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2 Volumina Jurassica

2 2006

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Oyster and brachiopod stable isotopes across the Triassic/Jurassic boundary and in the Early Jurassic

Korte C., Hesselbo S., Jenkyns H., Dietl G., Dulai A., Vörös A.

Volumina Jurassica

2006

Vol. 4, no. 1

289-289

Oxygen and carbon isotope values have been obtained from oysters for the Triassic/Jurassic boundary section at Lavernock Point (Wales), and from brachiopods and oysters for different Hettangian, Sinemurian and Pliensbachian localities of South Germany and Hungary. Low-Mg-calcite brachiopods and oysters are particularly suitable for such studies because this carbonate phase is the most resistant to diagenetic alteration. Nevertheless, all fossils have been screened by chemical and optical techniques (optical microscope, scanning electron microscopy, trace element analyses) to evaluate the isotope data for diagenetic change, and only samples with Mn content less than 250 ppm and Sr content more than 400 ppm, complemented by well preserved textures under SEM, were considered in this study. For the Triassic/Jurassic boundary (TJB) the carbon isotope values are at about 2.5‰ in the lower Langport Member, increase to 4.5‰ in the lowest Blue Lias and decrease subsequently to 1.5‰ just below the Planorbis Zone. The data remain low with variations between 1.5 and 2.5‰ up to the Liassicus Zone. These results correspond to the organic carbon isotope trend for the Triassic/Jurassic boundary section at St Audrie's Bay (Hesselbo et al. 2002). Oxygen isotope values increase from -0.5‰ in lower Langport Member to 0‰ at the base of the Blue Lias, decrease in the Blue Lias down to -1.5‰ just below the Planorbis Zone and change in parallel with the organic and inorganic carbon-isotope trends. The δ ¹ ⁸O values indicate decreasing seawater temperature with increasing δ ¹ ³C in the Langport Member and increasing water temperatures of about 6°C in the lower Blue Lias. The distinct warming trend occurred during the "main" TJB negative excursion. Carbon and oxygen isotope values from Hettangian, Sinemurian and Pliensbachian brachiopods and oysters, as well as from some complementary belemnites, show similar values and trends compared to the data compilation by Jenkyns et al. (2002). Carbon isotope values are between 1 and 2‰ in the Hettangian and Early Sinemurian followed by an increase of about 1‰ during the Sinemurian, a nearly 3‰ decrease in the Early Pliensbachian and higher δ ¹ ³C values (˜2.5‰) in the later Pliensbachian.

Apparent sea-level change at times of extraordinary sediment supply: integrated facies analysis and chemostratigraphy of Early Toarcian Oceanic Anoxic Event, Peniche, Portugal

Hesselbo S., Jenkyns H., Duarte L., Veiga de Oliviera L.

Volumina Jurassica

2006

Vol. 4, no. 1

171-172

The Toarcian Oceanic Anoxic Event in the Early Jurassic was a time of major environmental change characterized by widespread deposition of organic-rich black shales in both deep- and shallow-marine settings. Facies evidence, mainly from the European area, has been used to argue for a major sea-level rise in concert with oceanic anoxia, extinction of certain groups, and perturbation of the carbon cycle. However, in a number of locations, such as in the Lusitanian Basin, Portugal, it has also been argued previously that the start of the event was characterized by sea-level fall rather than rise, as evidenced by the initiation of gravity-flow deposition. In this study we combine new marine and terrestrial carbon-isotope and lithological data to suggest an alternative model for development of facies patterns in this basin, whereby an abrupt increase in sediment flux from the hinterland led to slope instability and gravity-flow deposition that lasted as long as the carbon-cycle perturbation. This interpretation is supported by existing osmium and strontium isotope data, which have been interpreted to indicate a significant continental transient increase in continental weathering fluxes. In effect this event provides a good example from the rock record where the assumption of constant sediment supply is demonstrably false, but in which the facies patterns may still be predictable on a variety of scales.