Stable organic carbon isotope stratigraphy ... - Wiley Online Library

Geochemistry Geophysics Geosystems

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Volume 6, Number 6 25 June 2005 Q06010, doi:10.1029/2004GC000850

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

ISSN: 1525-2027

Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic Jochen Erbacher and Oliver Friedrich Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Postfach 51 01 53, Alfred-Benz-Haus, Stilleweg 2, 30641 Hanover, Germany ([email protected])

Paul A. Wilson and Heather Birch Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, European Way, Southampton SO14 3ZH, UK ([email protected])

Jo¨rg Mutterlose Institut fu¨r Geologie, Mineralogie und Geophysik, Ruhr-Universita¨t Bochum, Universita¨tsstrasse 150, D-44801 Bochum, Germany ([email protected])

[1] 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 total organic carbon (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 d13Corg values at the base of the CTBI, which is followed by an interval of elevated d13Corg values and a subsequent decrease. Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere. Components: 4631 words, 4 figures. Keywords: carbon isotopes; Leg 207; oceanic anoxic events; ODP. Index Terms: 4999 Paleoceanography: General or miscellaneous; 9609 Information Related to Geologic Time: Mesozoic. Received 24 September 2004; Revised 7 February 2005; Accepted 14 March 2005; Published 25 June 2005. Erbacher, J., O. Friedrich, P. A. Wilson, H. Birch, and J. Mutterlose (2005), Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic, Geochem. Geophys. Geosyst., 6, Q06010, doi:10.1029/2004GC000850.

1. Introduction [2] 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 Copyright 2005 by the American Geophysical Union

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 d13C excursions in contemporaneous seawater which 1 of 9


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erbacher et al.: carbon isotope stratigraphy

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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) Paleogeographic reconstruction for 90 Ma modified after Barron et al. [1981] with locations of Demerara Rise and the other sections discussed.

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 draw-down of atmospheric CO2 [Arthur et al., 1988; Schlanger et al., 1987]. The most prominent and widespread of these d13C excursions is the one paralleling the OAE 2 near the Cenomanian-Turonian boundary. The isotope excursion as well as the distribution of organic-rich 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]. [3] Here we present four high-resolution total organic carbon (TOC) and d13Corg records across the OAE 2 from Ocean Drilling Program (ODP) Sites recently drilled along a depth transect on Demerara Rise, off Suriname (western tropical 2 of 9


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Atlantic, Figure 1; see also auxiliary material1). 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 mid-Cretaceous 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.

2. Cretaceous Black Shales on Demerara Rise [4] Demerara Rise is a submarine plateau off the coast of Suriname that stretches northward and gently dips toward the abyssal plain and distal Orinoco Fan in the NW [Shipboard Scientific Party, 2004] (Figure 1). Cretaceous to Holocene shallowmarine 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 predominantly 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 redrilled during Leg 207 as Site 1 Auxiliary material is available at ftp://ftp.agu.org/apend/gc/ 2004GC000850.

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1257 also recovered the Cretaceous organic-rich shales [Hayes et al., 1972]. Samples from Site 144 were used in a number of studies focusing on paleoceanographic and source rock studies of the mid-Cretaceous Atlantic [Kuypers et al., 2002; Norris et al., 2002; Sinninghe Damste´ and Ko¨ster, 1998; Stein et al., 1986; Wilson et al., 2002].

3. Methods [5] Because of the large number of samples involved, analyses were undertaken in three laboratories. Data from Sites 1258 and 1261 were 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 massspectrometry protocols. In Hanover we used a Finnigan Delta XL IR, in Bochum a Finnigan Delta S, 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. [6] Depths for Sites 1258, 1260 and 1261 are in meters composite depth (mcd), following the shipboard splice between the different holes drilled at each site [Shipboard Scientific Party, 2004]. Samples analyzed from Site 1259 are exclusively from Hole A. Consequently, the depth for this site is given in meters below seafloor (mbsf).

4. Results: D13Corg and TOC Curves 4.1. Site 1258 [7] 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 d13Corg values between 28 and 27% (between 437 and 427 mcd, 3 of 9


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Figure 2. Total organic carbon (TOC, in red) and d13Corg data for Sites 1258, 1260, 1261, and 1259 (mcd, meters composite depth; mbsf, meters below seafloor).

Figure 2). The d13Corg values rise rapidly at 426 mcd to between 23 and 21% and remain heavy until 422 mcd. Between 422 and 415 mcd d13Corg 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. [8] Organic carbon values vary between 2 and 29% and show the highest values during the carbon isotope excursion interval (Figure 2).

4.2. Site 1260 [9] 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 d13Corg 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. [10] 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.

4.3. Site 1261 [11] Although Site 1261 is the shallowest Site of the Demerara Rise depth transect, the sediments 4 of 9


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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]. The d13Corg values below the excursion interval fluctuate around 28% (Figure 2). The values start to increase at 637 mcd and heavy values between 23 and 21% persist until 630 mcd where a gradual decrease of the d13Corg values begins. Site 1261 recovered the most expanded (9 m) CTBI of the Leg 207 cores (from 637 to 628 mcd). [12] 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%.

4.4. Site 1259 [13] At Site 1259 dark organic-rich shales unconformably overly a tidally influenced mudstone (546 mbsf) [Shipboard Scientific Party, 2004]. Heavy d13Corg 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 artifact 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%. [14] The late onset of black shale deposition at Site 1259 points to a paleowater depth shallower than at Site 1261 as the deposition of openmarine dark shales there began well below the CTBI. Thus the black shale sequence at Demerara Rise appears to be transgressive confirming a

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Late Cenomanian sea level rise [Hardenbol et al., 1998].

5. Site to Site Correlation [15] The high-resolution d13Corg 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 labeled 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). [16] 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. [17] The correlations of the d13Corg 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).

6. Chemostratigraphic Correlation With Other OAE 2 Sections [18] The d13Corg 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, Tunisia [Nederbragt and Fiorentino, 1999] (Figure 4). This allows the records to be stratigraphically correlated and discriminate regional from global isotopic signals. 5 of 9


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Figure 3. Correlation of the d13Corg 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.

[19] 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 welldefined as ‘‘A,’’ ‘‘C,’’ and ‘‘D,’’ is tentatively correlated to similar troughs of the d13Corg record from Eastbourne and the d13Ccarb 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’’ in the sense of Paul et al. [1999] at Eastbourne and the other sections. The following trough (‘‘Trough Interval’’ in the sense of 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. [20] 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 6 of 9


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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.

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). [21] 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 d13Corg 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].

7. Average Sedimentation Rates [22] A number of studies have published cyclostratigraphies across the CTBI [Caron et al., 1999; Kuhnt et al., 1997, 2001; Prokoph et al., 2001; Scopelliti et al., 2004] resulting in estimates for the duration of OAE 2 between 320 and 400 kyr. 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 by Tsikos et al. [2004]). Accordingly, the excursion interval which was suggested to define OAE 2 must be longer than 400 kyr. 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 kyr. The upper and lower limits of the calculations of Prokoph et al. [2001] lie in the middle of 7 of 9


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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 S. Voigt et al. (Chronology of short-termed sealevel, carbon cycle, and climate variations during the Cenomanian-Turonian Boundary Event in NW Europe, submitted to Cretaceous Research, 2005) resulted in a duration of 400 kyr 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 kyr 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 kyr 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.

Acknowledgments [23] 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. J.E., O.F., and J.M. would like to thank the Deutsche Forschungsgemeinschaft (DFG), project ER 226/2-1 and MU 667/25-1, for funding this project.

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