Fossilized sea cow teeth suggest Earth was surprisingly wet and warm about 50 million years ago, a new study says.
"They're one of the forgotten groups of marine mammals," said study co-author Mark Clementz, a biologist at the University of Wyoming. "Whales get all of the attention."
Like all mammals, the gentle giants can maintain a constant core body temperature. This makes sea cows ideal for measuring Earth's past climates, because their fossils' chemical makeup isn't distorted by varying temperatures in their surroundings.
Sea Cows as Ancient Record-Keepers
Clementz and his colleague, Jacob Sewall of Pennsylvania's Kutztown University, used sea cows as biological marine data logs to investigate changes in water chemistry and the environment of ancient Earth.
The team examined variations in oxygen isotopes found in the hard tooth coating, or enamel, of sea cows that lived between 50 million years ago and the present.
Isotopes are variations of chemical elements that all have the same number of protons but different numbers of neutrons, and thus different masses.
Over their lifetimes, marine animals absorb the two most abundant oxygen isotopes found in seawater molecules, oxygen 16 and oxygen 18, and incorporate them into their bodies.
By measuring the oxygen isotope ratio in marine fossils, scientists can estimate the isotope ratio of the water that the animals lived in and then re-create the patterns of evaporation and rainfall—or the water cycle—on Earth at that time.
Fossil Teeth Point to Rainier Past
Clementz and Sewall used a small dental drill to gather enamel scrapings from about 150 sea cow skulls.
The pair then measured the oxygen isotope ratio from each skull. They found that the difference between oxygen 16 and oxygen 18 in sea cows living at low latitudes 50 million years ago—during Earth's Eocene period—was about two times higher than expected.
Rainfall contains more oxygen 16 than oxygen 18, because water molecules containing the lighter isotope evaporate more readily. In turn, oxygen 16-rich rain that falls into coastal waters is eventually absorbed into the sea cows' bodies.
The higher than expected concentration of oxygen 16 in the sea cow enamel suggests these low latitudes were receiving a lot more precipitation in the Eocene than they do today, Clementz said.
A more active water cycle also means a hotter Earth, because warmer atmospheres can hold more water vapor, he added.
Steamy Ancient Earth Fits Previous Findings
The new findings are consistent with other studies that showed the Eocene was a much balmier place, with forests at higher latitudes, warmer tropical waters, and no major ice sheets at the poles. (See a prehistoric time line.)
Scientists have done similar re-creations of Earth's ancient water cycle using the oxygen isotope ratios in other fossils of tiny animals, according to Gabriel Bowen, a paleoclimatologist at Purdue University in West Lafayette, Indiana.
However, most animals can't regulate their body temperatures like sea cows do, so the isotope ratio of tooth enamel is influenced by the temperature of their surroundings. Other animals therefore don't provide as good of a climate record, explained Bowen, who co-authored an accompanying news article about the finding. Both the article and study were published this week in the journal Science.
"The hotter the water gets, the less discriminating the [oxygen-incorporation] reaction is," Bowen said.
Sea Cow Study May Help Predict Future Warming
As Earth heats up again due to human-caused global warming, the sea cow findings could help predict how the water cycle will change in coming decades, study co-author Clementz noted.
This study shows "that we can't really make the assumption that the conditions that we think are stable and uniform today are always going to be so," he said.
"Particularly in the future, as greenhouse gases continue to rise, there might be some surprises in store for us in how it'll play out."
Purdue's Bowen agreed that the evidence seems to suggest the Earth's water cycle will become more active with climbing temperatures.
But he cautioned that there is still "no consensus on how much more intense [the cycle may get] or where the effects will be felt the most."