LETTERS
Warm saline intermediate waters in the
Cretaceous tropical Atlantic Ocean
OLIVER FRIEDRICH1,2 *, JOCHEN ERBACHER1 , KAZUYOSHI MORIYA2 , PAUL A. WILSON2
AND HENNING KUHNERT3
1
Bundesanstalt für Geowissenschaften, Stilleweg 2, 30655 Hannover, Germany
National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK
3
Center for Marine Environmental Sciences, Universität Bremen, Klagenfurter Strasse, 28359 Bremen, Germany
* e-mail: [email protected]
2
Published online: 30 May 2008; doi:10.1038/ngeo217
During the mid-Cretaceous period, the global subsurface oceans
were relatively warm, but the origins of the high temperatures
are debated. One hypothesis suggests that high sea levels and the
continental configuration allowed high-salinity waters in lowlatitude epicontinental shelf seas to sink and form deep-water
masses1–3 . In another scenario, surface waters in high-latitude
regions, the modern area of deep-water formation, were warmed
through greenhouse forcing4 , which then propagated through
deep-water circulation. Here, we use oxygen isotopes and Mg/Ca
ratios from benthic foraminifera to reconstruct intermediatewater conditions in the tropical proto-Atlantic Ocean from 97 to
92 Myr ago. According to our reconstruction, intermediate-water
temperatures ranged between 20 and 25 ◦ C, the warmest ever
documented for depths of 500–1,000 m. Our record also reveals
intervals of high-salinity conditions, which we suggest reflect an
influx of saline water derived from epicontinental seas around
the tropical proto-North Atlantic Ocean. Although derived from
only one site, our data indicate the existence of warm, saline
intermediate waters in this silled basin. This combination of
warm saline intermediate waters and restricted palaeogeography
probably acted as preconditioning factors for the prolonged
period of anoxia and black-shale formation in the equatorial
proto-North Atlantic Ocean during the Cretaceous period.
The middle of the Cretaceous period (∼120–80 Myr ago) is
widely interpreted to represent the best example of a long-lived
greenhouse interval from the entire geological record with high
global temperatures deep-ocean temperatures at least 10 ◦ C warmer
than today5 , high atmospheric carbon dioxide levels6 and minimal7 ,
possibly transient8 , continental ice budgets. Characteristic of this
greenhouse interval is the deposition of abundant organic-rich
sediments in the geological record, many of them finely laminated
and well known as some of the world’s most important petroleum
source rocks. Some of these black shales are widely, if not
globally, distributed. Among the global Oceanic Anoxic Events
(OAEs), OAE 2 is the most prominent, lasting approximately
500 kyr and falling during the interval of peak sustained global
warmth of the past 150 Myr (refs 5,9). It is associated with
significant turnover within numerous biotic groups, and a
prominent carbon isotope increase in marine records10,11 . Other
black shales are mainly present in the Atlantic–Tethyan realm and
recent results show that the equatorial proto-North Atlantic was,
for some reason, particularly prone to anoxia with black-shale
deposition at Demerara Rise spanning over 20 Myr of Albian to
Santonian time11 .
Numerous studies have focused on the origin of OAEs and
the cause of mid-Cretaceous warmth. However, with the exception
of some modelling experiments, comparatively few studies have
concentrated on the issue of water-mass formation and ocean
circulation during this peak greenhouse interval. Furthermore,
most palaeoceanographic studies of the mid-Cretaceous are
limited9,12 by problems of diagenetic alteration of carbonate
sediments and calcareous microfossils and/or extremely low
sampling resolution. Thus, controversy has developed regarding
the importance of ocean circulation mode to the question of
the warmth of the Cretaceous world. The hypothesis of a mode
of water-mass formation and oceanic circulation, to some extent
driven by or at least influenced by the sinking of warm, saline waters
in low-latitudinal regions with extreme net evaporation is deeply
rooted in the palaeoceanographic literature about the Cretaceous
period and the early Cenozoic era1–3 . Numerical circulation model
experiments for a restricted Atlantic basin during the Albian have
simulated formation of warm, saline intermediate- to deep-water
masses along the northern coastlines of South America and Africa13 .
