Cave ‘growth band’ find sparks climate history rethink

By Lee I-chia  /  Staff reporter

Fri, Oct 04, 2013 - Page 5

Growth bands within a speleothem — a cave structure formed by the deposition of minerals — do not always grow annually, according to a team of local researchers, which challenges the established hypothesis and provides new insight for climate change research.

Like the annual growth rings found in tree trunks, which can be used to track climate change by counting and analyzing the widths of the rings, speleothems were also found to have very thin bands called laminae, which were believed to be annual additions to stalagmites and other formations.

Scientists have used various dating techniques, such as radio carbon or luminescence dating, to estimate the age of speleothems and test the idea that their ages can be known by counting growth bands, but their results still showed deviation values ranging from dozens of years to hundreds of years.

In a National Science Council-funded project, a team led by National Taiwan University’s geosciences professor Shen Chaun-chou (沈川洲) and chief researcher Lin Ke (林可) said on Wednesday that it had made a breakthrough in high-resolution speleothem dating by using a single-lamina thorium-230 dating technique on a 300-year-old stalagmite from Xianren Cave (仙人洞) in China’s Yunnan Province and had successfully reduced the deviation value to a minimum of six months.

“Using a tiny sample weighing from 0.02 to 0.04g collected from the speleothem, we can achieve precise dating of [the formation of the ring] to an error of 0.5 years,” Shen said.

He said from the improved precision, they found “the bands did not always grow annually” and that the discovery of irregular formation of missing and false bands challenges the previous

hypothesis. This may have serious repercussions to the work focused on reconstructing the Earth’s climate history.

Simulations for predicting climate change are mainly designed based on analysis of the Earth’s climate history, he said, adding that therefore the climate history data made from previous not-so-precise speleothem growth band analysis may have errors and should be re-evaluated.