[1] 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-(d 34 S py ) and carbonate C-isotopic compositions (d 13 C carb ). 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 34 S-depleted hydrogen sulfide and 13 C-depleted dissolved inorganic carbon. The sulfide d
34S 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-d 13 C carb 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.Components: 5322 words, 5 figures.
In the agglomeration of Ho Chi Minh City (HCMC), South Viet Nam, located near the eastern edge of the Mekong Delta, both superficial Quaternary and deep Tertiary Alluvium may contain brackish or saline water from ground surface to great depths. The combination of two complementary geophysical methods, the Vertical Electrical Sounding (VES) for shallow investigation, and the Magnetotelluric Sounding (MTS) for deep investigation, gives a more complete electrical image of the sedimentary series from the surface to the deep crystalline basement. It turned out to be very efficient in delineating the ground‐water contamination in both superficial and deep aquifers. The combined results reveal a close correlation of the extent of the contamination with the paleo‐geomorphology of laterites, and subsequently with the paleorelief during the Tertiary and Quaternary periods. A simple guide for hydrogeological investigation is proposed in connection with the present topography and the Tertiary tectonics.
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