Similarly, δ18 O data from Albian and Cenomanian molluscs and
radiaxial fibrous calcite cements have been interpreted to record
warm, saline water masses originating from the ancestral Gulf
of Mexico14 . Yet the nonlinearity of the equation of state for
sea water means that the sensitivity of density to changes in
salinity is small at high temperatures. Furthermore, other model
experiments conclude4 that the global picture of water-mass
formation must be dominated by sinking at high latitudes,
and further petro-geochemical analysis15 of molluscs and marine
cements calls into question the δ18 O-based interpretations of the
Gulf of Mexico data because of diagenetic alteration.
To investigate the hydrography of the low-latitude proto-North
Atlantic, we generated stable isotope records in glassy9
foraminiferal calcite (see the Methods section) from over 100 m of
black shales recovered from Ocean Drilling Program sites 1258 and
1260 at Demerara rise, which have present water depths of 3,192 m
and 2,549 m respectively (Supplementary Information, Fig. S1).
On the basis of seismic stratigraphy and sediment composition,
these sites record the history of intermediate waters during the
Cenomanian age (palaeodepths of ∼1,000 m at site 1258 and
∼500 m at site 1260 (refs 16,17)). We generated a chronology
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LETTERS
Age
(Myr)
Stage
–30
δ 13 Corg (‰ VPDB)
–28
–26
–24
δ 18 Ocarb (‰ VPDB)
–22
–5
–4
–3
–2
–1
0
92.5
Turonian
92.0
5
93.0
93.5
OAE 2
4
94.0
94.5
3
95.5
Cenomanian
95.0
MCE
Decreasing benthic
foraminiferal
abundances
and diversities
96.0
2
96.5
Site 1258
1
97.0
Site 1260
–30
–28
–26
–24
δ13 Corg (‰ VPDB)
–22
36
32
28
24
20
Palaeotemperature (°C)
16
12
Figure 1 δ 18 O for planktic (black) and benthic foraminifera of sites 1258 (orange) and 1260 (red). See the Methods section for palaeotemperature calculation. δ 13 Corg
partly from ref. 11. Planktic foraminifera: Hedbergella delrioensis (open black circles, site 1258); benthic foraminifera (site 1258, this study and ref. 7; site 1260, this study):
Bolivina anambra (horizontal crosses), B. cf. incrassata (open downward triangles), Gavelinella dakotensis (open squares), G. intermedia (diagonal crosses), Gavelinella spp.
(asterisks), Lenticulina spp. (open upward triangles), Neobulimina albertensis (circles), Osangularia schloenbachi (open diamonds) Praebulimina prolixa (filled diamonds),
Tappanina sp. 1 (filled squares), mixed benthics (filled downward triangles). Grey fields: 90% of data around interval mean. MCE: Middle Cenomanian event.
for these sequences using new and published11 organic carbon
isotope (δ13 Corg ) data together with a recently published calcareous
nannofossil biostratigraphy18 (Fig. 1). Our benthic oxygen isotope
record is of much higher resolution than any previously published
data set for the mid-Cretaceous and indicates that intermediate
waters at Demerara Rise were extremely warm throughout the Early
and Middle Cenomanian (Fig. 1, Supplementary Information,
Fig. S2). Assuming reasonable, conservative estimates for seawater
δ18 O (see the Methods section), we estimate intermediate-water
temperatures for Demerara Rise between 20 and 25 ◦ C—the
warmest yet reported from well-preserved benthic foraminiferal
454
calcite of the mid-Cretaceous and far exceeding today’s water
temperatures at comparable depths (4–7 ◦ C (ref. 19)).
Our benthic oxygen isotope record exhibits noticeable
stratigraphic structure and these data fall into five time intervals
on this basis (Fig. 1). During the first of these intervals
(∼97.3–96.5 Myr), benthic δ18 O shows a relatively narrow spread
about a mean of about −2.25h Vienna PeeDee Belemnite (VPDB).
The mean benthic δ18 O in interval 2 (∼96.5–95.5 Myr) is only
slightly higher but the spread of data is noticeably wider, especially
at site 1258 (Fig. 1). For interval 3 (∼95.5–94 Myr), mean benthic
δ18 O is noticeably higher than for any other part of our record with
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Site 1258
Site 1260
Age
(Myr)
94.4
0
Benthics
(n × 10 3 /g)
1
δ 18O benthic
(‰ VPDB)
2
–3
δ 18O benthic
(‰ VPDB)
Benthics
(n × 10 3 /g)
–2
–1
0
1
2
–3
–2
–1
Tappanina sp.1
94.6
B. anambra
B. cf. incrassata
94.8
3
Mixed benthics
95.0
95.2
95.4
95.6
P. prolixa
G. dakotensis
G. intermedia
95.8
96.0
2
N. albertensis
96.2
96.4
Figure 2 Benthic foraminiferal abundances and δ 18 O values of sites 1258 and 1260 for the time interval between 94.4 and 96.4 Myr. Abundances as individuals per
gram dried sediment. Intervals 2 and 3 and isotopic markers are the same as in Fig. 1.
the lowest values recorded in interval 4 on the run up to OAE 2.
Benthic δ18 O values in the remainder of our record (interval 5)
show a narrow spread about a mean of about 2.0h VPDB. The data
comprising this record do not come from a single species, but the
sample-to-sample consistency and the similar values recorded by
different taxa in the same samples indicate that the variability seen
cannot be attributed to taxon-dependent isotopic offsets (Fig. 2).
Instead, we interpret the changes in our record (Fig. 1) to indicate
real environmental signals. These signals are large and require
substantial shifts in the δ18 O of sea water and/or temperature of
Cretaceous sea water. For example, the shift in mean benthic δ18 O
across the interval 2/3 boundary requires an increase in seawater
δ18 O of about 1h or a cooling of about 4 ◦ C. Similarly, within
interval 2 at site 1258, particularly well-defined, high-amplitude
variations in benthic δ18 O are seen on a timescale of about
100 kyr that require changes in intermediate-water δ18 O of about
2–2.5h or of temperature by about 8 ◦ C. Despite these pronounced
changes in our benthic record, very little variation is seen in
our accompanying planktic record (Fig. 1). This observation
indicates7 that the benthic δ18 O record cannot be explained in
terms of whole-ocean changes in seawater δ18 O (for example, due
to glaciation), but leaves the origin of the large changes in benthic
δ18 O unexplained.
To help decipher the origin of the large changes in benthic
δ18 O seen in our records, we have generated faunal records for
the key time intervals (top of 2, base of 3, Fig. 2). This data
set reveals large shifts in benthic foraminiferal abundance within
the late stages of interval 2 and a rapid decline to near-zero
abundance on the run up to the interval 2/3 boundary (Fig. 2). This
transition is slightly earlier and sharper in the deeper site (1258).
We interpret the high-amplitude changes in abundance within
interval 2 to reflect a combination of changes in food availability
and oxygenation at the sea floor. Yet, changes in food availability
cannot have contributed to the marked decline in foraminiferal
abundance near the interval 2/3 boundary because this would
require a total loss of productivity—something that is inconsistent
with the high total organic carbon content of these sediments.
Thus, this signal must predominantly reflect decreased sea-floor
oxygenation. These changes in faunal abundance occur in tandem
with pronounced shifts in benthic δ18 O, where low abundances
are coincident with high benthic δ18 O and vice versa, and this
relationship is particularly clear at the deeper site (site 1258, Fig. 2).
The sign of this relationship points to pronounced increases in local
seawater δ18 O and salinity (not cooling) as the cause of the high
benthic δ18 O intervals because benthic foraminiferal assemblages
are relatively insensitive to temperature.
To test our interpretation that much of the structure in our
benthic δ18 O record within intervals 2 and 3 reflects pronounced
changes in local seawater δ18 O, which reflects salinity rather than
temperature, we measured Mg/Ca ratios in benthic foraminiferal
calcite, which are a proxy for calcification temperature. Although
the foraminiferal calcite in our samples is extremely well preserved,
the tests are small in size and rare in many samples (Figs 1,2).
Therefore, it was not possible to generate a high-resolution record
using conventional techniques. Instead, we measured Mg/Ca
ratios in a small subset of samples using a laser-ablation method
(Table 1). This limited data set cannot be used to evaluate shortterm environmental change, and absolute temperatures cannot be
calculated with accuracy because of the uncertainty associated with
the Mg/Ca ratio of Cretaceous sea water20 and species-specific Mg
partitioning between sea water and foraminiferal calcite. However,
the data available show only modest variation with the highest
Mg/Ca ratios indicated for interval 3—a pattern consistent with
the interpretation that the comparatively high benthic δ18 O values
during interval 3 are attributable to the local incursion of warm
high-δ18 O, high-salinity waters (rather than colder waters).
To assess the probable mechanism of formation of the
postulated high-δ18 O, high-salinity waters, it is important to
consider the tectonic configuration of the Atlantic Ocean during the
mid-Cretaceous. During the Late Cenomanian, the central North
Atlantic was an isolated basin (see Supplementary Information,
Fig. S1) with limited exchange of intermediate and deep waters
occurring with adjacent basins. Connection to the Tethys Ocean
was restricted by continental terranes21 , the Equatorial Atlantic
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LETTERS
Table 1 Comparison of Mg/Ca with δ 18 O for three distinct intervals. (Depth in metre composite depth (mcd), age in million years (Myr), Mg/Ca ratios and δ 18 O data
versus VPDB. Interval numbers refer to those shown in Fig. 1; s.d., standard deviation; n of ind., number of measured individuals; holes, number of individual holes
ablated; B. anambra, Bolivina anambra ; N. albertensis, Neobulimina albertensis.)
Depth
(mcd)
Age
(Myr)
Mg/Ca
species
Mg/Ca
n of ind.
Mg/Ca
holes
Mg/Ca
sample mean
Mg/Ca
sample s.d.
δ 18 O
sample (h)
417.25
417.6
439.375
440.11
467.67
468.32
92.58
92.65
94.83
94.89
96.96
97.00
B. anambra
B. anambra
B. anambra
B. anambra
N. albertensis
N. albertensis
3
3
5
5
5
5
6
10
18
16
21
23
5.84
8.82
11.13
9.12
8.20
10.22
2.19
1.98
3.36
1.99
2.46
3.19
−2.69
−1.88
−1.13
−1.06
−2.15
−2.09
Gateway and the Caribbean Gateway allowed exchange of only
surface waters22,23 , whereas connection to the Arctic Ocean was
blocked until the opening of the Greenland–Iceland–Norwegian
Sea during the Oligocene epoch24 . Hence, the intermediate- to
deep-water masses of the central Atlantic Ocean at this time
would have been formed locally within the basin. One mode of
water-mass formation within the basin must presumably have
involved winter cooling at the highest latitudes available. Tectonic
reconstructions, however, show that the basin extended only
to the mid-latitudes (∼40–45◦ N, Supplementary Information,
Fig. S1). On the basis of brachiopod δ18 O data and observed
latitudinal gradients in the preservation of organic material in
the Atlantic Ocean, Cenomanian southwestern European shelf seas
are proposed as a potential source region contributing to protoNorth Atlantic intermediate and deep waters25 . Our data suggest
the operation of an extra mode of water-mass formation—lowlatitude evaporation-led modification of waters in epicontinental
basins and/or on shelf areas. Our sites can be excluded as a source
for this type of water formation owing to the increased gradient
between planktic and benthic foraminiferal calcite during interval 3
relative to 2 and 4. More likely source regions for the proposed highsaline waters would have been the contemporaneous epicontinental
seas of northern South America and northern Africa. Many
were the sites of contemporaneous evaporite formation (see
Supplementary Information, Fig. S1). Some of the Cretaceous
shelf areas closest to Demerara Rise are well known to have
become marginal anoxic basins accumulating vast deposits of
organic-carbon-rich sediments during intervals of sea-level high
stand. Sedimentological and palynofloral data from, for example,
eastern Venezuela, point to semi-arid climate conditions, which are
proposed to result in high salinities and low oxygen contents in
shallow shelf areas26 .
It seems probable that the main reason for the extreme
warmth of the Cretaceous ocean interior was the warmth of
surface waters at mid- to high-latitude sites of deep convection4,25 .
