Researchers Create New Weathering Index to Measure Rainfall in Ancient Ecosystems

Understanding climate and rainfall through geologic time is crucial because they relate to plant and animal evolution, ecosystem function and the hydrological cycle, including erosional and depositional events.

The paper appears in the May edition of the journal Geology.

Dr. Lee Nordt, professor of geology and dean of the College of Arts and Sciences at Baylor, and Dr. Steve Driese, professor and chair of the department of geology at Baylor, have created a unique equation that uses soil characterization data and whole-soil geochemical analyses to show that estimates of rainfall can be improved using a new geochemical “proxy,” called CALMAG. The Baylor researchers said the previously used chemical index of alteration minus potassium (CIA-K) proxy was giving less precise estimates of ancient rainfall and in many cases, under estimating the amount of rainfall. The new proxy created by the Baylor researchers works well for ancient soils buried and turned into rock known as paleosols, and specifically high clay-content paleosols known as paleo-Vertisols. Vertisols are arguably the most common fossil soil type, preserving much information about the ecosystems.

“What we discovered is by looking at the principles of these chemical compounds, that CIA-K estimated rainfall somewhat effectively, but also for the wrong reason, or not for the reasons originally purported, and for sure, not for most soil types we see in the rock record,” Nordt said. “The difference is mainly that CALMAG estimates higher rainfall amounts than CIA-K and with greater statistical reliability. This is important because we use past climate and ecosystem response relationships to predict what might happen in the future with global change.”

The paper also gives for the first time the underlying geochemical principles that explain why this and other equations relate to rainfall in the first place. Nordt said rather than documenting the hydrolysis of minerals common in many other soils, the CALMAG equation principally tracks the flux of calcium and magnesium sourced from calcium carbonate and detrital clay in the Vertisols.