Paper No. 9
Presentation Time: 8:00 AM-12:00 PM
NEGATIVE d13C CARBON ISOTOPIC ANOMALY IN CONTINENTAL STRATA AT THE TRIASSIC-JURASSIC BOUNDARY IN EASTERN NORTH AMERICA (NEWARK BASIN, PENNSYLVANIA, USA)
Late Triassic and Early Jurassic lacustrine strata of rift basins in eastern North America record the Triassic-Jurassic (Tr-J) boundary (~200 Ma) and allow events surrounding this critical juncture to be calibrated in time using Milankovitch cyclostratigraphy. We conducted stable carbon isotope analyses on samples of vascular plant-dominated bulk organic matter across the Tr-J boundary from Newark basin of southeastern PA, and found an abrupt initial negative d13C excursion of about 5 per mil, synchronous with the previously reported faunal and floral mass extinctions, "fern spike", and Ir anomaly. This is followed upsection by a positive excursion of about 7 per mil, approaching the base of the oldest basalt flow of the voluminous Central Atlantic Magmatic Province. These carbon isotopic variations from continental facies at the Tr-J boundary site strikingly parallel those from marine records indicating that the observed signal appears to be global. Specifically, the Newark lacustrine variations resemble the "initial isotope excursion" described from Tr-J marine and paralic facies at St. Audrie's Bay, England. Tying the carbon isotopic data to Milankovitch stratigraphy, the Newark excursion is restricted to less than 10 ky. This geologic "instant" at the Tr-J boundary is consistent with an abrupt mechanism for the initial isotopic variation. The initial carbon isotopic excursion may be sufficiently distinctive to allow identification of the Triassic-Jurassic boundary, even where palynological preservation is poor.