Hautmann 2012 End Triassic Libre

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Extinction: End-Triassic

Mass ExtinctionMichael Hautmann, Palaontologisches Institut und Museum, Zu rich, Switzerland

One of the five greatest mass extinction events in Earthshistory occurred at the end of the Triassic, c. 200 million years ago. This event ultimately eliminated conodonts

and nearly annihilated corals, sphinctozoan sponges andammonoids. Other strongly affected marine taxa includebrachiopods, bivalves, gastropods and foraminifers. Onthe land, there is evidence for a temporal disturbance of plant communities but only few plant taxa finally dis-appeared.Terrestrialvertebrates alsosuffered but timingandextent of this extinction remains equivocal. Thecauseof theend-Triassic mass extinction wasprobablylinked tothe contemporary activity of the Central Atlantic Mag-matic Province, which heralded the breakup of thesupercontinent Pangaea. Possible kill mechanisms asso-ciated with magmatic activity include sea-level changes,

marina anoxia, climatic changes, release of toxic com-pounds and acidification of seawater. Remarkably,long-term effects on marine biota were rather differentbetween ecological groups: a nearly instantaneousrecoveryof level-bottomcommunities is contrastedby thevirtual absence of reef systems for nearly 10 million yearsafter the extinction event.

IntroductionAlthough the study of mass extinction events has been akey topic of palaeontological research since the seminal

work of Alvarez et al . (1980) on the end-Cretaceouscatastrophe, the mass extinction event at the end of theTriassic held itsattribute as the least wellunderstood of themajor diversity depletions (Bambach et al ., 2004) untilrecently. In the past years, however, research progress wasimmense, and today theend-Triassic biotic crisis is actuallyamong the best-understood events of sudden diversity

decline in the geological past. One reason for this progressin knowledge is the documentation of an unexpectedlylarge volume of magmatic rocks genetically related to the

Central Atlantic Magmatic Province (CAMP)andan exactdetermination of their radiometric ages, which indicate asudden onset of massive volcanic extrusions synchronouswith the disappearance of many taxa and ecosystems at theend of the Triassic. Additionally, detailed work on sedi-mentology, geochemistry and extinction patterns inTriassicJurassic (TJ) boundary sections worldwide hasprovided palaeontological data that are in accordance withpredictions of volcanogenic extinction scenarios. Con-sequently, discussion todaynearlyexclusively concentrateson volcanogenically induced kill mechanisms. Becauseassociated changes of many environmental parameters canbe estimated relatively precisely and are in a similar orderof magnitude than projected environmental changescaused by human activity, the study of the end-Triassicmass extinction event provides a unique test-case for pre-dicting future changes in biosphere. See also : Diversity of Life

The Last Days of PangaeaThe transition from the Triassic to the Jurassic period ismarked by a pivotal plate tectonic change: The super-continent Pangaea, which had persisted for nearly 150 Ma,was affected by strong rift tectonics, nally leading to theopening of the central Atlantic in the middle Jurassic.Concomitant with rift tectonics was violent magmaticactivityin an area in excessof 10000000 km 2 on either sideof the rift zone (Marzoli et al ., 2011). Estimates of theerupted volcanic rocks of this CAMP indicate a volume of 2300000km 3 (McHone, 2003), making the CAMP one of the largest igneous provinces in Earths history. Magmaticactivity probably took place already in the Rhaetian (LateTriassic) with sill and dyke emplacement (Ruhl andKu rschner, 2011), followed by massive extrusive volcanismthat started near-synchronously in the circum-Atlanticbasins. Critically, the peak activity of the initial lava owswas at 201.8 + 0.7 Ma, which within error limits is indis-tinguishable from radiometric ages of the TJ boundary

(201.21 Ma; Schoene et al ., 2010; Marzoli et al ., 2011).

Advanced article

Article Contents. Introduction

. The Last Days of Pangaea

. Stratigraphy and Sedimentological Record across theT J Boundary

. Extinction Patterns

. Postulated Causes of Extinction

. Recovery from the End-Triassic Mass Extinction

Online posting date: 15 th August 2012

eLS subject area: Evolution & Diversity of Life

How to cite:Hautmann, Michael (August 2012) Extinction: End-Triassic MassExtinction. In: eLS. John Wiley & Sons, Ltd: Chichester.DOI: 10.1002/9780470015902.a0001655.pub3

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Subsequent magmatic pulses have been recorded up to10 Ma after the TJ boundary, but these were minor incomparison with the initial eruptions (Nomade etal ., 2007;

Marzoli et al ., 2011). Although volcanism has alreadyearlier been identied as a possible cause for the end-Triassic mass extinction (e.g. Courtillot, 1994; Wignall,2001), it was only after documentation of the large extentand exact radiometric ages of the volcanic rocks thatvolcanogenic extinction scenarios have become dominant.See also : Continental Drift

Stratigraphy and SedimentologicalRecord across the T J BoundaryContinuous marine sections across the TJ boundary arenotoriously rare (e.g. Morante and Hallam, 1996), andtheircorrelation has long been complicated by the lack of aglobal boundary stratotype section and point (GSSP).Recently, however, the Kuhjoch section in the KarwendelMountains (Northern Calcareous Alps, Austria; Figure 1)has been chosen as the GSSP for the base of the Jurassicand hence for the denition of the TJ boundary (Morton,2008). The index fossil dening the base of the Jurassicin this section is Psiloceras spelae tirolicum (Hillebrandtand Krystyn, 2009; Figure2 ), which rst occurs c. 6 m abovethe lithological boundary between the Rhaetian Ko ssen

Formation and the Tiefengraben Member of the Kendl-bach Formation (Hillebrandt and Krystyn, 2009). Thisdenition places the TJ boundary stratigraphically well

above main extinction horizon that is located just above theKo ssen Formation, but this is merely a technical aspect.

Figure 1 Global boundary stratotype section and point (GSSP) for the base of the Jurassic, Kuhjoch section (Karwendel Mountains, Austria), illustrating theabrupt interruption of carbonate sedimentation on top of the late Triassic Kossen Formation. Note that strata are overturned, that is, Late Triassic limestonesare above the claystones of the Triassic Jurassic transition. The first occurrence of Psiloceras spelae and thus the base of the Jurassic is stratigraphically c. 6 mabove the top of the Kossen Formation close to the lower limit of the photograph, notably postdating the extinction event. K. Kment (Bad Tolz) for scale.

Photo by the author.

Figure 2 Holotype of Psiloceras spelae tirolicum (Hillebrandt and Krystyn,2009), the index fossil for the base of the Jurassic. Scale bar represents1 cm. Photo courtesy of A. von Hillebrandt, Berlin.

Extinction: End-Triassic Mass Extinction

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Correlation of the GSSP section with other TJ boundarysections is facilitated by a negative carbon isotope excur-sion of global extent, which in the GSSP section occurs at

the very base of the Tiefengraben Member (Ruhl et al .,2010). In combination with the last appearance datum of the late Rhaetian index fossil Choristoceras marshi atthe top of the Ko ssen beds (Hillebrandt and Krystyn,2009), an easily applicable stratigraphical framework isprovided, allowing correlation of the GSSP with palaeo-geographically distant sections. An intriguing aspectemerging from interregional correlation of marine sectionsis the coincidence of the initial negative carbon isotopeexcursion with a gapin carbonate sedimentation just abovethe extinction horizon ( Figure 3). It has been proposed thatextinction, negative carbon isotope excursion and inter-ruption of carbonate sedimentation have a common cause:the injection of huge amounts of isotopically light carbondioxide (CO 2 ) from the magmatic activity of the CAMP,possibly added by release from thermally dissociatedmarine gas hydrates (Hautmann, 2004). Predicted envir-onmental consequences of increased atmospheric CO 2

include global warming (possibly following an initialcooling event due to volcanogenic sulfur dioxide (SO 2 )emission) and ocean acidication due to the hydrolysis of

CO 2 and SO 2 , temporarily depressing the saturation stateof seawater with respect to calcium carbonate minerals (seelater discussion). See also : Geological Time: Principles

In continental settings, stratigraphically well-constrainedsections spanning the TJ transition are even rarer than intheir marine counterparts. Possibly thebest-studiedsectionsare from the north-eastern United States (Newark andHartfort Rift Basins), which combine the record of contin-ental sediments andintercalated volcanicrocks.On thebasisof sections from this area and their chemostratigraphiccorrelation with marine sediments, Whiteside et al . (2010)suggested that the end-Triassic mass extinction event begansynchronous on land and in the sea and simultaneous withtheoldest CAMPeruptions. A sudden perturbation of plantcommunities (although not leading to signicant extinctionof taxa) has also been described from east Greenland, pos-sibly taking place synchronously with the marine extinction(McElwain etal ., 2009). A contrary view was put forward by

