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1 Department of Earth and Environmental Sciences, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES CITED |
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5–10 °C, higher rates of continental weathering, elevated extinction rates, and large-scale perturbations to the global carbon cycle. The major OAEs also overlapped temporally the emplacement of large igneous provinces. However, despite being known as OAEs, the extent of seawater anoxia at those times is undefined and the causative processes remain unclear. Here we show how changes in seawater molybdenum isotope ratios (a proxy for seawater anoxia) during the Toarcian (Early Jurassic) OAE define the onset and expansion of oxygen deficient conditions. Our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically, broadly in line with precession-driven changes in 
13 Corg. Despite their intermittent occurrence over geological history, OAEs have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day.
Key Words: Toarcian • Jurassic • molybdenum • oceanic anoxic event • carbon cycle
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES CITED |
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13C) of up to –7
in all biospheric reservoirs (Jenkyns and Clayton, 1997; Hesselbo et al., 2000, 2007; Röhl et al., 2001; Cohen et al., 2004). A recent high-resolution study shows that the entire 13Corg excursion lasted 200 k.y. and included four separate shifts in 13Corg that were paced by astronomical precession (Kemp et al., 2005; Kemp, 2006; Cohen et al., 2007). Although still debated (Bailey et al., 2003; McElwain et al., 2005; van de Schootbrugge et al., 2005; Svensen et al., 2007), the most likely explanation for these observations involves the sudden and repeated dissociation of large amounts of methane hydrate (Hesselbo et al., 2000; Cohen et al., 2004; Kemp et al., 2005). Proxies for local paleoredox (Raiswell et al., 1993) indicate that euxinic conditions persisted throughout the OAE in parts of northwestern Europe, while the presence of organic compounds such as isorenieratane and methyl isobutyl maleimide (Schouten et al., 2000; Pancost et al., 2004; van Breugel et al., 2006) shows that euxinia sometimes extended into the marine photic zone. Despite extensive study, however, it has hitherto not been possible either to quantify the extent of marine anoxia and/or euxinia (i.e., whether global or regional) or to define the relationship between the onset and duration of reducing conditions and the major perturbation to the global carbon cycle. As a consequence of these limitations, the precise nature of the processes that gave rise to the extreme and detrimental environmental conditions during the Toarcian and other OAEs remains unclear.
The redox state of seawater is reflected in the isotopic composition of hydrogenous molybdenum (Mo) that is incorporated into organic-rich marine sediments (Siebert et al., 2003; Arnold et al., 2004; Poulson et al., 2006). In principle, therefore, past changes in seawater redox may be determined from well-preserved geological samples that contain Mo that is predominantly hydrogenous in origin. Molybdenum is a redox-sensitive trace metal that occurs in modern seawater as the chemically inert molybdate ion MoO42– (Emerson and Huested, 1991). Under oxidizing marine conditions, Mo is slowly removed from the water column by incorporation into ferromanganese phases and other authigenic material. The preferential fractionation of lighter Mo isotopes into oxic phases results in their Mo isotope composition being isotopically light (98/95Mooxic –0.7), while seawater is consequently enriched in the heavy isotopes (Siebert et al., 2003; Arnold et al., 2004). Present-day 98/95 Moseawater is 2.3 and appears to have been constant over the past 60 m.y. when sampled at a resolution of 1–3 m.y. (Siebert et al., 2003). When conditions are euxinic ([H2S] >100 µM), dissolved molybdate is converted quantitatively to MoS42– without isotopic fractionation (Tossell, 2005). The isotopic composition of Mo deposited under these conditions therefore represents the isotopic composition of Mo in seawater; for modern sites of euxinic deposition 98/95Moeuxinic 
?98/95Moseawater ? 2.3 (Siebert et al., 2003; Arnold et al., 2004). The 98/95Mo values of samples from open-ocean sites such as continental margins, where [H2S] is substantially lower (between 0.1 µM and 100 µM), fall in an intermediate range that is generally close to 1.6 (McManus et al., 2006; Poulson et al., 2006).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES CITED |
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98/95Mo values, [Mo] and [Re] abundances, and the total organic carbon (TOC) content of a suite of early Toarcian marine sedimentary rocks. These organic-rich mudrocks were deposited in the Cleveland Basin, which was located within the transcontinental Laurasian Seaway, which extended from 30°N to 60°N and was open to the Tethyan and Boreal oceans (Bjerrum et al., 2001). Samples were collected at approximately millennial-scale resolution across the interval that is broadly considered to represent the Toarcian OAE, and at lower resolution below and above, from sections that are now exposed in Yorkshire, UK. Details of sample locations, analytical methods, and data are provided in the GSA Data Repository and Table DR1.1 Our new data are correlated accurately with 13Corg data for the same exposures (Figs. 1 and 2), and the temporal variations in the measured geochemical parameters are constrained by reference to the high-resolution floating astronomical time scale derived from these exposures (Kemp et al., 2005; Kemp, 2006). By analogy with Re, which for most of the organic-rich samples analyzed in this study is predominantly hydrogenous (>95%) in origin (Cohen et al., 1999), the Mo is also predominantly hydrogenous. Furthermore, high-quality Re-Os isochrons obtained from samples from this section demonstrate that there has been minimal diagenetic alteration of the redox-sensitive elements in these samples (Cohen et al., 1999, 2004).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES CITED |
