In Scientific Reports today, UConn researchers report a novel approach to reconstructing ancient climates using analyses of organic compounds in sediments and soils.
This method was developed by former UConn postdoctoral scientist Yvette Eley (now in the Department of Geography, Earth and Environmental Sciences at the University of Birmingham, U.K.) and assistant professor Michael Hren in the UConn Center for Integrative Geosciences. Their new approach makes use of organic compounds found in the waxy, lipid-rich cuticle of plants. These waxy surfaces are critical to plant survival, as they minimize water loss and provide protection from factors such as UV radiation.
The distribution of organic compounds in leaf waxes records information about their growing environment. For instance, when confronted with stressful conditions such as shortage of water, plants can respond by changing the distribution of organic compounds in their leaf wax to combat water loss and improve their chances of survival. Various environmental parameters can therefore result in plants with different distributions of lipids, and these profiles can reveal a lot about the climate those plants were growing in.
Once incorporated into the soil, these organic compounds can be preserved over tens to hundreds of millions of years, offering the potential to quantify changes to regional and global moisture budgets on geologic timescales. The leaf wax lipids are extracted from soils and sediments, which are complex mixtures containing, among many other components, weathered rock, minerals, and decayed plant materials that have accumulated over time.
"Looking at soil today, you're observing the integrated history of all the plant matter that went into forming that soil over the course of hundreds to thousands of years," says Hren.
In the past, various methods have been used to give a snapshot of environmental conditions at a point in time, such as analyzing stable isotopes in mammal bones and teeth, or looking at the chemistry of ice cores. However, all methods have limits to the information they can provide.
Eley and Hren investigated the relationship between leaf wax biomarker profiles and modern climate in a series of soils from North and Central America. A clear relationship began to emerge regarding leaf wax lipid distribution profiles and atmospheric moisture, suggesting that it is possible to use the distribution of leaf wax lipids to identify changes in moisture availability in the past.
This new approach represents a significant addition to the paleoclimate scientist's toolkit, as atmospheric moisture is a parameter that has been challenging to estimate over long periods of Earth history, until now.
With today's increasing CO2 levels, scientists know there is going to be climate change in the future, but it has not been clear how that may affect regional moisture patterns.
"One of the huge gaps in the past is we didn't have great quantitative records of moisture," says Hren. "We're now managing to get a really nice glimpse of the whole ecosystem and how it's responding."
As the researchers focus on the biomarker profiles of soils, they are capturing an integrated chemical signature of a whole ecosystem preserved in ancient soils and sediments.
Hren says they found that the distribution of organic compounds preserved in soils of these ecosystems seems to be strongly related not just to relative humidity, but also to the difference between how much water is in an air mass and how much the air mass can hold, or what is known as the vapor pressure deficit.
Once the researchers established this relationship using modern data, they applied the method to sediments dating back to between 16.5 and 12.4 million years from a well studied area in Spain. They were able to reconcile their lipid-based reconstruction of vapor pressure deficit with existing stable isotope and fossil data for the area, highlighting the utility of this new tool.
Says Eley, "The hope is that we'll be able to use this approach to tackle key questions about changing moisture availability over time."
Past, present, and future
Hren and Eley are now applying this method to a range of other ancient terrestrial sediments, to investigate the relationship between changes in past climate and atmospheric moisture. They hope to use insights from these studies, which reconstruct temperature and moisture availability over many millions of years of Earth's history, to advance understanding of the global changes in environmental conditions anticipated in the coming decades.
"By looking into the past, we're trying to understand the potential for future change," says Hren. "This is a powerful tool as we move forward."
The ultimate hope is that data generated by this new leaf wax biomarker proxy will improve knowledge of past climate responses to CO2, and fill in the gaps - like missing pieces of a puzzle - in spatial reconstructions of paleoclimate during past warm periods of earth history. This in turn will feed into climate predictions of the long-term future of our planet.
This work was supported by a National Science Foundation grant NSF-EAR-1338256, and the College of Liberal Arts and Sciences Dean's office*****