Sudden Productivity Collapse Associated with the Triassic-Jurassic Boundary Mass Extinction

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Isotopic analyses were performed in the University of Washington laboratory of P. Quay. Isotope analyses were done with two methods, both ultimately calibrated versus National Bureau of Standards–19 (NBS-19), for ¹³C = 1.95‰ versus the Vienna PeeDee Belemnite (VPDB) standard and for ¹⁵N = 0.0‰ versus air nitrogen. The first anaylsis (sealed-tube combustion with subsequent measurement on a Finnigan MAT 251 mass spectrometer) has a reproducibility of ± 0.08‰ for organic standards. The second analysis [elemental analyzer–continuous-flow isotope ratio mass spectrometry (EA/CFIRMS)] used a Carlo Erba NC2500 interfaced through a Finnigan CONFLO II to a Finnigan Delta XL mass spectrometer. Reproducibility in this system averages ±0.12‰ for organic standards and homogenous natural samples. The EA/CFIRMS analyses were calibrated as follows: For each sample, δ¹⁵N and δ¹³C were measured versus a pulse of 99.999% pure standard gas injected into the mass spectrometer source immediately before or after the sample pulse eluted from the EA. Because the isotope ratios obtained were dependent on mixing ratios of carrier gas and dilutant in the CONFLO, the ratios were normalized to a known organic standard run with the samples under the same conditions. To verify that the corrections were normalized properly, each run also contained four aliquots of a natural sample whose ratios were known from sealed-tube combustion. This natural sample had a precision identical to the organic standard (±0.12‰) and the average of four determinations was within a range of ±0.1‰ of its correct value. Each autosampler run contained seven organic acid standards, four natural sample standards, and 40 samples and blanks. Blanks typically were less than 1% of the sample amount. Samples were prepared for analysis by grinding, followed by weighing into silver boats (measuring 5 mm by 9 mm) and acidification with 20 μl of 50% HCl. After air drying overnight at 50°C, the sample boats were sealed and measured as follows: Isotope analyses were performed by EA/CFIRMS, using a Carlo Erba NC2500 interfaced through a Finnigan CONFLO II to a Finnigan Delta XL mass spectrometer. Sample isotope ratios were normalized in each run to the values obtained for an organic standard with known isotope ratios calibrated in sealed-tube combustions versus NBS-19 for δ¹³C = 1.95‰ versus VPDB and for δ¹⁵N = 0.0‰ versus air nitrogen. Precision in this system averages ±0.12‰ for organic standards and homogenous natural samples. Accuracy, as measured by including repeats of a natural sample of known isotopic ratio in each run, was ±0.10‰. All isotope ratios are expressed in delta notation, or parts per thousand deviation from VPDB, where δ¹³C = {[(¹³C/¹²C)_sample/(¹³C/¹²C)_VPDB] − 1} × 1000.

Carter E. S., Mem. Geol. (Lausanne) 11, 175 (1993);

. 13.  E. S. Carter, unpublished results of 2000 sampling at Kennecott Point.

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Supported by a grant from NSF (H. Lane, program administrator) to P.D.W. and by Geological Survey of Canada Project number 870070.