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Geography, travels provide clues to history

Yucatan's sink holes and caves conjure up images of ice age glaciers, celestial collisions, and a bad day for dinosaurs

I adjusted my snorkel and pushed off into the crystal clear pool at the entrance to the Grande Cenote. The cool water is a relief from the oppressive heat of the Yucatan jungle and I float on my back for a while, looking up at stalactites hanging from the roof of the cave. When my eyes have adjusted to the dim light I can look down through my mask at stalagmites projecting up from the bottom, 10 feet below me.

It makes no sense. Stalagmites are formed by droplets of water dripping off the roof, evaporating where they fall, and leaving behind calcium deposits that gradually build up into spikes resembling upside-down icicles. But the water in the Grande Cenote is part of an underground river system only a few feet above sea level, and stalagmites don't form under water.

Flashback:

On a hike into Singing Pass I pause for a snack before starting my climb up Piccolo. A large black boulder provides a back rest and a patch of shade. Unlike the angular chunks of green andesite and slabs of shale that have crumbled off Whistler and Piccolo the boulder is a well rounded chunk of amphibolite weighing several tons. Like the stalagmites in the Grande Cenote it is out of context. There is nothing like it on any of the nearby mountains. It doesn't belong here.

Flashback – Way Back:

About 65 million years ago a giant asteroid slammed into the earth. A cloud of dust and debris spread far and wide, blocking out the sun around the globe, dousing the plants with acid rain, and making life so unbearable for the dinosaurs that most of them packed it in, just rolled over and left the place to a few hardy warm-blooded mammals who managed to shiver through the crisis.

As unlikely as it seems there actually is a connection of sorts between these three random snippets of geo-cosmic trivia. But first, back to the Grande Cenote. It's one of hundreds of cenotes – steep-sided sink holes – that are scattered across the Yucatan Peninsula of Mexico. A flatter, less inviting bit of real-estate is hard to imagine. Made entirely of porous limestone and covered with scrub jungle, the Yucatan has no surface drainage – no rivers, no streams, not even a trickle. But the cenotes, which extend below the water-table, are filled with clear, fresh water. Rain that falls on the peninsula seeps into the porous limestone and flows out to sea through a system of subterranean fissures and caves.

The Maya, who arguably developed the most complex and advanced urban civilization of their time, used the cenotes as a source of water for their cities. They also revered them as openings to an underworld inhabited by vindictive Gods and a multitude of fearsome supernatural beings. The more sacred of these cenotes were used exclusively for religious rituals – places where offerings of jewelry, pottery and, on special occasions, young children were tossed in to placate the Gods.

Recent exploration by scuba divers has yielded a plethora of trinkets, bones, and human skulls from several sacred cenotes. And while no irate Gods or other ephemeral beings were encountered, the divers discovered an underworld almost as exotic as that imagined by Mayan priests a thousand years earlier.

Using only mask and snorkel, and constrained by an aversion to tight spaces, my own look into this vast underground labyrinth was little more than a tentative peek. But those less inclined to claustrophobia have, with the aid of scuba gear, charted many kilometres of submerged caves with all the stalactitic and stalagmitic adornments that lure spelunkers into conventional caverns. The flooded passageways were clearly formed when the caves were dry. In fact there is evidence that some were visited, and possibly even occupied, by pre-Mayan humans. Yet most of them are now underground rivers that discharge their fresh water into the sea through openings below the present surface of the ocean.

For a clue to this seeming contradiction we need look no farther than the big boulder of amphibolite perched enigmatically high up on the slope of Whistler Mountain.

After finishing my snack and continuing on toward Flute I passed dozens of other exotic boulders that had been carried in from somewhere else – erratics that had hitched a ride on the back of a long departed glacier and been left stranded when the ice melted. The melting began about 10,000 years ago when things were starting to warm up after several millennia of unrelenting cold. During the height of that long Pleistocene chill much of the northern hemisphere was encased in ice – ice so thick that it tied up enough of the earth's total water reserves to lower sea level an average of 100 metres. So while Whistler and its environs were locked in the grip of a global ice age the balmy Yucatan stood high above the surface of the depleted ocean. That's when its labyrinth of solution caverns and cenotes formed, and it was not until the ice melted and returned its water to the ocean that the Yucatan caves were flooded.

So why, you might ask, did the ancient Yucatan drainage form a scattering of cenotes and a labyrinth of solution caverns rather than a conventional system of canyons and gullies to carry its runoff to the sea? The answer lies in the extreme porosity of the Tertiary limestone that forms the Yucatan platform. Rather than running off on the surface, rainwater seeped into cracks and crevices and, over the centuries, these passageways were enlarged by solution to become the present day caverns and sink holes. Which brings us to the asteroid connection.

Long before a crater was discovered scientists speculated that the great Cretaceous/Tertiary (K/T) extinction was the result of an asteroid or comet impact with the earth. Fossils recorded not only the sudden demise of the dinosaurs but the disappearance of 70 per cent of all life-forms on earth some 65 million years ago. And precisely at the K/T boundary there is a world-wide layer of iridium-bearing clay or dust. Iridium, a rare element on earth, is fairly abundant in some meteorites. So the theory went that the celestial body that caused the K/T apocalypse was itself vaporized on impact and its iridium-rich remains scattered across the earth. But the "smoking gun," the impact crater needed to clinch the case, remained elusive for decades.

Ironically it was the view from space, an asteroid’s view, that led to the discovery of Chicxulub Impact Crater. In 1990 satellite images showing a string of cenotes draped in a perfect semicircle across the northern end of the Yucatan Peninsula were noticed by NASA researchers working on Mayan archaeology. Intrigued and curious, they assembled the existing gravity and magnetic data and went on to show that the string of cenotes corresponds to the southern rim of a giant crater, 180 km across and almost a kilometre deep. Subsequent drilling revealed fused rock and shock structures consistent with an impact origin and Chicxulub in now generally accepted as ground-zero in the K/T disaster.

During the 65 million years since it was formed the Chicxulub impact crater has been gradually filled in, covered over, and hidden beneath limestone deposits that now form the Yucatan platform. But, like the cracks surrounding the pothole in my driveway where the asphalt covers a soft spot, the limestone over the rim of Chicxulub crater cracked and fissured. These provided the initial openings that grew into the ring of cenotes and caves that first identified the huge buried crater where, 65 million years ago, an errant asteroid changed forever the course of life on earth.

It seems a long stretch from glacial erratics on Whistler to cenotes and impact craters in the Yucatan, but the last time I hiked into Singing Pass I found myself looking for oddball boulders and thinking about the connections. In fact the more I travel the more I realize how small and vulnerable our planet is.

Virgil Sharpton of the Lunar and Planetary Institute sees the global environmental collapse caused by the Chicxulub impact as a test of the earth's response to changes in the atmosphere. He writes "...in addition to the dust and ash that could interfere with sunlight reaching Earth's surface, the Chicxulub impact event probably changed the global atmospheric composition in ways not unlike those that result from modern human activities."

A sobering thought.