How did Great Dying that ended the Premian era occur?

long environmental problems, not a sudden catastrophe, might have led to the Great Dying that ended the Permian era 250 million years ago, says a new study. The event wiped out 90 per cent of marine creatures and 70 per cent of the land dwellers. Whether these extinctions were caused by meteor impacts, massive volcanic eruptions or long-term pressures is still a contentious issue and the new findings revive the argument that mass extinctions were mostly slow protracted crises.

Two us scientists claim the Permian, and the following Triassic era extinction episodes might have been caused by a steady deterioration of the global environment, fuelled by volcanic eruptions and global warming. Their research, published in the November issue of the journal Geology, could help scientists predict the effects of climate change on today's marine life.

Researchers from the University of Southern California, Los Angeles, reached the conclusion after studying fossils of bryozoans--coral-like marine creatures--from the two epoch. They found that bryozoan diversity in the deep ocean started to decrease about 270 million years ago, and fell sharply in the 10 million years before the mass extinction that marked the end of the Permian era. Whatever caused the Permian extinction likely came from the deep ocean and was very slow, thus questioning a gradual oceanographic process, as opposed to a rapid event, says Catherine Powers, lead writer of the paper. The researchers say warming oceans and declining oxygen levels, possibly caused by enormous volcanic eruptions, must have released hydrogen sulphide--a toxic gas--into the ocean. If large amounts of hydrogen sulphide escaped into the atmosphere, it would have killed most forms of life and also damaged the ozone shield, the researchers say. However, paleontologist Subhendu Bikash Bardhan of Jadavpur University says generally mass extinctions require two kinds of causes: long-term pressure on the ecosystem and a sudden catastrophe at the end of the period of pressure. Two factors acting alone is not enough for a significant increase in the extinction rate, he says.