used to estimate past ocean temperatures might be based on a flawed
assumption, according to new research.
true, it would mean our ancient seas were far cooler than previously
calculated, and our planet's current warming trend is even more
extraordinary than we thought.
team of scientists from some of Europe's leading research institutes
has taken a critical look at a chemical process that has served as a
proxy for determining the temperatures of oceans millions of years in
the most solid of scientific models rests on fairly well-reasoned
this case, the method for calculating temperature was based on the
thought that temperatures were preserved perfectly inside tiny marine
organisms called foraminifera.
the exact ratio of oxygen-18 to oxygen-16 in the calcite of the
organisms' exoskeletons varies with the isotope concentrations in the
environment – a factor that was determined by things like acidity
and salinity – and the water's temperature.
if we determine the differences in the oxygen isotopes in fossils, we
have a record of the temperatures as they were when they lived a
little over 100 million years ago.
tells us the temperature of the deeper parts of the ocean at the
tropics were about 15 degrees Celsius warmer than today.
it turns out things might not be quite so straightforward.
appeared to be perfectly preserved fossils are in fact not,"
says Sylvain Bernard, a mineralogist from the French National Center
for Scientific Research.
now suggests the ratio of oxygen-18 to oxygen-16 in the buried marine
life might not be quite as stable as thought.
test how the chemistry of the calcite in the foraminifera's shells
might continue to change over time, the researchers placed a sample
of the organisms in artificial sea water that contained just isotopes
then cranked the temperature to simulate the heat generated by being
buried beneath a pile of sediment and used a device called a
nanoscale secondary ion mass spectrometer (or NanoSIMS) to analyse
changes in the calcite's oxygen ratios.
enough, the equilibrium shifted, changing the ratios.
means that the paleotemperature estimates made up to now are
incorrect," says Bernard.
at face value, it implies the waters probably weren't all that much
warmer than today. The discovery also helps resolve a paradox that
has hinted at an inconsistency in the most favoured models.
the oxygen isotope method, ocean temperatures in the tropics during
the warm Cretaceous period weren't all that different to the surface
temperatures at the poles.
other models on climate and ocean currents don't gel with this
shallow gradient, hinting at a problem
addition, analysing magnesium isotopes in the foraminifera fossils
instead of oxygen suggests the sea surface temperatures at higher
latitudes were also colder than estimated.
all of the changes our planet has experienced over the past 100
million years, our oceans have remained pretty stable as far as
temperature goes. Cast in that light, today's rapid global warming
trend is even more dramatic than we'd thought.
next step for researchers is to go back to the drawing board on
existing data and see exactly what difference the changes make to
revisit the ocean's paleotemperatures now, we need to carefully
quantify this re-equilibration, which has been overlooked for too
long," says geochemist Anders Meibom from the Swiss Federal
Institute of Technology in Lausanne.
that, we have to work on other types of marine organisms so that we
clearly understand what took place in the sediment over geological
truly are living in an extraordinary period of our planet's history.