Eos, Vol. 85, No. 6,10 February 2004

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1 Eos, Vol. 85, No. 6,10 February 2004 EOS EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION Drilling Probes Past Carbon Cycle Perturbations on the Demerara Rise PAGE 57,63 Ocean Drilling Program (ODP) Leg 207 recently cored sediments on the Demerara Rise at ~9 N in the tropical Atlantic and successfully recovered continuous records of the Paleocene/Eocene Thermal Maximum (PETM),the Cretaceous/ Tertiary boundary (K/T),and Cretaceous Ocean Anoxic Events (OAEs).The Demerara Rise, north of Suriname and French Guyana, South America, is an ideal drilling target because expanded sections of Cretaceous- and Paleogene-age deep-sea sediments are shallowly buried and exist with good stratigraphic control in expanded sections. Furthermore, the northwestern escarpment of the Demerara Rise offered the possibility of recovering sediments along a paleo-depth transect.the paleogeographic position of the Demerara Rise lies within the core of the tropics in a location near the Equatorial Atlantic Gateway between South America and Africa, which is believed to have played an important role in controlling changes in global climate during the Cretaceous. The fate of fossil fuel carbon, which is presently being added to the atmosphere and contributes to global climate change, is one of the major concerns of modern society Learning from fossil archives of climate change will help us better understand the potential consequences of human-induced changes to the global carbon cycle. Spectacular examples of rapid (1 k.y to 1 m.y) and massive perturbations of the global carbon cycle and extreme changes in Earth's climate occurred during the Cretaceous and Paleogene periods; for example, the OAEs and the PETM. High-resolution paleoceanographic records from ocean drill sites, particularly from the tropics, which are so important in driving global ocean-atmospheric circulation, are required to understand the causes and effects of these global events. Geologic History of the Demerara Rise The Demerara Rise stretches -380 km along the coast of Suriname, and is -220 km wide BY JOCHEN ERBACHER, DAVID MOSHER, MITCHELL MALONE, AND THE ODP LEG 207 SCIENTIFIC PARTY from the shelf break to the northeastern escarpment. Most of the plateau lies in shallow water (-700 m),but the northwestern margin is a gentle ramp covered by a nearly uniform drape of pelagic sediment that reaches water depths of 3000 to 4000 m. Leg 207 drilling focused on the northwestern extent of this plateau, in 1800 to 3400 m water depth (Figure 1). The Demerara Rise is built on rifted continental crust of Precambrian and early Mesozoic age. Tectonic reconstruction of the equatorial Atlantic places the Demerara Rise south of Dakar, Senegal, prior to the rifting of Africa from South America, and was one of the last areas in contact with West Africa during the opening of the equatorial Atlantic. Rifting processes and related transform faulting separated the Guinea and Demerara plateaus along an east-west-striking fault system during the earliest Cretaceous. Barremian basalts have been recovered in industry wells from the eastern Demerara Rise, suggesting that rifting began in the Early Cretaceous.The first known marine sediments on the Demerara Rise are Neocomian in age. The northern edge of the plateau was thought to have subsided rapidly and reached water depths of nearly 2000 m by late Cenomanian time [Arthur and Natland, 1979]. Seismic Stratigraphy Seismic reflection profiles over the Demerara Rise show several distinct sedimentary packages (Figure lb); the lowermost is part of the pre- Cenomanian synrift sequence and demonstrates extensive deformation through extensional and translational faulting, thrusting, and folding. A pronounced regional angular unconformity (reflector C; Figures lb and lc) separates this sequence from overlying Cenomanian and younger sediments. This sequence is more or less flat-lying and not tectonically deformed. An erosional unconformity (reflector A on Figures lb and lc) is apparent in this section across the northwestern region of the Demerara Rise. It appears to be late Oligocene to early Miocene in age. Neogene sediments (calcareous ooze) are thin or absent from the distal portions of the plateau as a result, but they thicken inboard. VOLUME 85 NUMBER 6 10 FEBRUARY 2004 PAGES Lithostratigraphy Five drill sites were completed (Sites ) as part of the ODP Leg 207 program that included 13 drill holes. The sites constitute a depth transect ranging in water depths from 1900 m down to 3200 m. Most sites were drilled to the "C" unconformity, and in some cases, samples were recovered from the synrift sequence below the unconformity Based on extensive sample recovery in the drill holes, the sedimentary succession is divided into four broad styles of deposition synrift elastics, marine "black shales," pelagic