As discussed in an earlier chapter, many marine animals—clams, for example—burrow downward into the sediment beneath, dragging down younger material from the surface and bringing older material back up, an effect known as bioturbation. When these disturbed sediments eventually harden into rock, the fossils that they contain, as well as any iridium and tektite layers, are stretched out over a broader range than the one in which they were deposited. Studies of bioturbation in modern sediments have shown that material can be moved up and down by many centimeters, equivalent to tens of thousands of years.
Reworking is a similar, but mechanical, process in which a layer of sediment, and the biota it contains, is deposited in the sea or in a streambed but, before it is hardened into rock, is stirred by waves or currents, and redeposited. As with bioturbation, such disturbance before sediments are consolidated mixes them up and causes some of the temporal information to be lost.
Both bioturbation and reworking raise fossils from the dead, like zombies, and redeposit them higher in the section than they deserve to be, in younger rocks than those in which the organisms actually lived. This makes it appear that the organisms lived longer than they did and thus causes a sudden extinction to appear gradual, or not to have happened at all. For example, Tertiary sedimentary rocks sometimes contain foraminifera that some specialists believe lived only in the Cretaceous; to be present in Tertiary sediments, therefore, the forams must have been reworked. Others believe that instead these forams survived the K-T event and lived on, into the Tertiary, in which case they were not killed off by an impacting meteorite and the Alvarez theory is undercut. (In Chapter 9 we see how paleontologists go about trying to solve this particular puzzle.)
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