New
research from the University of Missouri indicates that Atlantic
Ocean temperatures during the greenhouse climate of the Late
Cretaceous Epoch were influenced by circulation in the deep ocean. These
changes in circulation patterns 70 million years ago could help scientists
understand the consequences of modern increases in greenhouse gases.
“We are
examining ocean conditions from several past greenhouse climate intervals so
that we can understand better the interactions among the atmosphere, the oceans,
the biosphere, and climate,” says Kenneth MacLeod, professor of geological
sciences in the College
of Arts and Science. “The
Late Cretaceous Epoch is a textbook example of a greenhouse climate on Earth,
and we have evidence that a northern water mass expanded southwards while the
climate was cooling. At the same time, a warm, salty water mass that had been
present throughout the greenhouse interval disappeared from the tropical Atlantic.”
The
study found that at the end of the Late Cretaceous greenhouse interval, water
sinking around Greenland was replaced by surface water flowing north from the South Atlantic. This change caused the North
Atlantic to warm while the rest of the globe cooled. The change
started about five million years before the asteroid impact that ended the
Cretaceous Period.
To
track circulation patterns, the researchers focused on “neodymium,” an element
that is taken up by fish teeth and bones when a fish dies and falls to the
ocean floor. MacLeod says the ratio of two isotopes of neodymium acts as a
natural tracking system for water masses. In the area where a water mass forms,
the water takes on a neodymium ratio like that in rocks on nearby land. As the
water moves through the ocean, that ratio changes little. Because the fish take
up the neodymium from water at the seafloor, the ratio in the fish fossils
reflects the values in the area where the water sank into the deep ocean.
Looking at changes through time and at many sites allowed the scientists to
track water mass movements.
While
high atmospheric levels of carbon dioxide caused Late Cretaceous warmth,
MacLeod notes that ocean circulation influenced how that warmth was distributed
around the globe. Further, ocean circulation patterns changed significantly as
the climate warmed and cooled.
“Understanding
the degree to which climate influences circulation and vice versa is important
today because carbon dioxide levels are rapidly approaching levels most
recently seen during ancient greenhouse times,” says MacLeod. “In just a few
decades, humans are causing changes in the composition of the atmosphere that
are as large as the changes that took millions of years to occur during
geological climate cycles.”
The
paper, “Changes in North Atlantic circulation
at the end of the Cretaceous greenhouse interval,” was published in an online edition
of Nature Geoscience.
