Ice Age Ocean Circulation Reacted To, Did Not Cause, Climate Change At Glacial Boundaries

Both ice sheet volume and the “global carbon budget,” the amount of carbon stored in deep ocean reservoirs compared to that on the earth’s surface, changed before ocean circulation patterns changed, according to evidence from deep sea cores taken from the South Atlantic. Thermohaline (heat and salt) ocean circulation changes were found to have occurred 1,000-3,000 years after carbon shifts in each case.

The Lamont Doherty Earth Observatory team, which included Steve Goldstein, Alexander Piotrowski (now a postdoctoral student at Cambridge University), Sidney Hemming, and Richard Fairbanks, identified a chemical marker, isotopes of the rare earth element neodymium (Nd), that they believe reveals the progression of ancient ocean circulation changes more unequivocally than did carbon isotope ratios used to study the chronology of these ancient climate changes previously.

The Nd markers provided evidence that changes in the ocean’s thermohaline circulation rate came later than both carbon budget shifts and changes in ice sheet volume at the start and end of the last ice age. In other words, the ocean’s “conveyor belt” system did not trigger the changing conditions of cold and ice on the surface of Earth but rather responded to them.

“This was an unusual use of neodymium (Nd) isotopes, which are more often applied to magnetism and continent-mantle evolution studies,” explains co-author Steve Goldstein. “Our study illustrates its great potential for the study of the history of climate change.” Goldstein is a geochemist who is Associate Professor of Earth and Environmental Sciences and Senior Researcher at LDEO.

Ice ages are driven by changes in the amount of heat that arrives at the poles from the sun. The carbon cycle shifts were likely caused by the decline of plant life on the planet’s surface because of the cold and the advancing ice sheets. The ocean’s circulation system amplifies the effect of the sun’s heat through warmth brought to high latitudes by the Gulf Stream, whose saltiness affects how fast it sinks and begins the deep water arm of the global ocean “conveyor belt” circulation back to the South.

Ocean circulation changes “amplified” the climate trends that started the advance of continental ice sheets 70,000 years ago, making it colder in the high latitudes, as well as those that caused the retreat of ice sheets that ended the most recent ice age about 15,000 years ago, making it warmer.

The deep sea core showed that ocean circulation also changed during several smaller abrupt periods of warming during the last ice age, but these showed no obvious time sequence among the proxies for ice sheet growth, carbon cycle, and ocean circulation, leaving open the possibility that ocean circulation changes could in fact have been the trigger of these warmings.

The neodymium in ocean water comes from weathering of the continental rock. The isotope ratios of dissolved neodymium “dye” the ocean water, Goldstein explains, and the Nd in the Pacific (where more molten rock is brought up from the earth’s mantle in volcanic activity) has a different signature than that in the Atlantic. The amount of North Atlantic Deep Water, as determined by Nd isotopes in a deep sea sediment core, can thus be used to trace ocean circulation. The team plans to continue to investigate the usefulness of neodymium as a proxy for ocean circulation in studying the history of climate change, using cores from other locations and depths.