Knowledge of climate variability in deep time often rests on measurements of the heavy oxygen isotope 18O in the calcite shells of benthic foraminifera that accumulated on the seafloor over hundreds of millions of years. The resulting benthic delta-18O record combines signals from deep-ocean temperature and continental ice volume and has been central to reconstructing both the glacial cycles of the past 2.6 million years and the largely ice-free warmth of the early Cenozoic between about 65 and 34 million years ago.
In the mid Oligocene, around 28 million years ago, large, regular oscillations in benthic oxygen isotopes with a 110,000-year rhythm were interpreted as evidence for strong glacial cycles and pronounced waxing and waning of the Antarctic ice sheet. Those reconstructions implied that Antarctic ice volume repeatedly varied to as much as roughly 90 percent of its modern extent in response to orbital forcing.
A team led by researchers from the University of Bergen now reports that these large isotope swings are better explained by abyssal temperature variability than by major changes in Antarctic ice volume. Lead author Dr Flavia Boscolo-Galazzo notes that temperatures in the very deep ocean have traditionally been regarded as relatively stable on multimillennial timescales because waters thousands of meters below the surface are partly isolated from climatic drivers acting at the ocean-atmosphere interface.
Using clumped-isotope palaeothermometry, the scientists reconstructed temperatures from benthic foraminiferal calcite at about 4,000 meters depth in the Southern Ocean. The measurements show temperature fluctuations of up to 4 degrees Celsius that occur in step with the benthic delta-18O variations and with changes in Earth's orbital eccentricity, indicating a climatic forcing of abyssal conditions.
"This is an important finding as it shows that, even at such depths, ocean temperature can change significantly in response to climate variability. For this reason, oxygen isotopes from the deep ocean can no longer be interpreted as an indicator of changes in ice volume without independent temperature reconstructions," explains Dr Flavia Boscolo-Galazzo. The new temperature reconstructions for the abyssal Southern Ocean, together with computer models, suggest that the ice volume of the Antarctic ice sheet was relatively stable during the Oligocene.
Geological evidence indicates that during the Oligocene the Antarctic continent likely stood higher above sea level than today and that the ice sheet probably did not extend into the surrounding ocean. The researchers propose that this configuration may have reduced direct contact between the ice sheet and warmer ocean waters, limiting the response of Antarctic ice volume to deep-ocean temperature swings.
For their analyses, the team used material from deep-sea drill cores obtained through international ocean drilling programs such as the International Ocean Drilling Programme and its predecessors, with cores archived in the Bremen Core Collection. Fossil benthic foraminiferal shells were extracted from the sediments and their chemical composition was measured at the FARLAB clumped-isotope facility at the University of Bergen.
The authors conclude that their findings improve understanding of how the climate system operates under conditions warmer than the present, including the links between orbital forcing, deep-ocean temperature, and continental ice sheets. The project received funding from the European Research Council and the Norwegian Research Council and involved scientists from institutions in Germany, Norway, the United Kingdom, Italy, Spain, and Poland.
Research Report:Oligocene deep ocean oxygen isotope variations primarily driven by temperature.
Related Links
MARUM - Center for Marine Environmental Sciences, University of Bremen
Beyond the Ice Age
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
