Another Life: How a ‘smell’ can transform the sea

An overnight deluge poured a great swathe of peat moss from the mouth of the mountain river, smudging the sea to sepia the whole length of the strand. Looking out to a wind-blurred horizon, I found the polytunnel unperturbed but plastered quite prettily with leaves torn off the trees.

I thought fondly of Ireland’s twitchers, so keen now to crouch on clifftops, bracing their telescopes to focus on the migrant seabird parade.

Forced close to the coast in an autumn gale and skimming low above the waves could be great and sooty shearwaters among Manx and Cory’s shearwaters, not to mention the odd pomarine skua and petrels of obscure, gadfly rarity. It is all breathlessly tense and thrilling, if you have young blood to warm your toes.

Some birds fly enormous distances over thousands of kilometres of featureless sea. Shearwaters breeding on Irish islands, for example, spend winter off the coast of Argentina. They make the return trip annually over lives lasting half a century or more, but how on earth do they navigate ?

Magnetic sense helps many species, but now there seems to be another just as good for the "tube nosed" seabirds, such as shearwaters, petrels, fulmars and albatrosses. Not only on migration but also in foraging, to bring food back to chicks hatched on islands, their acute sense of smell creates an "odour map" to direct their journeys. (You can read about it at iti.ms/2cWCimE.)

The main scent they detect is dimethylsulphide (DMS), a sulphur compound released as an aerosol when algal phytoplankton, the tiny plant food of the ocean food chain, is eaten by zooplankton, the sea’s infant animal life. This in turn is the food of most fish, so many seabirds are guided by the hints of DMS to likely locations of their prey.

Phytoplankton grows best at seamounts and steep shelves where the nutrients that it needs well up from the seabed, so the deep relief of the seabed is echoed on the surface by the rise of DMS and shapes key features of an "olfactory landscape". (You can read about this at iti.ms/2d3yjCN.)

The rich plankton of the northeast Atlantic attracts the European study of DMS and other trace gases. The high volume produced at the edge of the continental shelf has engaged researchers at the atmospheric observatory of NUI Galway, perched bleakly at Mace Head in Connemara.

There are more important roles for DMS than drawing maps for seabirds. Acting locally, its recycling of sulphur from the ocean to the land helps to fertilise the grassland of Ireland. For the planet, it helps to form clouds when its aerosol particles become nuclei for droplets of water.

The whiteness, or albedo, of the clouds’ upper surface reflects the radiation of the sun, which helps to regulate the planet’s surface temperature. DMS is one of the feedback mechanisms by which, as in James Lovelock’s brilliant construct of Gaia, Earth works to maintain its envelope of life.

But plant plankton has another role, at least equally important: it takes carbon dioxide from the atmosphere to use in photosynthesis. When algae die and drift down from the surface, they take some carbon with them. In one proposition for “fixing” global warming, growing masses more algae by fertilising the ocean could reduce the urgent burden of man-made carbon dioxide.

Over great stretches of the southern polar ocean, iron is a missing nutrient for plankton growth. Some early laboratory models suggested that for every tonne of iron dust spread on the sea up to 110,000 tonnes of carbon might be removed from the air.

From the 1990s onwards, a dozen small-scale experiments were tried. One, described in Nature in 2000, did, indeed, grow a “massive bloom” of plankton, raising the output of DMS and drawing on carbon dioxide, but “downward export of biogenic carbon was not increased”.

Along with scientific interest came that of commercial entrepreneurs, hoping to eventually profit from trading in carbon credits. Planktos, a would-be enlistment of business to virtuous purpose, set sail for the south Pacific in 2007 with a substantial cargo of powdered rust but ran out of money on the way. It had met strong opposition, not least from those arguing that such geoengineering ventures distracted from the chances of international carbon-dioxide agreement.

Iron-fertilised growth of algae in one stretch of ocean could, it has been suggested, exhaust other mineral nutrients that would normally be circulated to feed algae elsewhere.

However, new research (at iti.ms/2d3yuhr) shows vast trails of iron and other nutrients falling from big icebergs, more than 18km long, calving from Antarctica and triggering month-long algal blooms up to 10 times that size. As climate change breaks off more icebergs, Gaia may choose to trade up the connection.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks