Environmental changes across the Triassic–Jurassic boundary and coeval volcanism inferred from elemental geochemistry and mineralogy in the Kendlbachgraben section (Northern Calcareous Alps, Austria)

Pálfy, József and Zajzon, Norbert (2012) Environmental changes across the Triassic–Jurassic boundary and coeval volcanism inferred from elemental geochemistry and mineralogy in the Kendlbachgraben section (Northern Calcareous Alps, Austria). Earth and Planetary Science Letters, 335-36. pp. 121-134. ISSN 0012821X

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Abstract

The end-Triassic extinction (ETE), one of the five largest Phanerozoic mass extinctions, is associated with rapid and severe environmental change, but existing data permit alternative models of causation. Volcanism in the Central Atlantic Magmatic Province (CAMP) has been proposed as the main trigger, but direct evidence for this linkage is scarce. To help constrain scenarios for the ETE and other Triassic– Jurassic boundary (TJB) events, we obtained a temporally highly resolved, multidisciplinary dataset from the Kendlbachgraben section in the Northern Calcareous Alps in Austria. The section belongs to the same paleogeographic unit (Eiberg Basin) and share similar stratigraphy with the recently selected base Jurassic Global Stratotype Section and Point at Kuhjoch. Micromineralogic study of the topmost bed of the Rhaetian K¨ ossen Formation revealed pseudomorphs of altered, euhedral pyroxene and amphibole crystals. Their well-faceted morphology is consistent with their origin from distal mafic volcanic ash fallout. Spherical grains were also observed in the same bed, likely representing clay-altered volcanic glass. Clay minerals of this bed include lowto medium-charged smectite and Mg-vermiculite, both typical alteration products of mafic rocks. The same bed yielded a rare earth element pattern that differs from all other levels in an enrichment of heavy REEs, hinting at some minor contribution from mafic magmatic material. These features from a layer that was deposited very near to the TJB are interpreted as direct evidence of CAMP volcanism, coeval or immediately preceding the ETE and the initial negative carbon isotope anomaly. The kaolinite-dominated clay mineral spectrum of the overlying boundary mudstone records intensive weathering under hot and humid greenhouse conditions. Redox-sensitive minor and trace elements do not support the development of widespread anoxia in the studied section. Although pyrite is common in several layers, framboid size indicates formation within a reductive zone, below the sediment/water interface, rather than in an anoxic water column. Our data provide a direct link between uppermost Triassic marine strata and CAMP-derived material. They support scenarios where CAMP volcanism induced climate and other environmental change, which in turn triggered the ETE and that is also reflected in the carbon isotope anomalies.

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