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Palaeogene−Evidence for rapid climate change in the Mesozoic

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10.1098/rsta.2003.1240

Evidence for rapid climate change in theMesozoic–Palaeogene greenhouse world

By Hugh C. Jenkyns

Department of Earth Sciences, University of Oxford,Parks Road, Oxford OX1 3PR, UK

Published online 22 July 2003

The best-documented example of rapid climate change that characterized the so-called ‘greenhouse world’ took place at the time of the Palaeocene–Eocene bound-ary: introduction of isotopically light carbon into the ocean–atmosphere system,accompanied by global warming of 5–8 ◦C across a range of latitudes, took placeover a few thousand years. Dissociation, release and oxidation of gas hydrates fromcontinental-margin sites and the consequent rapid global warming from the inputof greenhouses gases are generally credited with causing the abrupt negative excur-sions in carbon- and oxygen-isotope ratios. The isotopic anomalies, as recorded inforaminifera, propagated downwards from the shallowest levels of the ocean, imply-ing that considerable quantities of methane survived upward transit through thewater column to oxidize in the atmosphere. In the Mesozoic Era, a number of similarevents have been recognized, of which those at the Triassic–Jurassic boundary, in theearly Toarcian (Jurassic) and in the early Aptian (Cretaceous) currently carry thebest documentation for dramatic rises in temperature. In these three examples, andin other less well-documented cases, the lack of a definitive time-scale for the inter-vals in question hinders calculation of the rate of environmental change. However,comparison with the Palaeocene–Eocene thermal maximum (PETM) suggests thatthese older examples could have been similarly rapid. In both the early Toarcian andearly Aptian cases, the negative carbon-isotope excursion precedes global excess car-bon burial across a range of marine environments, a phenomenon that defines theseintervals as oceanic anoxic events (OAEs). Osmium-isotope ratios (187Os/188Os) forboth the early Toarcian OAE and the PETM show an excursion to more radiogenicvalues, demonstrating an increase in weathering and erosion of continental crustconsonant with elevated temperatures. The more highly buffered strontium-isotopesystem (87Sr/86Sr) also shows relatively more radiogenic signatures during the earlyToarcian OAE, but the early Aptian and Cenomanian–Turonian OAEs show thereverse effect, implying that increased rates of sea-floor spreading and hydrothermalactivity dominated over continental weathering in governing sea-water chemistry.The Cretaceous climatic optimum (late Cenomanian to mid Turonian) also showsevidence for abrupt cooling episodes characterized by episodic invasion of borealfaunas into temperate and subtropical regions and changes in terrestrial vegetation;drawdown of CO2 related to massive marine carbon burial (OAE) may be implicatedhere. The absence of a pronounced negative carbon-isotope excursion preceding the

One contribution of 14 to a Discussion Meeting ‘Abrupt climate change: evidence, mechanisms andimplications’.

Phil. Trans. R. Soc. Lond. A (2003) 361, 1885–19161885

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1886 H. C. Jenkyns

Cenomanian–Turonian OAE indicates that methane release is not necessarily con-nected to global deposition of marine organic carbon, but relative thermal maximaare common to all OAEs. ‘Cold snaps’ have also been identified from the Mesozoicrecord but their duration, causes and effects are poorly documented.

Keywords: Palaeocene–Eocene thermal maximum; oceanic anoxic events;Jurassic; Cretaceous; climate change

1. Introduction

Although the Mesozoic Era has generally been regarded as dominantly ice freebecause of the presence of tropical to subtropical fauna and flora at high latitudes,there is some evidence for relatively cool periods, possibly characterized by the localpresence of polar ice (Kemper 1987; Frakes et al . 1992; Herman & Spicer 1996; Price1999) and, more equivocally, by glacio-eustatic changes in sea level (Markwick &Rowley 1998; Stoll & Schrag 2000; Gale et al . 2002). There are few data for theTriassic (although general circulation models suggest an arid Pangaea with no landice (Hay et al . 1994)), but long-term climatic change during the Jurassic–Cretaceousinterval, on a scale of stages, has long been recognized, based on faunal, floral andsedimentary evidence (see, for example, Hallam 1985, 1994; Frakes et al . 1992; Sell-wood & Price 1994; Deconto et al . 2000; Rees et al . 2000). Only since the adventof high-resolution carbon- and oxygen-isotope stratigraphy in the 1980s (Scholle &Arthur 1980) have rapid changes in global temperature and carbon cycling beenidentified in the Mesozoic: such changes are not normally resolvable using classicalsedimentary and palaeontological methods. In this account, I examine those intervalsof Jurassic–Cretaceous time where, at least in the context of resolvable time for theMesozoic Era, abrupt shifts in climate have been recognized. Such a discussion nec-essarily involves some consideration of oceanic anoxic events (OAEs), recognized asassociated with thermal maxima (Jenkyns 1999; Grocke 2002); the implied contextis that of a ‘greenhouse world’ characterized by relatively high levels of atmosphericCO2.

Much of the evidence for rapid climate change in the Palaeocene–Mesozoic inter-val has been recently interpreted in terms of possibly catastrophic introduction ofmethane into the ocean–atmosphere system from dissociation of gas hydrates. Thesecrystalline substances, composed of gas, typically methane and water, are stableunder certain pressure–temperature conditions, gas concentrations and water activi-ties, and are known to be present within the sedimentary prisms that lie along manypresent-day continental margins at a few hundred metres depth below the sea floor(Kvenvolden 1993). Below the crystalline hydrate, free gas is commonly present. Themethane, which derives largely from microbial activity within the sedimentary pileacting upon organic matter that has escaped oxidation by dissolved oxygen, nitrate

Figure 1. Diagram, modified from Dickens (2001b), illustrating the potential role of methane gas hydratesin the global carbon cycle. Methane is generated within continental-margin sediments due to bacterialfermentation of organic matter, and the gas becomes held locally as crystalline gas hydrates below thesea floor. Dissociation of gas hydrates, related either to warming of the bottom waters and/or tectonicdisruption of the sedimentary pile, leads to release of methane gas, much of which may transit the watercolumn to be oxidized in the shallow warm levels of the ocean and the atmosphere.

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and sulphate, constitutes a major component of the global carbon cycle (figure 1).The geochemical fingerprint of methane is singular in that it is characterized byextremely low carbon-isotope ratios (δ13C ∼ −60 %% or less): rapid introduction ofthis material into the oceans and atmosphere, and its subsequent near-immediateoxidation to CO2, leaves an obvious geochemical trace as well as potentially leadingto global warming. Such effects, on millennial time-scales, have been postulated forthe Quaternary of the Californian continental margin, based on both carbon-isotopechanges in foraminifera (Kennett et al . 2000) and biomarkers in the sediments thatindicate the former presence of aerobic and anaerobic methanotrophs (Hinrichs etal . 2003). Here the focus is on similar but apparently much larger events many tensof millions of years in the past.

2. Limitations to stratigraphic resolution

Only marine sediments can be studied using classic biostratigraphical means. Becausemuch of the Triassic, at least in North America and Europe, is developed in con-tinental facies or in sediments that contain little in the way of stratigraphicallyuseful marine fossils, most attention has focused on the Jurassic and Cretaceous forinvestigation of major environmental change. Marine carbonate sediments and theircontained fossils are also amenable to detailed geochemical analysis that may aid inpalaeoclimatic reconstructions. Ammonites particularly have been used to generatea high-resolution Jurassic time-scale with an estimated mean duration of 5 Ma for astage, 860 ka for a zone, 375 ka for a subzone and 130 ka for a faunal horizon (Cal-lomon 2001); the resolution is less impressive for Cretaceous sequences (Callomon1995). These figures derive from calibration against a radiometric time-scale such asthat of Gradstein et al . (1994) or Palfy et al . (2000). These time-scales primarilyrely upon absolute-age determinations of igneous components in volcanogenic sedi-ment interbedded with fossiliferous sequences. Interpolation between these chrono-metrically fixed points on the time-scale is accomplished by using biostratigraphicalsubdivisions based on first and last occurrences of index species.

Calcareous nannofossils have been increasingly used in recent years and, althoughnot invariably yielding such high resolution as ammonites, their abundance in mostJurassic marine carbonates, makes them an invaluable biostratigraphical resource(Bown 1999; Mattioli & Erba 1999). For Cretaceous sediments ammonites, nano-fossils and planktonic foraminifera are all locally employed. Various geochemicalparameters can also be used to correlate and date Mesozoic sequences once the datahave been calibrated against a biostratigraphically well-dated reference section. Mostused to date is the strontium-isotope curve (McArthur 1994; Jones et al . 1994a, b;Veizer et al . 1999; Jenkyns et al . 2002), where certain time-intervals characterized byrapid changes in sea-water strontium-isotope ratios allow temporal resolution downto the ammonite zonal or subzonal level. Carbon-isotope curves are also useful forcorrelation where possessed of characteristic shape and form (Scholle & Arthur 1980;Veizer et al . 1999; Jenkyns et al . 2002).

Cyclostratigraphy, involving the recognition of orbital-climatic signatures in sedi-mentary rocks, of both continental and marine character, potentially offers the high-est degree of temporal resolution, as well as helping to calibrate the absolute dura-tion of subzones, zones and stages. Studies of the non-marine Upper Triassic of theNewark Basin, eastern USA (Olsen & Kent 1999), have proven successful, as have




Rapid climate change in the greenhouse world 1889

studies of Jurassic–Cretaceous cyclic shelf and pelagic sediments in many outcropsworldwide and in many deep-sea cores (see, for example, Fischer 1986; Gale et al .1999; Herbert 1999; Hinnov & Park 1999; Weedon et al . 1999). Recognition of theprecession cycle (ca. 20 ka) offers a time-scale comparable with that achieved in theglacial epochs of the Tertiary.

Another means of measuring sedimentary rate in marine sediments derives fromdeterminations of helium isotopes in pelagic sediments. Such sediments have unusu-ally high 3He/4He ratios related to the input into the oceans of micro-meteoriticinterplanetary dust particles (Ozima et al . 1984). The assumption that the cosmicinput is constant through time can be checked by normalization to the 3He con-centration and calibrating the isotopic data against a section for which the massaccumulation rates of sediment are independently known (Marcantonio et al . 1995,1996). Studies show that the assumption of constant accumulation rates of extrater-restrial 3He is broadly correct, at least over time-scales of thousands of years, andsuch data can hence be applied to sediments that contain evidence of rapid climatechange (Farley & Eltgroth 2003).

Finally, varved sediments, if a convincing case can be made for their interpretationas annual, potentially offer resolution on the subdecadal scale. Some organic-richMesozoic sediments have been interpreted in this light (see, for example, Cope 1998),a conclusion in general accord with biostratigraphical data, but there is no reliableway of confirming the suggested durations. All the above techniques may be usedto shed light on the duration of local and global change but in many instances thesequences that contain encoded climatic data are not amenable to dating at thelevel of resolution required to allow cross-comparison of datasets from different areasand unambiguously separate causes and effects. Furthermore, stratigraphic gaps andcondensed sections, if unrecognized, can cause gradual geochemical changes to appearrapid, and lead to erroneous interpretations. Here I adopt the approach of taking,as a model, a well-constrained period of rapid environmental change during theTertiary and then identify similar but less well chronologically constrained events inMesozoic time. Particular attention is paid to the early part of the Toarcian stage ofthe Jurassic, an interval of time much investigated chemostratigraphically in recentyears, and for which there exists a considerable amount of palaeoclimatic data.

3. The Palaeocene–Eocene thermal maximum (PETM) as a modelfor rapid climate change in the greenhouse world

One of the most significant warming events in the Earth’s history has been iden-tified as having taken place during the Palaeocene–Eocene transition some 55 Ma(Kennett & Stott 1991; Zachos et al . 1993): this is the so-called PETM (figure 2).Benthic foraminifera from Pacific and Atlantic deep-sea sites show a negative shift(> −1 %% ) in oxygen-isotope values (δ18O) that signifies an increase in bottom-watertemperatures of 5–6 ◦C; similar records from planktonic foraminifera suggest warm-ing by up to 8 ◦C at high latitudes and possibly a lesser amount around the Equator(Bains et al . 1999; Zachos et al . 2001; Pearson et al . 2001). Coincident with thisshift to lower oxygen-isotope values is a ca.−3 %% step-wise negative carbon-isotopeexcursion, generally interpreted as a response to episodic dissociation and oxidationof isotopically light (δ13C ∼ −60 %% ) methane gas hydrates from continental margins




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somewhere in the world ocean (Dickens et al . 1995, 1997; Dickens 2000, 2001a): themass of carbon involved is estimated to be between 1000 and 2000 GtC.

That the whole of the ocean–atmosphere reservoir was affected by the geochemicalchanges at the Palaeocene–Eocene boundary is illustrated by the recognition of thesame dramatic drop in carbon-isotope values in palaeosol carbonates, mammaliantooth enamel and pollen from land plants (Koch et al . 1992; Stott et al . 1996; Beerling& Jolley 1998; Cojan et al . 2000). Furthermore, pollen in northwest Europe showsa change to a more tropical species. Major biotic changes accompanied this event,particularly involving extinction of benthic foraminifera, diversification of planktonicforaminifera and an increase in abundance of dinoflagellates in both Northern andSouthern Hemispheres (Thomas & Shackleton 1996; Kelly et al . 1996; Crouch et al .2001; Kelly 2002). Mammalian faunas were also affected as high-latitude migrationcorridors opened in response to global warming (see, for example, Clyde & Gingerich1998).

Although investigations of the PETM initially stressed warming of bottom watersas the trigger for dissociation of gas hydrates (Dickens et al . 1995, 1997; Dickens2000), the close correspondence between the structure of high-resolution δ18O andδ13C curves across this interval implies effectively coincident global warming andan influx of isotopically light carbon to the ocean–atmosphere system (figure 2).This association has led to the suggestion that the introduction of methane and itsoxidation product CO2 to the atmosphere precipitated much of the global rise in tem-perature (Bains et al . 1999). Dissolution of pelagic carbonate, due to elevation of thelysocline as the CO2 content of the oceans rose, accompanied this major palaeoenvi-ronmental event (Dickens et al . 1995, 1997). Removal of excess CO2 may have beenaccomplished by an increase in marine biological productivity, as well as an increasein carbon burial at the sea floor, causing drawdown of the greenhouse gas (Bains etal . 2000). Such an increase in productivity may be demonstrated by enhanced valuesof biogenic barium in sediments deposited during the PETM: concentrations of thiselement are known to correlate with carbon flux in deep-sea sediments (Schmitz etal . 1997). However, some authors link the presence of excess barium at this level torelease of Ba-rich waters from gas-hydrate reservoirs (Dickens et al . 2003).

Figure 2. Isotopic analyses of foraminifera across the Palaeocene–Eocene boundary from Ocean DrillingProgram Site 690 (Maud Rise, Southern Ocean) plotted against depth (metres below sea floor (mbsf));data from Kennett & Stott (1991) and Thomas et al . (2002). Note the ca. 8 ◦C warming in the upperlevels of the ocean implied by the change in oxygen-isotope values. Even though bioturbation may haveintroduced some stratigraphic error, the very-high-resolution sampling shows a gradual warming (fromlevel 1) registered in surface-dwelling foraminifera followed by erratic warming of the thermocline (asindicated by δ18O ratios of foraminifera just below and above level 2). The benthic record is more sparseand problematic, and provides no evidence for warming prior to any significant change in carbon-isotoperatios, but significant warming of the bottom waters may have taken place after temperatures rose in thethermocline. The δ13C chemostratigrahy shows very clearly that isotopically light carbon was introducedfirst into the shallow ocean, as recorded in the carbon-isotope values of surface-dwelling foraminifera(level 2) and that the signal progressively propagated downwards into the thermocline (level 3 andabove) and then into deeper levels. If the shift to relatively light δ13C values records the signature ofoxidized methane then the oxidation process must have taken place primarily in the shallow ocean andatmosphere. Level 2 records a coincident negative carbon-isotope shift and dramatic warming of near-surface waters. Solid squares, surface-dwelling planktonic; crosses, thermocline-dwelling planktonic; opencircles, benthic.




1892 H. C. Jenkyns

At least some if not all nutrients for any such accelerated production were probablyderived from increased continental weathering, as indicated by an osmium-isotopeanomaly that characterizes the PETM (Ravizza et al . 2001). The transient increase inthe 187Os/188Os ratio implies that continental crust, with its relatively high averageosmium-isotope ratio, suffered enhanced weathering through increased temperatureand rainfall. The residence time of osmium in the oceans (103–104 years) is shortenough to make it an ideal tracer for rapid climate steps that have forced changes inthe hydrological cycle affecting the quantity and ionic concentration of fluvial inputto the oceans, but is long enough to give what appears to be a globally uniformsignal (Peucker-Ehrenbrink & Ravizza 2000). Although, on a global scale, there isno universal correlation between fluvial dissolved load and temperature, a signifi-cant number of present-day rivers show a fivefold increase in weathering rates forevery 5 ◦C rise (Gaillardet et al . 1999). Evidence for an increase in temperatureand/or rainfall across the Palaeocene–Eocene boundary is given by the rise in kaoli-nite/illite and smectite/illite ratios seaward of Antarctica (Robert & Kennett 1994).A dramatic rise in the kaolinite to illite/smectite ratio is also recorded from the samestratigraphic level in New Jersey, Spain and the North Sea, implying tropical weath-ering in these relatively low-latitude sites (Gibson et al . 1993; Knox 1998). There issome palaeontological evidence that much of the new nutrient supply was preferen-tially sequestered in near-continent environments (Bralower 2002; Kelly 2002). Thepresence of organic-rich and/or laminated sediments in a number of localities acrossthe Palaeocene–Eocene boundary suggests the former presence of poorly oxygenatedbottom waters, resulting from increased organic-carbon flux to the sea floor and/oroxidation of methane (Dickens 2001a; Gavrilov et al . 2003).

The time-scale for the PETM, as elucidated by cyclostratigraphy of the most strati-graphically expanded and complete section known (from the Maud Rise, SouthernOcean), specifically through the identification of the two modes of the precessioncycle, suggests that the initial δ13C and δ18O anomalies developed over 2 ka or lessand persisted for some 210–220 ka (Norris & Rohl 1999; Rohl et al . 2000): the ini-tial drop in δ13C of 1 %% takes place over a mere 6 cm of section representing lessthan a thousand years (Kelly 2002). High-resolution centimetre-scale chemostrati-graphic studies of the same section on single-specimen foraminifera of planktonicsurface-dwelling, thermocline-dwelling and benthic habitat (figure 2) show that thecarbon- and oxygen-isotope anomaly propagated downwards from the near-surfacewaters, implying both that much of the dissociated methane reached the shallowlevels of the ocean and probably bubbled into the atmosphere prior to oxidation andthat warming of the deeper waters followed a temperature increase over the surfaceof the planet (Thomas et al . 2002). Oxygen-isotope data from this section recorda gradual warming of the surface and thermocline waters of ca. 2 ◦C prior to theputative methane-dissociation event and dramatic temperature rise (ca. 4 ◦C). Thismore subtle and gradual warming event has been attributed to regional volcanismin the North Atlantic and/or Caribbean (Eldholm & Thomas 1993; Bralower et al .1997). An alternative hypothesis, championed particularly by Katz et al . (2001),links the critical phase change within the gas-hydrate body to seismic activity caus-ing mechanical disruption of the sedimentary prism in a number of places along thecontinental margin of eastern North America.

Confirmation of the cyclostratigraphic time-scale for the onset of the PETM, with asuggested duration of a few kyr for the initial carbon- and oxygen-isotope anomalies,





is given by the relative abundance of 3He, assuming a constant flux of this extra-terrestrial isotope to the sea floor (Farley & Eltgroth 2003). The recovery phase,as illustrated by δ18O and δ13C plots against thickness for ODP Site 690 in theSouthern Oceans, looks to be considerably slower than the onset (figure 2). However,the helium-isotope data indicate an acceleration of sedimentation rate following theinitial isotopic events and hence indicate a more rapid return to pre-excursion iso-topic values than indicated by the use of sedimentary thickness as a linear proxy fortime. Widespread dissolution of pelagic carbonate during the PETM itself may havereduced net sedimentation rates relative to those that characterized the return tomore normal oceanographic conditions.

4. Rapid climate change during Permo–Triassic boundary time?

The Permo–Triassic boundary (ca. 248 Ma, after the time-scale of Gradstein et al .1994) is characterized by a mass extinction and is spanned by sedimentary successionscommonly rich in organic matter that indicate widespread anoxic conditions fromthe shelf to the deep sea (Hallam & Wignall 1997; Isozaki 1997). In many parts ofthe world the deposits that cross the boundary exhibit a series of apparently abruptnegative carbon-isotope excursions (ca. −3 %% in δ13Ccarbonate in some cases) in bothcarbonate and marine and terrestrial organic matter (see, for example, Holser &Margaritz 1987; Baud et al . 1989; Margaritz et al . 1992; de Wit et al . 2002). Releaseof methane from dissociation of gas hydrates has recently been suggested as a possiblecause for these isotopic disturbances (Krull & Retallack 2000; de Wit et al . 2002).However, as noted by Grossman (1994), the antiquity of Permo–Triassic depositsand their consequent diagenetic overprint means that there are no reliable oxygen-isotope data available for this interval. Hence, any change in global temperatures atthe onset of the Mesozoic Era, either gradual or rapid, and any possible relationshipwith the carbon cycle, is as yet unknown. Notable, however, is an association withthe formation of a large igneous province, namely the Siberian Traps, suggestingthat effusion of volcanogenic CO2 may have had a significant impact on climate atthis time (Renne et al . 1995), and indeed could have triggered dissociation of gashydrates through warming of bottom waters.

