Jean Guex

Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level

New high-precision U/Pb geochronology from volcanic ashes shows that the Triassic-Juras- sic boundary and end-Triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial fl ood volcanism in the Central Atlantic Mag- matic Province to <150 ka. This narrows the correlation between volcanism and mass extinc- tion by an order of magnitude for any such catastrophe in Earth history. We also show that a concomitant drop and rise in sea level and negative δ 13 C spike in the very latest Triassic occurred locally in <290 ka. Such rapid sea-level fl uctuations on a global scale require that global cooling and glaciation were closely associated with the end-Triassic extinction and potentially driven by Central Atlantic Magmatic Province volcanism.

Good Genes and Good Luck: Ammonoid Diversity and the End-Permian Mass Extinction

“Rapid” Recovery The Permian-Triassic extinction 252 million years ago was Earths most severe biotic crisis since the Precambrian and is thought to have depressed diversity in its wake for millions of years. Brayard et al. (p. 1118; see the Perspective by Marshall and Jacobs) show, however, that ammonoids, a large group of marine organisms that were severely affected, recovered remarkably quickly. Only 1 million years after the extinction, ammonoids had recovered to levels higher than in the Permian, compared with the 10-million-year biotic recovery period for other benthic organisms. The Triassic recovery seems to include several cycles, but the immediate recovery of ammonoids may have left them as one of the most diverse groups in the earliest Triassic. In contrast to other groups, ammonoid diversity recovered within 1 million years of the end-Permian extinction to levels higher than before. The end-Permian mass extinction removed more than 80% of marine genera. Ammonoid cephalopods were among the organisms most affected by this crisis. The analysis of a global diversity data set of ammonoid genera covering about 106 million years centered on the Permian-Triassic boundary (PTB) shows that Triassic ammonoids actually reached levels of diversity higher than in the Permian less than 2 million years after the PTB. The data favor a hierarchical rather than logistic model of diversification coupled with a niche incumbency hypothesis. This explosive and nondelayed diversification contrasts with the slow and delayed character of the Triassic biotic recovery as currently illustrated for other, mainly benthic groups such as bivalves and gastropods.

Radiolarite ages in Alpine-Mediterranean ophiolites: Constraints on the oceanic spreading and the Tethys-Atlantic connection

The history of continental breakup and oceanic spreading of the Alpine Tethys is defined by a revision of isotopic and biochronologic ages of 65 stratigraphic sections located in the Alps, Apennines, Betic Cordillera, Rif, and central Atlantic and a reinterpretation of the stratigraphic sequences of surpraophiolitic radiolarites. The biochronology of radiolarites is revised by using the deterministic approach known as the unitary association method. During the early Bajocian (unitary association zone, UAZ 3) radiolarite sedimentation began at the continental margin. Biochronologic ages determined in the lowermost radiolarites in basinal sequences of Tethyan margins are synchronous and mark a regional change in sedimentation regime in the Alpine Tethys. The onset of oceanic spreading of the Alpine Tethys is dated by isotopic methods as Bajocian, and is consistent with the timing of the structural evolution of the continental margins. The earliest fragments of Tethyan oceanic crust are characterized by the associations of ophiolites with deep-sea sediments, and coarse reworked sediments including platform and continental basement fragments. The earliest ophiolites also show geochemical affinities with synrift and transitional mid-oceanic-ridge basalts. The oldest radiolarites on oceanic crust are so far dated as Bathonian (UAZ 6) and are located in the Gets nappe (western Alps), in the Balagne nappe (Corsica), and in the central Atlantic (Deep Sea Drilling Project [DSDP] Site 534A). The oldest remnants of Alpine Tethyan crust have been identified in weakly metamorphosed cover nappes that occupy an external tectonic position in the Alpine orogenic belts, as compared to the main ophiolitic sutures. Thus, the older relics of oceanic lithosphere were the first to be accreted and transported onto the foreland during the collision. Siliceous sedimentation during the early Bajocian is correlated with westward deep-water circulation in the Alpine Tethys related to the opening of deep seaways between Laurasia and Gondwana. In the central Atlantic no radiolarites, but thin radiolarian-rich layers, were deposited during the earliest Bathonian (UAZ 6). The similarity between radiolarian faunal assemblages and ages in the Northern Alps, Gets nappe, Betic Cordillera, and Site 534 (DSDP Leg 76) suggest a Middle Jurassic connection between the Alpine Tethys and central Atlantic. Biochronologic and isotopic ages currently indicate that oceanic spreading of the Alpine Tethys began during the Bajocian and continued until the Kimmeridgian.

Mode and Rate of Growth in Ammonoids

In this chapter we discuss the mode and rate of growth in ammonoids, focusing primarily on postembryonic growth. We first discuss the general mode of growth and then describe the ontogenetic sequence of growth stages. These stages are recognized on the basis of changes in morphology. For example, a graph of the increase in size of whorl width versus shell diameter in an individual reveals changes through ontogeny that pinpoint the end of one growth stage and the beginning of another. We next discuss the overall rate of growth through ontogeny and establish a generalized growth curve. In this discussion, we refer to other cephalopods whose rate of growth is known. Fluctuations in the rate of growth that are superimposed on this growth curve are indicated in ammonoids by the presence of such shell features as varices and constrictions.

Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations

Abstract An analysis is presented of the diversity and faunal turnover of Jurassic ammonites related to transgressive/regressive events. The data set contained 400 genera and 1548 species belonging to 67 ammonite zones covering the entire Jurassic System. These data were used in the construction of faunal turnover curves and ammonite diversities, that correlate with sea-level fluctuation curves. Twenty-four events of ammonite faunal turnover are analyzed throughout the Jurassic. The most important took place at the Sinemurian-Carixian boundary, latest Carixian-Middle Domerian, Domerian-Toarcian boundary, latest Middle Toarcian-Late Toarcian, Toarcian-Aalenian boundary, latest Aalenian-earliest Bajocian, latest Early Bajocian-earliest Late Bajocian, Early Bathonian-Middle Bathonian boundary, latest Middle Bathonian-earliest Late Bathonian, latest Bathonian-Early Callovian, earliest Early Oxfordian-Middle Oxfordian, earliest Late Oxfordian-latest Oxfordian, latest Early Kimmeridgian, Late Kimmeridgian, middle Early Tithonian and Early Tithonian-Late Tithonian boundary. More than 75 percent of these turnovers correlate with regressive-transgressive cycles in the Exxon, and/or Hallam′s sea-level curves. In most cases, the extinction events coincide with regressive intervals, whereas origination and radiation events are related to transgressive cycles. The turnovers frequently coincide with major or minor discontinuities in the Subbetic basin (Betic Cordillera).

Towards accurate numerical calibration of the Late Triassic: High- precision U-Pb geochronology constraints on the duration of the Rhaetian

Numerical calibration of the Late Triassic stages is arguably the most controversial issue in Mesozoic stratigraphy, despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end-Triassic mass extinction. Here we report new chemical abrasion–isotope dilution– thermal ionization mass spectrometry zircon U-Pb dates for volcanic ash beds within the Aramachay Formation of the Pucara Group in northern Peru that place precise constraints on the age of the Norian- Rhaetian boundary (NRB) and the duration of the Rhaetian. The sampled ash bed–bearing interval is located just above the last occurrence of the bivalve Monotis subcircularis, placing this stratigraphic sequence in the uppermost Norian, perhaps ranging into the earliest Rhaetian. Zircon U-Pb dates of ash beds constrain the deposition age of this interval to be between 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma, providing precise constraints on the age of the NRB. Combined with previously published zircon U-Pb dates for ash beds bracketing the Triassic-Jurassic boundary, we estimate a duration of 4.14 ± 0.39 m.y. for the Rhaetian. This ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end-Triassic mass extinction.

A new scenario for the Domerian - Toarcian transition

In contrast to the majority of recently published hypotheses, we believe that the main trigger for early Toar- cian anoxia is neither increased primary productivity during the Tenuicostatum and Falciferum Zones nor sudden me- thane hydrate degassing close to the transition between these two zones. In our opinion, this peculiar paleoceanographic episode is linked to a major, though short-lived, regression at the end of Upper Domerian. Sea-level fall resulted from sudden cooling due to increased volcanic activity. This generated global thermal insulation and subsequent glaciation. The regression is responsible for a major hiatus over NW-European epicontinental seas and is later followed by the well-known Lower Toarcian transgression. The interval corresponding to this hiatus allowed vegetation to colonise vast newly emerged surfaces. The leaching and drowning of the accumulated organo-humic matter then triggered the anoxic cycle at the transgressive maximum, concomitant with a global warming. Un nouveau scenario pour le passage Domerien - Toarcien

A morphogenetic explanation of Buckman's law of covariation

In this paper, we demonstrate that Buckmans law of covariation, describing the cases of extreme variability observed in ammonoids, can be explained in a simple way by analysing the internal shell geometry. This geometry can be characterized by the amount of lateral and ventral curvature of the shell which controls the thickness of the mantle and the concentration of morphogens present in the shell-secreting epithelium. The most salient ornamentation is present where the whorls are most curved, shells with slight angular bulges often being spinose or carinate and flat ones being almost smooth. These observations agree with a morphogenetic model based on Meinhardts reaction -- diffusion mechanisms [Meinhardt 1995].

Diversity of radiolarian families through time

The examination of radiolarian biodiversity at the family level through Phanerozoic time reveals some general trends known in other groups of organisms, especially among plankton, while some other trends seem to be quite peculiar. The Permian /Triassic crisis that is one of the most important in the evolution of marine organisms, is marked in radiolarian assemblages by the extinction of two orders (Albaillellaria and Latentifistularia) towards the end of the Permian, and mostly by the tremendous diversification of Spumellaria and Nassellaria in the early-mid Triassic. Radiolarian diversity increased from Cambrian to Jurassic, remained quite stable during the Cretaceous and has decreased slightly since then.

Sea-level changes and ammonite faunal turnover during the Lias/Dogger transition in the western Tethys

The aim of this paper was to investigate the possible connections between ammonite faunal turnover and the eustatic events recorded in Tethyan sequences during the middle Toarcian/early Bajocian time interval. For this we have analysed the biostratigraphic ranges, at the subzone level, of approximately 600 ammonite species belonging to 160 genera from several selected sections of the western Tethys (Mediterranean and Submediterranean provinces). The analysis of taxon ranges enabled us to plot curves for ammonite faunal turnovers, inter-subzonal distance, and diversity. Comparing the mentioned curves with Tethyan sequences [Hardenbol et al., 1998], we find that sea-level changes correlate well with origination and extinction events and faunal diversity. Most of the faunal turnovers correlate with stratigraphic events. Extinction events with their corresponding decrease in diversity correlate with regressive intervals and with major or minor sequence boundaries. Origination events and their corresponding increase in diversity were clearly connected with transgressions in Tethyan sequences. In several cases, the major sequence boundary and the subsequent transgressive phase correlate with major ammonite faunal turnover, whereas minor or medium sequence boundaries generally gave rise to minor or medium turnovers.