A. Hallam

Mass extinctions and sea-level changes

Abstract Review of sea-level changes during the big five mass extinctions and several lesser extinction events reveals that the majority coincide with large eustatic inflexions. The degree of certainty with which these eustatic oscillations are known varies considerably. Thus, the late Ordovician and end Cretaceous extinctions are associated with unequivocal, major regressions demonstrated from numerous, widespread regions. In contrast, the multiple, high frequency sea-level changes reported for the Frasnian–Famennian crisis (based on the supposed depth-preferences of conodont taxa) have little support from sequence stratigraphic analyses, which reveals the interval to be one of highstand. The end Permian mass extinction has long been related to a severe, first order lowstand of sea level [Newell, N.D., 1967. Revolutions in the history of life. Geol. Soc. Am. Spec. Pap. 89, 63–91.] based primarily on the widespread absence of latest Permian ammonoid markers, but field evidence reveals that the interval coincides with a major transgression. Newells hypothesis that marine extinctions are related to shelf habitat loss during severe regression remains tenable for the end Guadalupian and end Triassic extinction events but not for other crises. Rapid high amplitude regressive–transgressive couplets are the most frequently observed eustatic changes at times of mass extinction, with the majority of extinctions occurring during the transgressive pulse when anoxic bottom waters often became extensive. The ultimate cause of the sea-level changes is generally unclear. A glacioeustatic driving mechanism can only be convincingly demonstrated for the end Ordovician and end Devonian events. At other times, it is speculated that they may relate to the widespread regional doming (and subsequent collapse) caused by the impingement of superplumes (and ultimate eruption) on the base of the lithosphere.

Anoxia as a cause of the Permian/Triassic mass extinction: facies evidence from northern Italy and the western United States

Facies, faunal and geochemical evidence from the Permian/Triassic boundary sediments of the Dolomites and Idaho indicates a major anoxic event in the earliest Triassic. In both regions, the basal beds consist of finely laminated micrites with common syngenetic pyrite. The only fauna consists of occasional bedding plane assemblages of Lingula or Clacaria, a typical lower dysaerobic assemblage. This is a level where previous studies have shown a major negative carbon isotope excursion and a cerium anomaly. In the Dolomites, the pyritic micrite directly overlies strata containing a diverse and typically Permian marine fauna of algae, foraminifers (including fusulinids) and articulate brachiopods, implying an abrupt extinction in contradiction to many previous views. Sequence stratigraphic analysis of the Dolomite boundary sediments reveals a minor sequence boundary in the late Permian followed by extremely rapid transgression leading to the development of the relatively deep water pyritic micrite — a maximum flooding surface at the Permo-Triassic boundary. A further pulsed deepening in the lower Griesbachian, recorded in both the Dolomites and Idaho, lead to the widespread establishment of dysaerobic facies. It is clear that most of the extinctions occurred at the erathem boundary although the subsequent failure of the marine benthos to fill the empty ecospace in the ensuing Griesbachian may have been due to the widespread development of dysaerobic conditions.

A review of Mesozoic climates

There is overwhelming evidence, based on the distribution of distinctive sediments and fossils and oxygen isotope data, that the climate of the Mesozoic world was appreciably more equable than that of today, with no polar ice caps, but precise quantitative data are not available. Except for an episode of late Cretaceous cooling there is no good documentation of any significant change in global temperature distributions through the era. The distribution of coals and evaporites, together with other criteria, indicates a pattern of humid arid and zones appreciably different in important respects from that of today. During the Triassic and Jurassic, western Pangaea in low to middle latitudes was largely arid, but in the early Cretaceous the lands on the margin of the newly opening Central Atlantic and western Tethys experienced a humid climate. By late Cretaceous times arid zones had become very restricted in extent. Because of insufficient suitable data, attempts at climatic modelling have had only modest success, and only to a limited extent can the major long-term changes in climate between the Permian and the present be explained in terms of changing geography. The most probable explanation of Mesozoic equability is an increased atmospheric CO2 content. A number of enigmas remain, such as the existence of flourishing forests in polar palaeolatitudes. Whereas for the late Cenozoic short-term climatic changes can be related successfully to variations in the geometry and mechanics of the earth-sun system, there is a long way to go before comparable success can be claimed for the Mesozoic.