5. Rapid warming during Triassic–Jurassic boundary time

The Triassic–Jurassic boundary (ca. 200 Ma, after the time-scale of Palfy et al . 2000)is marked by a major mass extinction and is characterized by palaeobotanical evi-dence for global warming. Such evidence derives from studies of changes in leaf mor-phology to more dissected forms as well as decreases in stomatal density and stomatalindex in fossil plant cuticle from Greenland and Sweden: the changes observed havebeen taken to imply a fourfold increase in atmospheric CO2 over this interval (McEl-wain et al . 1999). An increase in abundance of the thermophilic pollen Classopollis(Corollina) is noted in Europe and eastern North America for this interval (Fowell& Olsen 1993; Hesselbo et al . 2002). A number of marine sections across the bound-ary also show relatively high values of organic carbon (Ward et al . 2001; Cohen &Coe 2002a), but the major feature of chemostratigraphic interest is an abrupt neg-ative carbon-isotope excursion or excursions (of −2 to −3 %% depending on materialanalysed) in marine carbonate, organic matter and terrestrial wood (McElwain et al .




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1999; Ward et al . 2001; Palfy et al . 2001; Hesselbo et al . 2002). Sparse oxygen-isotopedata, only available for bulk carbonate, locally at least, show a negative excursionaccompanying that in δ13C, but the relative roles of palaeotemperature rise versusaddition of diagenetic calcite in governing final δ18O values are difficult to decipher(Palfy et al . 2001). The lack of exact definition of and recognition of the systemboundary in different sections also hampers integration of chemostratigraphic data(Guex et al . 2002).

Unlike the PETM, osmium-isotope ratios across the Triassic–Jurassic contact froma section in Somerset, England, show relatively non-radiogenic values, except atthe boundary itself where one, possibly spurious datapoint indicates an abrupt rise(Cohen & Coe 2002a). Sparse strontium-isotope data suggest an increase in 87Sr/86Srvalues at this level, interrupting a longer-term fall, perhaps implying a brief increasein radiogenic flux of this element from the continents (Jones et al . 1994a; Veizeret al . 1999; Korte et al . 2003). However, the Triassic–Jurassic boundary representsan interval where 87Sr/86Sr datasets, deriving from different materials and fromdifferent laboratories, are conflated, so the increase in values may not be real. Moregeochemical data across this boundary are needed to determine whether or not thereis evidence for a relative increase in continental weathering and/or fluvial influx tothe oceans over this period of geological time.

Both Hesselbo et al . (2002) and Cohen & Coe (2002a) suggest volcanism fromthe Central Atlantic Magmatic Province as an important factor behind the Triassic–Jurassic boundary events, acting to raise temperatures through effusion of CO2 andchanging the osmium-isotope ratio of sea water. The negative carbon-isotope excur-sion was initially interpreted as related to marine-productivity collapse or input ofvolcanogenic CO2 carrying a mantle signature (δ13C ∼ −7 %% ), but dissociation ofgas hydrates as a possible additional mechanism has been recently suggested (Palfyet al . 2001). Detailed modelling by Beerling & Berner (2002) suggests that methanedissociation may be required to explain the carbon-isotopic and documented tem-perature response. They suggest that volcanism from the Central Atlantic MagmaticProvince released some 8000–9000 GtC as CO2 and that dissociation of gas hydratereleased another 5000 GtC as methane. In this model the initial negative carbon-isotope excursion is assumed to take place over ca. 70 ka, in line with other putativemethane-release events.

6. Rapid warming at the onset of the early Toarcian (Jurassic) OAE

Qualitative evidence for a brief period of global warmth during the early Toarcianstage of the early Jurassic (ca. 183 Ma, from the time-scale of Palfy et al . (2000))is given by palynological data from the former Soviet Union, showing a relativeincrease in the abundance of Classopollis (Corollina) pollen derived from a warmth-loving conifer (Vakhrameyev 1982), and an increase in conifer-derived biomarkersfrom the northwest margin of Australia (van Aarssen et al . 2000). Low-resolutionoxygen-isotope and Mg/Ca ratios in belemnites from the former Soviet Union alsoindicate a relatively warm Toarcian stage (Berlin et al . 1967; Naydin & Teys 1976;Yasamanov 1981). Belemnite δ18O values, however, commonly tend to be rather scat-tered, even when careful screening for possible diagenetic effects has been undertaken.These fossils, however, are relatively robust and easy to extract from consolidatedrock, and currently still offer the most hopeful skeletal source of palaeoceanographic





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Figure 3. Carbon-isotope stratigraphy across the Toarcian stage of the Lower Jurassic: data fromthe Mochras Borehole, Wales (Jenkyns & Clayton 1997; Jenkyns et al . 2001). The abrupt steppednegative excursion, seen in both bulk carbonate and organic matter, is interpreted as the recordof the rapid, albeit episodic introduction of isotopically light carbon into the ocean–atmospheresystem from gas-hydrate dissociation (Hesselbo et al . 2000).

information for the Jurassic Period (Jenkyns et al . 2002). A number of Lower Toar-cian sequences of carbon-rich black shales in the British Isles and Italy, investigatedfor their bulk-rock light-isotope stratigraphy, show a δ13C and δ18O minimum inthe falciferum zone, as do the accompanying belemnites: the negative δ13C excur-sion, however, occurs between levels of relatively high carbon-isotope values and thusessentially divides two positive carbon-isotope excursions (figure 3 (Jenkyns & Clay-ton 1986, 1997; Jenkyns et al . 2002)). Because of the obvious co-variance of δ13C andδ18O values over the interval of the excursion the signals were initially attributed todiagenesis (see, for example, Marshall 1992); their regional reproducibility and pres-ence in both bulk carbonate and skeletal calcite, however, demonstrates that this isnot the case. The rapid fall in carbon-isotope values is everywhere positioned belowthe principal stratigraphic level of carbon enrichment in the black shales.

High-resolution sampling and analysis of sections in Yorkshire, northeast England,by McArthur et al . (2000) illustrates a fall in δ18O values of ca. − 2 %% and a risein Mg/Ca ratios in belemnites from the uppermost tenuicostatum zone (semicela-tum subzone) passing to a minimum and maximum, respectively, in the early tomid exaratum subzone of the falciferum zone (figure 4), a phenomenon also recordedfrom northern Spain (Rosales et al . 2001). Both these geochemical parameters indi-cate temperature increase, in the case of the oxygen-isotope ratios by 10 ◦C or more,




1896H

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.Jenkyns

graphic log

0.7071 0.7072 8 12 16 20 5 10−4−3−2−10 −32 −30 −28 −26

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ammonitebiostratigraphy

zone

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using the palaeotemperature equation of Anderson & Arthur (1983) for an ice-freeworld. This figure is so large that it is commonly assumed that marine waters innorthern Europe were freshened, and their δ18O values lowered, by significant fluvialinput (Sælen et al . 1996). There is no calibration for the exact temperature controlof Mg/Ca ratios in belemnites because they are an extinct group, although use of anavailable nonlinear equation relating Mg/Ca to temperature in foraminiferal calcite(Rosenthal et al . 1997) yields a considerably lesser degree of warming. Identifica-tion of the thermal maximum in the early to mid exaratum subzone is supportedby high-resolution palynological studies from coastal sections in Yorkshire that placethe Classopollis (Corollina) ‘spike’ at this level (Wall 1965). Most striking is thecorrelation between the assumed rapid temperature rise and the equally rapid butstepped fall in carbon-isotope values, registered in marine organic carbon (figure 4),including a range of biomarkers (Schouten et al . 2000), terrestrial higher-plant mate-rial and marine carbonate from pelagic and shelf-sea sites (Jenkyns & Clayton 1986,1997; Hesselbo et al . 2000; Jenkyns et al . 2002). This relationship recalls exactlythat of the PETM and indeed can be interpreted in an identical way.

