The Rock Doc: Cave formations another record of climate change

Some of the interesting features of certain caves are stalactites and stalagmites, the columnlike features that hang down from the ceiling and are built up from the floor. Humans have known of their existence since time immemorial, but it’s only in recent years we’ve realized they have a story about climate to tell us.

As reported recently in Science Express, researchers led by a team at the Georgia Institute of Technology studied four stalagmites from Borneo. The stalagmites are made of calcite, a relatively soft mineral made of calcium, carbon and oxygen. It’s estimated that the stalagmites in Borneo grow at a rate of about three-eighths of an inch every thousand years.

Stalactites and stalagmites grow in caves because rainwater penetrates the ground, dissolving a little bit of mineral matter as it moves. When it gets to a cave, the water drips down from the ceiling and onto the floor. Just a little at a time, minerals like calcite are precipitated as the water evaporates a smidgen. The stalactites and stalagmites in Borneo grew over the course of 100,000 years.

What’s especially interesting about the Borneo stalagmites is that the calcite in them has variations in the mineral’s oxygen atoms. Different oxygen atoms on Earth have different atomic weights. The differences in the oxygen weights in the stalagmites reflect different oxygen atoms in the groundwater of an area. That, in turn, is a function of variations in oxygen weights of rainwater that fell in Borneo over millennia.

The researchers investigating the stalagmites cut them in half lengthwise. They took small samples at the center of the stalagmites less than an eighth of an inch apart. The samples show two things: The rainwater of Borneo was influenced by the rapid climate change patterns known from the North Atlantic and called Heinrich events. But the Borneo samples don’t show evidence of being influenced by a different set of northern rapid climate change events, those called the Dansgaard-Oeschger oscillations.

We know a lot about natural climate change from studying ice cores from Greenland and Antarctica, but we’ve had much less clear evidence of climate variations from low-latitude regions. That’s why the Borneo stalagmites are important.

Other turns in history show up in the cave samples. They reflect changes when the mega-volcano in Indonesia at Lake Toba erupted around 75,000 years ago. Those were dark days throughout the region, and weather was affected, as shown by the stalagmite record.

The take-away lesson of the stalagmite record in the tropics is similar to what we see in the ice core records at the poles of the planet. Change is woven into the warp and weft of Earth processes, and climate is always changing, both in terms of precipitation and temperature. In short we are riding a bucking bronco and we’ve recently added to the risks we face by perturbing the greenhouse gas concentrations in the atmosphere.

Hold on to your hats!