The smallest fragments sink first followed by larger pieces later As the pumice floats away from the volcano, its bubbly structure becomes waterlogged. The smallest fragments sink first, followed by larger pieces later. "There is a pattern of pumice contained in the deep-sea sediments that could reveal the pattern of pumice-drift prior to sinking," explains Jokiel.Downstream of the volcano, a pumice trail is laid across the ocean floor, with bigger and bigger pieces sinking down as time goes by. Geologists often discover particles of pumice in sediment cores drilled from the sea floor. Dating and chemically analysing the source of these pieces of pumice could reveal the direction of ancient ocean surface currents.Where was the spaghetti junction of ocean currents in the past and has Hawaii always been off the beaten track? These are the kind of questions that are important to climate scientists and pumice could help to answer them all. "Nature has given us a major natural experiment, putting large amounts of drift tracers [pumice] into the ocean so that we can study the currents."Given that pumice can be reliably traced to its origins, it may even be possible, using pumice, to study how ocean currents moved in the past. "Coral can raft on to many different types of floating objects, from pumice to large drifting trees and logs and even man-made flotsam such as discarded shoes," says Jokiel. Christmas Island sits at a "spaghetti junction" of ocean highways, while Hawaii, by comparison, sits right out in the marine equivalent of the sticks.And pumice is not the only form of transport that coral colonies use to move around the globe. By contrast pumice is fairly rare on Hawaii and they could only identify three sources: the South Sandwich Islands, Mexico and Krakatau.Back at the outdoor aquaria at the Hawaii Institute of Marine Biology they discovered that coral was just as happy to grow on pumice as it was to grow on its normal bedrock of carbonate. This showed that theoretically there was no problem with coral hitching a lift across the ocean on a piece of pumice.Next they investigated the diversity of coral species around each island. The reef around Christmas Island is bursting with different colours, shapes and sizes of coral, reflecting the 81 species that live there. Meanwhile, Hawaii has a much more restricted range with only 50 different species of coral.Piecing this evidence together has led Jokiel and Cox to believe that the increased diversity of coral around Christmas Island reflects the fact that the island lies in a confluence of oceanic currents, allowing new species of coral to hitch across the ocean on miniature rafts of pumice.
"We identified that the pumice on Christmas Island came from the western Pacific Ocean (Krakatau), the south-western Pacific Ocean (Tonga Trench), the east Pacific Ocean (Mexico), the South Atlantic Ridge and one other unknown source," says Jokiel. They have also been experimenting with growing coral on pieces of pumice to see if the coral can manage to cling on.Christmas Island lies about 1,000 miles south of Hawaii and its beaches are littered with pieces of pumice. During the last two years they have been collecting pumice samples from both Hawaii and Christmas Island and working out where it has come from. For example, the pumice from the 1883 eruption of Krakatau was particularly adventurous: after the eruption, pumice drifted both west - across the Indian Ocean - and east - through the Sunda Strait. Ten months later, Krakatau pumice was washing up on the shores of Zanzibar over 6,000 miles away, and after one year, it had reached Durban in South Africa. Much of the pumice going east got caught up in large eddies and swirled around the Maldives and Sri Lanka for two years. |
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