But our data show that high-δ18 O, high-salinity water masses
existed at least sporadically over Demerara Rise during the Late
Cenomanian. Data of this type from one locality cannot be
used to determine the regional extent or global significance of
such water masses. However, the mid-Cretaceous was a time of
extremely high eustatic sea levels27 resulting in vast epicontinental
seas during the Cenomanian and Turonian—potentially favourable
locales for the sporadic formation of low-latitude warm and saline
water masses akin to those that form today during the summer
on the Bahama Bank complex28 , with potential implications for
our understanding of ocean circulation in a greenhouse world.
We interpret the existence of warm saline waters at depth on
Demerara Rise to the unique plate tectonic configuration of the
mid-Cretaceous proto-North Atlantic, the extremely high sea levels
and the greenhouse climate of the Cenomanian. We postulate
that delivery of these dense water masses acted, together with
456
Interval
number
Mg/Ca
interval mean
δ 18 O
interval mean (h)
δ 18 O
interval s.d.
5
7.33
−1.97
0.35
3
10.13
−1.56
0.52
1
9.21
−2.56
0.35
the restricted and silled nature of the basin, to precondition the
region in favour of anoxia, helping to explain the exceptionally long
history of laminated black shale and ultimately hydrocarbon source
rock formation.
METHODS
AGE MODEL
We use the numerical ages of ref. 29. Tie points are the beginning of the
carbon isotope excursion paralleling the Middle Cenomanian event (lower part
Acanthoceras rhotomagense and upper part Cunningtoniceras inerme ammonite
zone25 ; 95.7 Myr), the last occurrence of the nannofossil marker Corollithion
kennedyi18 (94.1 Myr) and the initiation of the OAE 2 positive carbon isotope
excursion, estimated as ∼300 kyr before the Cenomanian/Turonian boundary
(93.8 Myr (ref. 25)). At site 1258, a sharp decline of carbon isotope values in
the uppermost part of OAE 2 points to the existence of a short-term hiatus11 .
Accordingly, our Turonian data from site 1258 are plotted against age by
correlating the carbon isotope values to site 1260.
ISOTOPE AND Mg/Ca MEASUREMENTS AND PALAEOTEMPERATURE RECONSTRUCTION
Foraminiferal tests from Demerara Rise are generally extremely well preserved
(‘glassy’), lacking internal cements and recrystallization9 (see Supplementary
Information, Fig. S3). A few intervals including most parts of the OAE 2,
however, show significant calcite infilling of the shells. Tests from these intervals
are not used in this study. The small numbers of individuals and shifts in
assemblages made it necessary to generate species-specific records using
multiple taxa (see Supplementary Information, Tables). Different habitat effects
are believed to explain a part of the strong scattering in our data. Furthermore,
it is possible that the mechanism of changes in the bottom-water mass, as
proposed in this study, also operated on short timescales influencing the benthic
oxygen isotope values. Hence, we focus mainly on the long-term changes of the
stable isotope record.
Stable isotope measurements were obtained from single taxon separates
using between 5 and 50 individuals mainly from the fractions 63–125 µm and
125–250 µm. Measurements were carried out using a Finnigan MAT 251 mass
spectrometer at the Leibniz-Labor (Kiel, Germany), coupled online to the
Carbo-Kiel device I for automated CO2 preparation. External precision is better
than 0.07h. The data are given in the usual δ-notation and refer to the VPDB
standard as established using the NBS20 carbonate isotope standard.
Palaeotemperature calculations are based on equations (1) of ref. 30,
assuming a δ18 Ow of Late Cretaceous sea water of −1.0hSMOW for an ice-free
world31 and that the foraminiferal calcite was formed in isotopic equilibrium
with Cretaceous sea water. This universal application of the mean value for δw
is appropriate for deep waters (and thus for most of the presented data), but
will lead to an underestimation of SSTs calculated from planktic foraminiferal
δ18 O values due to latitudinal gradients for δw of the surface ocean9 .
