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Volcanic ash reveals chaos-causing seaweed’s journey

Published: 28 May 2024
Sandy beach with white ash on sand
Volcanic ash at Carlisle Bay, Barbados, on the sand and on washed-up sargassum

Scientists have used volcanic ash and ocean models to track the journey of huge mats of seaweed floating across the Atlantic and causing chaos in the Caribbean.

They found chemical traces of volcanic ash from the eruption of a volcano on St Vincent, in St Vincent and the Grenadines in the Caribbean, on sargassum seaweed that washed up four months later in Jamaica – 1,700 kilometres away.

It confirms that sargassum seaweed – which washes up by the ton on Caribbean and West African shorelines every year – spends months making its way to Caribbean shorelines.

The research team also analysed the chemical make-up of the sargassum to determine its potential to be used as fertiliser, or turned into biomaterials including cardboard and paper. But they confirmed that its possible uses are limited due to a high arsenic content.

The researchers from the University of Southampton, the University of York, and the University of the West Indies in Jamaica and Barbados have published their latest findings in a new paper.

Robert Marsh, Professor of Oceanography and Climate at the University of Southampton and co-lead author of the paper, said: “The sargassum beaching around Jamaica in late summer 2021 carried distinct traces of the volcanic ash that settled upon it around four months earlier, just to the east of St Vincent. This novel volcanic tag confirmed that sargassum arrives each summer at Jamaican beaches after a months-long journey drifting with currents from the central tropical Atlantic.”

Tropical beach with piles of brown seaweed on the sand
Sargassum washed up at Hastings beach, Barbados

What is sargassum?

Sargassum is a prolific seaweed that is swamping Caribbean and West African shorelines every year, causing environmental and economic harm. It washes up on beaches in such huge quantities that it blocks fishing boats, disrupts turtle nesting sites, threatens tourism, and releases smelly toxic gases that impact human health.

Since 2011, sargassum has flourished to form the ‘great Atlantic sargassum belt’ – a 9,000km-long macroalgal bloom, visible from space. Its growing prolificity is thought to be partly due to warming seas.

Scientists have been exploring its potential for sustainable reuse, as thousands of tons of sargassum end up in landfill every year. 

Dr Thierry Tonon, from the Department of Biology at the University of York and co-lead author of the paper, said: “Understanding sargassum’s response to environmental conditions is crucial for unlocking its biology and potential value. With the great sargassum belt also receiving additional nutrients from Sahara dust that blows across the Atlantic, huge quantities of the seaweed washing up on coastlines looks set to become the new normal.”

Professor Mona Webber, from the University of the West Indies, added: “It is very important for Caribbean islands being affected by sargassum inundation to be able to benefit from its valorisation. Understanding how the sargassum we collect in Jamaica has changed en route to our shores, and factors that could affect especially the arsenic content, will propel us towards safe use of the algal biomass.”

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