EARLY TRIASSIC BLACK SHALES FORMED VIA THE PRESERVATION MODEL

Most Early Triassic black shales probably formed via the preservation model rather than the productivity model. Gaps in Early Triassic chert and phosphate deposition support this suggestion as do two lines of reasoning. One is that upwelling zones were rare and weak in an Early Triassic stratified ocean. The other is that nutrient input to oceans was probably too low to generate productivity-based black shale.

Deep sea stratification and anoxia were probably facilitated by warm, saline bottom water that developed in the Permian and led to a slowing and perhaps a cessation of thermohaline circulation. Late Permian evaporite deposition is likely to have freshened surface waters enough that the increasingly warm Late Permian polar surface waters were no longer able to sink. These conditions intensified in the Early Triassic. Furthermore, the rapid loss of Late Permian forests associated with the end-Permian extinction probably generated an influx of fresh water as water stored in forest biomass, soils, and associated groundwater entered the oceans. This relatively sudden influx of fresh water to the oceans may have been contributed to the rapid transgression that occurred across the Permo-Triassic boundary.

Nutrient availability was minimal in the Early Triassic. Nutrient input to oceans resulting from silicate weathering in the Late Permian waned as mountain building in central Pangea ended and desert belts expanded and monsoonal climates developed in the tropics. Decimation of forests and related soil loss at the end of the Permian further reduced levels of silicate weathering and dissolved nutrient runoff.

Conditions favoring preservation-type anoxia probably persisted at least until silicate weathering increased as a result of the mid-late Triassic collisions of South China and Cimmeria with Pangea and the Middle Triassic return of coniferous forests.