Journal of the Geological Society, London, Vol. 155, 1998, pp. 619–626. Printed in Great Britain

Oceanic plateau formation: a cause of mass extinction and black shale depositionaround the Cenomanian–Turonian boundary?

ANDREW C. KERRDepartment of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK (e-mail: ack2@le.ac.uk)

Abstract: The Cenomanian–Turonian boundary (90.4 Ma) represents a major period of worldwideenvironmental disturbance. The physical manifestations of this are: elevated atmospheric and oceanictemperatures; a significant sea-level transgression; and a period of widespread anoxia, leading to theformation of oceanic black shales, and the extinction of 26% of all genera. Elevated ä13C values andenrichment of trace elements in Cenomanian–Turonian boundary sediments, combined with a reductionin 87Sr/86Sr, also imply a severe environmental perturbation. At this time oceanic crustal production ratesreached their highest level of the last 100 million years. This was principally caused by extensive meltingof hot mantle plumes at the base of the oceanic lithosphere, and the development of vast areas (up to1#106 km2) of thickened oceanic crust in the Pacific and Indian Oceans. The anomalous volcanismassociated with the formation of these oceanic plateaux may have been responsible for the environmentaldisturbances c. 90 Ma. These eruptions would also have resulted in the emission of large quantities of CO2

into the atmosphere, leading to global warming. Additionally, the emission of SO2, H2S, CO2 andhalogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate,and further release of CO2. This run-away greenhouse effect was probably put into reverse, by the declineof the anomalous volcanic activity, and by increased (CO2-driven) productivity in oceanic surface waters,leading to increased organic carbon burial, black shale deposition, anoxia and mass extinction in theocean basins.

Keywords: Cretaceous, volcanism plumes, mass extinctions, black shale.

Although the existence of large continental flood basaltprovinces has been acknowledged for some time, it is onlycomparatively recently that their counterparts, the oceanicplateaux have been recognized within the ocean basins (e.g.Kroenke 1974; Coffin & Eldholm 1994). In the same waythat the formation of flood basalt provinces can thickenthe continental crust (by eruption, intrusion and underplatingof magma), the formation of oceanic plateaux can alsocreate thickened (>10 km) oceanic crust. These large igneousprovinces (Fig. 1) probably formed by decompression meltingof anomalously hot mantle, possibly as much as 200–300)Chotter than ambient upper mantle, in a relatively short periodof time, generally less than 2–3 Ma (Coffin & Eldholm 1994).Continental provinces appear to have formed throughout mostof geological history (e.g. Campbell & Griffiths 1992; Nisbetet al. 1993). In the oceans, however, plateaux are not easilypreserved, because they are either removed completely bysubduction, or the uppermost portions are ‘scraped off ’(obducted or accreted) on to the continental margins (Ben-Avraham et al., 1981; Abbott & Mooney 1995).

The potential catastrophic environmental effects of the rapideruption of continental flood basalts have been extensivelydocumented, particularly in relation to the eruption of theSiberian Traps c. 250 Ma (e.g. Renne et al. 1995) and theDeccan Traps c. 65 Ma (e.g. Hallam 1987). These two eventscontributed significantly to the two biggest mass extinctionevents of the last 250 Ma; those at the Permian–Triassicboundary and the K–T boundary. What has received substan-tially less attention is the potential environmental influence ofthe formation of oceanic plateaux.

This paper will focus primarily on the Cenomanian–Turonian boundary (90.4 Ma; Harland et al. 1990) which wasmarked, particularly within the ocean basins, by a period of

severe environmental disturbance, including a significantmass extinction event. As will be shown, this boundary alsomarks the initiation of a period of extensive oceanic plateauformation and increased mid-ocean ridge volcanism. Potentiallinks between this volcanism in the oceans and the environ-mental catastrophe that befell the oceans and marine life inlate Cenomanian and Turonian times (c. 88–91 Ma) will beexplored.

