Association of 34S‐depleted pyrite layers with negative carbonate δ13C excursions at the Permian‐Triassic boundary: Evidence for upwelling of sulfidic deep‐ocean water masses

Abstract

A marine Permian‐Triassic boundary (PTB) section at Nhi Tao, Vietnam, contains a series of at least 9 pyritic horizons characterized by concurrent decreases in pyrite S‐ (δ34Spy) and carbonate C‐isotopic compositions (δ13Ccarb). The first and largest of the events that precipitated these pyritic horizons was coincident with the Late Permian mass extinction, while subsequent events were generally smaller and occurred at quasiperiodic intervals of ∼20 to 30 ka. A near complete lack of organic carbon to drive bacterial sulfate reduction in sediment pore waters, among other considerations, argues against a diagenetic control for these relationships. Rather, the covariant patterns documented herein are most easily explained as the product of recurrent upwelling of anoxic deep‐ocean waters containing 34S‐depleted hydrogen sulfide and 13C‐depleted dissolved inorganic carbon. The sulfide δ34S record of the study section represents a mixture of a small amount of isotopically heavy authigenic pyrite (formed via in situ bacterial sulfate reduction) with a generally larger quantity of isotopically light syngenetic pyrite precipitated within the water column during upwelling episodes. Although upwelling of toxic deepwaters has been invoked in earlier studies as a mechanism for the Late Permian marine mass extinction, this is the first study to (1) document patterns of pyrite‐δ13Ccarb covariation that strongly support upwelling as a major process at the PTB and (2) provide evidence of multiple, quasiperiodic upwelling events that may reflect reinvigoration of global‐ocean overturn following a prolonged interval of Late Permian deep‐ocean stagnation.