They had found, accidentally, that a raremetal, iridium, suddenly became very abundant exactly at the boundary and then slowly fell away.
他们偶然发现一种稀有金属铱的含量在边界处突然变得很丰富，然后又慢慢地消失了。
2
This sediment layer is enriched in the raremetal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth.