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Samples from strata spanning the Triassic–Jurassic boundary at the GSSP at Kuhjoch and at Kendlbachgraben were studied by NAA, XRF and combustion analysis to determine Ir levels and associated geochemistry. The results are compared to previously determined carbon isotope stratigraphy at these sections. Ir concentrations in the limestones of the Kössen Formation at Kuhjoch are very low (< 10 pg/g) below the top of the formation, rising to 26 pg/g, in the T-bed at the top of the Eiberg Member. The Tiefengraben Member of the Kendlbach Formation exhibits higher concentrations of Ir in general relative to the strata below. The shift to higher levels is abrupt at the base of the member, coinciding with a decrease in carbonate content. Concentrations of 60 to 80 pg/g are typical through the entire thickness of the Schattwald Beds and into the gray Tiefengraben Member strata, peaking at 145 pg/g. Concentrations decline to 30–40 pg/g above 680 cm from the formation base, coinciding with increasing carbonate content. The analyses from the Kendlbachgraben section compare well with those from Kuhjoch, with similar difference in Ir concentration between the Kössen and Kendlbach formations. In both sections, the initial increase in Ir corresponds to the initial carbon isotope excursion. Concentrations of redox-sensitive elements indicate transient reducing conditions during deposition of the uppermost Kössen Formation, but oxidizing conditions during Tiefengraben Member deposition. The Al/Ti ratio indicates more intense weathering during deposition of the lowermost 20 cm of the Tiefengraben Member, but otherwise consistently humid to sub-humid climate prevailed during deposition. The primary control on Ir concentration in the sampled section is formation lithology, although there are variations within the Tiefengraben Member that are independent of carbonate content. Enrichment of Ir at the top of the T-bed is associated with a redox boundary, but the cause of other variations is undetermined. [ABSTRACT FROM AUTHOR]

Abstract: A detailed profile of Ir concentrations in continental strata of the Blomidon Formation ostensibly spanning the Triassic–Jurassic boundary confirms the existence of multiple anomalies, with a peak Ir concentration of 450 pg/g. The total amount of Ir deposited exceeds 4 ng/cm2, a value that requires an external source other than typical terrestrial sediment. Of the 10 other elements measure, only Zn and organic carbon are correlated to Ir. The organic carbon appears to be concentrated in mm-thick kerogenous laminae in greenish-grey to dark grey sediment layers that also host the Ir anomalies. The stratigraphic distribution of Ir and probably Zn appears to be controlled largely by redox boundaries conditions, with Ir probably forming organo-metallic complexes. We find no specific geochemical or sedimentological evidence for an extraterrestrial source for this enrichment, although we cannot exclude an impact origin. However, the close spatial and temporal relationship of the Ir anomalies in the Fundy and Newark basins to the basalts of the Central Atlantic Magmatic Province suggests alternative working hypotheses. Potentially, the volcanic rocks sourced the Ir through post-eruptive fluid mobilization into the surrounding sediments. Conversely, mantle outgassing and aerosol deposition during early stages of the eruptions may have been the source for the Ir anomalies. At this time, the data are insufficient to eliminate any of these hypotheses conclusively. [Copyright &y& Elsevier]