Interpreting a climate record from 10,000-year-old migrating waters

Researchers confirm that waters migrating from the surface can take many tens of thousands of years to reach the water table

A team of researchers at Lawrence Livermore National Laboratory in California has confirmed that in drier regions, waters migrating from the surface can take many tens of thousands of years to reach the water table. Since such waters began their underground migration at the time of the last ice age, they hold a scientific and historical record of global climate change. Their results were reported in the inaugural issue of the electronic publication, Vadose Zone Journal www.vadosezonejournal.org, published by the Soil Science Society of America.

These scientists applied one of the largest super computers to study the effect of climate on water movement through this section underneath the surface, called the vadose zone. They used computer simulations to consider how this water chemically interacts with the rocks it migrates through. Because the type and abundance of minerals varies with changes in rock type, they can determine the chemical composition and how fast it changes at any point along the flow path of the water.

In arid environments, the water table is hundreds of meters deep. Although it has been assumed it would take many years for water to migrate to these deep water tables, it has only been recently that evidence has supported this assumption. Studies of the concentrations of conservative tracers and isotopes extracted from waters in the vadose zone have suggested some of these migrating waters may be 10,000 to 100,000 years old. Glassley and his team of researchers, thanks to the super computer’s simulations, now have the proof that these assumptions are valid.

“We were also interested in how big an effect changes in surface temperature and amount of rainfall would have on the water chemistry,” noted William Glassley, leader of the team. To their surprise, these climate changes had a measurable impact on the water chemistry, even after thousands of years and after migrating hundreds of meters through the vadose zone.

“What this implies is in principle, one could use a combination of water temperature, water chemistry, abundance of water, and isotopic signatures to reconstruct past climate conditions on a regional scale on most continents. This is one of the things needed to test and verify global climate change models,” said Glassley.

To interpret such a climate record, however requires conducting highly detailed computer simulations of a large amount of data describing the properties of the rocks on a scale not usually measured. There are also a few properties, such as how much surface area of a mineral the migrating water would travel through, that still cannot be established. Nevertheless, the results of these simulations indicate it may not be too long before records of climate change can be constructed for the last 100,000 years.

Vadose Zone Journal, www.vadosezonejournal.org, is an electronic, peer-reviewed, international publication launched in August 2002 by the Soil Science Society of America (SSSA), with the Geological Society of America as cooperator. The research and assessment needs of the vadose zone have grown in response to the pressure of increasing human impacts, prompting this new publication for a diverse range of scientists and engineers. The mission of the Vadose Zone Journal is to disseminate information about the physical, chemical and biological processes operating in this zone and to facilitate science-based decision making and sustainable management of the vadose zone.

As a startup, Vadose Zone Journal will be quarterly, with two issues during the 2002 volume. The public and scientific community have free trial access to the Journal through the end of this year. Visit www.vadosezonejournal.org to view the first issue, subscribe, or to view submission guidelines.