Mg/Ca measurements were carried out using a New Wave UP 193
solid-state laser ablation system with 193 nm wavelength, coupled to a Finnigan
Element 2 sector field inductively coupled plasma mass spectrometer at the
University of Bremen. The calibrations are based on the NIST 612 glass standard
reference material. For each sample, the final Mg/Ca ratio was calculated by
averaging the measurements of all ablated holes from all specimens (maximum
of five specimens per sample and five holes per specimen, see Table 1 for
number of ablated holes). The relative standard deviation for Mg/Ca based
on 10 measurements on the NIST 612 was 4.5%; the standard error of the
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LETTERS
average was 1.4%. The latter value gives an estimate of the precision of the
foraminiferal Mg/Ca data. On the basis of the uncertain Mg/Ca of Cenomanian
sea water, possible temporal changes in seawater Mg/Ca composition and the
unknown relation between ambient water temperature and the Mg/Ca of
extinct foraminifera, we refrain from calculating palaeotemperatures based on
this proxy and discuss inferred changes in only relative temperature.
Received 7 December 2007; accepted 29 April 2008; published 30 May 2008.
References
1. Brass, G. W., Southam, J. R. & Peterson, W. H. Warm saline bottom waters in the ancient ocean.
Nature 296, 620–623 (1982).
2. Arthur, M. A. & Natland, J. H. Carbonaceous sediments in the North and South Atlantic; the role of
salinity in stable stratification of Early Cretaceous basins. Maurice Ewing Ser. 3, 375–401 (1979).
3. Chamberlin, T. C. On a possible reversal of deep-sea circulation and its influence on geologic
climates. J. Geol. 14, 363–373 (1906).
4. Bice, K. L. & Marotzke, J. Numerical evidence against reversed thermohaline circulation in the warm
Paleocene/Eocene ocean. J. Geophys. Res. 106, 11529–11542 (2001).
5. Huber, B. T., Norris, R. D. & MacLeod, K. G. Deep-sea paleotemperature record of extreme warmth
during the Cretaceous. Geology 30, 123–126 (2002).
6. Bice, K. L. et al. A multiple proxy and model study of Cretaceous upper ocean temperatures and
atmospheric CO2 concentrations. Paleoceanography 21, doi:10.1029/2005PA001203 (2006).
7. Moriya, K., Wilson, P. A., Friedrich, O., Erbacher, J. & Kawahata, H. Testing for ice sheets during the
mid-Cretaceous greenhouse using glassy foraminiferal calcite from the mid-Cenomanian tropics on
Demerara Rise. Geology 35, 615–618 (2007).
8. Bornemann, A. et al. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science
319, 189–192 (2008).
9. Wilson, P. A., Norris, R. D. & Cooper, M. J. Testing the Cretaceous greenhouse hypothesis using glassy
foraminiferal calcite from the core of the Turonian tropics on Demerara Rise. Geology 30,
607–610 (2002).
10. Leckie, R. M., Bralower, T. J. & Cashman, R. Oceanic anoxic events and plankton evolution: Biotic
response to tectonic forcing during the mid-Cretaceous. Paleoceanography 17,
doi:10.1029/2001PA000623 (2002).
11. Erbacher, J., Friedrich, O., Wilson, P. A., Birch, H. & Mutterlose, J. Stable organic carbon isotope
stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic. Geochem.
Geophys. Geosyst. 6, doi:10.1029/2004GC000850 (2005).
12. Wilson, P. A. & Norris, R. D. Warm tropical ocean surface and global anoxia during the
mid-Cretaceous period. Nature 412, 425–428 (2001).
13. Poulsen, C. J., Barron, E. J., Arthur, M. A. & Peterson, W. H. Response of the mid-Cretaceous global
oceanic circulation to tectonic and CO2 forcings. Paleoceanography 16, 576–592 (2001).
14. Woo, K. S., Anderson, T. F., Railsback, L. B. & Sandberg, P. A. Oxygen isotope evidence for
high-salinity surface seawater in the mid-Cretaceous Gulf of Mexico: Implications for warm, saline
deepwater formation. Paleoceanography 7, 673–685 (1992).
15. Wilson, P. A. & Dickson, J. A. D. Radiaxial calcite: Alteration product of and petrographic proxy for
magnesian calcite marine cement. Geology 24, 945–948 (1996).
16. Erbacher, J. et al. Proc. Ocean Drilling Program, Initial Reports Vol. 207 (Ocean Drilling Program,
College Station, 2004).