Northern Alps

(a) (b) (c) (e)(d)

1 m

1 m

1 m

1 m

1 m

Southern Alps SW England Nevada Peru

Panthalassaocean

CAMP

e

d

c ab

T e t h y s o c e a n

SandstoneSiltstone

Limestone

Unconformity(hiatus)LatestChoristoceras

MarlyLimestone

Marl

Shales

Articulated bivalvesShell debrisFirstPsiloceras

Black shales

Slumping

Initial 13Cexcursion

Figure 3 Triassic Jurassic boundary sections from Europe, North America and South America, showing sedimentology, onset of negative d13 C excursion,and biostratigraphically important ammonoid occurrences ( Choristoceras and Psiloceras ). Note synchronous interruption of carbonate sedimentation,

indicated by stippled line. Reproduced from Hautmann et al . (2008a) by permission of Schweizerbartsche Varlagsbuchhandlung.




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Lucas et al . (2011), who found that extinction of terrestrialvertebrates in continental sediments of the southern Col-orado Plateau preceded marine extinction.

Extinction PatternsThe end-Triassic mass extinction event is best documentedin the marine record. On the basis of Sepkoskis 2002compilation of marine animal genera, Bambach et al .(2004) calculated that 46.8% of Rhaetian genera did notsurvive into the Hettangian. Yet, extinction was not evenlydistributed among clades ( Figure 4). Most severely affectedwere the conodonts, which failed to survive the end of theTriassic. In ammonoids, all Triassic groups became extinctwith two exceptions: (1) Choristoceratidae, which mayhave locally survived in the Hettangian before they nallydisappeared and (2) Phylloceratina, which nally gave riseto the Jurassic diversication of ammonids (Guex et al .,2004; Hillebrandt and Krystyn, 2009).

Other prominent victims of the end-Triassic extinctionevent were reef-building taxa ( Figure 5). Although coralreefs locally reappeared already during the Hettangian(Kiessling et al ., 2009), it was not after a lag phase of 8 10 Ma that reef systemsfully re-established (Stanley,2006).Flu gel (2002) indicated an extinction of 96% of all generaof scleractinian corals (74/77 genera) and 91% of sphinc-tozoan sponges (53/58 genera), the two most prominentgroups of Rhaetian reef builders. However, Kiessling et al .(2009) criticised that these high values may partly reect

sampling failure and the lack of taxonomic standard-isation. On the basis of a sample-standardised approachusing the Paleobiology Database (http://paleodb.org/cgi-bin/bridge.pl), Kiessling et al . (2009) estimated an extinc-tion of 45% of scleractinian genera across the TJ boun-day, but these authors also noted that this value stillrepresents thehighest extinction in thegeologicalhistory of the clade.

With a 71%loss of genera, articulated brachiopods werealso among the most severely affected groups, yet the soletwo genera of inarticulated bachiopods survived the crisis(Hautmann et al ., 2008a). See also : Brachiopoda

Bivalves suffered a 40% loss of marine genera (Haut-mann etal ., 2008a)and thus slightly less than theaverage of marine genera. Infaunal taxa suffered more than epifaunaltaxa (McRoberts and Newton, 1995), but this might be anepiphenomenon of an increased extinction of taxa withcompletely aragonitic shells in comparison with taxa thathad calcitic outer shell layers (50% versus 30% extinction;Hautmann et al ., 2008a), because infaunal bivalves areinvariably aragonitic, whereas epifaunal bivalves havepredominantly calcitic outer shell layers.

Gastropod extinction is difficult to estimate becausetaxonomically important characters such as the proto-conch are seldom preserved, but it has been suggested thatthe end-Triassic crisis might have been an even moreimportant caesura in the history of this class than the end-

Permian mass extinction (Batten, 1973).

