Geology

Weighing Sverdrup Sediment

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Science  11 Jul 2008:
Vol. 321, Issue 5886, pp. 176
DOI: 10.1126/science.321.5886.176b

The huge extinction at the end of the Permian period (∼252 million years ago) is marked globally by a large drop in the carbon isotope ratio (13C/12C) of organic matter preserved in sedimentary rocks. Low values persist for millions of years after the extinction, in part marking the delayed recovery of Earth's biota. In addition to providing a marker of the event, the magnitude and abruptness of the drop are important constraints on the likely cause(s) and pace of the extinction. Many sections show a relatively abrupt decrease, though some of these may be partly compressed by erosion; the Permian was a time when most of the land masses were assembled together in one supercontinent (Pangea), and sea level fluctuated markedly. Yet some sections seem to show a more complicated or gradual decrease. Grasby and Beauchamp describe several isotope records preserved across the Sverdrup Basin, Arctic Canada, which is now known to contain thick sections of Permian and Triassic rocks. Sections at the margin of the basin, where some erosion is documented to have occurred, show an abrupt carbon isotope drop at the boundary. Sections in the center, which seem to record continuous deposition and a thicker boundary layer than most other sections globally, show a more gradual decline over about 3 m, after a period of relative stability that is not well resolved elsewhere. Although the sections thus provide important detail on the extinction record and perhaps a period just before it, detailed dates are not yet available to calibrate absolute rates of change.— BH

Chem. Geol. 10.1016/j.chemgeo.2008.05.005 (2008).

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