Climate–ocean coupling off North-West Africa during the Lower Albian: The Oceanic Anoxic Event 1b

Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p a l a e o

Climate–ocean coupling off North-West Africa during the Lower Albian: The Oceanic Anoxic Event 1b Peter Hofmann a,⁎, Isabel Stüsser a, Thomas Wagner b, Stefan Schouten c, Jaap S. Sinninghe Damsté c a b c

Institute for Geology and Mineralogy, University of Cologne, Zülpicher Str. 49a, 50674 Köln, Germany School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom Department of Marine Organic Biogeochemistry, NIOZ Royal Netherlands Institute for Sea Research, Post Office Box 59, 1790 Den Burg, The Netherlands

A R T I C L E

I N F O

Article history: Received 28 November 2007 Received in revised form 21 February 2008 Accepted 26 February 2008 Keywords: Oceanic Anoxic Event 1b Black shale Sea surface temperatures Upwelling system

A B S T R A C T High-resolution records of organic and inorganic geochemical proxies document environmental changes during Oceanic Anoxic Event 1b for the Albian upwelling system at DSDP Site 545 Mazagan Plateau off NW Africa. Sea surface temperature estimates based on TEX86 analyses show an abrupt rise by ~ 3 °C concurrent with a more than 2-fold increase in accumulation rates for organic carbon and siliciclastic sediment components. Geochemical grain-size indicators (Si/Al and Zr/Al) indicate the influx of overall finer grained sediment during the event. Our data suggest that the warming phase associated with OAE 1b resulted in an attenuation of the NE trade wind system and a weakening of local upwelling conditions. Higher amounts of precipitation over NW Africa and an associated higher continental run off may have supplied nutrients to the eastern North Atlantic and fostered the production of organic matter and black shale deposition. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The Cretaceous is characterized by repetitive and isochronous perturbations of the global carbon cycle, commonly referred to as Oceanic Anoxic Events (OAEs, Schlanger and Jenkyns, 1976; Jenkyns, 1980; Arthur et al., 1990; Bralower et al., 1994; Jenkyns, 2003), which resulted in the episodic and widespread deposition of organic matter (OM)-rich marine black shale. The lower Albian OAE 1b has recently received increasing attention (e.g., Bréhéret, 1994, 1997; Erbacher and Thurow 1998; Erbacher et al., 1999; Ogg et al., 1999; Erbacher et al., 2001; Barker et al., 2001; Hofmann et al., 2001; Kuypers et al., 2001; Holbourn and Kuhnt, 2001; Kuypers et al., 2002; Leckie et al., 2002; Herrle, 2002, 2003; Herrle et al., 2003, 2004; Tsikos et al., 2004, Friedrich et al., 2005, Wagner et al., 2007) although it is probably one of the less significant OAEs in terms of perturbation of the global carbon cycle. Different definitions for OAE 1b are entrenched in the literature. Leckie et al. (2002) consider OAE 1b as a period that ranges from the upper Aptian to lower Albian. In contrast Erbacher et al. (2001) and Herrle et al. (2003) applied the term OAE 1b more restricted to the lower Albian Paquier event. The Paquier event was identified in parts of the Atlantic Ocean and the Tethys realm (e.g., Coccioni and Galeotti, 1993; Bréhéret 1994, 1997; Erbacher et al., 2001; Barker et al., 2001; Kuypers et al., 2001; Herrle et al., 2003; Tsikos

⁎ Corresponding author. Fax: +49 221 4705149. E-mail address: [email protected] (P. Hofmann). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.02.014

et al., 2004; Wagner et al., 2007). The Paquier event (OAE 1b sensu stricto) is the shortest of the Cretaceous OAEs. It was estimated based on orbital tuned time models to have lasted for approximately 45 ka (Ogg et al., 1999; Erbacher et al., 2001; Herrle et al., 2003), while the Valaginian Weissert Event is thought to have lasted for ~ 2.3 Ma (Sprovieri et al., 2006), the Aptian OAE 1a between ~ 400 ka and 1 Ma (Herbert, 1992; Fiet, 2000), the upper Albian OAE 1d around 160 ka (Bornemann et al., 2005) and OAE II at the Cenomanian/Turonian boundary for ~ 850 ka (Sageman et al., 2006). OAE 1b is associated with a distinct negative shift in the oceanic and atmospheric carbon isotope reservoirs (Herrle et al., 2004; Wagner et al., 2007, 2008) and, at least locally, exceptionally high contribution of OM of non-thermophilic, marine archaea in regions of the subtropical and temperate ocean (Vink et al., 1998; Kuypers et al., 2001; 2002; Tsikos et al., 2004). Also documented for OAE 1b are a distinct rapid increase in ocean surface temperatures (Erbacher et al., 2001; Herrle et al., 2003; Wagner et al., 2008) and severe perturbations in the hydrological balance, e.g. for the southern parts of the North Atlantic Ocean and the West European coastal regions of the Tethys Ocean (Erbacher et al., 2001; Herrle, 2003; Wagner et al., 2007). The short duration, the occurrence of a highly specialized biota and the perturbation of the hydrological regime of tropical regions distinguish OAE 1b from other Cretaceous OAEs and argue for different mechanisms and feedbacks of the atmosphere–land–ocean system. The cause of the observed perturbation of the climate system during OAE 1b is still under dispute. Recent work by Wagner et al. (2008) documents a synchronous perturbation of the marine and

158

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

atmospheric carbon pools as shown by carbon isotope shifts with similar magnitude recorded in bulk carbonate, bulk organic matter, in higher plant long-chain n-alkanes and marine C27 sterenes. Coupled atmosphere–land–ocean modeling (Wagner et al., 2007) suggest the release of isotopically light CO2 into the atmosphere to be the probable cause of the observed negative carbon isotope excursion, potentially by destabilization of gas hydrates during early Albian times. Volcanogenic CO2 emission as a trigger for OAE 1b appears to be unlikely because of unrealistically high amounts of greenhouse gases required to explain the observed negative isotope excursion (Wagner et al., 2008). The modeling results indicate a moderate climate perturbation with global average warming at the onset of the event in the order of 0.3 °C followed by global cooling of about 0.8 °C throughout the event. In this study we explore the effects associated with the warming pulse during OAE 1b on the lower Cretaceous ocean–land system off the North-West African coast exemplified by DSDP Site 545 (Fig. 1). We present new continuous, millennial-scale marine and terrestrial geochemical proxy records and compare them with a high-resolution sea surface temperature record derived from TEX86 (Schouten et al., 2002; Wagner et al., 2008) to investigate the relationships between African climate, surface water thermal structure and deep ocean carbon burial. 2. Geological setting During the early Albian DSDP Site 545 was located at about 20°N (Hay et al., 1999). The analyzed marine sediments were deposited on the slope of the rifted African continental margin off shore Morocco at a paleowater depth of approximately 2000 m (Fig. 1; Hinz et al., 1984). The adjacent African continent was characterized during the Albian by