The comprehensive compendium of Loeblich and Tap-pan (1988) suggest extinction of c. 35% of Foraminiferagenera, which is below the average of marine extinction but

still notably high (cf. Bambach et al ., 2004). In a thoroughregional study, Cle mence et al . (2010) documented eco-logical changes in foraminifer communities across the TJboundary, showing a decreasing abundance and diversityof calcareous taxa and a corresponding increase of agglu-tinated forms, and a change in feeding strategies fromdeposit to detritus feeders and bacterial scavengers.See also : Foraminifera

Data on the extinction of Radiolaria across the TJboundary remains controversial. On the basis of localstudies, a severe mass extinction of this group has beenproposed (e.g. ODogherty et al ., 2010), which, however,does not appear on the global scale. On the basis of reviseddata, Kiessling and Danelian (2011) calculated merely a17% loss of radiolarian genera at the end of the Triassic,which increases to still moderate 29% if short-term sur-vivors were counted as victims. Moreover, Kiessling andDanelian (2011) demonstrated that this moderate extinc-tion is even lower than Triassic background extinction inthe Radiolaria, and that extinction rates of Radiolariaactually declined from the Triassic to the Jurassic. See also :Radiolaria

Marine reptiles suffered extinction of thalattosaurs,nonplesiosaurian sauropterygians (e.g. Placodontia andNothosauria) and nonparvipelvian ichthyosaurs near theend of the Triassic, but some of these taxa have their lastappearance datum already in the Norian, suggesting some

temporal offset in comparison to the end-Rhaetian mainextinction event (Benson and Butler, 2011). In contrast,shes probably passed the TJ transition without majorfaunal changes (McCune and Schaeffer, 1986).

On the land, plant communities suffered temporal eco-system perturbations across the TJ boundary (McElwainet al ., 2009), but only few taxa became nally lost (Ash,1986; Kelber, 1998). In fact, Rhaetian and Early Jurassic(Liassic) oras are extremely similar in their taxonomiccomposition and have traditionally been subsumed asRhaetoliassic oras.

Although it is generally agreed that Jurassic tetrapodcommunities differed signicantly from their Triassic pre-cursors, it is debated whether theprincipal changeoccurredalready at the end of the Carnian (Benton, 1993), near theend of the Triassic but preceding marine extinction (Lucaset al ., 2011), or synchronous with the extinction of othergroups around the TJ boundary (Olsen et al ., 2002).

Postulated Causes of Extinction

OverviewBefore consensus was reached that the principal cause of the end-Triassic mass extinction was linked with CAMP-volcanism (e.g. Whiteside et al ., 2010), a variety of other

scenarios has been proposed, including the impact of an




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extraterrestrial body (Olsen et al ., 2002), climatic changes(Fabricius et al ., 1970), sea-level changes (Hallam, 1981)and marine anoxia (Hallam, 1981; Hallam and Wignall,2000). Whereas evidence for an asteroid impact syn-chronous with theend-Triassic mass extinction rests chieyon a modest Iridium anomaly from a single locality with

poor age control, climatic changes, uctuation of sea-level

and marine anoxia can all been incorporated in volcano-genic extinction scenarios (e.g. Pa lfy,2003).Additional killmechanisms resulting from extraordinary magmaticactivity include acid rain (Pa lfy, 2003), emission or gener-ation of toxic gases (McHone, 2003; van de Schootbruggeet al ., 2009) and ocean acidication (Hautmann, 2004;

Hautmann et al ., 2008a, b). In spite of the wide acceptance

0%(c)

(b)

(a)

Polychaeta (excl. serpulids)

Radiolaria

(Proteinaceous 0 % [0/1])

15.4% [6/39] Agglutinated

47.6% [10/21]Low-Mg calcitic:

High-Mg calcitic:53.8% [7/13]

Aragonitic: 66.7% [6/9]

CaCO 3: 53.5% [23/43]

All: 35% [29/83]

Foraminifera

30.2% [16/53]Calcitic:

Aragonitic: 50% [29/58]

All: 40.5% [45/111]

Bivalvia (autolamellibranchiate genera)

(Organophosphatic: 0% [0/2])

Calcitic: 71.4% [40/56]

Brachiopoda

91.4% [53/58]

Sphinctozoa

96.1% [74/77]

Scleractina Average marine extinction [46.8%]

[3/22]

[12/69]17.4%

13.6%

50% 100%

All: 69% [40/58]

Figure 4 Extinction of genera in taxa with different skeletal physiology. (a) Hypercalcifying taxa with aragonitic and/or high-Mg calcitic skeletal mineralogyand little physiological control of biomineralisation. (b) Extinction in groups with variance in skeletal material, demonstrating increasing extinction risk fromnoncalcareous skeletons to low-Mg calcitic, high-Mg calcitic and aragonitic skeletal material. (c) Extinction of taxa with noncalcareous skeletons. Error barsindicate 95% binomial confidence intervals. Reproduced with slight modifications from Hautmann et al . (2008a) by permission of Schweizerbartsche Varlagsbuchhandlung.




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of volcanism as the ultimate cause of the end-Triassic massextinction, there is a lively discussion on which of thesepossible kill mechanisms had the largest impact on thelatest Triassic biota.

Apart from sea-level changes, which possibly reect alithosphere bulging-collapse couplet in response to mag-matic activity (Pa lfy, 2003), all currently discussed killmechanisms are directly or indirectly related to volcanicgas emissions. The well-known chemical composition of basaltic lavas including the content of volatiles allowsdirect calculation of absolute volatile emissions from thevolume of magmatic rocks ( Table 1 ; McHone, 2003).However, these values do not directly translate intoatmosphericconcentrations,because atmospheric lifetimesfor most of these gases are much shorter than the totalinterval of volcanic degassing.Nevertheless, the short-livednature and large extent of the main extrusion events con-centrated at the TJ boundary (Marzoli et al ., 2011) sug-gest a signicant atmospheric build-up of each of thesegases. For CO 2 , additional release from thermally dis-sociated marine gas hydrates has been postulated in orderto explain changes in the carbon isotope record (Beerlingand Berner, 2002).Actual CO 2 palaeoconcentrations in theatmosphere have been inferred from changes in the sto-matal density of land plants and on the basis of geo-chemical methods. A recent palaeobotanic estimate

suggests a maximum CO 2 concentration of 2750 ppmv(Bonis et al ., 2010), whereas maximum concentrationsbetween 4400 (Schaller et al ., 2011) and nearly 6000 ppmv(Yapp and Poth, 1996) have been suggested based on car-bon isotope compositions of the Fe(CO 3 )OH componentin pedogenic oolithic goethites. See also : Global CarbonCycle

Sea-level changesThe idea that sea-level falls might have caused marineextinction events goes back to Newell (1967), who noted arepeated coincidence between sea-level lowstands and

marinemass extinctions. The rationalebehind this model is

that regressions reduce the area of shallow marine habitatswhere marine biodiversity is concentred, and thus thenumber of marine taxa according to the well-known spe-ciesarea relationship. However, it has been doubtedwhether the effect of reduced shelf areas was sufficient toexplain the magnitude of mass extinction events, not thelast because many regressions in Earths history were notassociated with signicant marineextinctions (see review inHallam and Wignall, 1997). A regressiontransgressioncouplet during the TJ transition has been described formany palaeogeographically distant areas and rst beenproposed as a possible cause of the TJ extinction event byHallam (1981) and Hallam and Goodfellow (1990).However, Hallam (1981) added that the spread of anoxicbottom waters during the Hettangian transgressionmight have had a bigger impact on marine extinction thanthe preceding, relatively modest regression (see later dis-

cussion). See also : Sea Level Change

Marine anoxiaIn the 1990s, scenarios of widespread marine anoxia as theultimate kill mechanism in many of the big Phanerozoicmass extinction event replaced the prevalence of sea-level-related extinction scenarios. For the end-Triassic massextinction, Hallam and Wignall (1997) summarised evi-dence for the spread of anoxic bottom waters during thebasal most Jurassic as a possible cause of this event.Wignall (2001) later incorporated the anoxia model involcanogenicextinction scenarios by suggesting that globalwarming in response to volcanogenic CO 2 exhalationscaused a decrease in the equator-to-pole temperature gra-dient and thus in oceanic circulation, ultimately leading tooxygen decient bottom waters. However, detailed strati-graphic analyses have shown that early Jurassic blackshales in the western United States notably postdate theextinction event (Guex et al ., 2004). In the Northern Cal-careous Alps, laminated shales near the extinction horizonare supercially suggestive for anoxicdysoxic conditionsbut actually contain assemblages of epifaunal and shallowburrowing bivalves that indicate well-oxygenated con-ditions (McRoberts et al ., in press). Moreover, Ruhl et al .(2010) have shown that increased organic matter in theseshales was from terrestrial rather than marine sources.

Thus, whereas marine anoxia may have locallycontributed

Table 1 Estimation of volatile emission from CAMP-volcanism

GasTotalemissionfrom CAMP(tons)

CO 2 5.19E+12SO 2 2.31E+12F 1.11E+12Cl 1.58E+12H 2 O 3.65E+13

Source : McHone (2003).

