The sight of an octopus caught by a neighbour and put in a tub is one of Mark Dion's earliest memories. "I just thought it was so cool," says the artist raised on Cape Cod and his weird, yet worldly work hinges on framing the things that lurk "naturally" in the environment. Now 38, he is the world's leading exponent of found object compositions.
For his latest work, the Tate Thames Dig, he and his volunteer beachcombing team have grubbed through the river's slime and ooze at low tide for things to exhibit in one of his trademark curiosity cabinets. The two dig sites are Millbank, opposite the Tate Gallery which stands on the site of a huge 19th-century prison from which convicts were transported to Australia, and Bankside, which housed theatres, brothels and bear and bull-baiting pits in Shakespeare's time, and where the Tate's extension is nearly finished.
When I visited the dig the mud was like quicksand. Thick drops of rain fell and it was a sweat hauling buckets across the bridge to the Tate's storage hut. We could have talked in the gallery restaurant but Dion chose to sit on a plank outside the hut.
In his boiler suit, he looks like a cross between Woody Allen and Bill Gates. So far he has found an array of things including a plastic fridge magnet, animal claws and an undeveloped roll of film. He explains the inspiration for the dig. "It was the feeling that the river isn't really integrated in London. At certain points - I've seen it in New Bedford, the city I come from, and I've seen it here in cities like Liverpool - the city turns its back on the waterfront and moves in another direction and that's always seemed to me tragic.
"But the Metropolitan Museum in New York is integrated into its site in Central Park successfully and at the Tate at Bankside there's an opportunity for integrating the river. The Thames is a natural system which the city is entirely in debt to."
He values flotsam and jetsam as "links to concepts, philosophy made concrete. They guide you through the past, through ideas, through imagining other systems of value. Even the smallest item is a keyhole to another time". Dion intends grouping the finds of the Thames Dig visually - in terms of size and shape, for example - rather than chronologically.
He rejects a linear model of history. "I want to show the continuity of history instead of seeing our moment in time and history leading up to us. This is just one stop on a bus that's gonna continue far beyond us and I'm tracing these artifacts which are a shadow of how the city's grown and developed.
The river of course physically mixes history so Dion's approach is apt. His eye has been sharpened by decades of beachcombing. He can hardly walk down a street without discovering something and he rarely stops hunting and travelling. In the back of his mind, he is already plotting a future exhibition at the Carnegie Museum of Art in Pittsburgh. For his best-known work, A Yard of Jungle (1992), displayed in the Rio Museum of Modern Art, he went to the Amazon. And he made erhaps his most remarkable find, a pair of Renaissance shoes, turned up during a dig at the 1997 Venice Biennale.
His jetsetting seems glamorous but Dion shrugs. He obviously misses his wife, also an artist back home in their "Unabomber-style" Pennsylvania shack. He finds it hard to pin down the values which guide and …