Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of
Demerara Rise, Western Tropical Atlantic
Jochen Erbacher, Oliver Friedrich, Paul A. Wilson, Heather Birch & Jörg Mutterlose
Abstract
Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon rich
laminated shales on Demerara Rise (western tropical Atlantic). High-resolution organic
carbon isotope and TOC records are presented, which span the Cenomanian-Turonian
boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites
oriented along a NW striking depth transect. These records represent the first high-resolution
carbon isotope records across OAE 2 from the South American margin of the tropical
Atlantic. Due to the scarcity of age significant fossils, the main purpose of this study was to
develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the
depth transect and to tie this to biostratigraphically well-defined sections in the Western
Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England) and western Tethys
(Oued Mellegue, Tunisia). All four sections studied document a 6‰ increase of δ13Corgvalues at the base of the CTBI which is followed by an interval of elevated δ13Corg-values and
a subsequent decrease. Our results supply an important stratigraphic base for subsequent
palaeoceanographic studies on Late Cenomanian to Early Turonian sediments from
Demerara Rise and elsewhere.
Introduction
Stable carbon isotope stratigraphy has become a powerful tool for the stratigraphic
correlation of Cretaceous open marine pelagic sediments [e.g. Arthur et al., 1988; Gale et al.,
1993; Jenkyns et al., 1994; Voigt and Hilbrecht, 1997; Weissert et al., 1998] and is especially
useful for the correlation of the so-called Oceanic Anoxic Events (OAEs) [Arthur et al., 1990;
Jenkyns, 1980]. These OAEs are often characterized by positive δ13C-excursions in
contemporaneous seawater which have been interpreted to be the result of enhanced burial
of organic carbon in marine sediments either through an increase in marine productivity or an
increase in the preservation of organic matter during anoxic conditions, resulting in a drawdown of atmospheric CO2 [Arthur et al., 1988; Schlanger et al., 1987]. The most prominent
and wide-spread of these δ13C-excursions is the one paralleling the OAE 2 near the
Cenomanian-Turonian boundary. The isotope excursion as well as the distribution of organicrich sediments is truly global and both have been described from numerous outcrops and
deep sea cores around the world [e.g. Arthur et al., 1988; Hasegawa, 1997; Holbourn and
Kuhnt, 2002; Pratt and Threlkeld, 1984; Thurow et al., 1992; Tsikos et al., 2004].
Here we present four high-resolution total organic carbon (TOC) and δ13Corg-records across
the OAE 2 from Ocean Drilling Program (ODP) Sites recently drilled along a depth-transect
on Demerara Rise, off Suriname (western tropical Atlantic, Figure 1). There, ODP Leg 207
recovered expanded sections of Albian to lower Campanian organic-rich sediments including
some of the mid-Cretaceous Oceanic Anoxic Events [Erbacher et al., 2004; Shipboard
Scientific Party, 2004]. The black shale unit is a lateral equivalent of important midCretaceous source-rocks in basins west of Demerara Rise (e.g. Canje Formation, Guyana;
La Luna Formation, Venezuela and Colombia) [see Bralower and Lorente, 2003 for an
overview]. As in these formations, index fossils used for classic biostratigraphic zonations are
rare or absent at Demerara Rise. Consequently one of the main objectives of our study is to
correlate the detailed chemostratigraphic records across OAE 2 with similar records of
biostratigraphically well-defined sections such as the Cenomanian-Turonian stratotype
section in Pueblo (Colorado, USA, [Kennedy et al., 2000]), sections in Europe (Eastbourne,
England) and Tunisia (Oued Mellegue), in order to establish a stratigraphic framework for the
CTBI of Demerara Rise.
Cretaceous black shales on Demerara Rise
Demerara Rise is a submarine plateau off the coast of Suriname that stretches northward
and gently dips towards the abyssal plain and distal Orinoco Fan in the NW [Shipboard
Scientific Party, 2004] (Figure 1). Cretaceous to Holocene shallow-marine to pelagic
sediments overly Precambrian to Early Mesozoic continental crust. The late Albian to early
Campanian organic-rich sediments are often expressed as distinctly laminated black shales.
