The Permian-Triassic (P-T) extinction is documented geochemically in a marine sequence deposited in a basinal setting at Williston Lake, northeastern British Columbia, by using elemental and isotopic organic geochemical data from well-preserved sedimentary rocks. The δ13C values of kerogens in the rocks exhibit a sudden shift at the P-T boundary from latest Permian values of -29‰ ± 1‰ (PDB) to a minimum of -32.6‰ 2 m above the P-T boundary and then back to the Permian value 4 m above the P-T boundary. After considering various factors, we conclude that reduced surface-water primary productivity following the P-T mass extinction is largely responsible for the observed δ13C shift. The abruptness of the δ13C shift in a sequence of continuous deposition argues that the strong pulse of extinction at the P-T boundary was sudden rather than gradual. Marine primary productivity did not recover until at least 50 to 100 ka after the time of the P-T boundary, so a higher atmospheric pCO2 in the earliest Triassic may have resulted from buildup of dissolved CO2 owing to reduced photosynthetic carbon demand in the surface water.