Typhoons may explain how small islands shift huge amounts of carbon into the ocean each year. Credit: Xinhua/Photoshot

Researchers in Taiwan have discovered that typhoons can pump large amounts of carbon from inland areas into the deep sea, potentially affecting the global carbon cycle and marine ecosystems.

Small islands such as Taiwan constitute only 3% of the world's landmass, but are responsible for more than 35% of terrestrial-carbon flux to the sea. Exactly how they are able to dump this much carbon into the world's oceans has been a mystery until now.

To investigate the mechanism, marine geologist James Liu at National Sun Yat-sen University in Kaohsiung, Taiwan, and his colleagues studied the Gaoping River, a mountainous river in southern Taiwan with a source 4,000 metres above sea level in the Central Mountain Range. The river flows into the South China Sea through the Gaoping Submarine Canyon, a 260-kilometre undersea gorge southwest of Taiwan that eventually merges with the Manila Trench, more than 3,000 metres deep off the west coast of the Philippines.

"With an elevation drop of 7,000 metres, you can get some very interesting phenomena," says Liu, who presented the findings on 29 June at China's First Conference on Deep-Sea Research and Earth Systems Science, in Shanghai.

Sinking feeling

The researchers noted unusually brown, sediment-laden flow at the river mouth after cyclones, and wondered where in the sea that sediment eventually ended up. Such sediments contain fresh organic carbon (from plants and animals) as well as inorganic carbon (from soil and minerals). In a pilot study, the researchers set out traps that were designed to collect settling particles and record the time of collection as the current passed over the canyon floor.

"It's a very clever system that captures sedimentation processes in the making," says Dai Minhan, a marine ecologist at Xiamen University in Xiamen, Fujian province, in southeastern China. Most sedimentation studies either analyse existing deposits in ocean basins or try to simulate the process either in the laboratory or by using computer modelling.

With an elevation drop of 7,000 metres, you can get some very interesting phenomena. ,

Liu and his colleagues studied four typhoons that took place between July and September 2008. After each of the first two typhoons, they found sediment from the river in their deepest traps, at a depth of 650 metres.

Fresh water is less dense than seawater, so discharge from rivers normally sinks no deeper than a couple of metres. But, says Liu, if the water in the Gaoping River after a typhoon is dense with sediment, it gushes down the mountain and plunges much deeper into the canyon than expected.

Such hyperpycnal flows have been reported in, for example, a submarine canyon on the edge of the Gulf of Lions in the northwestern Mediterranean1. They tend to take place in winter when cooling and evaporation make the river water denser than seawater. "Our study is probably the first to document such phenomena for warm waters," says Liu.

Everything must go

Of the four typhoons, the first two — Kalmaegi and Fung Wong — flushed a large amount of terrestrial sediments into the canyon, resulting in strong signals in the traps, whereas the other two — Kammuri and Nuri — left few traces. "Kalmaegi and Fung Wong probably carried away all the sediment that could be carried away," says Liu.

"Although typhoons last only a few days at a time, their impact on terrestrial-carbon export can be huge," says Liu. The team found that typhoon Kalmaegi dumped 30,000 tonnes of carbon into the ocean in one day, about 15% of all the carbon exported from the river's watershed in that year; 75% of that was transported further into the South China Sea by ensuing currents.

"The findings add an unprecedented dimension to carbon flux, because previous research on typhoons has largely focused on the shallow part of the ocean," says Dai.

To get a better idea of the discharge process, Liu and his colleagues plan to leave more sediment traps in several locations along the canyon, at varying distances and depths from the river mouth. They are trying to improve the traps, so that they can take time the arrival of sediment more accurately.

Meanwhile, the team is studying the effects of typhoon Morakot, which wreaked havoc in southwestern Taiwan in August 2008. Morakot dumped a whole year's worth of precipitation in the Gaoping River basin over four days. "We are expecting to see a greater carbon flux than typhoon Kalmaegi," says Liu.

With the frequency and intensity of tropical cyclones increasing as a result of climate change, "there is a greater need to understand how cyclones impact land–ocean interactions than ever before", says Liu.