These are characterized by sometimes very high contents of TOC (up to 29% in the
Cenomanian to Turonian), phosphatic nodules and well-preserved fish debris. Shipboard
Rock Eval pyrolysis analyses indicate a pre-dominantly marine origin for the organic matter
of the shales even in the shallowest sites [Shipboard Scientific Party, 2004]. There was a
consistent deepening upward trend during accumulation of the black shales, from shelf
(Cenomanian) to upper bathyal (early Campanian) water depths. The top of the black shale
sequence shows a transition through a condensed glauconite-rich, bioturbated claystone into
overlying pelagic calcareous chalk of Late Campanian age. Deep Sea Drilling Project Site
144, situated at the northern edge of Demerara Rise and re-drilled during Leg 207 as Site
1257 also recovered the Cretaceous organic rich shales [Hayes and Pimm et al., 1972].
Samples from Site 144 were used in a number of studies focussing on palaeoceanographic
and source rock studies of the mid-Cretaceous Atlantic [Kuypers et al., 2002; Norris et al.,
2002; Sinninghe Damsté and Köster, 1998; Stein et al., 1986; Wilson et al., 2002].
Methods
Because of the large number of samples involved, analyses were undertaken in three
laboratories. Data from Sites 1258 and 1261were generated in Hanover, data from Site 1259
in Bochum and data from Site 1260 in Southampton. Bulk sediment wt.% TOC was
measured by standard elemental analyzer methods using an ELTRA CS 500 (Hanover) and
Carlo-Erba C-N machines (Bochum and Southampton). Similarly, the carbon isotope data
were produced by established mass-spectrometry protocols. In Hanover we used a Finnigan
Delta XL IR, in Bochum a Finnigan Delta S, Bochum and in Southampton a GV Instruments
IsoPrime continuous flow machine. All results are expressed as standard d values with
respect to the PDB standard. Further details of the analytical methods used can be obtained
from the authors upon request.
Depths for Sites 1258, 1260 and 1261 are in metres composite depth (mcd), following the
shipboard splice between the different holes drilled at each site [Shipboard Scientific Party,
2004]. Samples analysed from Site 1259 are exclusively from Hole A. Consequently, the
depth for this site is given in meters below seafloor (mbsf).
Results: δ13Corg- and TOC-curves
Site 1258
Site 1258 (present water-depth of 3192 mbsl) lies at the deep end of the Demerara Rise
depth transect. The Cenomanian-Turonian boundary interval (CTBI) as defined by the
pronounced positive carbon isotope excursion occurs between 422 and 426 mcd. The
lithology of the investigated section comprises finely laminated dark shales with occasional
beds of phosphatic nodules, stringers of very dark homogenous shales, and rare
concretionary limestone nodules. No significant lithological changes are observed around the
excursion interval. The interval starts with regularly alternating δ13Corg-values between -28
and -27‰ (between 437 and 427 mcd, Figure 2). δ13Corg-values rise rapidly at 426 mcd to
values between -23 and -21‰ and remain heavy until 422 mcd. Between 422 and 415 mcd
δ13Corg-values vary from -24 to -27‰ and are generally heavier than below the excursion
interval. Significant peaks of up to -24‰ occur at 421 and around 416 mcd.
Organic carbon values vary between 2 and 29% and show the highest values during the
carbon isotope excursion interval (Figure 2).
Site 1260
At a present water-depth of 2549 mbsl Site 1260 serves as an intermediate site on the depth
transect. The CTBI at Site 1260 is between 426.4 and 425 mcd and thus thinner than at Site
1258. Lithologically, the interval is very similar to Site 1258, although phosphatic nodules are
not as common as in the deeper site. The carbon isotope across the CTBI show a very
similar pattern to those at Site 1258, with a pronounced rise of the values at 426.5 mcd and
heavy carbon isotope values until 425 mcd. The decrease of δ13Corg-values above the
excursion interval at 425.5 is not as steep as in Site 1258 which points to the existence of a
hiatus in this interval at Site 1258. The upper part (above 425.5 mcd) of the investigated
interval is again very similar to Site 1258 with carbon isotopes being heavier than the preexcursion values and a pronounced peak of 2.5‰ around 424 mcd.
Organic carbon values vary between 1 and 22% and show the highest values during the
carbon isotope excursion interval where a marked drop of the TOC values is present as at
Site 1258.
Site 1261
Although Site 1261 is the shallowest Site of the Demerara Rise depth transect, the
sediments drilled during Leg 207 indicate a deeper position during the mid-Cretaceous than
Site 1259. The deposition of organic-rich shales at Site 1261 starts in the upper
Cenomanian. These overlie sandstones of an Albian to Cenomanian age [Shipboard
Scientific Party, 2004]. Again, no lithological expression of the CTBI was observed. The
organic-rich shales are finely laminated and they alternate with occasional, potentially
diagenetic limestone beds [Shipboard Scientific Party, 2004]. δ13Corg-values below the
excursion interval fluctuate around -28‰ (Figure 2). The values start to increase to between
-23 and -21‰ at 637 mcd and heavy values persist until 630 mcd where a gradual decrease
of the δ13Corg −values begins. Site 1261 recovered the most expanded (~9 m) CTBI of the
Leg 207 cores (from 637 to 628 mcd).
Organic carbon values vary between 1 and 19% and show the highest values during the
carbon isotope excursion interval. Only one sample at the base of the CTBI shows high
organic carbon values of 29%.
Site 1259
At Site 1259 dark organic-rich shales unconformably overly a tidally influenced mudstone
(546 mbsf) [Shipboard Scientific Party, 2004]. Heavy δ13Corg-values (-22 to -21‰) at the very
base of the shales suggest a beginning of open marine sedimentation at Site 1259 during the
upper part of the excursion interval (Figure 2). The isotope values show a sharp but gradual
decrease to values between -28 and -27‰ from 545 to 544 mbsf. These values are ~0.5 ‰
lighter than at the other Sites, a potential artefact of the involvement of different laboratories
in this study. Four prominent positive carbon isotope peaks (up to -24.5‰) are observed
between 531 and 525 mbsf. Organic carbon values vary between 1 and 23% and show the
highest values during the carbon isotope excursion interval. The interval above ~522 mcd
was not investigated at the other sites but documents very high TOC-values of up to 36%
TOC which are paralleled by increasing carbon isotope values of up to -23.8‰.
The late onset of black shale deposition at Site 1259 points to a palaeowater depth shallower
than at Site 1261 as the deposition of open-marine dark shales there began well below the
CTBI. Thus, the black shale sequence at Demerara Rise appears to be transgressive
confirming a Late Cenomanian sea-level rise [Hardenbol et al., 1998].
Site to Site Correlation
The high-resolution δ13Corg-records produced allow for a detailed chemostratigraphic
correlation among the four sites (Figure 3). The most distinctive feature at all sites is the
pronounced positive carbon isotope excursion (~6.5‰), which describes the CTBI. Despite
this broad interval a number of distinct peaks and troughs within and above the excursion
interval can be correlated between the sites. The onset of the excursion interval is observed
in Sites 1258, 1260 and 1261 and here labelled as A. The sections at Sites 1258 and 1261
both document a short but distinctive peak with values of -24 to - 23‰ followed by a short
trough of 3‰ (B). Higher up in the sections, values rise to numbers between -23 and -21‰
(C). After another short decrease, values rise again to the heaviest values of the excursion
interval. The stable carbon isotopes remain heavy on a plateau (-23 to -21‰) to reach a last
maximum seen in all sites studied (D). Above Interval D Sites 1260 and 1259 both show a
short positive peak during the general decrease of the isotope values (E).
Above the CTBI at least two peaks can tentatively be correlated between the sites. Interval F
is a 3‰ shift that is well developed at Sites 1258 and 1260. Interval G marks three distinct
shifts of ~3‰ which are observed at the Sites 1258, 1259 and 1261.
The correlations of the δ13Corg-curves among all sites clearly show that a number of
condensed intervals and hiatuses are present in the successions. The very sharp decline of
isotope values in the top of the CTBI at Site 1258 points to a significant hiatus in this interval
(Figure 3).