17. Nederbragt, A. J., Thurow, J. & Pearce, R. Sediment composition and cyclicity in the mid-Cretaceous
at Demerara Rise, ODP Leg 207. Proc. ODP Sci. Res. 207,
doi:10.2973/odp.proc.sr.207.103.2007 (2007).
18. Hardas, P. & Mutterlose, J. Calcareous nannofossil biostratigraphy of the Cenomanian/Turonian
boundary interval of ODP Leg 207 at the Demerara Rise. Rev. Micropaléontol. 49, 165–179 (2006).
19. World Ocean Atlas 2001. <http://www.nodc.noaa.gov/OC5/WOA01F/tsearch.html> (2001).
20. Wilkinson, B. H. & Algeo, T. J. Sedimentary carbonate record of calcium-magnesium cycling. Am. J.
Sci. 289, 1158–1194 (1989).
21. Dercourt, J., Ricou, L. E. & Vrielynck, B. Atlas Tethys Paleoenvironmental Maps (CCGM, Paris, 1993).
22. Meschede, M. & Frisch, W. A plate-tectonic model for the Mesozoic and early Cenozoic history of the
Caribbean plate. Tectonophysics 296, 269–291 (1998).
23. Otto-Bliesner, B. L., Brady, E. C. & Shields, C. Late Cretaceous ocean: Coupled simulations with the
National Center for Atmospheric Research Climate System Model. J. Geophys. Res. 107,
doi:10.1029/2001JD000821 (2002).
24. Wold, C. N. Palaeobathymetric reconstruction on a gridded database: The northern North Atlantic
and southern Greenland-Iceland-Norwegian Sea. Geol. Soc. Lond., Spec. Publ. 90, 271–302 (1995).
25. Voigt, S., Gale, A. S. & Flögel, S. Midlatitude shelf seas in the Cenomanian-Turonian greenhouse
world: Temperature evolution and North Atlantic circulation. Paleoceanography 19,
doi:10.1029/2003PA001015 (2004).
26. Erlich, R. N., Villamil, T. & Keens-Dumas, J. Controls on the deposition of Upper Cretaceous organic
carbon-rich rocks from Costa Rica to Suriname. AAPG Mem. 79, 1–45 (2003).
27. Haq, B. U., Hardenbol, J. & Vail, P. R. Chronology of fluctuating sea levels since the Triassic. Science
235, 1156–1167 (1987).
28. Wilson, P. A. & Roberts, H. H. Density cascading: off-shelf sediment transport, evidence and
implications, Bahama Banks. J. Sedim. Res. 65, 45–56 (1995).
29. Hardenbol, J. et al. Mesozoic and Cenozoic sequence chronostratigraphic framework of European
basins. SEPM Spec. Publ. 60, 3–13 (1998).
30. Bemis, B. E., Spero, H. J., Bijma, J. & Lea, D. W. Reevaluation of the oxygen isotopic composition of
planktonic foraminifera: Experimental results and revised paleotemperature equations.
Paleoceanography 13, 150–160 (1998).
31. Shackleton, N. J. & Kennett, J. P. Paleotemperature history of the Cenozoic and the initiation of
Antarctic glaciation: oxygen and carbon isotope analysis in DSDP sites 277, 279, and 280. DSDP Init.
Rep. 29, 743–755 (1975).
Supplementary Information accompanies this paper on www.nature.com/naturegeoscience.
Acknowledgements
We are grateful to the Leg 207 Shipboard Scientific Party, W. Hale, N. Westphal, K. Noeske and S. Feller.
D. Panten, N. Andersen and H. Erlenkeuser are thanked for measuring stable isotopes. Our paper
benefited from discussions with H. Brumsack, A. Forster, P. Hardas, A. Hetzel, J. Mutterlose, R. Norris
and P. Sexton. This research used samples provided by the Ocean Drilling Program. ODP is sponsored
by the US National Science Foundation and participating countries under the management of Joint
Oceanographic Institutions. Financial support for this study was provided by the German Research
Foundation (to O.F., J.E. and H.K.), KAKENHI (to K.M.) and UK ODP NERC (to P.A.W.).
Author information
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Correspondence and requests for materials should be addressed to O.F.
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