semi-arid conditions as indicated by the formation of evaporite deposits along the West African coastal regions (Pletsch et al., 1996 and references therein; Ufnar et al., 2004). Enhanced accumulation of siliceous skeletons (radiolarians and sponge spicules), combined with a sharp increase in benthic foraminifera and fish debris and a concurrent drop in planktic foraminifera diversity (Leckie, 1984) and the presence of sterol ethers (Wagner et al., 2008) provide evidence for upwelling and associated nutrient-rich surface waters during the latest Aptian through middle Albian at Site 545. During the early Albian DSDP Site 545 was located in the easterly trade wind belt which probably fuelled an upwelling cell off the West African coast (Poulsen et al., 1999; Handoh et al., 2003). 3. Sample material and methods Albian sediments at DSDP Site 545 are about 110 m thick and consist mainly of green nannofossil claystones composed chiefly of illite and mixed-layer clays (Shipboard Scientific Party Leg 79, 1984). The OAE 1b covers a black shale interval extending from 389.85 to 390.65 m below sea floor (Leckie et al., 2002; Herrle, 2002) which is generally faintly laminated. The top 15 cm are mottled, with burrows up to 0.5 cm in diameter. The OAE 1b black shale is embedded in bioturbated, light gray-weathering, nannofossil-rich marls. At DSDP Site 545, the Paquier event was identified based on the presence of organic matter-rich sediments (organic carbon content N2%; Wagner et al., 2008) associated with a negative bulk carbonate carbon isotope excursion in the Hedenbergella planispira planktic foraminiferal zone and the Prediscosphera columnata NC8b nannoplankton subzone (Fig. 1; Leckie, 1984; Leckie et al., 2002; Herrle, 2003; Herrle et al., 2004). The carbon isotope excursion characteristic for OAE 1b occurs at DSDP Site 545 in core 42 (Figs. 1 and 2) and is defined by a distinct

Fig. 1. Albian stratigraphy, paleowater depth and paleogeography of DSDP Site 545 (modified after Hinz et al., 1984; Hay et al., 1999; Herrle, 2002).

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

negative carbon isotope excursion from approximately 2‰ prior to the OAE 1b interval to on average 0.75‰ during the event (Fig. 2). The termination of the OAE interval is characterized by a gradual return to isotopic values in the order of 1.2‰ (Herrle, 2002; Wagner et al., 2007). Site 545 is specifically suitable for a detailed climate study because stratigraphic control and sedimentary boundary conditions are reasonably well constrained (Leckie et al., 2002; Herrle, 2002). We focused our investigation on six meters of Core 42 known to contain the OAE 1b black shale interval (Fig. 1; Leckie et al., 2002; Herrle, 2002). Sampling of the core was performed continuously whenever possible in 1 cm-increments, to generate a high-resolution record comprised of 550 samples in total. Samples were oven-dried at 35 °C and powdered prior to analyses. Total carbon, total sulfur and total organic carbon (TOC) concentrations were measured for every sample with a LECO CS-300 instrument, the latter (TOC) after removal of carbonates with hydrochloric acid. Rock Eval pyrolysis, TEX86 determination and X-ray fluorescence spectrometry analysis was conducted on a reduced sample set (80, 59 and 204 samples, respectively). Pyrolysis was performed with a Rock Eval 3 instrument following the procedure outlined in Peters (1986). Inorganic sediment chemistry was determined by wavelengthdisperse X-ray fluorescence spectrometry (XRF) following the procedure described in Hofmann et al. (1999). The TEX86 index was determined using fossil membrane lipids originating from marine crenarchaeota (Schouten et al., 2002) using high-performance liquid chromatography-mass spectrometry following the procedure outlined in Wagner et al. (2008) and instrumental condition described in Schouten et al. (2007). Accumulation rates for Al, Si, K, and organic carbon (OC) were calculated based on sedimentation rates derived

159

from an orbital tuned time model for DSDP Site 545 elaborated in Wagner et al. (2007). 4. Results 4.1. Organic matter content and quality Total organic carbon concentrations (TOC) pre and post dating OAE 1b as defined by the negative isotope excursion fluctuate between 1 and 2.5% (Fig. 2). At the onset of the OAE 1b isotope excursion, TOC rapidly increases by about 1% from 1.5 to 2.5%. The event itself is characterized by a gradual increase in TOC up to 4.9% followed by a gradual return to pre OAE values in the top ~ 10 cm of the black shale interval. Hydrogen indices (HI), an indicator of the hydrogen-richness of the bulk OM and at high values commonly associated with mainly algal and bacterial biomass, follow changes in TOC and range from 190 to remarkably high 530 mg HC/g TOC. The Rock Eval oxygen indices (OI), a measure of oxygen-richness of the bulk OM with high values supporting terrestrial OM but alternatively an indicator of oxygenation at times of OM preservation, range from 90 to 240 mg CO2/g TOC (Fig. 2). Oxygen indices show an overall inverse relationship with HI values exceeding 300 mg HC/g TOC and OI falling below150 mg CO2/g TOC across the OAE interval. 4.2. Sea surface temperatures Studies in modern oceans have shown that TEX86 mostly records sea surface temperatures (SST; Schouten et al., 2002; Kim et al., 2008) though in some upwelling areas, e.g. Santa Barbara Basin (Huguet et al., 2007), the TEX86 tends to underestimate actual SSTs (Kim et al.,

Fig. 2. Summary of the bulk carbon isotope record, the sea surface temperature development (TEX86) and the amount and bulk composition for organic matter for the OAE 1b containing interval at DSDP site 545. The δ13Ccarb data are from Herrle (2002) complemented with data from Wagner et al. (2007). Note the distinct offset in the δ13Ccarb curve which marks the onset of OAE 1b. TEX86 data from 389.5 to 391 mbsf are from Wagner et al. (2008). The broken line in the TEX86 column is thought to reflect the overall 1 °C increase in the base line of the SST record. T-cycles 1–3 are TEX86 defined cyclic SST fluctuations. Estimates for the time content of the T-cycles are in the order of 100–200 ka based on the sedimentation rates displayed in Fig. 5 (Wagner et al., 2007).