Igneous events in the Cenomanian and Turonian

Oceanic plateauxWith advances in 40Ar/39Ar radiometric dating, a much moreprecise picture of the age and duration of eruption of largeigneous provinces is emerging. Table 1 summarizes availableage dates and, where possible, estimated erupted volume andareal extent of the provinces which are late Cenomanian orTuronian in age. Figures 1 & 2 show present-day and a 90 Maplate reconstruction of these provinces. The most extensivevolcanism around this period occurred in the oceans and inparticular the Pacific Ocean (Figs 1 & 2). The largest eruptiveevent is today preserved as the Caribbean–Colombianoceanic plateau (Kerr et al. 1997a) and makes up most of theCaribbean plate along with extensive accreted terranes inColombia (Kerr et al. 1997b) (Fig. 1). It is widely believedthat this province formed in the eastern Pacific, possibly atthe start-up of the extant Galápagos hot-spot (Duncan &Hargraves 1984; Kerr et al. 1997a) (Fig. 2). The preservedremnants of the plateau have been moved into their presentlocations by plate-tectonic motions because the plateauoriginally formed on the eastward-moving Farallon plate(Duncan & Hargraves 1984). The volume of lava erupted has

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been estimated at 4#106 km3, with the most intense volcanismoccurring between 89 and 91 Ma (Table 1). The mean eruptionrate was therefore of the order of 2 km3 a"1.

Broken Ridge is a part of the Kerguelen Plateau in theIndian Ocean (Figs 1 & 2) which covers an area of about0.5#106 km2 (Coffin & Eldholm 1994; Table 1). The dates

obtained from Broken Ridge are all from dredge samples(Duncan 1991) and so probably represent the ages of some ofthe top-most flows. Thus, the bulk of the edifice may be olderthan the 88–89 Ma recorded from the dredge samples. Thevolume of lava erupted has been estimated at 2 million km3,and the mean eruption rate was approximately 1–2 km3 a"1.

Fig. 1. Locations of some of the biggestlarge igneous provinces at the presentday (after Saunders et al. 1992). Known90 Ma provinces are shown in largerbold type.

Table 1. Age, eruptive volume and areal extent of anomalous Cenomanian–Turonian volcanism

Province

Age range(Ma)

40Ar/39ArMean age

(Ma)

Volumeerupted

(106 km3)

Arealextent

(106 km2) References

Caribbean/Colombian 87.1–91.7 90.1 c. 4 1.0 Kerr et al. (1997)Kerguelen (Broken Ridge) 88.1–89.2 88.7 c. 2 0.51 Coffin & Eldholm (1994), Duncan (1991)Part of Ontong Java 89.8–94.5 90.8 c. 2 Eastern portion Mahoney et al. (1993),

0.5 (estimate) Tejada et al. (1996)Madagascar 84.4–90.7 87.6 ?1–2 <1 Storey et al. (1995)

Fig. 2. Reconstructed map at 90 Ma(after Owen 1983) showing the locationof 90 Ma large igneous provinces (gradedshading). Older parts (124–110 Ma) ofthese provinces are shown as stippledornament. Also shown are the knownlocalities of Cenomanian–Turonianboundary black shales (Arthur et al.1987; Herbin et al. 1987; Summerhayes1987).

620 A. C. KERR

Although the northern part of the Ontong Java plateauappears to have formed at about 122 Ma, the extent of thec. 90 Ma phase of volcanism is poorly known. 40Ar/39Ar agesof 93.9&1.4 and 88.2&1.1 Ma were obtained by Mahoneyet al. (1993) from lavas from an ODP (Leg 130) drill hole (site803) and Tejada et al. (1996) have found a similar range of agesfrom an obducted part of the plateau in the Solomon Islands.Due to the largely submarine nature of this plateau it is quitedifficult to estimate the areal extent of the 90 Ma lavas.Nevertheless, Tejada et al. (1996) have suggested that theeastern lobe of the plateau (Fig. 1) may have been the focus ofthe 90 Ma phase of volcanism. If this model is accepted, then aconservative estimate of the areal extent of this phase would beabout 0.5#106 km2. The erupted volume of lava in this 90 Maphase may therefore have been as much as 2#106 km3.Eruption rates are difficult to estimate, due to a lack ofaccurate dates, but they probably average about 1 km3 a"1.

A potentially large continental/oceanic flood basalt provinceassociated with the Marion hot-spot and the break-up ofMadagascar and India, also formed c. 90 Ma (Fig. 2). Todaythese basalts are preserved along the 1500 km length of the eastcoast of Madagascar, and as submarine fragments on theMadagascar Plateau (Storey et al. 1995) (Fig. 1). Storey et al.(1995) have suggested that the province had a probableoriginal area of around 1#106 km2, a significant proportionof which probably erupted in late Cenomanian and Turoniantimes.

Kaiho & Saito (1994) estimated that the contribution(intrusive and extrusive) to oceanic crustal production ratesfrom oceanic plateaux in Cenomanian–Turonian times wasc. 11#106 km3 per million years. However, because recentdating has revealed that oceanic plateau volcanism at this timewas more widespread than previously thought, oceanic plateauproduction rates were probably significantly greater than thisfigure, and were perhaps closer to 20#106 km3 per millionyears. It is estimated that the total erupted volume of plume-related lava around the Cenomanian–Turonian boundary wasof the order of 8–10#106 km3. The available 40Ar/39Ar agessuggest that most of the volcanism occurred between 88.5 and91.0 Ma. This works out at a mean eruption rate of between 3and 4 km3 a"1, whereas in contrast the mean present-dayeruption rate from plume-related oceanic volcanoes is about0.5 km3 a"1 (White 1993).

Ocean ridge volcanismIn late Cenomanian and Turonian times there was also anincrease in mid-ocean ridge volcanism (Schlanger et al. 1981;Larson 1991; Kaiho & Saito 1994). This increase was probablydue to an increase in the length of the global ridge system,since at this time South America was separating from Africaand the southern North Atlantic was beginning to open.

Both Larson (1991) and Kaiho & Saito (1994) have calcu-lated production rates of oceanic crust (plume-related andmid-ocean ridge) for the last 100–150 Ma. Around theCenomanian–Turonian boundary (Larson 1991) has estimatedthat 31#106 km3 of oceanic crust were produced per millionyears, whereas Kaiho & Saito (1994) estimate that57#106 km3 of oceanic crust were produced (the latter figureis higher because it includes crustal production in the nowsubducted Tethys). Average estimated oceanic crust pro-duction rates for 110–80 Ma are shown in Fig. 3, and it can beseen that these rates peak around 90 Ma. For comparison,

c. 20#106 km3 of lava have been erupted in the oceans overthe past million years (Larsen 1991; Kaiho & Saito 1994).

These oceanic crustal production rates highlight the fact thatthe Cenomanian–Turonian boundary was marked by intense,anomalous predominantly oceanic volcanism, relative to boththe rest of the mid–late Cretaceous (110–80 Ma), and thepresent day. In the next section some other characteristics ofthe Cenomanian–Turonian boundary will be reviewed.

Fig. 3. Graphs showing how various parameters discussed in thetext vary from 110 to 80 Ma. The dotted horizontal line representsthe Cenomanian–Turonian boundary. Sources of data: CO2, Arthuret al. (1987); Relative intensity of anoxia, Jenkyns (1980); 87Sr/86Sr,McArthur et al. (1993); ä13C of pelagic limestone and sea surfacetemperature, Arthur et al. (1985); Sea level and Oceanic crustproduction, Larson (1991), Kaiho & Saito (1994).

OCEAN PLATEAUX AND MASS EXTINCTIONS 621

Geochemical and stratigraphic characteristics of theCenomanian–Turonian boundary eventThe most significant features of the Cenomanian–Turonianboundary event are the near-ubiquitous worldwide (Fig. 2)occurrence of a horizon of black, organic-rich shales, anextinction episode (Jenkyns 1980; Schlanger et al. 1987) andthe relatively short duration of this event (<1 Ma; Arthur et al.1987).

