Carbon migration induces the formation of carbon-enriched layer in the austenitic weld metal and carbon-denuded layer in the Cr-Mo steels base metals.

With increase of heating temperature and duration of heating time, the depth and concentration of phosphorous-enriched layer in uphill diffusion increases continuously.

This sediment layer is enriched in the rare metal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth.