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Minerals of Climate Past

How minerals preserved evidence of drought conditions over 1,000 years ago

Paleoclimate data is hard to come by. There are few circumstances in which ancient climate conditions create a record for scientists to later find. Some of the best sources are undisturbed layers of material that were deposited regularly over time like tree rings, giving us a window into the past.

A rare opportunity lies in Laguna Minucúa in central Mexico. This half-acre, isolated pond formed in a sinkhole in the mountains of Sierra Madre del Sur, Oaxaca. Luckily for researchers, it meets the criteria to be a rich source of paleoclimatic information. It has no rivers running into or out of the lake, keeping the bottom relatively undisturbed. Layers upon layers of sediment settled neatly to the lakebed, beautifully preserving in their layers a record of past climate conditions.

A team of scientists sampled this small lake, pulling a 5.6-meter-long core out of the lakebed representing the past 4,500 years of sediment buildup. To make sense of those layers, scientists turned to minerals.

A piece of montmorillonite from the mineralogy collection at NHMLA.
A piece of montmorillonite from the mineralogy collection at NHMLA.

Minerals hold a wealth of information within their dazzling, complex structures. A product of their environment, they form under specific conditions, safely storing clues scientists can use to retrace Earth’s past.

“Minerals are a key component to understanding what’s happening on the surface of the Earth,” explained Mineralogy Associate Curator Aaron Celestian. “The hard part is finding a place where minerals get trapped and then aren’t disturbed.”

Celestian helped identify key minerals contained within the lake’s sediment core and decode the conditions that led to their deposit. Of particular interest were montmorillonite, kaolinite, and illite. The ratios of these clay minerals tell a story.

It starts with montmorillonite, a mineral formed from weathering volcanic rocks. Montmorillonite gets swept into the lake by wind or rain, where it stays suspended in the water before slowly settling to the bottom. This takes time, and the water must be still, undisturbed by stormy weather. Once montmorillonite falls to the lakebed, over long periods of time it would react with water, resulting in related clay minerals kaolinite or illite. This gradual process can be interrupted by an influx of sand and silt following a storm, which quickly settles to the bottom and covers freshly laid montmorillonite, preventing further reactions with water that would otherwise yield kaolinite or illite. In this way, the layers of these three minerals preserve records of long dry periods punctuated by rainstorms.

A: This is a close-up view of a 1-meter section of the lake core sample. Notice the visible striped layers of different minerals. B: A close-up of a boundary between two layers of sediment. C: A close-up of a layer with quarts, feldspar, and grains of charcoal.

The study published in Quaternary International found evidence of a significant drought in the area between 1,160 and 1,280 years ago. Interestingly, this time period overlaps with the collapse of the nearby Mixteca society in Monte Albán, adding some climate context to the area’s rich cultural history and the demise of a regional culture.

This Zapotec urn from the Museum’s archaeology collection is similar to those found in the region near Monte Alban. Between 1,700-2,100 years old, it represents lightning deity Cocijo, according to the glyph in the center of his headdress.

Celestian looks to other sources for mineralogical climate data as well. Part of a team sampling a glacier in Peru, he is analyzing minerals in the snow and how they vary from year to year. Because some minerals speed up glacial melt, knowing the mineral makeup and history of the ice could inform efforts to preserve and restore this environment, especially as climate change takes its toll and nearby human activities introduce new mineral pollutants that settle into the snow.

Minerals have much to tell us about our home planet, and intriguingly, the possibilities aren’t restricted to the pull of Earth’s gravity. Some of these methods could have other worldly applications, including understanding the climate history of other planets or searching for signs of ancient life on Mars.

Kaolinite is on display in the Gem and Mineral Hall.

 

 

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