then diffuse via addition-fragmentation chain
transfer of midchain functional groups. These processes lead to photoinduced
plasticity, actuation, and equilibrium shape changes without residual stress. Such
polymeric materials are critical to the development of microdevices, bioma-
terials, and polymeric coatings.
Cross-linked, gelled polymers have an Binfinite’’
molecular weight and are described as ther-
mosets, implying a network that cannot be
melted or molded (1). This description is true
for most chemically cross-linked polymers;
however, several cross-linked networks are
known to undergo bond cleavage or depolym-
erization at high temperatures or under various
chemical or other treatments (2). Although
such treatments are useful for recycling pur-
poses,thereisanassociateddegradationinthe
mechanical properties of the polymers. BCrack-
healing[ networks, such as those that use
groups in the polymer backbone able to un-
dergo thermoreversible Diels-Alder reactions
(3), are able to relieve stress without mechan-
ical degradation. However, this reaction must
be performed at elevated temperatures, mak-
ing it unsuitable in thermally sensitive applica-
tions such as dental composites. Internal stress
buildup during polymerization is typical when
shrinkage occurs. This stress decreases the ul-
timate mechanical properties of the cured poly-
mer, which is highly detrimental in fields such
as polymeric coatings, fiber-reinforced compos-
ites, and dental materials, or it may introduce
birefringence, unwanted in optical materials.
Additionally, given that the equilibrium shape
of conventional cross-linked polymers is de-
fined by the shape at gelation, stress relief
would enable a material to be Bmolded[ and
subsequently destressed, allowing for arbitrary
equilibrium shapes to be attained after cure.
We describe a covalently cross-linked net-
work that is able to undergo photomediated,
reversible cleavage of its backbone to allow
chain rearrangement for rapid stress relief at
ambient conditions without mechanical prop-
erty degradation. The key to this reversible
backbone cleavage is addition-fragmentation
chain transfer. Reaction diffusion of radicals
through the cross-linked matrix occurs initially
by reaction of a radical with an in-chain func-
tionality, forming an intermediate, which in
turn fragments, reforming the initial function-
ality and radical. Allyl sulfides have been used
as efficient addition-fragmentation chain trans-
fer agents (4–6). This addition-fragmentation
process alters the topology of the network, but
the polymer chemistry and network connec-
tivity remain unchanged. In the absence of
radical termination events or other side reac-
tions, the number of allyl sulfide groups, and
hence network strands, remains unchanged
(Scheme 1), although relaxation of the stresses
in each bond is facilitated by the alternating
cleavage and reformation reactions.
The monomers used to produce the net-
works are shown in Scheme 2. The base net-
work studied was formed from a stoichiometric
mixture of pentaerythritol tetra(3-mercapto-
propionate) (PETMP) and triethyleneglycol
divinylether (TEGDVE), which produces a
rubbery network with a glass transition tem-
perature (T
g
) of about –25-C. This monomer
systemwasmodifiedbytheadditionofvarying
concentrations of the ring-opening monomer
2-methyl-7-methylene-1,5-dithiacyclooctane
(MDTO) (7) as a comonomer. Addition of a stoi-
1
Department of Chemical and Biological Engineering,
University of Colorado, Boulder, CO 80309, USA.
2
School of Physics and Materials Engineering, Monash
University, Clayton, Victoria 3800, Australia.
*To whom correspondence should be addressed.
E-mail: christopher.bowman@colorado.edu
Scheme 1. Reaction mechanism for chain trans-
fer within the polymer backbone.
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