"We see here how dynamic the climate-ocean system is and that the response to change is not always what we would expect," said Adina Paytan of the University of California, Santa Cruz. "We need to keep this in mind when considering future climate and other anthropogenic changes, like ocean acidification, and their impact on ocean ecosystems and resources."
The researchers believe that the changing climate and expansion of ice sheets between 13 and 8 million years ago caused a drop in sea level and changes in the amount and type of weathering occurring on land, which in turn had an impact on the calcium cycle. Weathering can occur when carbon dioxide and rain in the atmosphere combine to form carbonic acid, which reacts with calcium carbonate rocks to form calcium bicarbonate. In the ocean, on the other hand, sedimentation of the mineral calcium carbonate is a massive carbon sink.
Today the amount of calcium in the ocean depends on the balance between input from rivers and hydrothermal sources and removal by sedimentation and alteration of oceanic crust. The researchers believe that at the time of the changes in calcium isotope ratios, the amount of calcium ions in the ocean may have been up to twice the current value.
"We decided to reconstruct the seawater calcium isotope record because it gives an integrated view of the links between processes such as weathering and ocean circulation and the carbon cycle and climate," explained Paytan. "Specifically, we chose to look at calcium isotope ratios in the mineral barite because it is well preserved in layers of deep sea sediments which can be dated precisely to reconstruct changes in ocean chemistry over long periods of time," added Griffith.
Now the team, including colleagues from the Carnegie Institution of Washington, US Geological Survey, Yale University and Wesleyan University, plans to look at the calcium cycle during two other periods of abrupt climate change when the carbon cycle was perturbed. They'll study the Eocene-Oligocene Transition around 34 million years ago when ice sheets expanded and the Paleocene-Eocene Thermal Maximum around 55 million years ago when a rapid rise in temperature was caused by a massive release of carbon.
The researchers reported their work in Science.