Figure 5 Dendroid scleractinian corals ( Retiophyllia sp.) in Rhaetian reef limestone (Parvadeh, Lut Desert, east-central Iran). Photo by the author.




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to marine mass extinction, it is unlikely that they were themain or sole cause of the end-Triassic crisis.

Toxic elements and compoundsFluorine (F) is highly poisonous and may directly killorganisms around the volcanically active areas (McHone,2003).Moreover, large ejections of halogens (Cl andF) canleadto ozone depletion (Sigurdsson,1990) and, by reactionwith H 2 O, make rainfall acidic,thus attaining interregionalto global environmental impact. Table1 shows the amountsof F and Cl presumable released to the atmosphere byCAMP, which are clearly high enough for harming life if released within a short time interval. Moreover, van deSchootbrugge et al . (2009) found an enrichment of poly-cyclic aromatic hydrocarbons near the TJ boundary interrestrial settings of north-western Europe, which they

interpreted as the result of incomplete combustion of organic matter by ascending magma. They suggested thatthese toxic compoundsin addition to SO 2 have contributedto the extinction event. However, the actual role of toxicelements and compounds in the end-Triassic extinctionevent is hard to test because no particular selectivity inextinction pattern can be predicted.

Short-term coolingAlthough SO 2 is a greenhouse gas, it rapidly forms sulfateaerosols in the atmosphere that absorb or backscattersunlight (e.g. Wignall, 2001). The net effect is that SO 2causespronounced climatic cooling, provided thatSO 2 was

ejected high enough into the atmosphere for allowing glo-bal dispersion. Although the residence time of sulfur-basedaerosols in the atmosphere is short (Pyle et al ., 1996), theenormous amount of sulfur ejections form CAMP ( Table 1 )suggest that it might have played an important role in theend-Triassic mass extinction (McHone, 2003; Guex et al .,2004). In support of the cooling hypothesis, Kiessling etal .(2007) found a somewhat higher extinction quota for tro-pical genera, although they concluded that latitudinalpreferences were not a dominant factor in the end-Triassicmass extinction.

Global warming

The well-known greenhouse effect of CO 2 is diametricallyopposed to the cooling effect of sulfur-based aerosols.Because both effects operate over very different timescalesdue to the much longer atmospheric residence time of CO 2in comparison to sulfate aerosols, it is predicted that long-term global warming followed initial cooling after majorvolcanic events (Wignall, 2001). Surprisingly, little efforthas been undertaken to quantify temperature changesacross the TJ boundary by means of oxygen isotopesexcept for the studies of Hallam and Goodfellow (1990)and Morante and Hallam (1996), which however usedoxygen data from carbonates that were probably diag-enetically altered, as stated by these authors. Evidence for

increased temperatures, therefore, rests chiey on the

quantication of palaeo-CO 2 concentrations in theatmosphere, which suggest a sharp increase up to severalthousand ppmv (see earlier discussion). However, no

spreadof tropical taxa into high latitudes in the wake oftheextinction event has been documented so far. Thus,although global warming currently is a likely hypothesis, itstill awaits conrmation by geochemical and palaeonto-logical data.

Ocean acidificationIncreasing atmospheric CO 2 concentrations are partlycounterbalanced by increased hydrolysis of this gas in theseawater. In the present-day global carbon cycle, c. 40% of the industrial CO 2 release is taken up by the oceans (Zeebeet al ., 2008). This uptake changes seawater chemistry byenhancing the hydrogen ion concentration of seawater, aprocess known as ocean acidication (Caldeira andWickett, 2003). A direct effect of ocean acidication is adecrease in the concentration of the carbonate ion andconsequently a decrease in the saturation state of seawaterwith respect to calcium carbonate minerals. Present-dayseawater would become undersaturated with respect toaragonite at CO 2 concentrations between 1200 and1700 ppmv and additionally with respect to calcite between1900 and 2800 ppmv (Feely et al ., 2004). Clarifying thepotential harm of decreased carbonate saturation on bio-calcifying organisms is an ongoing research agenda inexperimental biology and oceanography (e.g. Orr et al .,2009), but relatively few studies have dealt with examples