Hesselbo et al . (2000) deemed the following two factors to be particularly relevant:synchronous volcanic activity, in this case from the Karoo–Ferrar igneous province,likely leading to increased flux of volcanogenic CO2 and gradual global warming ofnot only the atmosphere but also intermediate and deep waters; and accelerated rift-related tectonic activity in the European and adjacent regions. Such effects could havedestabilized a gas-hydrate reservoir lying below the sea floor and led to a significantflux of methane to the ocean–atmosphere system, followed by its rapid oxidation andsubsequent global temperature rise. In the case of the Toarcian, the size of the hydratereservoir has been estimated as 1500–2700 GtC by Hesselbo et al . (2000) and as5000 GtC by Beerling et al . (2002): these discrepancies arise from differing estimatesof the absolute magnitude of the carbon-isotope excursion. Careful comparison ofthe initial carbon-isotope drop in terrestrial and marine carbon shows the former tobe locally greater than the latter by ca. − 2 %% . Although this difference could reflecta different degree of carbon-isotope fractionation between higher-plant material andplanktonic organic matter, dependent on a number of variables, it is equally possiblethat dissociated methane and its oxidation product were more rapidly mixed with thesmaller carbon reservoir in the atmosphere, producing an initial amplified responsecompared with that in the oceans with its larger dissolved carbon reservoir.

Figure 4. Composite chemostratigraphic profiles of Yorkshire (northeast England): sections across theToarcian stage of the Lower Jurassic. Shaded band indicates that part of the section characterizedby major geochemical anomalies. 87Sr/86Sr, Mg/Ca and δ18O values derive from belemnite calcite(McArthur et al . 2000). δ18O values converted to palaeotemperatures using the equation of Ander-son & Arthur (1983) for an ice-free world where δ18Osmow = −1 %% . Total-organic-carbon (TOC) andδ13C values are from the Hawsker Bottoms section, near Whitby, Yorkshire (Jenkyns et al . 2001). Abso-lute age (Ma) and timing of eruption of Karoo–Ferrar Large Igneous Province (Palfy et al . 2000; Palfy& Smith 2000). Note the abrupt increase in temperature, as registered by the Mg/Ca and δ18O valuesof the belemnites coincident with the abrupt fall in δ13C values of the organic matter. The most signifi-cant geochemical changes took place during the semicelatum subzone of the tenuicostatum zone and theexaratum subzone of the falciferum zone. Graphic log from Hesselbo & Jenkyns (1995); detailed bios-tratigraphy from Howarth (1992); data compilation from Woodfine (2002). apyr = apyrenum subzone;hawsk = hawskerense subzone; palt = paltum subzone; c = clevlandicum subzone; tn = tenuicostatumsubzone; semi = semicelatum subzone.




1898 H. C. Jenkyns

The time-interval of the first major negative carbon-isotope shift and the accom-panying change in δ18O and Mg/Ca ratios in belemnite carbonate occupies a smallfraction of an ammonite subzone, suggesting a period considerably less than 100 kyr(average subzone length being computed as 375 ka (Callomon 2001)). Counting ofsub-millimetre-scale laminae through the black shales that represent this criticalinterval in Yorkshire, on the assumption that they are annual and that there are nogaps in the section, gives a minimum duration of 5 ka for the initial dramatic fall inδ13Corg values (−2 %% over less than 22 cm of sediment) and a total duration for thenegative shift of ca. 70 ka (Hesselbo et al . 2000).

The other major feature of chemostratigraphic interest is the regional pattern ofenrichment in organic carbon, which rises to a maximum in the mid exaratum sub-zone, post-dating the abrupt rise in temperature. Because organic-rich shales of thisage are recognized worldwide, and associated with a broad positive carbon-isotopeexcursion over the whole of the tenuicostatum–falciferum zone interval (albeit inter-rupted by the aforementioned negative carbon-isotope excursion: figures 3 and 4), amajor disturbance in the global carbon cycle is indicated. The phenomenon of globalexcess marine carbon burial and consequential positive carbon-isotope excursionrelated to preferential sequestration of the lighter isotope, 12C, has been describedas an OAE: in this case the Posidonienschiefer Event, named after the character-istic black shales in Germany (Jenkyns 1985, 1988, 1999). The sedimentary recordof such events in open-marine sections reveals a change in the nature of the marineplankton from dominantly carbonate (coccoliths) to dominantly organic (bacteria,dinoflagellates, etc.) and siliceous (radiolarians). Furthermore, there is geochemicalevidence for denitrification and sulphate reduction in the water column in a numberof areas, indicating anoxic to euxinic conditions extending from the sea floor to thephotic zone (Schouten et al . 2000; Jenkyns et al . 2001).

The exact relationship between relatively elevated temperatures and the OAE isnot established, but a working hypothesis suggests an accelerated hydrological cyclewith increased continental weathering and flux of nutrients to the oceans and inten-sified wind-driven upwelling, together driving increased marine organic productivityand carbon burial (Jenkyns 1999). Temperature rise may have stemmed from twofactors: volcanogenic CO2 from the Karoo–Ferrar large igneous province (Palfy &Smith 2000) and dissociation of gas hydrates. Several independent parameters sup-port the notion of increased continental weathering and run-off from latest tenuicosta-tum-zone to mid exaratum-zone time: a gradual rise in the strontium-isotope ratios(87Sr/86Sr) recorded in belemnite calcite and an abrupt rise in osmium-isotope ratios(187Os/188Os) in the black shales deposited over this interval (Jones et al . 1994a;Cohen & Coe 2002b). Both of these geochemical tracers indicate enhanced flux intothe oceans of radiogenic isotopes derived from continental crustal rocks: this effectis in accord with the assumption that the δ18O values of belemnites could have beenaffected by freshwater input, as suggested before for this interval of time (Riegel et al .1986; Jenkyns 1988; Prauss & Riegel 1989; Sælen et al . 1996; Bjerrum et al . 2001).The size of the osmium-isotope excursion is an order of magnitude greater than thatcharacterizing the PETM. Locally, as in southern Switzerland, pelagic sedimentsshow a dramatic ‘spike’ in the abundance of kaolinite in lower Toarcian sediments,equally suggestive of enhanced continental weathering (Deconinck & Bernoulli 1991).

Enhanced burial of organic matter during the anoxic event is credited with causingdrawdown of CO2, temperature fall and decreased organic productivity by seques-





tration of nutrients, thus returning the ocean–atmosphere system to equilibrium(Jenkyns 1999): the model is similar to that proposed by Bains et al . (2000) fortermination of the PETM. The time-scale for the later Toarcian cooling, if the strati-graphic record is not distorted by being unduly expanded, was considerably longerthan for the temperature rise (figure 4). Detailed investigation of cyclic pelagic sedi-ments from the Pliensbachian–Toarcian of north Italy indicate a change in the dom-inant orbital-climatic frequencies recognized, with a shift from precession to preces-sion/eccentricity in the latest Pliensbachian to precession/eccentricity and obliquityduring the enuicostatum/falciferum zones and obliquity only in younger strata (Hin-nov & Park 1999). This transition from one dominant frequency to another hencecorrelates with temperature change and the oceanic anoxic event, reinforcing theinterpretation of this interval of time as a period of global climatic reorganization.

