The collapse of the World Trade Center has left a chemical record on the bottom of the Hudson River.

It is possible that the new layer of sediment will become a permanent reminder of the tragedy for generations of geologists to come.

The terrorist attack on September 11, 2001, launched thousands of tonnes of ash and dust into the atmosphere.

Much of this settled on the neighbouring estuary of the Hudson, where scientists from the University of Massachusetts have discovered it has formed a distinct marker on the river bed.

The fires that burned at Ground Zero for weeks after the attack, coupled with pollution in water running off the site, also contributed to the layer of sediment.

Shifting sediment

Analysis linked it to the twin towers because it contained high levels of calcium, strontium and sulphur, which are key ingredients of gypsum, a commonly used building material.

Larry Clark, from the National Science Foundation's division of ocean sciences, said: "The destruction of the World Trade Center and the resulting deposition of dust and ash into the Hudson River have provided scientists with a definitive chemical signal."

There is no guarantee, however, that the Trade Center layer will remain so distinct forever.

The natural ebb and flow of currents in the Hudson estuary may redistribute the debris in years to come.

However, the scientists hope that such a strong chemical footprint will allow far more detailed study of what happens to deposits on the bottom of an estuary.

Then they can apply this knowledge to sediments discovered in other estuaries and on the ocean floor elsewhere.

Lead researcher Curtis Olsen, from the University of Massachusetts, said: "One of the tools available for finding out the fate of fine particles and contaminants released into estuarine systems is the measurement of geochemical 'tracers' that have known sources and histories of input into the system, such as those from the World Trade Center."