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98/95Mo, [Mo], and Re/Mo data, we divide the Toarcian succession in Yorkshire into four geochemically distinct intervals (Fig. 1). The lower part of Interval 1 (–13.00 m to –5.00 m) is characterized by the lowest 98/95Mo values (–0.5 to 0.0), and by very low [Mo] and generally high Re/Mo ratios (Fig. 1). In the upper part of this interval (between –5.00 m and –0.73 m), 98/95Mo values increase from –0.5 to 1.6, and [Mo] increases significantly. The following period (Interval 2) is associated with four abrupt shifts in 13Corg, and with the major sedimentological and geochemical changes that have previously been taken to characterize the Toarcian OAE (Jenkyns, 1985; Hesselbo et al., 2000; Cohen et al., 2004; Kemp et al., 2005) (Figs. 1 and 2). The 98/95Mo values in this interval decrease cyclically four times from a maximum of 1.6 by between 0.5 and 0.8, the magnitude and duration of the excursions increasing sequentially (Fig. 2). Each of these isotopic cycles is associated with a cyclic change in [Mo], while Re/Mo ratios remain relatively low and are close to the value for present-day seawater (cf. Crusius et al., 1996). Within Interval 3, 98/95Mo values are notably higher than in Interval 2, averaging 1.84 and reaching a maximum of 2.14 (Fig. 1). The [Mo] is also appreciably higher in Interval 3 than elsewhere in the succession, while Re/Mo ratios approach that of present-day seawater. Near the end of Interval 3 (at 19.50 m), [Mo] decreases abruptly and then remains relatively low throughout Interval 4. In Interval 4, 98/95Mo values are marginally lower than in Interval 3, averaging 1.59, and Re/Mo ratios are generally higher than in either Intervals 2 or 3. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES CITED |
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98/95Mo and 1/[Mo] (Fig. 3B). At the start of Interval 1, 98/95Mo values 
0 and low [Mo] are consistent with its being the only part of the succession where the depositional environment was oxic. The subsequent increase in [Mo] and 2 increase in 98/95Mo in the upper part of Interval 1 that are interpreted to reflect the onset of marine anoxia in the early Toarcian occurred over exactly the same period as the relatively gradual 2 decrease in 13 Corg (Fig. 1).
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98/95Mo similar to those that occurred periodically within Interval 2 would reflect a return to more oxic conditions of deposition and/or the predominance of terrigenous Mo (Siebert et al., 2003; Arnold et al., 2004; Poulson et al., 2006). However, this explanation is not valid here because Re/Mo ratios, which act as an indicator of local redox conditions (Crusius et al., 1996), support biomarker and other evidence for the persistence of euxinia across this interval in northwestern Europe (Raiswell et al., 1993; Schouten et al., 2000; Pancost et al., 2004; van Breugel et al., 2006) (Fig. DR1). Therefore the isotopic composition of the Mo that was incorporated into these highly reducing sediments records directly that of contemporaneous seawater. The decreases in 98/95Mo within Interval 2 are thus interpreted to have been caused by a severe decline in the areal extent of oxic deposition worldwide that reduced the magnitude of Mo isotope fractionation and lowered 98/95Moseawater repeatedly.
Despite being characterized by high TOC of as much as 15%, the [Mo] of samples from Interval 2 are unusually low for euxinic sedimentary deposits (Fig. 3A). However, the low [Mo] is not the result of basin restriction (cf. Algeo and Lyons, 2006), because the Cleveland Basin was open to both the Tethyan and Boreal oceans during the Toarcian (Bjerrum et al., 2001). These observations are nevertheless fully consistent with the highly efficient sequestration of Mo that would have occurred under widespread reducing conditions, which would have resulted in a significant reduction in both the global seawater Mo inventory and the Mo residence time (cf. Algeo, 2004). It is noteworthy that the four abrupt increases in [Mo] within Interval 2 occurred at exactly the same time as the four 13Corg shifts A–D (Fig. 2). If these prominent 13Corg shifts were caused by the periodic dissociation of large masses of methane hydrate (as suggested by Kemp et al., 2005), then the associated increases in [Mo] could reflect transient increases in the flux of Mo to the oceans resulting from elevated continental weathering rates (Cohen et al., 2004). Alternatively, or additionally, the increases in [Mo] may have resulted from the rapid dissolution of Morich ferro manganese deposits caused by the sudden expansion of marine anoxia.
