Traverse City Record-Eagle

Opinion

July 20, 2013

Rock Doc: Stalagmites speak of climate history

Some of the interesting features of certain caves are stalactites and stalagmites, the column-like 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 are found in caves because the rainwater of a region 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 lengthwise in half. 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.

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