chalks and calcareous claystones, and pelagic calcareous oozes. Synrift sediments consist of mixed lithologies, including claystone, siltstone, and sandstone. Sedimentary structures in the siltstone and sandstone from Sites 1259 and 1261 suggest a very shallow marine-to-tidal environment of deposition with an Albian to Cenomanian age. Above the "C" unconformity is a 56- to 96-mthick sequence of Cenomanian to Santonian laminated black shales (organic-rich claystone), which commonly contain spectacularly wellpreserved calcareous microfossils, and stringers of limestone and rare chert (Figure 3).The black shales represent the local equivalent of widespread organic-rich sedimentation in the southern part of the mid-cretaceous Central Atlantic [e.g.,er!ich etai,2000]. Within this sequence are dark intervals with extremely high organic content (up to 29 wt % TOC) (particularly in the Cenomanian-Turonian) that alternate with light-colored, laminated foraminiferal packstones and wackestones. The organic-rich claystones are characterized by sometimes very high content of well-preserved fish debris and phosphatic nodules. There was a consistent deepening upward trend during accumulation of the black shales, from shelf to intermediate water depths. The top of the black shale sequence transitions through a glauconite-rich bioturbated claystone into overlying pelagic calcareous chalk of Campanian age. Campanian to Paleogene sediments are calcareous chalks that include long intervals of relatively constant sedimentation and generally good microfossil preservation.they are interpreted as open marine pelagic sediments. Hiatuses and mass-flow deposits are present at various intervals, especially on the extreme flanks of the plateau. A thin (1-2 mm), finegrained, white layer composed of Cretaceous calcareous nannofossils, overlain by a ~1.5-cm interval composed of clay spherules that are up to 3 mm in diameter, marks the K/T boundary Above the spherule layer lies a dark gray clay layer low in carbonate. Carbonate content returns, but a subsequent increase in clay

2 Eos, Vol. 85, No. 6,10 February 2004 Fig. 1. (a) The locations of Ocean Drilling Pro gram Leg 207 sites on the Demerara Rise are shown, (b) Industry Multi Channel Seismic line C22II demonstrates the Early Cretaceous and older synrift sedimentary sequence with numerous extensional faults capped by a regional unconformity (Reflector C) and the Late Cretaceous and Paleogene sedimentary succession that was the drilling target of Leg 207. (c) This high-resolution MCS line shows an expanded and high-resolution image of Reflector C (the Albian unconformity) and the overlying sedimentary sequence. Reflector B' correlates with the top of the black shale sequence and Reflector B correlates with the K/T boundary. Reflector A is an unconformity produced by an apparent middle Eocene erosional event. Original color image appears at back of volume. 55 W W 54 W W

3 Eos,Vol. 85, No. 6,10 February 2004 W Site m Site m Site m Site m Site m (condensed) P/E Boundary K/T Boundary Fig. 2. Lithology of the Paleocene/Eocene Thermal Maximum (PETM) and Cretaceous/Tertiary (K/T) cores across the Demerara Rise paleoceanographic depth transect. Note the abrupt change from light, carbonate rich chalk to dark green and red claystones that mark the PETM. Site 1260 shows a 12-cm-thick laminated succession at the base of the event. The K/T Boundary is marked by a graded bed of clayey spherules that represent the altered fallout layer. Note the thin, white layer of calcareous nannofossil chalk at the base of the spherule bed that is interpreted as deposited after resuspension caused by the seismic shock of the impact event. Depths indicate present water depth. Original color image appears at back of volume. content further up-section marks the Paleocene/ Eocene boundary, with sediments returning to calcareous ooze and chalk above. A prominent submarine channel system and erosional surface developed on the Demerara Rise in the late Oligocene to early Miocene. This surface is at or near the present sea floor at a number of drill sites. At Site 1261, however, this surface is overlain by 370 m of Neogene calcareous ooze that appears to off-lap toward deep water and the extremities of the Demerara Rise. Critical Intervals Ocean Anoxic Events. OAEs represent major perturbances of the ocean system defined by massive and synchronous deposition of organic carbon. OAEs played a fundamental role in the evolution of Earth's climatic and biotic history. Arguably, between two and six OAEs occurred during the mid- and Late Cretaceous (OAE la-d,oae 2, and OAE 3; [Schlanger and Jenkyns, 1976; Arthur et a/., 1990; Erbacher et a/., 1996]),and these are particularly important because they have left records from shallow basins to the deep sea.the thick succession of Cenomanian to earliest Campanian black shales recovered on Leg 207 covers two of the major