160

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

Fig. 3. Aluminum normalized element ratios which are interpreted to reflect fluctuation in grain-size. Note the excellent co-variation in the Si/Al and Zr/Al ratios which are thought to represent the fluctuation in the relative abundance of quartz and zircon with respect to aluminum-rich clay minerals. The K/Al ratio is indicative for the compositional variations in the clay mineral assemblage. For T-cycles 1–3 and TEX86 data see Fig. 2.

2008). Our SST estimates based on TEX86 calculations for Site 545 range from ~29 °C to 33 °C (Fig. 2) and are in the same range as SST estimates for the Cretaceous oceans based on modeling and δ18O estimates from pristinely preserved planktic foraminifera (Poulsen et al., 2001; Wilson and Norris 2001; Norris et al., 2002; Huber et al., 2002; Schouten et al., 2003; Steuber et al., 2005; Bice et al., 2006; Forster et al., 2007a,b; Bornemann et al., 2008). Therefore, we interpret our TEX86 data as SSTs although actual Albian SSTs may have been even somewhat higher. The typical sample thickness in this study is 1 cm and averages a period of 0.5–1 ka depending on sedimentation rate. The SST-estimate curve in Fig. 2 (Tex86) represents, therefore, the averaged long term SST development off NW Africa. The SST profile reveals an overall warming trend by approximately 1 °C over the investigated interval with superimposed cyclic fluctuations. Moderate warm/cold cycles with a temperature range of 1.5 °C are clearly recognized in the lower, pre OAE part of the record (Figs. 2 and 3). This general SST trend is disrupted during OAE 1b. The onset of the event is marked by a rapid temperature increase by ~ 3.5 °C at its base (Wagner et al., 2008) which is more than twice as the observed temperature fluctuation in the pre OAE section. Sea surface temperatures remained high, between 32 °C and 33 °C, throughout the event and return to the general temperature trend with superimposed warm/cold cycles following the termination of OAE 1b (Fig. 2).

and illite as range between 2 and 3 (Weaver and Pollard, 1975). Si and Zr and, to a smaller degree, Si and Ti correlate significantly (Fig. 4). The elements Zr and Ti are typically concentrated in the heavy mineral fraction (Taylor and McLennan, 1985) as zircon, rutile or other Tibearing phases. The correlation of Si with Ti and Zr supports a common terrestrial source for the three elements. X-ray diffraction data from the clay mineral fraction revealed that illite (K-rich), and smectite (Al-rich) are the dominant clay minerals present in the lower Albian sediments of Site 545 (Chamley and Debrabant, 1984). Kaolinite, chlorite, palygorskite and mixed-layer minerals only occur

4.3. Composition of the clastic flux Compositional variations in the siliciclastic fraction across the studied interval are reconstructed from aluminum normalized metal ratios. Records of Si/Al and Zr/Al show cyclic low amplitude variations (Fig. 3) in the pre OAE part of the section. High Si/Al ratios of 3.5–5 support the presence of elevated amounts of quartz taking into consideration Si/Al ratios of the widespread clay minerals smectite

Fig. 4. Correlation of Zr, Ti, and Si. The good correlation suggests that all elements are delivered to DSDP Site 545 as part of the siliciclastic flux. For further information see text.

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

161

Fig. 5. Sedimentation rates and selected element accumulation rates indicative of the development of the siliciclastic flux and organic matter accumulation at DSDP Site 545. Sedimentation rates from Wagner et al. (2007, left column) are used for the calculation of accumulation rates. Al and OC flux rates are from Wagner et al. (2007). The gray-shaded area represents the OAE 1b interval.

Fig. 6. Variation of selected redox-sensitive elements and element ratios. The broken lines represent average shale values (AVS from Brumsack, 2006). V/Al ratios from 389 to 393 mbsf are from Wagner et al. (2007). Note that OAE 1b interval is not significantly enriched in redox-sensitive elements.

162

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

in lower concentrations. K/Al ratios may hence serve as a proxy for the relative abundance of illites over smectite. Illites are generally formed during sediment diagenesis and are often recycled as detrital sediment components whereas smectitic clays are found in hydromorphic soils or form during hydrolyzing climate conditions and thus represent moist climate conditions (Chamley, 1989; Weaver, 1989 and references therein). Illite abundance as indicated by relatively high K/ Al ratios follows the pattern of other detrial sediment components e.g. zirconium and quartz abundance (as revealed by Zr/Al and Si/Al ratios; Fig. 3) which are thought to be indicative for a coarsening in grain-size (Shimmield, 1992; Martinez et al., 1999; Moreno et al., 2001; Zabel et al., 2001). The onset of OAE 1b is marked by a drop in K/Al, Si/Al and Zr/Al ratios with Si/Al and Zr/Al ratios remaining low after the termination of OAE 1b. The synchronous increase in the flux of Al, Si, Zr and K in the OAE 1b interval indicates a significantly higher supply of detrital sediment components to Site 545 compared to pre and post OAE 1b conditions (Fig. 5). The concurrent drop in Si/Al, K/Al and Zr/Al ratios in the OAE interval is, hence, most likely caused by a higher supply of fine grained clay minerals at the expense of coarser grained sediment components. The observed accumulation pattern in conjunction with the concurrent shift in sediment chemistry to more aluminium-rich sediment components (clay minerals) suggests a change in the transport processes of the detrital sediment fraction. 4.4. Redox-sensitive elements Redox-sensitive trace elements show only moderate enrichment with respect to average shale values (AVS; Taylor and McLennon, 1985; Brumsack, 2006; Fig. 6) for the investigated interval. V/Al ratios are fairly low at values between 15 and 35 (Fig. 6). Ni/Al ratios fall slightly above AVS values, with moderate enrichment towards the top of the OAE 1b (Fig. 6). Zn/Al ratios are with few exceptions on average approximately twice as high as AVS in the investigated core section and are not significantly enriched in OAE 1b interval. The aluminum normalized trace element concentrations at Site 545 are generally low compared to other Albian black shales from the North Atlantic which range e.g., in V/Al ratios from 20 to 200 and Ni/Al ratios from 8 to 80 (Hofmann et al., 1999; Hofmann et al., 2001), but are similar to trace metal/aluminum ratios from modern upwelling environments (Brumsack, 2006). The correlation of Ni and Zn concentrations with TOC content (Fig. 7) but not with total sulfur content (S total; r2 = 0.26 and 0.19 respectively; not shown), suggests both elements behaved

Fig. 7. Correlation of Ni, Zn and TOC at DSDP Site 545. The good correlation suggests that Ni and Zn acted as micronutrients (for discussion see text).