The deposition of black shales is generally taken to indicatea period of widespread anoxia in the ocean basins, andseveral anoxic events (known as ‘oceanic anoxic events’) havebeen identified in the Cretaceous (Jenkyns 1980) (Fig. 3). Inaddition to this, the Cenomanian–Turonian boundary was atime of a major sea-level transgression (Fig. 3) (e.g. DeBoer1986; Hallam 1989) and is also marked by a positive carbonisotopic anomaly (ä13C excursion) of up to +4 to 5‰ (Fig. 3),indicating an increase in organic carbon burial rate (Arthuret al. 1987). A marked reduction in the 87Sr/86Sr of seawater which started before the Cenomanian–Turonianboundary (Fig. 3) reaches a maximum of 0.70753 in the lateCenomanian, and drops steadily until the mid-Turonian,before starting to rise.

Oxygen isotopes reveal that globally averaged surfacetemperatures around the Cenomanian–Turonian boundarywere 6–14)C higher than at present (Caldeira & Rampino1991; Kaiho 1994) and this global warming also affected theoceans (Barron 1983). The reason for this worldwide tem-perature rise appears to have been an increase in globalatmospheric CO2 content (Fig. 3) which may have been four toeighteen times higher than present-day levels (Arthur et al.1987).

In common with the K–T boundary, Cenomanian–Turoniansediments display enrichments in iridium (Ir). However, thedegree of enrichment is 1–2 orders of magnitude less thanthose observed in the K–T boundary sediments (Orth et al.1990, 1993). Additionally, trace element ratios between thesediments of K–T and Cenomanian–Turonian boundarysections are markedly different (Orth et al. 1993) (Table 2). Forexample, K–T boundary sediments have Cr/Ni ratios of c. 0.3,whereas Cenomanian–Turonian sediments possess ratios ofc. 3.0. Other trace elements such as Sc, Ti, V, Cr, Mn, Co, Ni,Pt and Au are enriched along with Ir (Leary & Rampino 1990;Orth et al. 1993). Table 2 shows that trace element abundancesand ratios found in the Cenomanian–Turonian boundary

sediments are broadly similar to plume-derived volcanicrocks and mid-ocean ridge basalts. For example, in maficvolcanic rocks Ni/Ir#104 values range from 70 to 190, andin Cenomanian–Turonian boundary sediments this ratioaverages 180. In contrast the K–T boundary sediments and C1chondrites possess Ni/Ir#104 values which are less than 3(Table 2).

One of the most significant aspects of the phenomenaobserved at this time is the extinction of a wide range oforganisms (e.g. Sepkoski 1986; Jarvis et al. 1988). Although,the Cenomanian–Turonian boundary does not represent afirst-order extinction event (Sepkoski 1986), 7% of families and26% of genera became extinct at this time, mostly affectingmarine micro- and macro invertebrates (Fig. 4) (Sepkoski1986; Bralower 1988; Jarvis et al. 1988; Hart & Leary 1991).Figure 4 shows percent extinction (genus level) for various

Table 2. Elemental ratios, Ir and Cr contents of Cenomanian–Turonian and K–T boundary sediments, C1 chondrite andvarious volcanic rocks

Ir (ppb) Cr (ppm) Cr/Ir#104 Ni/Ir#104 Cr/Ni Ti/Cr Yb/La

C–T sediments 0.07 365 500 180 2.8 12 0.3K–T sediments 41.6 371 0.91 2.7 0.33 5.2 0.08C1 480 2640 0.55 2.3 0.24 0.16 0.67MORB 0.07 323 460 190 2.6 24 0.8Gorgona komatiites 1.6 1200 75 70 2.5 2.0 2.3Siberian traps 0.14 240 170 100 1.9 39 0.2Reunion seamounts 0.1 200 200 150 2.1 28 0.6

Figures in bold indicate those values which are less than double, yet greater than half of the value for theCenomanian–Turonian (C–T) boundary sediments. Data sources: C–T sediments, Orth et al. (1993); K–T sediments,Alverez et al. (1980) (Danish sections); C1 chondrite, Anders & Ebihara (1982); MORB, Schilling et al. (1983)(mid-Atlantic Ridge); Gorgona komatiites, Brugmann et al. (1987) and Kerr et al. (1996); Siberian traps, Brugmannet al. (1993), Lightfoot et al. (1990) (average composition); Reunion seamounts, Fryer & Greenough (1992) (ODP Leg115).

Fig. 4. Bar graph comparing the pattern of extinction at theCenomanian–Turonian boundary with that at theCretaceous–Tertiary boundary (data from Sepkoski 1986, 1990) forvarious taxonomic groups of marine organisms, also shown is thetotal extinction percentage for all genera.

622 A. C. KERR

taxonomic groups of marine organisms at this time, based onthe extinction curves of Sepkoski (1986, 1990).

Although the overall extinction rate observed at theCenomanian–Turonian boundary is much less than that at theK–T boundary, other differences also exist between these twoextinction events. For example, in a study of benthic andplanktonic foraminifera Kaiho (1994) showed that at the K–Tboundary 90% of surface dwelling foraminifera and c. 10% ofdeep water foraminifera became extinct, whereas at theCenomanian–Turonian boundary the pattern of foraminiferalextinction was reversed, with less than 20% surface dwellingand c. 50% deep water foraminifera being wiped out.

The fact that the extinction event affected deep-ocean-dwelling organisms more than shallow-dwelling organisms,suggests that anomalous oceanic volcanism at theCenomanian–Turonian boundary may have played a sig-nificant role in the environmental disturbance and in theextinction event. In the following section possible linksbetween the stratigraphic and geochemical observations at theCenomanian–Turonian boundary, and the extensive (mostlyoceanic) volcanism, will be discussed.

Links between oceanic volcanism and environmentalperturbationPrevious workers on the Cenomanian–Turonian boundaryphenomena have not addressed the root cause of the environ-mental disturbance. For example, Jenkyns (1980) andSummerhayes (1987) have variously suggested that sea-leveltransgressions, warm climate or high surface water produc-tivity were in themselves the cause of the other phenomena,particularly black shale deposition. On a more general level,Hallam (1989) proposed that marine trangressions wereintimately associated with anoxia. Herbin et al. (1987) sug-gested that the observed phenomena could be explained byincreased mid-ocean ridge volcanism, while Arthur et al.(1987) postulated an ‘undocumented volcano-tectonic event’ asa cause. Schlanger et al. (1981), Vogt (1989), Larson (1991),Orth et al. (1993) and Kaiho & Saito (1994) have identifiedanomalous intraplate volcanism as a causal factor of one ormore of the phenomena observed at the Cenomanian–Turonian boundary and the following discussion builds ontheir work.

Physical effects of plume-related volcanismAs shown above 8–10#106 km3 of lava were erupted on to theocean floor around the Cenomanian–Turonian boundary(Table 1). One of the most obvious physical effects of mantleplume-related volcanism in the oceans will be to raise the sealevel (Larson 1991), both by the extrusion of lava on to theocean floor as well as by the ‘head’ of the hot, buoyant mantleplume uplifting the oceanic lithosphere and displacing seawater (e.g. Courtney & White 1986). This rise in sea levelprobably predates the onset of significant volcanism. Thus,in the case of the Cenomanian–Turonian boundary, theprogressive rise in sea level throughout the Late Albianand Cenomanian (Fig. 3) may reflect the build-up of theCaribbean, Ontong Java and Kerguelen plume headsbelow the oceanic lithosphere, prior to the onset of extensivevolcanism (Schlanger et al. 1981; Vogt 1989; Larson 1991).

Another implication of plume-related uplift is that oceanicplateaux may have erupted close to the ocean surface, and so

may have caused the disruption of important oceaniccirculation systems. This may be particularly true of theCaribbean–Colombian plateau which formed relatively closeto the only significant (Cenomanian–Turonian) oceanic gate-way between the Pacific and the opening Atlantic (Fig. 2). Asthe supply of deep, cold (polar-derived), oxygenated water tothe Atlantic came mostly from the Pacific (DeBoer 1986), it ispossible that circulation in the Pacific was disturbed to such anextent that cool, polar waters were not circulated to lowerlatitudes, resulting in increased oceanic anoxia. Additionally,increased circulation of hydrothermal fluids resulting fromCenomanian–Turonian volcanism would have led to increasedheat flow in the oceans and so could have contributed toanoxia, since the solubility of oxygen in seawater decreases by2% for every 1)C temperature rise (DeBoer 1986).

Chemical effects of plume-related volcanismThe positive ä13C anomalies at the Cenomanian–Turonianboundary reflect increased rates of organic carbon burial(Arthur et al. 1987) as a result of high productivity andmore effective preservation of organic material. In order toproduce increased productivity in surface waters, the supply ofnutrients into these waters must also be increased, and it isprobable that the upwelling of nutrient-rich waters in theCenomanian–Turonian was induced by volcanism (Vogt1989).

The elevated carbon dioxide contents may also beprincipally due to the increased volcanic activity at 88–92 Ma.Assuming, (a) that at least 8#106 km3 of basalt eruptedaround the Cenomanian–Turonian boundary (see Table 1) and(b) that between 1 and 2#1010 kg of CO2 are released fromeach cubic km of basalt (Caldeira & Rampino 1991) it can becalculated (assuming mild solubility of CO2 in water) that ofthe order of 1017 kg of CO2 would have been released as aresult of oceanic plateau volcanism in Cenomanian–Turoniantimes (cf. Arthur et al. 1987).

Plume-related volcanism also releases substantial amountsof SO2. Using the estimates of SO2 release by the Laki eruptionin Iceland (when 12.3 km3 of basalt erupted in 50 days;Thordarson et al. 1996) approximately 8#1016 kg SO2 mayhave been discharged into the oceans in the Cenomanian–Turonian, along with significant amounts of chlorine, fluorineand H2S. The net effect of the partial or near completedissolution of all these gasses in the oceans at this time wouldhave been to acidify the oceans. Arthur et al. (1987) havesuggested that the lack of carbonate at the Cenomanian–Turonian boundary may be the result of increased dissolutionby acidic seawater, so releasing CO2 to the atmosphere.

The addition of CO2 to the atmosphere, either directly orindirectly as a result of volcanic activity, would have resultedin a substantial degree of global warming. The solubility ofCO2 in seawater decreases by 4% for every 1)C temperaturerise (DeBoer 1986); as a result the warmer the oceans get, theless CO2 will dissolve in them. Thus, with this positive CO2

feedback mechanism it is possible that a ‘runaway green-house’ climate may have developed quite rapidly (Fig. 5).One way of reducing atmospheric CO2 levels is by increasingthe weathering of continental silicates, a process which con-sumes CO2. However, the elevated temperatures at this timesuggest that the rate of CO2 release was much greater thanits uptake by slow weathering processes. Increased atmos-pheric CO2 and the upwelling of nutrients from the deep

OCEAN PLATEAUX AND MASS EXTINCTIONS 623

ocean could have resulted in increased productivity in oceansurface waters (Fig. 5), so leading to the widespread depo-sition of black shales. However, the simplest way of reducingthe CO2 levels is to reduce the primary supply (i.e. thevolcanism) to the atmosphere.

Black-shale-hosted trace metals which were liberated byhydrothermal fluids from the submarine lava piles wereprobably incorporated into organic matter in the photic zone(Leary & Rampino 1990) and would have posed a significantthreat to Cenomanian–Turonian marine life. Wilde et al.(1990) have suggested that toxicity from increased concen-trations of trace metals may well have been a contributoryfactor to the demise of some marine organisms around theCenomanian–Turonian boundary.

