Oxygen isotopes and climate change

First, a fuller explanation of oxygen isotopes: “Oxygen isotopes fractionate due to very small differences in the kinetic energy of the two isotopes, so with increasing water temperatures more oxygen-16 are incorporated into the [foraminifera] calcite.

“Additionally, when the glacial ice melts (warmer climates), the highly negative oxygen isotope (enriched in oxygen-16s) water returns to the sea, lowering the oxygen isotopes value of the seawater (more O-16). So, during warmer climates, the combined effects of melting ice and warmer waters lower the oxygen isotopes composition of the seawater. The reverse happens during global cooling.”

Now back to the conundrum of using oxygen isotopes to determine past climate. Remember, two key variables control these values: ice volume and water temperature. Climate, as we learned, does not necessarily control the latter.

Pekar explains why this has led to problems in using oxygen isotopic records to determine climate, using this example:

“A point in case is the period between 26 and 16 million years ago, in which the oxygen isotope records ‘decoupled’ from both the Antarctic records and the atmospheric CO2 records. 

“The CO2 records show a long-term decrease [in temperature] between 40 and 24 million years ago, while the oxygen isotopic records indicate an abrupt warming occurred at  about 26 million years ago that continued into the early Miocene (16 million years ago).

“This was resolved by showing that significant changes in water mass distribution resulted in an increasing temperature gradient between deep waters from around Antarctica and from the other ocean basins.”

Scientists discovered this by calibrating the oxygen isotope records with global sea level estimates. “This resulted in the isotopic records being more in line with the sparse but consistent story the Antarctic records indicate, as well as [in agreement] with the CO2 estimates. 

“This also may be true for the middle Miocene (17-13 million years ago), in which there are a number of conflicting data sets in relating climate change. It should be noted that there are currently no sedimentary cores from the Ross Sea area for this period. The ANDRILL program is expected to recover these sedimentary archives of climate change for this region.”