Fig. 8. TOC vs Zr/Al crossplot. The Zr/Al ratio is used as a measure for grain-size and wind strength (for discussion see text). Note the trend of higher TOC values with increasing Zr/Al ratios in the pre OAE section (shaded area). The trend is disrupted for the OAE and most of the post OAE interval. Only samples representing the top part of the post OAE interval fall on the trend of the pre OAE sample set.

primarily as micronutrients at Site 545 and were embedded into the sediment via absorption to and complexation processes with organic matter (Tribovillard et al., 2006 and references therein). Anoxic, or even euxinic bottom water conditions which occurred during other Cretaceous OAEs (e.g., OAE 1a and OAE 2; e.g. Kuypers et al., 2004) and resulted in trace metal enriched black shale deposits (Brumsack, 2006) were probably not established during OAE 1b at Site 545. A drop in the abundance of benthic foraminifera in the OAE 1b interval at Site 545 (Herrle, 2002; Friedrich et al., 2005) indicates somewhat lower oxygen levels of the bottom waters than during the pre and post OAE periods even though strictly anoxic conditions with an oxygen content b ~ 0.2 ml O2/l H2O were never reached (Algeo and Maynard, 2004). 5. Discussion 5.1. Upwelling at Site 545 prior to OAE 1b Previous work suggested that Site 545 was located in an upwelling controlled environment during the early Albian (Leckie, 1984). Our new data seem to not only support this interpretation but add more detail into key ocean properties and mechanisms. The long term pre OAE 1b SST record is characterised by moderate cyclic fluctuations with an amplitude of ~1.5 °C (Fig. 2). These temperature fluctuations are comparable to the amplitude of Late Quaternary fluctuations in modern upwelling areas of NW Africa (Cape Blank; Sicre et al., 2000, 2001), however, offset by up to 10 °C higher values compared to the Holocene (Sicre et al., 2000, 2001). The TEX86 derived mean long term temperature range is between ~ 29 and 31 °C and, hence, similar to SST estimates for the OAE 1b interval at ODP Site 1049 located in the Eastern Atlantic (Schouten et al., 2003; Wagner et al., 2008) and in good agreement with modelled shallow marine mid-Cretaceous temperatures (Poulsen et al., 1999). The latter are thought to have ranged between 29 °C and 35 °C. The high SSTs at Site 545 reflect the all over significantly warmer global oceanic surface and bottom water temperatures of the mid-Cretaceous oceans (Poulsen et al., 2001; Wilson and Norris 2001; Norris et al., 2002; Huber et al., 2002; Schouten et al., 2003; Steuber et al., 2005; Bice et al., 2006; Forster et al., 2007a,b). The last warm/cold cycle (T-cycle 2 in Figs. 2 and 3) prior to OAE 1b in particular is characterized by an overall inverse relationship between mean SST and inorganic grain-size indicators (e.g., Zr/Al, Si/ Al and K/Al; Fig. 3). Previous work has shown that a large fraction of the siliciclastic sediment delivery to the Cretaceous Atlantic off NW

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

Africa can be attributed to eolian transport processes (Lever and McCave, 1983; Chamley, 1989). Increasing Zr/Al and Si/Al ratios during cooler SST phases at Site 545 may therefore reflect changes in wind intensity and higher transport capacity in conjunction with the advection of cooler bottom waters during periods of more intense upwelling which resulted in a rise of the paleo-thermocline. The overall inverse relationship between SST and wind intensity in the pre OAE interval is in good agreement with the existence of a trade winddriven upwelling system off West Africa in the Albian (Leckie, 1984; Poulsen et al., 1999; Herrle, 2002). The correlation of organic carbon and Zr content in the pre OAE interval (Fig. 8) further suggests a link of organic matter production, wind strength and upwelling intensity. Organic matter from this interval is characterized by HI values around 300 mg HC/g TOC and OI values N125 mg CO2/g TOC pointing towards poor preservation conditions and contributions of organic matter from marine and terrestrial sources. The occurrence of e.g., Δ4 and Δ5 sterenes, sterol ethers and crenarchaeota biomarker at low abundance as well as the presence of long-chain odd-numbered alkanes through the entire investigated interval also support a mixture of marine and terrestrial derived organic matter (Wagner et al., 2008). The production of marine organic matter at Site 545 is probably directly linked to upwelling conditions, whereas the terrestrial derived organic matter is more likely to reflect the background supply transported by runoff from the African margin to the adjacent ocean basin. Fairly low Al-normalized concentrations of redox-sensitive trace metals and the presence of benthic foraminifera throughout the OAE 1b containing interval at Site 545 (Friedrich et al., 2005) confirm that OM of the pre OAE period accumulated in an oxygenated water column, supporting results from coupled atmosphere–land–ocean modeling which suggest oxygen concentrations in the order of 100 µM (Wagner et al., 2007). 5.2. The Paquier event (OAE 1b) at Site 545 The upwelling regime which characterized the pre OAE interval was disrupted during the onset of OAE 1b. Most notably, mean SST rose by more than 3.5 °C over a short time period (estimated 1–3 ka; Wagner et al., 2008) to more than 33 °C, an increase more than twice as high as the amplitude of SST fluctuations observed for the pre OAE interval (1.5 °C). The observed rise in SSTs by 3.5 °C at DSDP Site 545 is an order of magnitude higher than the modelled global temperature rise for OAE 1b of 0.3 °C (Wagner et al., 2007), arguing for an amplification of the temperature signal by local feedback mechanisms, such as regionally reduced upwelling. The drastic increase in SST was accompanied by the onset of progressive burial of hydrogen-rich OM of marine origin at the sea floor. During OAE 1b the organic matter flux to the sea floor on average doubled compared to the pre OAE stage suggesting a substantial increase in surface water productivity and most likely improved preservation conditions (Fig. 5). A three phase development of organic matter accumulation is recognized for the OAE 1b interval. The initial phase comprises the onset of the negative carbon isotope excursion, a continuous increase in TOC values from 2 to ~3.5%, a gradual increase in HI values from 300 to 400 and a progressive drop in OI values 170 to below 100 (Fig. 2). The increase in HI and drop in OI values corresponds to a gradual reduction in the abundance of benthic foraminifera reported by Friedrich et al. (2005) for this interval and indicates a progressive drop in bottom water oxygenation levels and hence improving preservation conditions for organic matter. The second phase is characterized by TOC values N3.5%, HI values N400 and OI values ~100 (Fig. 2) as well as a very low abundance of benthic foraminifera (Herrle, 2002; Friedrich et al., 2005), suggesting lower bottom water oxygenation levels and good preservation conditions. A moderate increase in Stot values and Ni/Al ratios may also indicate improving preservation conditions (Fig. 6). The preservation and

