A spate of volcanic activity may have triggered environmental changes that led to widespread destruction of life in the oceans, according to a new report. Oxygen disappeared from much of the seas nearly 100 million years ago, wiping out one third of ocean life. Sulphur from volcanoes could have been the cause. Today's oceans may face a similar threat because of the warming of the seas and use of fertiliser. When the Earth erupted in a flurry of volcanic activity during the Cretaceous period, marine life briefly bloomed but took the oxygen from the sea, creating a dead zone wheremarine animals could not survive. Scientists are warning that in wide areas of ocean, the amount of oxygen is dropping, while localised dead zones are spreading because of agricultural fertiliser being flushed into the water. The volcanoes created a bloom in photosynthetic life near the surface. As that plankton sank, it fed a secondary boom among the bacteria below, consuming much of the oxygen in deeper waters. Unable to survive, the bacteria disappeared from the deep, along with 27% of all marine genera. But plankton at the surface thrived, and its remains began to accumulate on the seabed in greater quantities than normal. But the mechanism by which the volcanoes fertilised the ocean have remained mysterious. Until now the main suspect has been the carbon dioxide put into the atmosphere, changing the weather and washing more nutrients into the ocean. At the same time the world warmed and slowed the ocean circulation that usually replenishes oxygen in the deep. Evidence of mounting levels of sulphur in the half million years before the Ocean Anoxic Event that occurred 94.5 million years ago suggests a different explanation. Although sulphates are not a key nutrient for ocean life, the authors of the study, published in the journal Nature Geoscience, suggest a new mechanism by which its arrival could lead to a widespread bloom. "Sulphates help the ocean hang on to its phosphorous," said Professor Matthew Hurtgen, one of the authors. "Along with nitrogen and iron, phosphorous is a key limiting nutrient in the ocean. Without it phytoplankton cannot grow. But when massive volcanism delivered more, it changed the amount of phosphorus available, and drove these anoxic events." Phosphorus becomes scarce in the sea because of its affinity for iron oxide. Other geoscientists are not convinced. "Their model relies on adding seven times the amount of sulphate for 500 million years from prior to the start of the anoxic event," said Martin Palmer of the National Oceanography Centre. "But some evidence suggests the big volcanic kick began between nine and 23,000 years beforehand."