The Mo isotope data reported here enable us to estimate the changes in the areal proportion of sediments accumulating under highly reducing conditions during Interval 2. At the present day, Mo sequestration into highly reducing sediments accounts for 25% of the Mo removal flux from seawater, despite covering no more than 0.5% of the ocean floor. A relatively small decrease in the area of oxic sedimentation worldwide would therefore be accompanied by a large proportional increase in the area covered by reducing sediments, and by greatly enhanced levels of Mo removal from the oceans (Ling et al., 2005). At steady state, the periodic decreases in 98/95Moseawater within Interval 2 would have been consistent with an tenfold increase worldwide in the area of highly reducing sediment accumulation (Fig. DR2). This estimate is a minimum, however, because the relatively rapid periodic changes in Mo isotope composition show that steady state had not been attained. The estimate is also an approximation because there are no data on the extent of sediment accumulation that occurred under suboxic conditions in the Toarcian; in today's oceans, this process is thought to account for appreciable Mo sequestration from seawater (McManus et al., 2006). Nevertheless, if the expansion of reducing conditions as calculated for the Toarcian were to occur at the present day, then an area equivalent to most, if not all, of the world's continental shelf would become anoxic or euxinic.
Because conditions remained highly reducing within the Cleveland Basin throughout much of the overlying Interval 3, the shift to higher 98/95Moseawater values together with elevated [Mo] abundances in Interval 3 indicate that the areal extent of marine euxinia contracted appreciably and that the global Mo inventory had increased (Figs. 1 and 3). Throughout Interval 4, 98/95Moseawater values and [Mo] decreased while Re/Mo ratios increased, indicating that the reducing conditions within the local basin became less intense.
Our observations allow us to propose the following mechanism for the development of the Toarcian OAE. Environmental change during the Toarcian was initiated by the emplacement of the Karoo-Ferar large igneous province (Hesselbo et al., 2000; Pálfy and Smith, 2000; Cohen et al., 2004, 2007; Kemp et al., 2005). One plausible cause for the gradual decrease in 13Corg and for the associated environmental changes in the upper part of Interval 1 (Cohen et al., 2007), including the onset of reducing conditions as defined by this study, could have been the production of large quantities of thermo genic methane following the voluminous intrusion of sills into organic-rich sedimentary rocks (Beerling and Brentnall, 2007; Svensen et al., 2007). The first widespread expansion of marine reducing conditions (98/95Mo excursion I; Fig. 2) occurred at exactly the same time as the first abrupt shift in 13Corg (point A; Fig. 2), which was most likely the result of large-scale methane hydrate dissociation (Kemp et al., 2005). Subsequent shifts in 13Corg at stratigraphic levels B, C, and D, which are interpreted to have resulted from further large-scale pulses of methane hydrate dissociation (Kemp et al., 2005; Kemp, 2006; Cohen et al., 2007), took place at specific points during the subsequent fluctuations in marine reducing conditions represented by 98/95Mo excursions II, III, and IV (Fig. 2). The end of the widespread reducing conditions that marked the Toarcian OAE is clearly defined by the geochemical changes that occurred between Intervals 2 and 3.
This study demonstrates that large-scale perturbations to the global C cycle during the Toarcian OAE were intimately linked with rapid changes in ocean redox. Episodes of global warming and the widespread accumulation of organic-rich deposits during the Cretaceous OAEs and at the Paleocene-Eocene Thermal Maximum ca. 55.8 Ma show many features in common with the early Toarcian OAE (Schlanger and Jenkyns, 1976; Jenkyns, 1985; Erba, 2004; Cohen et al., 2007), and may have resulted from similar Earth processes. The integration of the Mo isotope system with other proxy indicators of environmental change is able to increase our understanding of the evolution of the oceans and atmosphere over Earth's history and has the potential to indicate how the ocean-atmosphere system might evolve in the future.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received for publication 21 September 2007
Revised manuscript received 9 November 2007
Manuscript accepted 10 November 2007
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). The succession is divided into four intervals on the basis of their geochemical characteristics: -13.00 m to -0.73 m (Interval 1), -0.73 m to 5.80 m (Interval 2), 5.80 m to 20.85 m (Interval 3) and 20.85 m to 35.00 m (Interval 4). Interval 2 lasted 