The increase in, and redistribution of, trace metals in theoceans at the Cenomanian–Turonian boundary may also havehad indirect effects on marine life at that time. The upwellingof trace metals and nutrients from the deeper ocean mayhave resulted in the enlargement of the trace metal-restrictedhabitat of deeper dwelling organisms (Wilde et al. 1990). Thismay have led to increased predation by deeper dwellingcreatures on those living in intermediate to shallow water.Thus, volcanically induced habitat expansion of some deeperdwelling organisms could have resulted in the demise of somespecies.

Did an asteroid impact cause the Cenomanian–Turonianevent?

With the ongoing vociferous debate on the causal mechanismof the K–T boundary extinction (asteroid impact v. Deccaneruption), it is instructive to ask if the observed phenomenaand mass extinction event at the Cenomanian–Turonian mightbe explained by an asteroid impact. Firstly, we have seen thatnot only was the K–T a more intense extinction event but italso affected more of the surface dwelling marine life than theCenomanian–Turonian event, implying that the root cause ofthe c. 90 Ma event was in the deep oceans. Secondly, Hart &Leary (1991) have proposed that the pattern of extinction atthe Cenomanian–Turonian boundary was not sudden andoccurred in predominantly stepwise fashion over a period of0.3 Ma. Such an observation is incompatible with the short-term effects of an asteroid impact. Thirdly, incompatible traceelement ratios in Cenomanian–Turonian boundary sedimentsdo not suggest an extraterrestrial source (Orth et al. 1993)and despite extensive searches, there have been no reportsof shocked-quartz from Cenomanian–Turonian boundarysections. Finally, the reduction in sea water 87Sr/86Sr from0.70752 to 0.70737 around the Cenomanian–Turonianalso supports a substantial volcanogenic hydrothermal input(87Sr/86Sr<0.7045) into the oceans.

Fig. 5. Flow diagram summarizing thepossible physical and chemicalenvironmental effects of the formation oflarge igneous provinces around theCenomanian–Turonian boundary (seetext for detailed a detailed description).

624 A. C. KERR

In summary, anomalous intraplate volcanism (oceanicplateau formation) would appear to be the most reasonableexplanation for the observed phenomena around theCenomanian–Turonian boundary, and many of the primaryand secondary effects of this volcanism on the biosphere whichhave been discussed in this section have been summarized inFig. 5.

Conclusions(a) The Cenomanian–Turonian boundary is characterized bya second-order mass extinction event, widespread anoxia inthe oceans, sea level transgression, elevated atmosphericand oceanic temperatures, enriched levels of trace elements,reduced 87Sr/86Sr values in the oceans and increased ä13Cvalues. Coincident with this environmental perturbation wasa significant increase in oceanic crustal production, chieflydue to the eruption of large mantle plume-related volcanicprovinces (over a period of 2–3 Ma) in the mostly in the PacificOcean.

(b) The eruption of these large igneous provinces wasresponsible for the disruption of the biosphere at theCenomanian–Turonian boundary both directly, due to volatileand trace metal-enriched hydrothermal fluid release, sealevel transgression and disrupted circulation patterns, andindirectly, due to elevated temperatures from increased CO2.Nutrient upwelling and increased CO2 in the atmospherewould have led to increased productivity which, along with thewarmer oceans, contributed to anoxia and mass extinction.

(c) The atmospheric build up of CO2 would have rapidlyled to a ‘runaway greenhouse’ climate. This greenhousescenario (increasing atmospheric and oceanic temperatures)was probably put into reverse by the increased organic pro-ductivity and the removal of carbon from the atmosphericreservoir, and its burial as organic carbon in black shales.

The Leverhulme Trust and Leicester University are thanked for thegenerous provision of a Research Fellowship. Discussions with A.Hallam, A. D. Saunders, J. Tarney, and R. Kent during theformulation of this paper are much appreciated. The twoanonymous journal referees are thanked for their constructivecomments which helped to improve the manuscript.

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Received 16 June 1997; revised typescript accepted 2 December 1997.Scientific editing by Nick Rogers.

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