7. The Callovian–Oxfordian boundary (Jurassic) ‘cold snap’

Oxygen-isotope ratios of belemnite rostra from localities in Russia indicate a dra-matic increase in δ18O values of ca. 1.5 %% across the Callovian–Oxfordian boundary(ca. 156 Ma, after the time-scale of Palfy et al . 2000), translating into a temperaturefall of 6–7 ◦C (Podlaha et al . 1998; Barskov & Kiyashko 2000). Similar isotopic trendsare recorded in belemnites from Poland (Gruszczynski 1998) and Britain (Jenkynset al . 2002). Palynological data from the North Sea also indicate a fall in tempera-ture (Abbink et al . 2001). Boreal ammonite faunas invaded Tethyan Europe duringthis interval (Fortwengler et al . 1997), suggestive of significant cooling. However, thescattered nature of the belemnite-derived isotopic data does not allow identificationof the rapidity of climate change nor whether or not such changes were global orlinked to movement of local water masses. Paradoxically perhaps, 87Sr/86Sr valuesat this time are the lowest recorded for Phanerozoic sea water (Jones et al . 1994b;Veizer et al . 1999), suggesting that hydrothermal cycling, presumably accompaniedby high rates of sea-floor spreading, subduction and CO2 effusion, was relativelymore important than continental weathering. Presumably, therefore, the sinks forCO2 were also operating at an accelerated pace with respect to other periods ofPhanerozoic time. The trigger for this particular ‘cold snap’ is unknown.

8. The mid-Oxfordian (Jurassic) negative carbon-isotope excursion

An abrupt negative isotope excursion was identified in the transversarium zone ofthe middle Oxfordian (ca. 155 Ma, after the time-scale of Palfy et al . 2000) from anumber of sections in France and Switzerland by Padden et al . (2001), and also inter-preted as due to dissociation of gas hydrate. Although relatively low oxygen-isotopevalues in bulk carbonates occur locally at the same stratigraphic level (Padden etal . 2002), there is no high-resolution record available that might yield informationon the rapidity of any associated climate change.

9. Rapid warming at the onset of the early Aptian (Cretaceous) OAE

Although the early Aptian OAE or Selli event is well documented globally by anextensive black-shale record with organic matter and radiolarian silica largely replac-ing coccolithic and planktonic foraminiferal carbonate (Coccioni et al . 1987; Sliter




1900 H. C. Jenkyns

−3 −2 −1 00 1 1 2 3 4−1 graphiclog

Ont

ong

Java

Pla

teau

Lat

eE

arly

Apt

ian

age (

Ma)

cabr

ibl

owi

zone

strati

grap

hic

thic

knes

s (m)

stage

foraminiferalbiostratigraphy

~120

13C(‰ PDB)

18O(‰ PDB)

δ δ

Figure 5. Carbon- and oxygen-isotope profiles through the equivalent of the Selli Level (lowerAptian), as exposed in northwest Sicily (Calabianca), where it is relatively carbonate-rich (Bel-lanca et al . 2002). Shaded band indicates the part of the section characterized by major geochem-ical anomalies. Note the synchronous shifts to lower δ18O and δ13C values in pelagic carbonatesjust below the black shale that registers the OAE. Dissociation and release of methane into theocean–atmosphere system, with concomitant rapid temperature rise, is implied. Rapid climatechange clearly preceded the OAE. Extrusion of the basalts of the Ontong Java Plateau in thePacific Ocean (Large Igneous Province) are approximately synchronous with these major dis-turbances in the global carbon cycle and could have caused enough global warming to triggerdissociation of gas hydrates and hence precipitate the concatenation of environmental effectsthat followed (Larson & Erba 1999). Lithological symbols as in figure 3.

1989, 1999; Bralower et al . 1993, 1994, 1999, 2002), and by its effect on planktonicbiota (Larson & Erba 1999; Leckie et al . 2002), any accompanying temperaturechange is poorly constrained, even though there is considerable evidence for globalcooling following the event. Like the early Toarcian, the early Aptian (around theboundary between the blowi and cabri planktonic foraminiferal zones; ca. 120 Ma,after Larson & Erba 1999) is characterized by a pronounced, abrupt and steppednegative carbon-isotope excursion, registered in deep- and shallow-marine carbon-ate, marine organic matter and terrestrial higher-plant material from Europe, NorthAmerica, Japan and the Pacific Ocean (Weissert & Lini 1991; Menegatti et al . 1998;Erba et al . 1999; Grocke et al . 1999; Jenkyns & Wilson 1999; Luciani et al . 2001;Ando et al . 2002; Bellanca et al . 2002).





In pelagic sections from Switzerland and Italy, and in a shallow-water carbonatesection in the Pacific, the negative δ13C excursion in carbonate and organic matteris characteristic of a level at the base of or just below the black shale, a stratigraphicarrangement similar to that of the Toarcian. Available oxygen-isotope data deriveonly from lithified carbonate and the datapoints in all sequences analysed to dateshow considerable scatter, suggestive of diagenetic overprinting. However, given thatin virtually all localities examined to date there is a clear shift (up to −2 %% ) to loweroxygen-isotope values coincident with the negative carbon-isotope excursion, a rapidtemperature rise is implied (figure 5 (Menegatti et al . 1998; Luciani et al . 2001;Bellanca et al . 2002)). Above the black shale, δ18O values generally become heavierand hence suggest temperature decline, a conclusion supported by changes in paly-nology, particularly a decrease in abundance of the thermophilic pollen Classopollis(Corollina) in an Italian section (Hochuli et al . 1999). By late Aptian times globalclimates were cool enough (less than 4 ◦C in polar regions) to foster the formationof glendonites (marine low-temperature hydrated polymorphs of calcium carbonate)in Arctic Canada and Australia (Kemper 1987; Frakes & Francis 1988; De Lurio &Frakes 1999). The changes in microfauna and nannoflora in the black shale of theSelli Event from Sicily suggest an increase in fluvial input just before the depositionof the organic-rich facies; and relatively high barium values in sediments higher in thestratigraphic column imply that an increase in productivity could have caused draw-down of atmospheric CO2, as suggested for the PETM (Bains et al . 2000; Bellancaet al . 2002).

The gas-hydrate hypothesis for the early Aptian negative carbon-isotope anomaly,suggested by Jahren et al . (2001), has been further developed by Beerling et al .(2002), who suggest release of ca. 3000 GtC from methane at the onset of the OAE. Aswith the Toarcian, the geochemical response to the input of isotopically light carbonis greater (by some 2–3 %% ) in the terrestrial as opposed to the marine organic-carbon(and carbonate) isotope record (Grocke 2002). This effect, related to increase in theCO2 content of the atmosphere by Grocke et al . (1999), may equally well be explainedby the lesser dilution of methane-derived CO2 in the atmosphere than in the oceans(Jahren 2002). Detailed organic geochemical work on lower Aptian black shales fromItaly and the Pacific basin illustrates the negative excursion in both terrestrial andmarine biomarkers (van Breugel et al . 2002), but it is not yet established whetherthere is any time lag between the two in registering the isotopic signal, although theresponse of shallow-water carbonates predates that in marine organic matter in onePacific site (Jenkyns & Wilson 1999). Biomarkers further suggest that photic-zoneeuxinic conditions existed locally in the water column during the early Aptian OAE.Whether gradual global warming or destabilization of continental-margin sedimentstriggered catastrophic release of gas hydrates is not established. Larson & Erba (1999)have noted that the Ontong Java large igneous province overlaps in age with the OAEand the negative carbon-isotope excursion, so a gradual influx of volcanogenic CO2 tothe ocean–atmosphere can be postulated to have caused enough warming of bottomwaters to destabilize gas hydrates. However, tectonic explanations for dissociationof the gas-hydrate layer have also been advanced (Jahren 2002). Unlike the earlyToarcian, strontium-isotope ratios from marine carbonates show a movement to lessradiogenic values in the early Aptian, beginning at the level of the negative carbon-isotope shift (Jones & Jenkyns 2001). The response of the strontium-isotope system inthis case is interpreted as dominated by a hydrothermal flux related to increased rates




1902 H. C. Jenkyns

of sea-floor spreading that accompanied the formation of submarine large igneousprovinces (for example, Ontong Java Plateau and Manihiki Plateaus), unlike thedominantly subaerial Karroo–Ferrar volcanics of the Toarcian whose extrusion wascoincident with rifting of the southern continents (Encarnacion et al . 1996).

