Researchers use minerals from ancient soils to reconstruct past climate

UNIVERSITY PARK, Pa. — When the  Paleocene ended and the Eocene began nearly 56 million years ago, Earth’s atmospheric carbon dioxide levels ranged between 1,400 and 4,000 parts per million (ppm). These carbon dioxide levels gave rise to sauna-like conditions across the planet, which scientists can now measure using tiny minerals called siderites.

“I’ve been obsessed with siderites since I started my postdoctoral studies more than 20 years ago,” said Timothy White, research professor in Penn State’s Earth and Environmental Systems Institute. “The minerals only form in wetland soils under the right conditions — it has to be totally saturated, and the soils cannot be frozen. Siderites are not well described in geologic literature, but I noticed that when they do appear, it’s at very discreet intervals and in really warm greenhouse episodes.”

White and researchers from ETH Zurich and CASP in Cambridge, U.K., used siderite minerals to reconstruct the climate at the Paleocene-Eocene boundary. The siderites, provided by White, came from 13 different sites in the northern hemisphere and cover a broad range of geographical latitudes from the tropics to the Arctic.

The researchers found that the mean annual air temperature at the equator where Colombia lies today was around 106 Fahrenheit. Farther north in Arctic Siberia, the average summer temperature was 73 F. They also discovered that the tropics and higher latitudes would have had very high atmospheric humidity levels. They report their findings in a recent issue of Nature Geoscience.

“This is the first tropical continental temperature estimate on land, and it agrees with the only other dataset out there that shows the tropical paleoclimate temperature of the ocean surface at 38 Celsius (104 F),” said Joep van Dijk, who completed the research for his doctoral dissertation at ETH Zurich. Van Dijk was co-advised by Stefano Bernasconi, ETH Zurich, and White.

The siderite that the scientists studied formed in oxygen-free soil environments that developed under dense vegetation in swamps, which were abundant along the hot and humid coastlines in the Paleocene and Eocene. The mineral is composed of iron, carbon and oxygen atoms. As siderite crystals grow, the temperature of the soil influences which carbon and oxygen isotopes become embedded in the crystal lattice.

The researchers used a new method called clumped isotope thermometry to measure the temperatures and intensity of precipitation at the time the crystals formed. The method enabled them to determine the concentration of carbonate molecules in the siderite, which contain the heavy oxygen-18 and carbon-13 isotopes. Warmer soils mean that fewer molecules in the crystals contain both rare isotopes. Based on this concentration, the researchers were able to determine the temperature of the soil and, in turn, the mean air temperature.

The oxygen isotopes, which penetrated the soil through rain, also hold information about moisture in the atmosphere. When water vapor condenses to rain, it removes the heavier oxygen-18 isotopes from the atmosphere. Because most evaporation on Earth takes place in the tropics and the moisture is then transported toward the poles, the isotopic signature in precipitation is distributed today in a predicable manner. Precipitation at high latitudes contains less oxygen-18 than in the tropics. Scientists can use the difference in oxygen-18 content between the equator and the pole as an indirect measure of the amount of moisture removed from the atmosphere.

“Our reconstruction of the climate based on the siderite samples shows that a hot atmosphere also comes with high levels of moisture,” said van Dijk. “We’ve never had experimental evidence of this in the geologic record prior to this study.”

The global moisture content in the atmosphere was much higher in the Paleocene and Eocene epochs than it is today. Water vapor also remained in the air longer because specific humidity increased at a greater rate than evaporation and precipitation. However, the increase in specific humidity was not the same everywhere. The researchers attributed this phenomenon to water vapor that was transported to these zones from the subtropics, where specific humidity rose the least. While evaporation increased, precipitation decreased. This resulted in a higher level of atmospheric water vapor, which ultimately reached the poles and the equator, carrying heat along with it.

These new findings suggest that today’s ongoing global warming goes hand in hand with increased transport of moisture and, by extension, heat in the atmosphere.

Atmospheric carbon dioxide levels today measure 412 ppm, up from the pre-industrial level of 280 ppm. If humans continue burning fossil fuels and emitting carbon dioxide into the atmosphere, scientists believe this number could reach 1,000 ppm by the year 2100.

Although the carbon dioxide content in the atmosphere was much higher 56 million years ago than it is today, increases in carbon dioxide occurred over millions of years, said van Dijk. Today the Earth system may respond a little differently to a quick rise in carbon dioxide levels, but the consequences in terms of temperature and humidity will be the same. This quick rise will pose challenges to ecosystems where we find food and medicine, and places like the subtropics will become uninhabitable, especially during summer heat waves.

“The Paleocene-Eocene boundary is what most paleoclimatologists think is the best analogue for where Earth is headed with its current carbon dioxide emissions trajectory,” said White. “I mean, literally an Earth with no polar ice caps, sea levels greatly elevated and crocodiles up in northern Greenland. It’s a very different world than where we live today.”

Other researchers included Stefano Bernasconi, ETH Zurich; Alvaro Fernandez, University of Bergen, Norway; Jeremy Caves Rugenstein, Colorado State University; and Simon Passey, CASP, Cambridge, U.K.

ETH Zurich and the Swiss National Science Foundation funded this study.