Chemostratigraphic correlation with other OAE 2 sections
The δ13Corg-records from Demerara Rise have been correlated to carbon isotope records of
the biostratigraphically well-defined sections from Pueblo, Colorado, USA [Pratt et al., 1993],
Eastbourne, England [Tsikos et al., 2004], and Oued Mellegue, Tunesia [Nederbragt and
Fiorentino, 1999] (Figure 4). This allows the records to be stratigraphically correlated and
discriminate regional from global isotopic signals.
The onset of the excursion interval (A) is present in all the sections we used for a correlation
and the potentially strongest tie point in our record. Interval B, although not as pronounced
and well-defined as “A”, “C” and “D”, is tentatively correlated to similar troughs of the δ13Corgrecord from Eastbourne and the δ13Ccarb-record from Oued Mellegue. “B” is not documented
at Pueblo which might be caused by the relatively low resolution of this isotope record. “C”
correlates to the peak of the “First Build-up” sensu Paul et al. (1999) at Eastbourne and the
other sections. The following trough (“Trough Interval” sensu Paul et al., 1999) again
corresponds to carbon isotope deceases in the other sections and is followed by a second
build up interval at Eastbourne and Pueblo, which is nicely present at Sites 1258 and 1260
(blue line in Figure 4). A characteristic feature of the isotope curves at Pueblo, Eastbourne
and Oued Mellegue is the so-called “Plateau” (Paul et al., 1999). Black shale deposition at
Site 1259 starts during the “Plateau”. The Plateau seems to be very condensed at Site 1260
and the Site 1258 section shows a pronounced negative excursion where the plateau should
be present. A similar trough is present in the Oued Mellegue section and even the section at
Eastbourne has a very short negative peak preceding the last isotope peak of the CTBI (our
“D”). This last peak or end of the “Plateau” is well developed in all the Demerara Rise Sites.
To date, none of the Demerara Rise carbon isotope events above the “Plateau” can clearly
be correlated to events elsewhere.
Most of the above correlations are only based on comparisons of the structures of the carbon
isotope curves and have little biostratigraphic control. However, due to the increasing
number of biostratigraphically well-defined carbon isotope curves elsewhere we believe that
a detailed correlation of our records to those discussed above is not too ambitious. The next
and consequent step is to tie the Demerara Rise sections to the planktic foraminiferal and
calcareous nannofossil zones from Pueblo, Eastbourne and Oued Mellegue (Figure 4).
Tsikos et al. (2004) correlated two major faunal events to the isotope curves in Pueblo,
Eastbourne, Gubbio and Tarfaya, Morocco. The last occurrence of the planktic foraminiferal
marker species Rotalipora cushmani falls in the trough interval of their records (see also
[Keller and Prado, 2004]). The first occurrence of the calcareous nannofossil marker species
Quadrum gartneri, which is very close to the Cenomanian/Turonian boundary, falls very
close to the onset of decreasing values in the upper part of the excursion interval
[Nederbragt and Fiorentino, 1999; Tsikos et al., 2004]. Both isotopic events are clearly
recognized in the records from Demerara Rise (Figure 4).
The magnitude of 6.5‰ of the C-T-excursion on Demerara Rise is 2.5‰ greater than in most
other C-T-sections with δ13Corg-records. The only other records that document increases of
6‰ are from DSDP Sites 367 and 368 Cap Verde Basin off West Africa [Arthur et al., 1988].
Arthur et al. (1988) attributed this greater magnitude to the high-productivity regime there due
to potential upwelling conditions causing increased CO2 depletion and a reduction of carbon
isotope fractionation. The conditions at Demerara Rise might have been very similar to those
in the Cap Verde Basin and thus explain the greater magnitude of our excursion. Similar
upwelling conditions for the Cenomanian/Turonian of Sites 367 and 144 (Demerara Rise)
resulting from a numerical model are suggested by [Handoh et al., 1999].
