The Grand Canyon is one of the Seven Natural Wonders of the World, known for its spectacular valleys and rock layers that span millions of years of Earth's history.
But beneath the surface, there are vast cave systems that may hold clues to better understand the future of climate change by studying nature's past.
A research team led by UNLV paleoclimatologist and professor Matthew Lachniet retrieved an ancient stalagmite from the floor of an undisturbed Grand Canyon cave.
By analyzing the mineral deposits' geochemistry, they were able to reconstruct precipitation patterns during the rapidly warming period following the last Ice Age, which lasted from about 14,000 to 8,500 years ago.
How stalagmites reveal past climate changes
ROBYN BECK/AFP via Getty Images
Stalagmites are cone-shaped formations that grow upward from the cave floor as water drips from the ceiling.
The water carries dissolved minerals, such as calcium carbonate, that accumulate over time and form layers.
By drilling a core sample from a stalagmite and measuring its chemical composition, scientists can infer how much water seeped into the cave at different times in the past.
One of the key indicators of past precipitation is the ratio of oxygen isotopes in the stalagmite.
The ratio of these isotopes depends on the source and temperature of the water.
For example, water that evaporates from the ocean is enriched in oxygen-16, while water that condenses in colder regions is enriched in oxygen-18.
By measuring the oxygen isotope ratio in the stalagmite, scientists can estimate where and when the water originated.
Another indicator of past precipitation is the amount of trace metals, such as magnesium and strontium, in the stalagmite.
These metals are derived from the bedrock and soil that the water passes through before reaching the cave.
The amount of these metals depends on how much water is available to dissolve them. More water means more dilution and less metals, while less water means more concentration and more metals.
By measuring the trace metal content in the stalagmite, scientists can estimate how much water infiltrated into the cave.
What the stalagmite reveals about past monsoon rains
The researchers found that the stalagmite showed increasing levels of water seepage into the cave between 14,000 and 8,500 years ago, during a period known as the early Holocene when temperatures rose throughout the region.
This suggested that there was more rainfall during this time, especially in summer.
Using a paleoclimate model, the researchers determined that this was likely caused by intensified and expanded summer monsoon rains in the U.S. Southwest and northwestern Mexico.
The summer monsoon is a pattern of increased thunderstorms and precipitation that typically occur between June and mid-September.
It is driven by atmospheric circulation patterns that depend on the temperature difference between land and ocean.
The researchers hypothesized that during the early Holocene, higher temperatures caused faster melting of winter snowpacks and faster evaporation of surface water, which increased moisture in the atmosphere.
This moisture was then transported by winds to higher elevations on the Colorado Plateau, where it condensed and fell as rain.
This enhanced summer rainfall contributed to groundwater recharge in the region, which fed into caves and aquifers.
The findings of this study have implications for understanding how future climate change may affect summer monsoon rains and groundwater resources in the region.
The researchers noted that most of the water currently infiltrating through the bedrock and into caves and aquifers comes from winter snowmelt.
However, during the early Holocene, when peak temperatures were only slightly warmer than today, both summer and winter moisture contributed to groundwater recharge.
The researchers suggested that future warming, which could cause temperatures to rise above those of the early Holocene, may also lead to greater rates of summer rainfall on the high-elevation Colorado Plateau and an intensifying North American monsoon.
This could have positive effects on groundwater recharge and availability, but also negative effects on flash floods and erosion.
The researchers also cautioned that their findings are based on one stalagmite from one cave, and that more studies are needed to confirm their results and explore regional variations.
They also pointed out that other factors, such as human activities and land use changes, may affect groundwater recharge and quality in ways that are not captured by their study.
The Grand Canyon is not only a natural wonder, but also a natural laboratory for studying past and future climate changes.
By investigating its ancient secrets hidden in caves, scientists can gain insights into how nature responds to warming temperatures and changing precipitation patterns.
This can help us prepare for and adapt to the challenges and opportunities that climate change may bring.
