Cycles in the growth and decay of Antarctica’s ice sheets once shaped marine biological productivity thousands of miles away in the subtropical ocean, according to new research led by scientists at the University of Wisconsin-Madison. The study, published in the Proceedings of the National Academy of Sciences, found that the obliquity cycle—a 40,000-year astronomical cycle tied to changes in Earth’s axial tilt—influenced ocean productivity in subtropical latitudes about 34 million years ago, when the Antarctic ice sheet was first expanding.

The finding surprised researchers because the 40,000-year cycle, while an important factor in the conditions at Earth’s poles, typically has a more limited influence on climate and ocean conditions near the equator.

“We generally expect other astronomical cycles to have a greater influence,” says Stephen Meyers, a professor of geoscience at UW-Madison and one of the study’s lead authors.