163

accumulation of organic matter was probably also enhanced by higher sedimentation rates during phase 2 of the OAE 1b development (Fig. 5). An increase in sedimentation rate effectively decreases the exposure time of organic matter to oxidants (oxygen, sulphate or nitrate) at the sediment water interface and thus reduces the rate of organic matter degradation (Canfield, 1989; Hedges et al., 1999). In the third phase representing the termination of OAE 1b, TOC values drop from N4% to less than 2%. Decreasing HI values and progressively increasing OI values, the onset of sediment bioturbation and an increase in the abundance of benthic foraminifera to pre OAE levels (Friedrich et al., 2005) document the re-establishment of oxygenated bottom water conditions. Even though oxygenation levels fluctuated during OAE 1b at Site 545 bottom waters were oxygen deficient during the event, but never anoxic indicated by the relatively low abundance of redox-sensitive trace metals. Modelling results predict a global increase in export productivity of more than 15 Tmol of marine particulate organic carbon per year during OAE 1b accompanied by a moderate reduction in deep ocean water oxygenation by about 10 µM to concentrations above 90 µM (Wagner et al., 2007) suggesting that both enhanced organic matter production and improved preservation conditions fostered the accumulation of organic matter. The increase in marine organic matter accumulation during OAE 1b at DSDP Site 545 was probably not caused by intensification of local upwelling conditions, even though the flux of alumo-silicates to Site 545 almost doubled (Al, Si and K flux; Fig. 5), because Si/Al, Zr/Al and K/Al ratios and, hence wind intensity is expected to have markedly dropped (Fig. 3). The co-variation between organic matter and Zr content characteristic for the pre OAE interval is not observed in OAE 1b interval (Fig. 8) supporting a decoupling of organic matter production from wind strength and resulting upwelling intensity. Preferred delivery of Al-rich clay minerals (low Si/Al, Zr/Al and K/Al ratios) in conjunction with rising SSTs during OAE 1b suggests warmer regional climate conditions leading to an accelerated hydrological cycle which fostered precipitation along the African continental margin and enhanced neo-formation of clay minerals. The attenuation or break down in (trade) wind strength for the North African region may have triggered a northward expansion of monsoonal circulation into subtropical latitudes stimulating even higher amounts of continental run off and enhanced the delivery of nutrients and clastic sediment to the adjacent ocean basins (Herrle et al., 2003). At DSDP Site 545 elevated nutrient fluxes resulting in a doubling of the sedimentary phosphorous flux during OAE 1b are observed (Wagner et al., 2007). High amounts of precipitation and fluvial discharge during the event are also reported for the North American continental margin (Erbacher et al., 2001) and the South France Basin (Herrle, 2002, Wagner et al., 2007), in line with our results. 5.3. The post OAE stage The termination of OAE 1b is marked by an almost synchronous drop in SSTs (approximately 2 °C) and TOC concentration, and a 0.5‰ shift towards heavier carbon isotopes in bulk carbonate. However, sea surface temperatures remained warmer than during the pre OAE period (Fig. 2). Pre OAE stage detrital flux levels were not reached before an estimated 25 ka after the termination of the event. The generally weak trade wind system characteristic for the OAE stage persisted in the post OAE stage as indicated by continuously low Si/Al, Zr/Al and K/Al ratios. Pre OAE levels of Si/Al and Zr/Al are only reached in the top 30 cm of the investigated interval where increasing TEX86 SSTs suggest the onset of a new temperature cycle which is, however, only partially recorded in core 42 (Fig. 3). The top 30 cm of the core display TOC/[Zr/Al] relationships similar to that of the pre OAE stage were increasing TOC values correspond to increasing Zr/Al ratios (Fig. 8) suggesting an overall return to pre OAE upwelling conditions for this interval.

164

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165

The observations described above show that the perturbation of the regional climate system over NW Africa which resulted in the attenuation of the trade wind system and weakening of upwelling conditions thus persisted considerably longer than the isotope excursion associated with OAE 1b providing evidence for a significant lag time in the response of the Cretaceous land–ocean climate system to atmospheric heating. A return to pre OAE upwelling conditions may have occurred approximately 25 ka after the event, but the reasons and mechanisms for this apparent time lag are currently only poorly understood. We can only speculate that the still weakened trade wind system may have resulted in continuously moist and warm conditions over the African continent fostering intense weathering and neoformation of clay minerals and an ongoing flux of aluminium-rich sediment to the North Atlantic. 6. Summary The record from DSDP Site 545 implies that rapid atmospheric warming even with a low overall magnitude (suggested to be 0.3 °C globally by model calculations) during greenhouse conditions had severe effects on the lower Albian atmosphere–land–ocean system of NW Africa affecting the coastal upwelling system and adjacent coastal regions. The heating of the atmosphere triggered a complex chain reaction which can be explained by a sequence of feedback reactions starting with the attenuation of the NE trade wind system, reduced upwelling intensity, sea surface warming, an acceleration of the hydrological cycle, higher amounts of continental run off associated with an increasing nutrient supply to the ocean and finally black shale deposition. Our study shows that even small perturbations of the global climate system during time periods of global greenhouse conditions can be greatly enhanced by local feedback mechanisms leading to a severe reorganization of the regional atmosphere–land– ocean system. Acknowledgments Constructive comments by O. Friedrich and an anonymous reviewer helped to improve this manuscript. This work was supported by the German Science foundation (DFG) through grants to T. Wagner and P. Hofmann. We are indebted to the International Ocean Drilling Project which allowed us to sample DSDP Site 545. TW recognizes the Royal Society-Wolfson Research Merit Award. References Algeo, T.J., Maynard, J.B., 2004. Trace-element behaviour and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems. Chemical Geology 289, 218–318. Arthur, M.A., Jenkyns, H.C., Brumsack, H.-J., Schlanger, S.O., 1990. Stratigraphy, geochemistry, and paleoceanography of organic carbon-rich Cretaceous sequences. In: Ginsburg, R.N., Beaudoin, B. (Eds.), Cretaceous Resources, Events and Rhythms. NATO ASI Series, Series C, vol. 304, pp. 75–119. Dordrecht. Barker, C.E., Pawlewicz, M.J., Cobabe, E.A., 2001. Deposition of sedimentary organic matter in black shale facies indicated by the geochemistry and petrography of high resolution samples, Blake Nose, western North Atlantic. In: Kroon, D., Norris, R.D., Klaus, A. (Eds.), Wester North Atlantic Paleogene and Cretaceous Palaeoceanography. Special Publications, vol. 183. Geological Society, London, pp. 49–72. Bice, K.L., Birgel, D., Meyer, P.A., Dahl, K.A., Hinrichs, K.U., Norris, R.D., 2006. A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations. Paleoceanography 21. doi:10.1029/2005PA001203. Bornemann, A., Pross, J., Reichelt, K., Herrle, J.O., Hemleben, C., Mutterlose, J., 2005. Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian Niveau Breistroffer black shales (Oceanic Anoxic Event, 1d SE France). Journal of the Geological Society, London 162, 623–639. Bornemann, A., Norris, D.R., Friedrich, O., Beckmann, B., Schouten, S., Sinninghe Damsté, J.S., Vogel, J., Hofmann, P., Wagner, T., 2008. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science 319, 189–192. Bralower, T.J., Arthur, M.A., Leckie, R.M., Sliter, W.V., Allard, D., Schlanger, S.O., 1994. Timing and paleoceanography of oceanic dysoxia/anoxia in the Late Barremian to Early Aptian (Early Cretaceous). Palaios 9, 335–369. Bréhéret, J.G., 1994. The mid-Cretaceous organic-rich sediments from the Vocontian zone of the French South-East Basin. In: Mascle, A. (Ed.), Hydrocarbon and

Petroleum Geology of France. The European Association of Petroleum Geoscientists Special Publication, vol. 4. Springer Verlag, Berlin, pp. 295–320. Bréhéret, J.G., 1997. L'Aptien et l'Albien de la Fosse vocontienne (bordures au bassin): Èvolution de la sédimentation et enseignements sur les événements anoxiques, 25. Publications de la Société Géologique du Nord, pp. 1–614. Brumsack, H.J., 2006. The trace metal content of recent organic carbon-rich sediments: Implications for Cretaceous black shale formation. Palaeogeography, Palaeoclimatology, Palaeoecology 232, 344–361. Chamley, H., 1989. Clay Sedimentology. Springer-Verlag, Berlin. Chamley, H., Debrabant, P., 1984. Mineralogical and geochemical investigation of sediments on Mazagan Plateau northwestern African margin (Leg 79, Deep Sea Drilling Project). In: Hinz, K., Winterer, E.L., et al. (Eds.), Initial Reports of the Deep Sea Drilling Project, vol. 79. U.S. Govt. Printing Office, Washington, pp. 497–506. Canfield, D., 1989. Sulfate reduction and oxic respiration in marine sediments: implications for organic carbon preservation in euxinic environments. Deep Sea Research 6, 121–138. Coccioni, R., Galeotti, S., 1993. Orbitally induced cycles in benthonic foraminiferal morphogroups and trophic structures distribution patterns from the Late Albian ‘Amadeus Segment’ (Central Italy). Journal of Micropalaeontology 12, 227–239. Erbacher, J., Thurow, J., 1998. Mid-Cretaceous radiolarian zonation for the North Atlantic an example of oceanographically controlled evolutionary processes in the biosphere? In: Cramp, A., McLeod, C., Lee, V., Jones, E.J.W. (Eds.), Geological Evolution of Oceanic Basins: Results from the Ocean Drilling Program. Special Publication, vol. 131. Geological Society, London, pp. 71–82. Erbacher, J., Hemleben, C., Huber, B.T., Markey, M., 1999. Correlating environmental changes during early Albian oceanic anoxic event 1b using benthic foraminiferal paleoecology. Marine Micropaleontology 38, 7–28. Erbacher, J., Huber, B.T., Norris, R.D., Markey, M., 2001. Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period. Nature 409, 325–327. Fiet, N., 2000. Calibrage temporal de l`Aptien et des sous-étages associés par une approche cyclostratigraphique appliquée à la série pélagique de Marches-Ombrie (Italie central). Bulletin de la Société Géologique France 171, 103–113. Forster, A., Schouten, S., Baas, M., Sinninghe Damsté, J.S., 2007a. Mid-Cretaceous (Albian–Santonian) sea surface temperature record of the tropical Atlantic Ocean. Geology 35, 919–922. Forster, A., Schouten, S., Moriya, K., Wilson, P.A., Sinninghe Damste, J.S., 2007b. Tropical warming and intermittent cooling during the Cenomanian/Turonian oceanic anoxic event 2: sea surface temperature records from the equatorial Atlantic. Paleoceanography 22, PA1219. doi:10.1029/2006PA001349. Friedrich, O., Nishi, H., Pross, J., Schmiedl, G., Hemleben, C., 2005. Millennial- to centennial-scale interruptions of the Oceanic Anoxic Event 1b (Early Albian, midCretaceous) inferred from benthic foraminifera repopulation events. Palaios 20, 64–77. Handoh, I.C., Bigg, G.R., Jones, E.J.W., 2003. Evolution of upwelling in the Atlantic Ocean basin. Palaeogeography, Palaeoclimatology, Palaeoecology 202, 31–58. Hay, W.W., DeConto, R.M., Wold, C.N., Wilson, K.M., Voigt, S., Schulz, A., Wold-Rossby, A., Dullo, W.-C., Ronov, A.B., Balukhovsky, A.N., Söding, E., 1999. Alternative global Cretaceous paleogeography. In: Barrera, E., Johnson, C. (Eds.), Evolution of the Cretaceous Ocean/climate System. Geological Society of America Special Paper, vol. 332. Geological Society of America, Boulder, CO, pp. 1–48. Hedges, J.I., Hu, F.S., Devol, A.H., Hartnett, H.E., Tsamakis, E., Keil, R.G., 1999. Sedimentary organic matter preservation: a test for selective degradation under oxic conditions. American Journal of Science 299, 529–555. Herbert, T.D., 1992. Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments. Earth Planetary Science Letters 112, 15–18. Herrle, J.O., 2002. Paleoceanographic and paleoclimatic implications on mid-Cretaceous black shale formation in the Vocontian Basin and the Atlantic: evidence from calcareous nannofossils and stable isotopes. Tübinger Mikropaläontologische Mitteilungen 27, 1–114. Herrle, J.O., 2003. Reconstructing nuticline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France). Marine Micropaleontology 47, 307–321. Herrle, J.O., Pross, J., Friedrich, O., Kößler, P., Hemleben, C., 2003. Forcing mechanism for mid-Cretaceous black shale formation: evidence from the Upper Aptian and Lower Albian of the Vocontian Basin (SE France). Palegeography, Paleoclimatology, Paleoecology 190, 399–426. Herrle, J.O., Kößler, P., Friedrich, O., Erlenkeuser, H., Hemleben, C., 2004. High-resolution carbon isotope records of the Aptian to Lower Albian from SE France and the Mazagan Plateau (DSDP Site 545): a stratigraphic tool for palaeoceanographic and paleobiologic reconstruction. Earth and Planetary Science Letters 218, 149–161. Hinz, K., Winterer, E.L., et al., 1984. Initial Reports of the Deep Sea Drilling Project 79, U.S. Government Printing Office, Washington. Hofmann, P., Ricken, W., Schwark, L., Leythaeuser, D., 1999. Coupled oceanic effects of climatic cycles from late Albian deep-sea sections of the North Atlantic. In: Barrera, E., Johnson, C. (Eds.), Evolution of the Cretaceous Ocean/climate System. Geological Society of America Special Paper, vol. 332. Geological Society of America, Boulder, CO, pp. 143–159. Hofmann, P., Ricken, W., Schwark, L., Leythaeuser, D., 2001. Geochemical signature and related climatic–oceanographic processes for early Albian black shales: Site 417D, North Atlantic Ocean. Cretaceous Research 22, 243–257. Holbourn, A., Kuhnt, W., 2001. No extinctions during Oceanic Anoxic Event 1b: the Aptian–Albian benthic foraminiferal record of ODP Leg 171. In: Kroon, D., Norris, R.D., Klaus, A. (Eds.), Wester North Atlantic Paleogene and Cretaceous Palaeoceanography. Geological Society Special Publication, vol. 183. The Geological Society, London, pp. 73–94.