Cyclostratigraphic studies of Aptian pelagic sediments in central Italy, investi-gated by Herbert (1992) and Fiet (2000), recognized precession and eccentricitycycles, allowing an estimated depositional duration of the lower Aptian black shaleof between 400 ka and 1 Ma, according to the interpretation adopted. The negativecarbon-isotope excursion and associated δ18O shift generally take place over a fewcentimetres or tens of centimetres of section, whereas the black shale is typically1–2 m thick. Such relationships imply palaeotemperature change on a time-scale oftens to thousands of ka (Grocke 2002).

10. The early Albian (Cretaceous) Paquier Event

The early Albian Paquier Event, first recognized by Breheret (1985), is characterizedby a laminated black shale of regional extent. Initially described from southeasternFrance (Vocontian Trough), with coeval correlatives recognized in southern Ger-many and Austria, its sedimentary record has now been traced through much of theTethyan-Atlantic region, including deep-sea drilling sites (Breheret 1997; Braloweret al . 1993; Leckie et al . 2002). This phenomenon clearly qualifies as an OAE butthe resultant black shales apparently lack the global distribution that character-izes the Toarcian OAE and its Cretaceous counterparts of the early Aptian andCenomanian–Turonian boundary. The Albian black shale is dated to the planispiraplanktonic foraminiferal zone and the tardefurcata ammonite zone (Strasser et al .2001; Herrle 2002; Herrle et al . 2003), with a likely absolute age of 111 Ma, after thetime-scale of Gradstein et al . (1994).

Although carbon- and oxygen-isotope values of single-species benthic and plank-tonic foraminifera in core material from the western Atlantic show little changethrough the black shale itself (Erbacher et al . 2001), stratigraphically just belowthis horizon there is a pronounced negative excursion in both carbon and oxygenisotopes. This signal is seen regionally in bulk carbonates from Atlantic drilling sitesand southern France (Herrle 2002; Grocke et al . 2002). Because the material fromthe Vocontian Trough contains a certain amount of organic matter and may carrya variable burial-diagenetic overprint (see, for example, Weissert & Breheret 1991),the isotopic data from the Atlantic (Mazagan Plateau and Blake Nose) are deemedthe more reliable. These data show a synchronous drop in δ13C and δ18O values ofbulk carbonate of ca. − 1 %% , suggesting an input of isotopically light carbon intothe oceans and a concomitant average surface-water temperature rise of 4–5 ◦C. Thecyclostratigraphic time-scale of Herrle (2002) and Herrle et al . (2003), based on therecognition of eccentricity, obliquity and precession cycles in the sedimentary section,suggests that the temperature rise took place over a period of 40 ka, with the initialmajor increase being much more rapid than this. However, considerable potentialerror must attach to these figures as they are based on computation of averagedsedimentary rates that may not be applicable during the events in question.

Significantly, perhaps, deposition of the regional black shale post-dates the carbon-isotope disturbance and temperature rise, suggesting that the gas-hydrate hypoth-esis is viable in this case also and that enhanced weathering, nutrient flux and ele-





vated organic productivity could have followed effusion of greenhouse gases intothe atmosphere and consequent rise in temperature (Grocke et al . 2002). The rela-tively parochial record of the black shale that defines the Paquier event implies anadditional regional control on the accumulation of organic matter. Detailed organic-geochemical studies of the organic matter show that it is entirely different from thoseof other Cretaceous black shales, being dominated by the remains of Archaea ratherthan phytoplankton and there is no biomarker evidence for a sulphidic water col-umn (Kuypers et al . 2002a). In an alternative to a global model, Erbacher et al .(2001) favour increased carbon burial related to water mass stratification caused byan increase in run-off into semi-restricted basins (Tethys and proto-Atlantic), anddraw parallels with the formation of Quaternary Mediterranean sapropels. Herrle(2003) suggests that a monsoonal climate prevailed and that the black shale wasdeposited under conditions of strongly fluctuating surface-water fertility.

11. Rapid temperature fall during the Cenomanian–Turonian(Cretaceous) climatic optimum?

The detailed carbon- and oxygen-isotope stratigraphy of the English Chalk, firstelucidated by Scholle & Arthur (1980), was revised and amplified by Jenkyns etal . (1994), who recognized a palaeotemperature maximum at around Cenomanian–Turonian boundary time (ca. 93 Ma, after Gradstein et al . (1994)). An oxygen-isotoperecord from cored material recovered from off northwest Australia similarly showeda climatic optimum at around the stage boundary, extending into the middle Tur-onian: the considerable stratigraphic extent of this section suggested that tempera-tures attained at this time were the highest of the last 115 Ma (Clarke & Jenkyns1999). Similar climatic trends, based on single-species planktonic and benthonicforaminifera, have been reported from southern high latitudes and the western NorthAtlantic (Huber et al . 1995, 2002), with middle bathyal water temperatures of thelatter region estimated at 20 ◦C around the Cenomanian–Turonian boundary. Wilsonet al . (2002) and Norris et al . (2002) estimate unusually warm peri-equatorial sea-surface temperatures (ca. 33–34 ◦C) based on oxygen-isotope analyses of particularlywell-preserved upper Cenomanian and Turonian planktonic foraminifera.

The currently available isotopic records do not allow for particularly high-resolu-tion climatic studies: however, invasion of boreal faunal elements in sections thatspan the Cenomanian–Turonian boundary is recognized in many European and someNorth African sections (Jefferies 1962, 1963; Kuhnt et al . 1986) and such changescorrelate with heavier oxygen-isotope values in the English Chalk (Jeans et al .1991; Jenkyns et al . 1994). Whether such faunal changes relate to local or regionalupwelling of cooler waters driving increases in organic productivity as implied forthe North Atlantic (Kuypers et al . 2002b), or southward movement of a boreal watermass, is not established, but some high-frequency climatic change is nonethelessindicated (Jarvis et al . 1988). Detailed cyclostratigraphic studies of north EuropeanChalk sections and shallow-marine to fluvial sandstones from southeast India suggesta number of synchronous rapid sea-level falls (on a time-scale of the long eccentricitycycle, ca. 400 ka) that Gale et al . (2002) tentatively interpret as glacio-eustaticallycontrolled. The possible existence of an Antarctic ice cap during a period of unusu-ally high global temperatures is only explicable if increased evaporation in the World




1904 H. C. Jenkyns

Ocean led to increased precipitation in polar regions and high-altitude sites existedwhere ice could accumulate.