A review of the broad pattern of Jurassic sea-level changes and their possible causes in the light of current knowledge

Abstract An assessment is made of the current state of research into Jurassic sea-level changes based on new insights into depositional patterns in relation to these changes and new stratigraphic information from several widely separated regions across the world. The bearing of this information on alternative models, respectively, involving sea-level rise interrupted by stillstands or falls, is then evaluated. The gross pattern appears to be one of more or less gradual rise of sea level through the period, interrupted by episodes of comparative stillstand rather than eustatic fall and several episodes of significant regression are shown to be the result of regional tectonics. Major episodes of eustatic rise took place in the early Hettangian, early Sinemurian, early Pliensbachian, early Toarcian, early and late Bajocian, middle Callovian and late Oxfordian to Kimmeridgian. A significant episode of rapid and very extensive regression, possibly global, took place at the end of the Triassic, but other regressive episodes, in the late Aalenian, Bathonian, late Oxfordian–Kimmeridgian and Tithonian, are clearly only regional in extent. There is no evidence for glacioeustasy and most if not all of the regional or global changes recognised can readily be related to plate tectonics. The events at the Triassic–Jurassic boundary seem to be the result of mantle plume activity centred on the Central Atlantic region.

Continental humid and arid zones during the Jurassic and Cretaceous

Abstract The global distribution of climatically significant criteria is surveyed for six successive time intervals: Hettangian-Toarcian, Aalenian-Callovian, Oxfordian-Tithonian, Berriasian-Barremian, Aptian-Cenomanian and Turonian-Maastrichtian. The criteria are primarily mineralogical—evaporites, coals, bauxites, ironstones and kaolinite—but include sedimentological and palaeontological aspects of facies, e.g. aeolian sands, fossil ferns and xerophytes, fresh-water invertebrates. In Early and Mid Jurassic times an arid zone can be distinguished in low- to mid-latitudes in western Pangaea, with humid zones elsewhere. In the Late Jurassic this arid zone spread northwards to embrace much of southern Eurasia. There followed in the Early Cretaceous a sharp reversal to more humid conditions over a large area extending from the eastern margin of North America to the Middle East, a major consequence of which was the widespread replacement of carbonates by coarse siliciclastic facies. By Late Cretaceous times humid conditions were well established also in North America and the South Atlantic margins, and the tropical arid zone had contracted to a minimum. The 87 Sr/ 86 Sr ratio of marine carbonates provides an independent monitor of continental runoff, which is related to (a) area and (b) extent of aridity. Changes in the ratio confirm that, taking sea-level stand into account, the Late Cretaceous was appreciably more humid than the Late Jurassic. These climatic changes are related primarily to the progressive breakup of Pangaea in the Late Jurassic and Cretaceous which, together with sea-level rise in the Mid to Late Cretaceous, caused a significant increase in maritime influence. This implies that monsoonal winds probably had a major role in promoting higher precipitation rates. A puzzle remains as to why there was a spread of aridity in the Late Jurassic. Finally, some implications for terrestrial plant evolution are noted.

Griesbachian (Earliest Triassic) palaeoenvironmental changes in the Salt Range, Pakistan and southeast China and their bearing on the Permo-Triassic mass extinction

Abstract Facies and faunal analysis from Pakistan and China show that the Permo-Triassic mass extinction of marine invertebrate faunas was associated with a spectacularly rapid Griesbachian transgression which lead to the widespread establishment of deep-water anoxic and dysoxic conditions. The extinction event was thus caused by habitat loss due to the extensive development of inhospitable conditions. The initial Griesbachian transgression in Pakistan produced extensive shallow, normal marine conditions in which Permian holdover taxa were able to survive until the development of dysaerobic facies in the late Griesbachian. The exceptionally complete sections of China show a three-phased deepening and extinction event beginning in the latest Permia. By the late Griesbachian a variety of dysaerobic and anaerobic facies were developed in all the regions studied. Several of these contain evidence for minimal sulphate reducing activity suggesting that marine productivity and thus organic matter flux to the sediments was very low in early Triassic seas.