Average Sedimentation rates
A number of studies have published cyclostratigraphies across the CTBI [Caron et al., 1999;
Kuhnt et al., 2001; Kuhnt et al., 1997; Prokoph et al., 2001; Scopelliti et al., 2004] resulting
estimates for the duration of OAE 2 between 320 and 400 kyrs. Two of these studies only
dated the duration of black shale deposition during the OAE 2 ([Caron et al., 1999], Wadi
Bahloul, Tunisia, [Scopelliti et al., 2004], Sicily). However, black shale deposition during OAE
2 is related to a number of local environmental factors and has proven to be diachronous
(see discussion in Tsikos et al. [2004]). Accordingly, the excursion interval which was
suggested to define OAE 2 must be longer than 400 kyrs. Nevertheless, Prokoph et al.
(2001) calculated the duration of the isotope excursion at Eastbourne and Pueblo by using
the data from Paul et al. (1999) and Pratt et al. (1993) which resulted in a length of the
excursion of 320 kyrs. The upper and lower limits of the calculations of Prokoph et al. (2001)
lie in the middle of the carbon isotope increase at the base and decrease at the top. Both of
these potential tie points are difficult to define in our curves and elsewhere. Especially as the
carbon isotope decrease at the top of OAE 2 has been shown to potentially depend on
various local factors and therefore differ significantly at different localities [Tsikos et al.,
2004]. A recent cyclostratigraphic study on a core that recovered an expanded CTBI at
Groebern, Germany by Voigt et al. (in review) resulted in a duration of ~400 kyrs from the
base of the CTBI excursion (our “A”) to the last peak of the excursion interval (our “D”). This
is longer than calculated by Prokoph et al. (2001) but lies in the range of the calculations for
Tunisia and Sicily. Assuming 400 kyrs as an appropriate duration for the interval between “A”
(onset of excursion) and “D” (end of the plateau) average sedimentation rates for the CTBI of
Site 1258 are ~1 cm per kyr, for the condensed interval of Site 1260 ~0.25 cm per kyr and for
Site 1261 ~1.5 cm per kyr. Due to the likely presence of a hiatus at the top of the CTBI at
Site 1258 the sedimentation rate there represents a minimum estimate. It is difficult to
calculate the sedimentation rate for Site 1259. However, as Site 1261 is considered to be a
complete section without obvious hiatuses, the recovery above Interval D would last for ~200
ky if we assume a more or less constant sedimentation rate across OAE 2. Accordingly, the
sedimentation rate at Site 1259 would be ~0.75 cm per kyr.
Acknowledgements
We thank Dieter Panthen, Annegret Tietjen, Jerome Beyris, Sarah Schaper, Stefan Feller,
Katrin Noeske, BGR, Dieter Buhl, Ulrike Schulte, Bochum and Mike Bolshaw, Shir Akbari,
SOC for technical support. Silke Voigt, Hugh Jenkyns and Ulrich Berner are thanked for
fruitful discussions. The paper definitely benefited from the constructive reviews of Harilaos
Tsikos and Ian Jarvis. This research used samples provided by the Ocean Drilling Program
(ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating
countries such as Germany and Great Britain under management of Joint Oceanographic
Institutions (JOI), Inc. JE, OF and JM like to thank the Deutsche Forschungsgemeinschaft
(DFG), project ER 226/2-1 and MU 667/25-1 for funding this project.
Figure captions
Figure 1: A: The four Leg 207 sites investigated in this study are in red. They are oriented
along a depth transect on Demerara Rise. B: Palaeogeographic reconstruction for 90 Ma
modified after Barron et al. (1981] with locations of Demerara Rise and the other sections
discussed.
Figure 2: Total organic carbon (TOC, in red) and δ13Corg-data for Sites 1258, 1260, 1261 and
1259 (mcd = metres composite depth; mbsf = metres below seafloor).
Figure 3: Correlation of the δ13Corg-records from Demerara Rise. Letters A to G mark peaks
and troughs in the carbon isotope records that were used for correlation between the sites.
Note different vertical scales.
Figure 4: Correlation of the highest resolution carbon isotope record from Demerara Rise
(Site 1258) to records from the biostratigraphically well-constrained sections at Pueblo
Eastbourne and Oued Mellegue (see text for references). The blue line marks the last
occurrence of R. cushmani in the sections. Letters A, B, C and D mark isotopic events that
were identified in the Demerara Rise records and elsewhere. The biostratigraphic column to
the left is based on the biostratigraphies of Tsikos et al (2004) and Nederbragt and Fiorentino
(1999) and not on the Demerara Rise sites where few biostratigraphic marker fossils occur.