The Cenomanian–Turonian boundary also records a major OAE, with marineblack shales now known to have been deposited in all major oceans and epiconti-nental seas, with an accompanying pronounced carbon-isotope excursion in marinecarbonate, organic matter and terrestrial higher-plant material (Schlanger & Jenkyns1976; Jenkyns 1980; Schlanger et al . 1987; Arthur et al . 1988; Gale et al . 1993; Vil-lamil & Arango 1998; Hasegawa 1997; Hasegawa et al . 2003). Unlike the early Toar-cian and early Aptian OAEs, there is no preceding negative carbon-isotope excur-sion that might suggest dissociation of gas hydrates as a mechanism for amplifyingglobal warmth. However, supply of volcanogenic CO2 from large igneous provinces(Caribbean province and Ontong Java Plateau) is, as with the early Toarcian andearly Aptian OAEs, indicated for the Cenomanian–Turonian boundary, with stimu-lation of plankton productivity by hydrothermally sourced trace metals an additionalpossible variable (Sinton & Duncan 1997; Kerr 1998). Biomarker evidence from NorthAtlantic cores suggests that much of the water column was sulphidic, with euxinicconditions extending high into the photic zone (Sinninghe Damste & Koster 1998;Kuypers et al . 2002b): sulphur-isotope data across this interval suggest that sulphatereduction and pyrite formation were globally significant enough to move δ34S to morepositive sea-water values (Arthur et al . 1985; Ohkouchi et al . 1999).

As with the early Aptian OAE, but unlike that of the Toarcian, 87Sr/86Sr ratiosbegin a move to less radiogenic values at the Cenomanian–Turonian boundary, sug-gesting that the geochemical signature of increased rates of sea-floor spreading andhydrothermal flux to the oceans dominated over that of continental weathering (Jones& Jenkyns 2001). Isotopic evidence from the Western Interior of the United States(Freeman & Hayes 1992), and biomarker evidence from Morocco and Atlantic drillingsites (Kuypers et al . 1999) together imply a rapid (ca. 60 ka, based on the cyclostrati-graphic interpretation of Kuhnt et al . 1997) fall in CO2 levels, related to massiveglobal carbon burial during the OAE itself. On the northern margin of Africa theresultant climate change apparently led to sudden changes in vegetation, adapted tolower atmospheric concentrations of CO2.

Relatively abrupt cooling phases, on a 250 ka time-scale, are also documented fromthe Late Turonian of northern Europe. Such events are characterized by regionalexcursions to heavier δ18O values in chalk facies and southward migration of borealfaunas (ammonites, echinoids, bivalves) in response to climatic deterioration (Voigt& Wiese 2000; Wiese & Voigt 2002). Whether these changes reflect movement oflocal water masses or record a global signal is not established.

12. Triggers for gas-hydrate release: tectonics or temperature?

Study of the Jurassic–Cretaceous isotopic record pinpoints gas-hydrate release asthe most likely cause of rapid climate change, on a scale of kyr, in the Mesozoicgreenhouse world. The most popular current theory for triggering the dissociation ofsuch gas hydrates is warming of oceanic bottom waters, an interpretation that findssome support in the high-resolution oxygen-isotope data from foraminifera depositedin the prelude to the PETM (Thomas et al . 2002) and is consistent with modelsof changing ocean circulation (Bice & Marotzke 2002). However, as noted above,





dissociation of gashydrates and injectionof oxidized methaneinto ocean/atmospheresystem: negative 13Cexcursion and rapid global warming

δ

local formationof salinity-

stratified waterbodies

fluvial and nutrient fluxto oceans increases:

187Os/188Os and 87Sr/86Sr values of

sea-water rise

hydrological cycleaccelerates and

weathering rates increase: CO2 is

drawn down

globaltemperature

rises:greenhouse

effect

excessvolcanogenicCO2 suppliedto oceans andatmosphere

eruption of large igneous provinces: if accompanied by

accelerated sea-floor spreading and hydrothermal circulation

87Sr/86Sr values of sea-water fall

windvelocitiesincrease

eustatic sea-levelrise increases shelf-

sea area andpromotes production

of warm salinebottom waters

planktonproductivity

increases

shelf-sea,continental-margin andequatorialupwellingintensifies

organic-carbonburial rate risesand 13C valuesof sea-water rise

OAE begins

δ

nutrientsdepleted

from ocean

excess CO2is drawn

down

productivity,carbon flux andcarbon burialrate declines

and 13C valuesof sea-water fall

OAE ends

δ

temperaturefalls: inversegreenhouse

effect

oxygen-minimumzone intensifies:denitrification

begins

organic-carbon fluxto sea floor increases15N values of organic

matter riseδ

oceanic sulphatereduction increasesand 34S values of

sea-water riseδ

O2 content ofocean–atmosphere

system rises

Figure 6. Diagram to illustrate possible relationships between volcanism, gas-hydrate dissocia-tion, climate change, OAEs and assumed geochemical responses. Amplified from Jenkyns (1999).

for both the PETM and the early Aptian event, tectonic causes have been enter-tained (Katz et al . 2001; Jahren 2002). Mesozoic–Palaeogene abrupt carbon-isotopeexcursions recognized to date occurred at the following times: Permo–Triassic bound-ary, Triassic–Jurassic boundary, early Toarcian, mid-Oxfordian, early Aptian, earlyAlbian and Palaeocene–Eocene boundary. Such dates certainly overlap with riftingphases of Pangaea, undoubtedly characterized by rotation of tilted fault blocks andmajor episodes of submarine slumping and redeposition along continental margins,potentially causing phase changes within any hydrate reservoir. Whether dissocia-tion of gas hydrates was the cause or the effect of slope instability remains an openquestion (see, for example, Haq 1998). Until more data can be assembled to focus onenvironmental conditions preceding these putative methane-release events, a tectoniccausal mechanism remains impossible to exclude, although such effects are presum-ably likely to be more local than regional. Possible linkages involving carbon cycling,




1906 H. C. Jenkyns

temperature changes and likely geochemical responses, but excluding tectonic effects,are illustrated in figure 6.

Work on Jurassic and Cretaceous palaeoceanography and palaeoclimatology has been supportedby the NERC and the EC over a number of years. I particularly acknowledge useful discussionswith S. Hesselbo, D. Grocke, S. Robinson and R. Woodfine in Oxford and thank them for theirhelp in the preparation of diagrams. Jerry Dickens cast an expert eye over figure 1. Tim Bralowerand Debbie Thomas courteously supplied data and images for figure 2. Harry Elderfield and NickMcCave are thanked for helpful reviews.

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Discussion

G. P. Weedon (Department of Environment, Geography and Geology, Universityof Luton, UK ). In studies of Mesozoic sediments, most work on rapid climate changehas derived from high-resolution sampling of intervals where organic carbon contentsare particularly high. So, is our knowledge of rapid climate change affected by the factthat only certain intervals have been studied at high resolution? With the generationof more high-resolution isotopic data, would more intervals of rapid climate changebe found?

H. C. Jenkyns. Almost certainly, yes, although there seems to be a genetic con-nection between climate change and deposition of organic-rich sediments. As morelong stratigraphic sequences are investigated with high-resolution sampling it is likelythat progressively more geologically rapid climatic events, involving abrupt negativeshifts in carbon- and oxygen-isotope ratios, will come to light. Once the most strati-graphically complete sequences have been identified for whatever geological intervalin whatever part of the world, time (for very detailed sampling) and expense (forappropriate analytical work) are the principal limiting factors in data generation.




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