Two Seismographs on One Shore: How Ice Cores Hear Different Oceans ✛
On the coast of Dronning Maud Land, two ice cores stand like two seismographs on one shore. One, FK17, was drilled on the crest of the Lokenryggen ice rise; the other, TIR18, on the crest of Hammaryggen. A hundred kilometers apart, they catch different tremors of the climate. One hears the Pacific, the other the Atlantic and Indian Oceans.
Snow that fell here over 250 years has compacted into ice and preserved the oxygen isotope ratio (δ18O), the ratio of heavy to light oxygen. This is a thermometer of the past: the warmer the air, the more heavy isotope in the snowflake. Scientists measured δ18O in the upper 120 meters of each core at 5-centimeter intervals using a PICARRO L 2130-i laser spectrometer. The result was time series from 1979 to 2010, cleaned of seasonal cycles and high-frequency noise.
Spectral analysis decomposed the signals into periodic components. FK17 showed sharp peaks at periods of 3, 4, 6, 8, and 10–12 years, while TIR18 showed broader peaks at 3, 5, 7, and 22–28 years. But matching rhythms do not yet say who controls whom. So the researchers applied causal analysis using the Liang–Kleeman method, which estimates the rate of information transfer from climate indices to δ18O, based on changes in Shannon entropy. This distinguishes cause from mere coincidence.
Causal analysis showed that FK17 is significantly influenced by El Niño, the Pacific Decadal Oscillation, the Southern Annular Mode, the Indian Ocean Dipole, and sea ice extent in the 0–20°E and 20–40°E sectors. For TIR18, the main influences are El Niño, the Pacific Decadal Oscillation, the Indian Ocean Dipole, the Atlantic Multidecadal Oscillation, and ice extent in the 20–40°E and 40–55°E sectors. The differences between the sites are linked to different air-mass sources: TIR18 receives air from more easterly and continental directions.
So two seismographs on one shore revealed that each hears its own ocean. Previously it was unclear which climate modes drive the low-frequency variations in δ18O in the coastal cores of Dronning Maud Land and why neighboring sites give different signals. Now it is clear: FK17 is affected by El Niño, the Pacific Decadal Oscillation, the Southern Annular Mode, the Indian Ocean Dipole, and sea ice in the western sectors, while TIR18 is affected by El Niño, the Pacific Decadal Oscillation, the Indian Ocean Dipole, the Atlantic Multidecadal Oscillation, and ice in the eastern sectors. The influence of the Indian Ocean Dipole has been identified for the first time. The method can be immediately applied to other proxies from these same cores and to any paleoclimate series where cause must be distinguished from correlation.
The two cores were drilled just a hundred kilometers apart, yet their isotopic spectra differ more than those of some cores separated by thousands of kilometers. The reason is that air masses reaching TIR18 come from the east and carry a more continental signal, while those reaching FK17 come from the west, over the Weddell Sea.