Scientists explain why the Earth cooled after the age of the dinosaurs
Seawater chemistry could have controlled Earth's climate for millions of years
Scientists say they are closer to solving one of the key climate questions: how the Earth went from a warm "greenhouse" world after the extinction of the dinosaurs to a planet with polar ice caps. According to a new study, calcium in ocean water may have played a crucial role.
An international team led by the University of Southampton found that over the past 66 million years, the concentration of dissolved calcium in seawater has more than halved. The authors of the paper believe that such a massive rearrangement of ocean chemistry may have changed the way the ocean "deals" with carbon, and ultimately led to a long-term decline in carbon dioxide (CO₂) in the atmosphere - one of the main greenhouse gases.
According to lead author, oceanographer and Earth scientist Dr David Evans, when calcium concentrations were higher, ocean processes may have contributed to less carbon being sequestered and CO₂ being more easily returned to the atmosphere. When calcium became less abundant, conversely, the system began to more efficiently "take up" CO₂ from the air. The researchers estimate that the associated cooling could be on the order of 15-20 °C on geological time scales.
To reconstruct changes in ocean chemistry, the scientists studied microscopic fossilised remains of foraminifera - tiny marine organisms whose calcareous shells accumulate in bottom sediments and retain the "chemical signature" of ancient seawater. Using these data, the team created a detailed record of the evolution of ocean composition and found a strong link between calcium levels in the sea and the concentration of CO₂ in the atmosphere.
The researchers then used computer modelling to test a possible mechanism: when calcium is high, it changes how much carbon is 'fixed' by marine life (such as corals and plankton) and how efficiently it is buried as calcium carbonate in bottom sediments. The decline in calcium may have shifted this balance so that carbon was increasingly "locked up" in sediments and atmospheric CO₂ was falling - as if someone had "cranked up the thermostat" of the planet.
The authors also compared the calcium trend with geological processes and came to the conclusion that it coincides in time with the slowing down of spreading of the ocean floor - the process of formation of new oceanic crust. In their version, the decrease in seafloor "production" may have altered the exchange of chemical elements between the oceanic crust and seawater, which led to a gradual drop in dissolved calcium.