Out on a Water Lily
The great dying-out of the dinosaurs is no longer a mystery, or so many scientists would have us believe. The accepted story is that catastrophe struck Earth about 66 million years ago. Most experts believe the evidence points to collision with a giant meteorite or meteorites; a few hold out for cataclysmic volcanic upheaval. Compromisers suggest the meteorite could have triggered the eruptions, so the planet caught a double whammy.
Eruption, impact, or both probably led to a harsh and long winter. That assumption followed logically from predictive studies of nuclear war. Computer models showed that the dust, smoke, and soot from an all-out war waged with atomic weapons would bring on a great freezing. Titanic volcanic eruptions or massive impacts---much less both---would similarly generate clouds of dust and set sooty fires. The net result would be a winter to end all winters, or at least to end many forms of life.
This explanation has been challenged by an article in the British journal Nature. Author Jack A. Wolfe, of the U.S. Geological Survey, builds on evidence gathered by others, but his main argument comes from a single long-gone pond in what is now Wyoming.
The sedimentary rock marking the ancient pond's bottom held thousands of fossil leaves from two kinds of water lilies. Leaf fossils from these plants are rare elsewhere; in modern water lilies, the leaves usually decompose before they fall to the bottom, and evidently that was true back then. Then as now also, sedimentation rates are low where water lilies grow, so it's unusual for enough mud to build up over fallen leaves to preserve them.
The sedimentary stone layer in which the leaves rested was different from other layers accumulated over the pond's long life. The fossil layer was clay. It contained broken bits and pieces of water lily roots, not the whole root systems to be expected if they had grown in that clay layer. Some of the fossil leaves were folded and broken in unusual ways. The clay layer also held masses of pollen from both kinds of plants.
From these clues, Wolfe reconstructed the water lilies' last summer. He deduces that the plants were killed suddenly during the blooming season. The season was shown by the pollen masses---they had been still attached to flowers too fragile to fossilize. The suddenness was proved by the broken roots---something had yanked them forcefully from the bottom. The nature of that something showed up in the damage to the leaves; that kind of breakage and folding came from freezing. The pond had frozen, trapping the leaves in ice and ripping up the still-attached roots.
When the ice melted, it dumped accumulated clay dust particles onto the pond bottom. Those particles, Wolfe believes, were the residue of a meteorite impact. The clay layer contains grains of shocked quartz, evidence of that sort of hit. The water lilies died in an impact winter.
But---and here is where Wolfe disagrees with others---it was a very short freeze, on the order of a few weeks. New plants began to grow, then were hit by another impact winter. That too was fairly short; he thinks the fossil plant evidence shows it could have been no longer than four months at most. Some plants were growing again by the following spring.
Yet something significant had happened; many of the old kinds of plants soon vanished, including the specific water lilies Wolfe studied. Was one extra winter enough to exterminate so many kinds of plants and animals? And what about the Southern Hemisphere, where dinosaurs and other now-extinct forms lived, but where it was winter when the meteorites hit? Wolfe's detective work in the rocks doesn't solve a mystery; it reopens one.