P. Hofmann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 262 (2008) 157–165 Huber, B.T., Norris, R.D., MacLeod, K.G., 2002. Deep-sea paleotemperature record of extreme warmth during the Cretaceous. Geology 30, 123–126. Huguet, C., Schimmelmann, A., Thunell, R., Lourens, L.J., Sinninghe Damsté, J.S., Schouten, S., 2007. A study of the TEX86 paleothernometer in the water column and sediments of the Santa Barbara Basin, California. Paleoceanography 22. doi:10.1029/2006PA001310. Kim, J., Schouten, S., Hopmans, E.C., Donner, B., Sinninghe Damsté, J.S., 2008. Global core-top calibration of the TEX86 paleothernometer in the ocean. Geochimica et Cosmochimica Acta 72, 1154–1173. Kuypers, M.M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R., Schouten, S., Sinninghe Damsté, J., 2001. Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event. Science 293, 92–94. Kuypers, M.M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R., Schouten, S., Sinninghe Damsté, J., 2002. Archaeal remains dominate marine organic matter from the early Albian oceanic anoxic event 1b. Paleogeography, Paleoclimatology, Paleoecology 185, 211–234. Kuypers, M.M.M., Lourens, L.J., Rijpstra, W.R.C., Pancost, R.D., Nijenhuis, I.A., Sinninghe Damsté, J.S., 2004. Orbital forcing of organic carbon burial in the proto-North Atlantic during oceanic anoxic event 2. Earth and Planetary Science Letters 228 (3–4), 465–482. Jenkyns, H.C., 1980. Cretaceous anoxic events: from continents to oceans. Journal Geological Society London 173, 171–188. Jenkyns, H.C., 2003. Evidence for rapid climate change in the Mesozoic–Palaeogene greenhouse world. Philosophical Transactions A of the Royal Society London 361, 1885–1916. Leckie, R.M., 1984. Mid-Cretaceous panktonic foraminiferal biostratigraphy off Central Morocco, Deep Sea Drilling Project Leg 79, Sites 545 and 547. In: Hinz, K., Winterer, E.L. (Eds.), Initial Reports of the Deep Sea Drilling Project, vol. 79. U.S. Government Printing, Washington, pp. 579–620. Leckie, R.M., Bralower, T.J., Cashman, R., 2002. Oceanic anoxic events and plankton evolution: biotic response to tectonic forcing during the mid-Cretaceous. Paleoceanography 17. doi:10.1029/2001PA000623. Lever, A., McCave, I.N., 1983. Eolian components in Cretaceous and Tertiary North Atlantic sediments. Journal of Sedimentary Petrology 53, 811–832. Martinez, P., Bertrand, P., Shimmield, G.B., Cochrane, K., Jorissen, F.J., Foster, J., Dignan, M., 1999. Upwelling intensity and ocean productivity changes of Cape Blanc (northwest Africa during the last 70 000 years: geochemical and micropalaeontological evidence. Marine Geology 158, 57–74. Moreno, A., Taragrona, J., Hendriks, J., Canals, M., Freudenthal, T., Meggers, H., 2001. Orbital forcing of dust supply to the North Canary Basin over the past 250 kyrs. Quaternary Science Reviews 20, 1327–1339. Norris, R.D., Bice, K.L., Magno, E.A., Wilson, P.A., 2002. Jiggling the tropical thermostat in the Cretaceous hothouse. Geology 30, 299–302. Ogg, J.G., Röhl, U., Geib, T., 1999. Astronomical tuning of Aptian–Albian boundary interval: oceanic anoxic event 1b through lower Albian magnetic polarity subchron M-2r. EOS Transactions 80, F491–F492. Peters, K.E., 1986. Guidelines for evaluating petroleum source rocks using programmed pyrolysis. American Association of Petroleum Geolologists Bulletin 70, 318–329. Pletsch, T., Daoudi, L., Chamley, H., Deconinck, J.F., Charroud, M., 1996. Paleogeographic controls on palygorskite occurrence in mid-Cretaceous sediments of Morocco and adjacent basins. Clay Minerals 31, 403–416. Poulsen, C.J., Barron, E.J., Johnson, C.C., Fawcett, P., 1999. Links between climatic factors and regional oceanic circulation in the mid-Cretaceous. In: Barrera, E., Johnson, C. (Eds.), The Evolution of Cretaceous Ocean/climate Systems. Geological Society of America Special Paper, vol. 133. Geological Society of America, Boulder, CO, pp. 73–89. Poulsen, C.J., Barron, E.J., Arthur, M.A., Pedterson, W.H., 2001. Response of the midCretaceous global oceanic circulation to tectonic and CO2 forcings. Paleoceanography 14 2000PA000579. Sageman, B.B., Meyer, S.R., Arthur, M.A., 2006. Orbital time scale and new C-isotope record for Cenomanian Turonian boundary stratotype. Geology 34, 125–128.

165

Schlanger, S.O., Jenkyns, H.C., 1976. Cretaceous anoxic events: Causes and consequences. Geolgie en Mijnbow 55, 179–184. Schouten, S., Hopmans, E.C., Schefuß, E., Sinninghe Damsté, J.S., 2002. Distributional variations in marine crenarchaeotal membrane lipids: a new tool for reconstructing ancient sea water temperatures? Earth and Planetary Science Letters 204, 265–274. Schouten, S., Hopmans, E.C., Forster, A., Van Breugel, Y., Kuypers, M.M.M., Sinninghe Damsté, J.S., 2003. Extremely high sea-surface temperatures at low latitudes during the middle Cretaceous as revealed by archaeal membrane lipids. Geology 31, 1069–1072. Schouten, S., Huguet, C., Hopmans, E.C., Sinninghe Damsé, J.S., 2007. Improved analytical methodology of the TEX86 paleothermometry by high performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry. Analytical Chemistry 79, 2940–2944. Shimmield, G.B., 1992. Can sediment geochemistry record changes in coastal upwelling paleo-productivity? Evidence from northwest Africa and the Arabian Sea. In: Summerhayes, C.P., Prell, W.E., Emeis, K.C. (Eds.), Upwelling Systems: Evolution Since the Early Miocene. Geological Society Special Publication, vol. 64. The Geological Society, London, pp. 29–46. Shipboard Scientific Party Leg 79, 1984. Site 545. Initial Reports of the Deep Sea Drilling Project, vol. 79. U.S. Government Printing Office, Washington, pp. 81–177. Sicre, M.-A., Ternois, Y., Paterne, M., Boireau, A., Beaufort, L., Martinez, P., Bertrand, P., 2000. Biomarker stratigraphic records over the last 150 kyrs off the NW African coast at 25°N. Organic Geochemistry 31, 577–588. Sicre, M.-A., Ternois, Y., Paterne, M., Martinez, P., Bertrand, P., 2001. Climatic changes in the upwelling region off Cap Blanc, NW Africa over the last 70 kyear: a multibiomarker approach. Organic Geochemistry 32, 981–990. Sprovieri, M., Coccioni, R., Lirer, F., Pelosi, N., Lozar, F., 2006. Orbital tuning of lower Cretaceous composite record (Maiolica Formation, central Italy). Paleoceanography 21, PA4212. doi:10.1029/2005PA001224. Steuber, T., Rauch, M., Masse, J.-P., Graaf, J., Malkoc, M., 2005. Low-latitude seasonality of Cretaceous temperatures in warm and cold episodes. Nature 437, 1341–1344. Taylor, S.K., McLennan, S.M., 1985. The Continental Crust, Its Composition and Evolution. Blackwell Science, Oxford. Tribovillard, N., Algeo, T.J., Lyons, T., Riboulleau, A., 2006. Trace metals as paleoredox and productivity proxies: an update. Chemical Geology 232, 12–32. Tsikos, H., Jenkyns, H.C., Walsworth-Bell, B., Petrizzo, M.R., Foster, A., Kolonic, S., Erba, E., Pemoli Siva, I., Baas, M., Wagner, T., Sinninghe Damsté, J.S., 2004. Organic carbon deposition in the Cretaceous of the Ionian Basin, NW Greece: the Paquier Event (OAE 1b) revisited. Geological Magazin 141, 401–416. Ufnar, D.F., González, L.A., Ludvigson, G.A., Brenner, R.L., Witzke, B.J., 2004. Evidence for increased latent heat transport during the Cretaceous (Albian) greenhouse warming. Geology 32, 1049–1052. Vink, A., Schouten, S., Sephton, S., Sinninghe Damsté, J.S., 1998. A newly discovered norisoprenoid, 2, 6, 15, 19, thetramethylicosane, in Cretaceous black shales. Geochimica et Cosmochimica Acta 62, 965–970. Wagner, T., Wallmann, K., Herrle, J.O., Hofmann, P., Stüsser, I., 2007. Consequences of moderate ~ 25,000 year lasting emission of light CO2 into the mid-Cretaceous ocean. Earth and Planetary Science Letters 259, 200–211. Wagner, T., Herrle, J.O., Sinninghe Damsté, J.S., Schouten, S., Stüsser, I., Hofmann, P., 2008. Rapid warming of Cretaceous surface waters in the subtropical North Atlantic in response to the emission of carbon to the atmosphere. Geology 36, 203–206. Weaver, C.E., 1989. Clays, muds, and shales. Developments in Sedimentology, 44. Elsevier, Amsterdam. Weaver, C.E., Pollard, L.D., 1975. The chemistry of clay minerals. Developments in Sedimentology, 15. Elsevier, Amsterdam. Wilson, P.A., Norris, R.D., 2001. Warm tropical ocean surface and global anoxia during the mid-Cretaceous period. Nature 412, 425–429. Zabel, M., Schneider, R.R., Wagner, T., Adegbie, A.T., de Vries, U., Kolonic, S., 2001. Late Quaternary climate changes in Central Africa as inferred from terrigenous input to the Niger Fan. Quaternary Research 56, 207–217.