The Permian–Triassic boundary at Nhi Tao, Vietnam: Evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform
The Permian–Triassic boundary at Nhi Tao, Cao Bang Province, Vietnam was sampled in a 7.5-m-thick outcrop section at high resolution (∼ 5 cm intervals) for chemostratigraphic and magnetic susceptibility analysis. The section consists entirely of slightly argillaceous limestone representing shallow-marine facies of the Jinxi Platform, one of several large carbonate platforms within the Nanpanjiang Basin, located on the southern margin of the South China Craton. Upper Permian strata (Beds 1–7) are mainly dark-gray, cherty fossiliferous wackestones and packstones containing a diverse open-marine fauna, whereas uppermost Permian and Lower Triassic strata (Beds 9 and higher) are medium-gray calcimicrobial framestones containing rare macrofossils. These facies are separated by a 12-cm-thick oolitic–pisolitic grainstone (Bed 8) that coincides with the disappearance of most Late Permian faunal elements as well as with the first appearance of various geochemical anomalies that continue into the Lower Triassic part of the section. This “Late Permian event horizon” is characterized by (1) an abrupt decline in total organic carbon to near-zero values, (2) the onset of a sustained decline in carbonate δ13C, and (3) the first of eight concentration peaks in pyrite sulfur. Significantly, each sulfide peak is associated with lower pyrite δ34S values as well as with the onset of a negative carbonate δ13C excursion (or the acceleration of an excursion already in progress). These chemostratigraphic relationships are consistent with multiple episodes of upwelling of sulfidic, 34S- and 13C-depleted deep-ocean waters onto the Jinxi Platform. The first upwelling event was the most intense and caused a drastic reduction in primary productivity and the demise of the Late Permian fauna; subsequent episodes were less intense but may have contributed to a delay in recovery of Early Triassic marine ecosystems. A ten-fold increase in magnetic susceptibility in Bed 9 may record the influx of fine detrital particles following destruction of terrestrial ecosystems and massive soil erosion. The terrestrial signal of the end-Permian catastrophe thus follows the marine signal with a 12-cm lag, which may reflect the time-of-transit of soil-derived particles across the Nanpanjiang Basin, suggesting that the marine and terrestrial crises in the Nhi Tao region occurred more-or-less synchronously. These observations suggest a model in which renewal of global-ocean overturn followed a prolonged interval of deep-ocean stagnation during the Late Permian, with upwelling intensity modulated by short-term (∼ 20 kyr) climate cyclicity.