OAEs: OAE 2, which spans the Cenomanian/ Turonian boundary, and the Coniacian to Santonian OAE 3. Specific identification and study of these intervals awaits isotopic and detailed micro-paleontologic analysis, but they likely correspond to the described intervals of very high organic carbon content (very dark layers) in the black shale sequence. Existing seismic lines do not support a silled basin model for the deposition of the black shales, but rather, an intensified oxygen-minimum layer impinging on the Demerara Rise. This interpretation is further supported by occasional glauconite-rich bioturbated intervals within the Coniacian, and especially, in the Santonian at the shallower sites marking the weakening or even retreat of the oxygen minimum zone. Thick fallout layers at the K/T Boundary. The ~1.5-cm interval of clay spherules over a thin (1-2 mm), fine-grained, white lamina of Cretaceous calcareous nannofossils marks the K/T boundary and is notable from three of the drill sites.the thickness of the spherule layer is the same across the plateau depth transect (Figure 2),suggesting the bed is not reworked but represents primary ejecta fallout from the event. The thin, white lamina is believed to represent sediment deposited after resuspension caused by the seismic shock of the K/T impact event.the overlying clay layer resulted from faunal extinctions and lack of productivity of calcareous and siliceous biota following the impact.the Demerara Rise is located km southeast of the Chixculub impact crater on the Yucatan peninsula.this evidence of the K/T impact and traces of impact ejecta in a K/T boundary section from northeastern Brazil [Albertao et al., 1994] are the only evidence of the event from the South American continent. The first tropical depth transect of the PETM. Global warming at the end of the Paleocene is believed to be the precursor of the dramatic events at the Paleocene/Eocene boundary producing the PETM. Models suggest that this

4 Eos, Vol. 85, No. 6, 1 0 February 2004 warming led to dissociation of methane gas hydrates of the o c e a n margins with a conse quent abrupt release of methane to the atmosphere, where it is oxidized to carbon dioxide [Dickens, 2000].This increase of atmospheric carbon dioxide led to even higher global temperatures, which served as an amplifier for the methane release. Recent investigations have shown that the injection of fossil carbon to the atmosphere probably occurred in a series of short, few thousandyear-long steps [Norris and Rohl, 1999]. The PETM record on the Demerara Rise shows a pronounced and sharp lithologic change from calcareous chalks to green, clay-rich beds (Figure 2).This change is documented in 10 cores at five sites across a depth transect of 1300 m present water depth, and records the hypothesized calcite dissolution associated with the sudden methane released during the PETM.The oxidation of methane to carbon dioxide decreased the ph of sea water, dissolved calcite, and hindered the precipitation of calcareous shells of benthic organisms [e.g., Dickens,2000].Accordingly no calcareous microfossils are present in the clay beds. These clay layers are massive at the shallow and deep sites of the transect and laminated at the intermediate sites.the re-occurrence of bioturbation is paralleled by the return of poorly preserved and tiny benthic foraminifers, followed by a distinct planktonic foraminiferal fauna typical for the PETM. At the Demerara Rise, the thickness of clay and clay-rich lithologies associated with the PETM is approximately 1.5 m. Given a sedimentation rate of about 1.5 c m / k. y, t h e duration of the event, including recovery of the system, was at least as long as 100,000 k.y Future A B Fig. 3. Typical fades types of Cretaceous black shales recovered during Leg 207. (a) This cyclic succession shows dark-laminated, organic carbon-rich claystones and light-laminated foraminiferal packstones. (b) Dark-laminated, organic-rich claystones with occasional lenses of foraminifers and phosphatic pebbles are shown. Studies The sediments recovered during ODP Leg 207 serve the unique potential of high-resolu tion studies of multiple critical boundaries and paleoclimatic/paleoceanographic events of the Cretaceous-Paleogene greenhouse world.the investigation of paleoceanographic proxy data of sediments and microfossils will contribute to a tremendous increase of knowledge about these events. However, understanding the role of Atlantic rifting and margin tectonics is critical to the timing of south Atlantic and north Atlantic o c e a n water mixing, and to establishing the paleo-water depths for the Demerara Rise, and thus, to understanding the paleoceanography of the Demerara Rise. Seismic and sedimentologic studies will provide criti cal linkages between drill sites, and lead to an understanding of the depth transect of the plateau.they will also allow extrapolation of all results