from the palaeorecord. Of these the end-Triassic event isprobably the best understood. Palaeobotanical proxiessuggest atmospheric CO 2 maxima of up to 2750 ppmv nearthe TJ boundary (see earlier discussion), which are in anorder of magnitude that suffice complete undersaturationof seawater with respect to both aragonite and calcite(Feely et al ., 2004). Berner and Beerling (2007) calculatedthe effect of CAMP-related CO 2 and SO 2 emissions onCaCO 3 saturation of seawater, concluding that completeCaCO 3 undersaturationof theworlds oceansover a periodof 2040 ka was possible, provided that the emissions wereshort-lived. In support of strongly decreased CaCO 3 sat-uration of palaeo-seawater are observations of a globalcarbonate gap above the extinction horizon in marinestrata worldwide ( Figure 3). Critically, this gap coincideswith a negative d 13 C excursion ( Figure 3) that indicates theinjection of isotopically light carbon from volcanism, witha possible addition from dissociated gas hydrates (Beerlingand Berner, 2002; Hautmann et al ., 2008a; / rne et al .,2011).

A biocalcication crisis in response to ocean acidi-cation has been demonstrated for marine phytoplankton(van de Schootbrugge et al ., 2007) and foraminifers(Cle mence et al ., 2010). In both groups, calcareous-walledtaxa declined during the crisis in favour of organic-walledand agglutinated species, respectively. The extinction pat-tern of marine invertebrates with respect to their skeletal

physiology was analysed by Hautmann et al . (2008a),




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conrming the prediction that taxa with calcareous skel-etons were at a higher extinction risk than taxa with non-calcareous skeletons, and that within the calcareous group

aragonite as a skeletal mineral was disadvantageous incomparison to the less soluble calcite ( Figure 4). Other fac-tors identied as determinants for increased extinction riskinclude hypercalcication and little biological control onbiocalcication, providing an explanation for the highextinction quota of reef-forming corals and sphinctozoansponges (Hautmann et al ., 2008a). The selectivity againsthypercalcifying taxa suggests that increased energeticcosts forbiomineralisation during times of reduced CaCO 3saturation were the principal agent of extinction in mac-roinvertebrates with calcareous skeletons, rather thanincreased susceptibility for dissolution. Reduction of skeleton/shell sizeand replacement of skeletal aragonite bycalcite therefore appear plausible as evolutionaryresponses of marine biocalciers to ocean acidication.This prediction has been conrmed in several groups of preadapted epifaunal bivalves (Hautmann, 2006), but ithas not yet been described from other clades straddling theTJ boundary.

Recovery from the End-Triassic MassExtinctionHallam (1996) reported a relatively slow recovery of themarine fauna in Europe, which he attributed to the

prevalence of oxygen-decient facies. This European pat-tern is contrasted by a nearly instantaneous recovery of level-bottom communities in southern Tibet (Hautmannet al ., 2008b), where no signs of environmental stress havebeen found. However, the ad hoc explanation that envir-onmental stress was the sole determinant for the onset of recovery is probably notthe complete account, because reef communities recovered only after a lag phase of 810 Ma(Stanley, 2006). It rather appears that apart from thepresence/absence of environmental stress there is also acritical threshold of the extinction quota that poses limitson ecosystem recovery, and that this limit was exceeded inthe case of reef-forming organisms during the end-Triassiccrisis (see Figure 4a versus Figure 4b for a comparisonbetween reef-organisms and typical level-bottom fauna).Additionally, Hautmann et al . (2008b) suggested that themore co-evolved ecological structure of reef communitiesin comparison to level-bottom communities was anotherdeterminant of the pace of recovery. See also : BioticRecoveries after Extinction

The importance of palaeorecord data like that from theend-Triassic crisis for global change scenarios becomesobvious by their comparison with results from experi-mentalbiology.On thebasisof a tank experiment, Fine andTchernov (2007, p. 1811) demonstrated that scleractiniancorals can survive and recover from decalcication afteracidication of the surrounding water, implying that

corals might survive large-scale environmental change,

such as that expected for the following century. However,this provoking conclusion does not account for the naturalecological context of corals, where a lack of calcication

would corals not only make more susceptible forpredationbut also disable them to maintain a growing reef structure,undoubtedly the principal basis of their present bio-diversity. The lesson from the end-Triassic mass extinctionthat recovery of reefs from oceanacidication may take upto 10 Ma is thus probably the more realistic prediction.See also : Global Change Contemporary Concerns

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Hautmann 2012 End Triassic Libre