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Geophysical Monograph, 70, 253-273, 1992.
Tsikos, H., H.C. Jenkyns, B. Walsworth-Bell, M.R. Petrizzo, A. Forster, S. Kolonic, E. Erba, I.
Premoli Silva, M. Baas, T. Wagner, and J.S. Sinnighe Damsté, Carbon-isotope
stratigraphy recorded by the Cenomanian-Turonian Oceanic Anoxic Event: correlation
and implications based on three localities, Journal of the Geological Society, London,
161, 711-719, 2004.
Voigt, S., and H. Hilbrecht, Late Cretaceous carbon isotope stratigraphy in Europe:
correlation and relations with sea level and sediment stability, Palaeogeography,
Palaeoclimatology, Palaeoecology, 134, 39-59, 1997.
Weissert, H., A. Lini, K.B. Föllmi, and O. Kuhn, Correlation of Early Cretaceous carbon
isotope
stratigraphy
and
platform
drowning
events:
a
possible
Palaeogeography, Palaeoclimatology, Palaeecology, 137, 189-203, 1998.
link?,
Wilson, P.A., R.D. Norris, and M.J. Cooper, Testing the Cretaceous greenhouse hypothesis
using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara
Rise, Geology, 30, 607-610, 2002.
Erbacher et al., Figure 4
Site 1258
W. archaeocretacea
Early Turonian
δ13Corg [‰]
NC
14
?
-28
-22
D
NC
11
Eastbourne (England)
δ13Ccarb [‰]
4
5
0
1
2
3
6
-28 -27 -26 -25 -24
NC
13
D
D
D
Hiatus
A
R. cushmani
-24
Pueblo (Colorado, USA)
δ13Corg [‰]
NC
12
Late Cenomanian
-26
Oued Mellegue (Tunesia)
δ13Ccarb [‰]
B
C
C
C
A
B
A
-25 - 24 -23 -22
δ13Corg [‰]
A
B
C
Erbacher et al., Figure 3
mbsf
Hole 1259 A (2354 mbsl)
-28
δ13Corg [‰]
-26
-24
-22
510
-28
-26
-24
Site 1260 (2549 mbsl)
-22
G
416
δ13Corg [‰]
mcd
mcd
414
δ13Corg [‰]
-28
-26
-24
δ13Corg [‰]
-28
-26
-22
-24
520
-22
525
G
612
616
424
F
422
D
Hiatus
426
428
430
432
434
436
A
B
C
F
535
624
E
425
426
424
A
D
C
540
628
D
632
636
A
B
640
427
644
428
429
G
530
620
420
515
604
608
423
418
mcd
Site 1261 (1899 mbsl)
Site 1258 (3192 mbsl)
648
Unconformity above
sandstone
C
545
E
Unconformity above
tidal flat deposits
D
Site 1258
TOC
[%]
0
5
10
15
20
25
30
-28
d C
[‰ ]
13
-26
Site 1260
TOC
[%]
-24
-22
0
5
10
15
20
-28
Erbacher et al., Figure 2
d C
[‰ ]
13
-26
-24
-22
416
423
418
420
424
422
426
m cd
m cd
425
424
428
426
427
430
428
432
434
429
436
Site 1261
TOC
[%]
0
5
10
15
20
25
30
-28
d C
[‰ ]
13
-26
-24
-22
0
508
605
510
512
610
514
516
615
518
520
522
m cd
mbsf
620
625
524
526
528
630
635
640
645
Hole 1259 A
TOC
[%]
530
532
534
536
538
540
542
544
546
10
20
30
40
-28
d C
[‰ ]
13
-26
-24
-22
Erbacher et al., Figure 1
A
10°0'0"N
water depth
value
-1000
metres
-5000
1258
1257/
DSDP 144
1259
1260
1261
9°0'0"N
55°0'0"W
54°0'0"W
B
Eastb
rn
u
o
e
lo
b
e
u
P
G
io
b
u
M
d
e
u
O
lg
D
raR
m
e
is