Eustatic cycles in the Jurassic

Abstract Stage-by-stage analysis of the shallow marine Jurassic sequences in northwest Europe reveals a number of widespread, synchronous cycles of deepening and shallowing water, independent of local tectonics and facies development. The more important cycles are shown to correlate with phases of world-wide transgression and regression and hence are attributed to eustatic changes of sea level. The most important times of sea-level rise were the Early Hettangian, Early to Mid-Toarcian, Early Bajocian, Late Bathonian to Early Callovian and Mid-Oxfordian, and the most important times of sea level fall in the Late Toarcian to Aalenian, Early Bathonian, Late Callovian and Late Tithonian/Volgian. Until late in the Oxfordian or early in the Kimmeridgian, the secular trend was towards rise in sea level (positive eustasy), with the amount of episodic falls failing to match the intervening rises; in the Tithonian this trend was checked or reversed. Of five eustatic models discussed, the one most favoured by the evidence involves a rapid rise of sea level followed by a longer phase of stillstand and then rapid fall. These changes are tentatively related to episodes of uplift and subsidence of oceanic ridges. The influence of eustasy on marine faunas is briefly discussed with reference to two important macroinvertebrate groups. The ammonites were a stenotopic group highly susceptible to extinction, especially at times of regression, when the development of endemic faunas was also favoured. Times of transgression were marked by migratory spread and evolutionary radiation. The bivalves were relatively eurytopic and little affected by short-term changes of sea level, but times of low sea level in the Early and Late Jurassic correlate with increased endemism.

Why was there a delayed radiation after the end‐Palaeozoic extinctions?

Data from widespread dysaerobic facies, carbon/sulphur ratios and cerium anomalies suggest that the early Triassic was a time when anoxic conditions spread widely over epicontinental seas. These conditions, associated with marine transgression following the latest Permian regression, are likely to be a prime cause of the mass extinction of Palaeozoic marine faunas. The occurrence of many Lazarus taxa in the Middle and Upper Triassic indicates, however, that the extinctions at the end of the Permian were less severe than has been widely assumed, and that the turnover from Palaeozoic to Mesozoic faunas was considerably extended in time, being finally accomplished only after the end‐Triassic mass extinction event.

The end-Triassic bivalve extinction event

One of the most important episodes of mass extinction of animal groups in the whole of the Phanerozoic took place in the latter part of the Triassic, but the time of major faunal turnover of the vertebrates preceded that of marine invertebrates by several million years. As shown by ammonites, brachiopods and conodonts, decline from a Carnian—early Norian diversity peak was followed by a significant extinction episode at the end of the period. This pattern is confirmed at generic and subgeneric level by the bivalve molluscs, probably the most abundant and diverse macroinvertebrate group. This paper analyses the bivalves in detail and attempts to seek a relationship of the extinction event to environmental changes as recorded by late Triassic facies. Stratigraphy of the youngest Triassic is briefly discussed. Following a recent recommendation, the Rhaetian stage is dropped and the Upper Norian (Sevatian) subdivided into three new ammonite zones. It is suggested that the classic northwest European “Rhaetic” correlates with the Kossen and Zlambach Beds of the Alps and corresponds at least approximately with the upper two of these zones. It is estimated that nearly half the existing bivalve genera and nearly all the species failed to survive the end-Triassic extinction event. The trigonioid, unionoid and hippuritoid orders were especially strongly affected but most of the pholadomyoid genera survived. Three major ecological categories are distinguished: a shallow neritic group, to which most taxa belong, a euryhaline group that could tolerate marginal marine and lagoonal conditions, and a deep neritic group, notably Halobia and Monotis. All groups were strongly affected by the extinction but species of the deep neritic group were apparently the most susceptible. Seven biogeographic regions are distinguished for the purpose of analysis and it is shown that provinciality is slight, but some important taxa are endemic to the Tethyan and Pacific regions. Endemic genera prove to have been much more subject to extinction than cosmopolitans. There are very few places in the world with a more or less continuous marine sequence of deposits across the Triassic-Jurassic boundary. Details are given of sections in the Alps, northwest Europe and Nevada and briefer mention made of others. With regard to late Triassic palaeogeographic changes, those involving climate were neglibible, but there was a more or less progressive regression of epicontinental seas following a Ladinian—early Carnian transgressive maximum, with a corresponding shallowing in the more continuously marine Tethyan region. That this first-order effect was due to a eustatic lowering of sea level is fairly certain, but the situation is less clear for second-order transgressive-regressive cycles. The late late Norian (“Rhaetian”) cycle is probably, however, the results of a minor eustatic event. By the end of the Triassic the sea had regressed substantially and the mass extinction event is though to be bound up partly with this phenomenon and partly with the phase of widespread anoxicity that occurred during the succeeding Hettangian transgression.

End-cretaceous mass extinction event: argument for terrestrial causation.

The end-Cretaceous mass extinctions were not a geologically instantaneous event and were selective in character. These features are incompatible with the original Alvarez hypothesis of their being caused by a single asteroid impact that produced a world-embracing dust cloud with devastating environmental consequences. By analysis of physical and chemical evidence from the stratigraphic record it is shown that a modified extraterrestrial model in which stepwise extinctions resulted from encounter with a comet shower is less plausible than one intrinsic to the earth, involving significant disturbance in the mantle.