High-resolution ammonite and carbon isotope stratigraphy across the Triassic-Jurassic boundary at New York Canyon (Nevada)

The Triassic-Jurassic boundary is generally considered as one of the major extinctions in the history of Phanerozoic. The high-resolution ammonite correlations and carbon isotope marine record in the New York Canyon area allow to distinguish two negative carbon excursions across this boundary with different paleoenvironmental meanings. The Late Rhaetian negative excursion is related to the extinction and regressive phase. The Early Hettangian delta(13)C(org) negative excursion is associated with a major floristic turnover and major ammonite and radiolarian radiation. The end-Triassic extinction-Early Jurassic recovery is fully compatible with a volcanism-triggered crisis, probably related to the Central Atlantic Magmatic Province. The main environmental stress might have been generated by repeated release of SO(2) gas, heavy metals emissions, darkening, and subsequent cooling. This phase was followed by a major long-term CO(2) accumulation during the Early Hettangian with development of nutrient-rich marine waters favouring the recovery of productivity and deposition of black shales. (C) 2004 Elsevier B.V. All rights reserved

Origin of sulfide replacement textures in lunar breccias. Implications for vapor element transport in the lunar crust

Lunar samples 67016,294, 67915,150, and 67016,297 represent clasts of Mg-suite and ferroan anorthosite lithologies that have interacted with a S-rich vapor. Numerous studies have speculated on the composition and source of these “fluids”, their capability for the transport of vapor-mobilized elements, and the scale and environment under which these types of process occurred. These models all assumed a Moon with a very “dry” mantle, crust, and surface. The olivine in these lithologies is partially to totally replaced by troilite and low-Ca pyroxene. The troilite makes up 30–54 vol% of the troilite + low-Ca pyroxene pseudomorphs after olivine. Other silicates and oxides in the assemblages have experienced post-magmatic reequilibration (pyroxene exsolution, recrystallization, “exsolution” of ilmenite in spinel). The troilite also occurs in veins cross cutting individual phases and metamorphic textures. The sulfide veining and replacement features are restricted to individual clasts and do not cut across the matrix surrounding the clasts, and thus predate the breccia-forming event. The proportion of troilite to low-Ca pyroxene and silicate chemistries indicate that simple reactions (such as olivine + S_2 ↔ low-Ca pyroxene + troilite + O_2) do not adequately represent the replacement process. The sulfides have compositions that are similar to those found in mare basalts. In particular, the sulfides generally are enriched in Co relative to Ni. Exsolution of Ni–Co–Cu in the sulfides is distinctly different between the breccias and mare basalts and suggests a different cooling or crystallization (melt versus vapor) history. The sulfur isotopic composition of the vein and replacement troilite ranges from approximately δ^(34)S = −1.0‰ to −3.3‰. Based on our observations, it appears that the model suggested by Norman et al. (1995) is the most appropriate for the origin of the troilite veining and troilite–pyroxene pseudomorphs after olivine. Our data add significant definition to this model. This process occurs in the relatively shallow lunar crust on a scale that involves vapor interaction with multiple plutonic lithologies of various ages and compositions. These reactions occur at distinct conditions of fS_2, fO_2, and temperature. The reacting vapor is S-rich, and perhaps low in H. The reduction of the oxides in the clasts was not a product of H-streaming as has been suggested for similar textures in lunar rocks, but more likely related to “S-streaming”. These vapors had the capability to transport chalcophile–siderophile elements. However, a proportion of the minor elements making up the troilite (Fe, Ni, Co) did come directly from the olivine being replaced. Further, there is evidence to suggest minor mobility of Mg from the olivine pseudomorphs into the adjacent pyroxene. One of the heat sources driving the transport of elements is closely tied to the emplacement of magmas into the shallow lunar crust. These intrusions were either the source for the S or provided heat to remobilized troilite already in the lunar crust. The process that drove the derivation of the S-rich volatiles was instrumental in fractionating the isotopic composition of S. The enrichment of S^32 in the vapor phase may be attributed to either the stable S species during degassing (COS, S_2 and CS_2) or the high-temperature partial breakdown of troilite in the shallow crust

Spathian (Lower Triassic) ammonoids from western USA (Idaho, California, Utah and Nevada)

The Early Triassic marine deposits are distributed over a large area in the Western United State and are very rich in ammonoids. The detailed bed by bed study of their stratigraphic distribution allowed us to present a new very precise biochronological framework of the Spathian stage (Middle to Late Olenekian). Nineteen new ammonoid species belonging to the genera Pseudosvalbardiceras ?, Prohungarites, Silberlingeria, Bajarunia, Hemilecanites, Arctomeekoceras, Xenoceltites, Nordophiceratoides, Sibirites, Columbites, Hellenites and Svalbardiceras and eighteen new spathian ammonoid genera (Courtilloticeras, Yvesgalleticeras, Marcouxia, Jeanbesseiceras, Tapponnierites, Gaudemerites, Deweveria, Ceccaisculitoides, Coscaites, Eschericeratites, Carteria, Goricanites, Tardicolumbites, Cowboyiceras, Nordophiceratoides, Glabercolumbites) have been described in a recent preliminary report by Guex et al. (2005) on the basis of unpublished material collected in the western USA (Idaho, Utah, Nevada and California). In addition, one new genus (Rudolftruempyiceras) and four new species are also described in the present work. The precise stratigraphic description of the collected sections is given in the present Memoir and the stratigraphic distribution of 88 species belonging to 51 genera is established herein. Twenty-three new biochronological horizons are defined thanks to these new data. The Cowboy Pass section (Utah) records a very interesting terrestrial (red beds and very shallow water deposits) transition between the marine Late Smithian and the Earliest Spathian faunas. That worldwide short lived regression followed by a major transgression fits the model proposed by Guex et al. 2001 and Morard et al. 2003 for the Pliensbachian - Toarcian transition: major volcanic SO2 emissions generating a short but major cooling and glaciation associated with an important sea level fall and large scale emersions, followed by a warming inducing a transgressive episode with some anoxic deposits

Late Early Triassic climate change: Insights from carbonate carbon isotopes, sedimentary evolution and ammonoid paleobiogeography

The late Early Triassic sedimentary-facies evolution and carbonate carbon-isotope marine record (delta(13)C(carb)) of ammonoid-rich, outer platform settings show striking similarities between the South ChinaBlock (SCB) and the widely distant Northern Indian Margin (NIM). The studied sections are located within the Triassic Tethys Himalayan belt (Losar section, Himachal Pradesh, India) and the Nanpanjiang Basin in the South China Block (Jinya section, Guangxi Province), respectively. Carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end-Permian biotic crisis. This study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover (extinction followed by a radiation) occurring around the Smithian-Spathian boundary. Based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity, we demonstrate that the late Early Triassic (Smithian and Spathian) was a time of a major climate change. More precisely, the end Smithian climate can be characterized by a warm and equable climate underlined by a flat, pole-to-equator, sea surface temperature (SST) gradient, while the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions. Moreover, sedimentary evidence suggests a transition from a humid and hot climate during the Smithian to a dryer climate from the Spathian onwards. By analogy with comparable carbon isotope perturbations in the Late Devonian, Jurassic and Cretaceous we propose that high atmospheric CO(2) levels could have been responsible for the observed carbon cycle disturbance at the Smithian-Spathian boundary. We suggest that the end Smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased CO(2) flux possibly originating from a short eruptive event of the Siberian igneous province. This increase in atmospheric CO(2) concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms, including ammonoids, in late Smithian oceans. (c) 2006 Elsevier B.V. All rights reserved