Unraveling the Cambrian Puzzle: Animals and the Rise of Oceanic Sulphate
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A little more than 500 million years ago, something put the evolutionary pedal to the metal, and the stately, subdued pace of animal life on Earth revved up. Alongside this spurt in speciation –– known as the Cambrian explosion — came a jump in the concentration of sulphate in the world's oceans.
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Don Canfield of the University of Southern Denmark and his collaborator, James Farquhar of the University of Maryland, have a theory to explain it, […] attributing the rise in sulphate to the onset of bioturbidity –– the burrowing, sluicing, pumping and mixing caused by masses of worms, clams, crustaceans and other animals that began to appear around this time in Earth's history.
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The planet has long been seen as a driver of evolution. But this, they say, is an important example of how animals can create a global geochemical signal that, in turn, twists evolution's corkscrew… Before these sea-floor animals began their steady churn, sulphate — arriving in seas in the run-off from rivers — would largely be turned into hydrogen sulphide by bacteria living in the ocean floor. The sulphide would then be converted to pyrite (FeS2), which, once buried, removes the sulphate from the system. Once bioturbation turned on, however, oxygen in the deep ocean could mix more freely with the sediments, allowing bacteria and other processes to recycle pyrite and turn it back to sulphate. This excess sulphate would have reached a saturation point, giving rise to the formation of gypsum deposits — a mineral that, along with sulphate levels, also happened to rise in the rock record around this time.
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The researchers roughly sketched changes in ocean sulphate concentrations through time by directly analysing tiny amounts of brine trapped in salt crystals… The researchers built a simple model for the effects of bioturbation and found that its output — both in the timing and the magnitude of the sulphate signals — matched what they were seeing in the data.
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Biogeochemist Tim Lyons says that the theory proposed by Canfield and Farquhar works well. But he points out that data gaps still need to be filled and specific problems must be worked out — for instance, explaining gypsum deposits that are far more ancient than the Cambrian era. Also, he says, the deep burrowing animals that would have done the most to mix oxygen into the sea floor didn"t come along until sometime after the Cambrian explosion — so the sudden rise in sulphate concentrations precisely at the Cambrian boundary is difficult to explain.
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Lyons thinks that increases in atmospheric oxygen could be at least partly responsible for the rise in oceanic sulphate without the invocation of animals. More oxygen, percolating from the atmosphere into the ocean, would on its own encourage the recycling of pyrite into sulphates. And higher oxygen levels in deep seas might encourage existing animals to migrate into them, where they could become the sea-floor blenders that Canfield and Farquhar propose. So, are animals the cause or the consequence of these epochal changes to the oxygen and sulphur cycles? Similar to the question of the chicken and egg, the answer might not be one or the other. "These things go hand in hand," Lyons says.