to a regional or ocean scale. Geochemistry and microbiology studies will reveal the processes of black shale deposition and decomposition of organic matter, and will b e particularly important in understanding the role of Cretaceous o c e a n i c anoxic events in the Earth's climate history and participating countries under management of the Joint Oceanographic Institutions, Inc. We thank the technicians, drilling operations team, and ship's crew who sailed on Leg 207 for their exemplary support. The Leg 207 ship board scientific party (including the three authors) are: Debora Berti, Karen Bice, Helen Bostock,Hans Brumsack,Taniel Danelian, Astrid Forster, Christine Glatz, Felix Heidersdorf, Jorijntje Henderiks,Thomas Janecek, Christopher Junium, Laurence Le Callonnec, Ken MacLeod, Phil Meyers, Jorg Mutterlose, Hiroshi Nishi, Richard Norris, J a m e s Ogg, Matthew O'Regan, Brice Rea, Philip Sexton, Helen Sturt,Yusuke Suganuma, Jurgen Thurow, Paul A.Wilson, and Sherwood Wise, Jr. References Acknowledgments Albertao, G. A., E. A. M. Koutsoukos, M. P S. Regali, M.Attrep,Jr.,and RPMartins, Jr., ( ), T h e Creta ceous-tertiary b o u n d a r y in southern low-latitude regions: Preliminary study in P e r n a m b u c o, north eastern Brazil, Terra Nova, 6, Arthur, M. A., H.-J. Brumsack, H.C., Jenkyns, and S. 0. Schlanger, ( ), Stratigraphy, g e o c h e m i s t r y and p a l e o c e a n o g r a p h y of organic carbon-rich Creta c e o u s s e q u e n c e s, in Cretaceous Resources, Events and Rhythms, edited by R. N. Ginsburg and B. B e a u d o i n, pp , Kluwer, Dordrecht. This research used samples and data provided by the O c e a n Drilling Program, which is spon sored by the U.S. National S c i e n c e Foundation Arthur, M.A.,and J. H. Natland, ( ), C a r b o n a c e o u s sediments in the north and south Atlantic: T h e role of salinity in stable stratification of Early Cretaceous basins, in Deep Drilling Results in the Atlantic Ocean: Continental Margins and Paleoenvironment, edited by M.Taiwani,WW. Hay, a n d W B. E Ryan, pp , AGU, Washington, D.C. Dickens,G. R., ( ), Methane oxidation during the Late P a l e o c e n e thermal m a x i m u m, Bull.Soc. Geol. France, 1 7 1, Erbacher, J., J.Thurow, and R. Littke, ( ), Evolution patterns of radiolaria and organic matter variations-a n e w a p p r o a c h to identify sea-level c h a n g e s in mid-cretaceous pelagic environments, Geol, 24, Erlich, R. N., O. Macsotay A. J. Nederbragt, a n d M. Antonieta Lorente, ( ), Birth and death of the Late C r e t a c e o u s "La Luna Sea," and origin of the Tres Esquinas phosphorites, J. South Am. Earth Sci., 75, Norris, R. D.,and U. Rohl, ( ), C a r b o n cycling and c h r o n o l o g y of climate warming during the P a l a e o c e n e / E o c e n e transition,nature, 401, Schlanger,S.O.,and H.C. Jenkyns, ( ), C r e t a c e o u s a n o x i c events: Causes and c o n s e q u e n c e s, Geol. Mijnb., 55,79-184,1976. Author Information J o c h e n Erbacher, Federal Institute for G e o s c i e n c e s and Natural Resources, Hannover, Germany; David Mosher, Natural R e s o u r c e s Canada, Halifax; Mitchell Malone, Integrated O c e a n Drilling Program,Texas A&M University, College Station; and the ODP Leg 207 Scientific Party For additional information, contact J o c h e n Erbacher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2,30655 Hannover, Germany;

5 Eos,Vol. 85, No. 6,10 February 2004 Page 57 NW SE Fig. 1. (a) The locations of Ocean Drilling Program Leg 207 sites on the Demerara Rise are shown, (b) Industry Multi Channel Seismic line C2211 demonstrates the Early Cretaceous and older synrift sedimentary sequence with numerous extensional faults capped by a regional unconformity (Reflector C) and the Late Cre taceous and Paleogene sedimentary succession that was the drilling target of Leg 207. (c) This high-resolu tion MCS line shows an expanded and high-resolution image of Reflector C (thealbian unconformity) and the overlying sedimentary sequence. Reflector B correlates with the top of the black shale sequence and Reflector B correlates with the K/T boundary. Reflector A is an unconformity produced by an apparent middle Eocene erosional event.

6 Eos, Vol. 85, No. 6,10 February 2004 Fig. 2. Lithology of the Paleocene/Eocene Thermal Maximum (PETM) and Cretaceous/Tertiary (K/T) cores across the Demerara Rise paleoceanographic depth transect. Note the abrupt change from light, carbonate rich chalk to dark green and red clay stones that mark the PETM. Site 1260 shows a 12-cm-thick laminated succession at the base of the event. The K/T Boundary is marked by a graded bed of clayey spherules that represent the altered fallout layer. Note the thin, white layer of calcareous nannofossil chalk at the base of the spherule bed that is interpreted as deposited after resuspension caused by the seismic shock of the impact event. Depths indicate present water depth. Page 63