Ancient Antarctic ice cycles impacted ocean productivity thousands of miles away

UW-Madison study links Antarctic ice sheet growth and decay to a 40,000-year rhythm in subtropical marine productivity.

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, recently 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.

Yet the researchers noted a strong, singular influence of the 40,000-year cycle on the ancient subtropical ocean's bioproductivity, across a 1-million-year interval of time that is associated with the first expansion of the Antarctic ice sheets around 34 million years ago.

"This tells us that bioproductivity is being influenced by a distant high-latitude process, through nutrient delivery to the lower latitudes," Meyers says.

The team arrived at this conclusion by analyzing chemical signals preserved in ocean sediment that record past biological productivity. The sediments were collected during ocean drilling expeditions from 2020-2022 aboard the now-retired scientific drilling vessel JOIDES Resolution. For decades, the vessel recovered ocean sediment cores to study Earth's oceans and their geological history, funded by the US National Science Foundation and 23 collaborating countries.

"The vessel has provided archives that ground huge scientific discoveries related to global climate events, evolution of life and plate tectonics," says Alexandra Villa, who co-led the research with Meyers as a PhD student at UW-Madison, and was a shipboard scientist on the drilling expedition. Villa is now a postdoctoral researcher at MARUM in Bremen, Germany and is continuing her research using science ocean drilling archives.

The sediment cores offered researchers a chance to reconstruct if and how life in subtropical oceans changed in response to dynamics of the Antarctic ice sheet thousands of miles away. To understand how Antarctic ice sheets might affect subtropical ocean life "it's first important to think about how ocean circulation is linked to bioproductivity," says Villa. "Today, about three-quarters of all marine bioproductivity north of 30 degrees south of the equator is supported by nutrients derived from Southern Ocean circulation - this is the ocean that surrounds Antarctica," says Villa. "The nutrient-filled Southern Ocean water sinks, then makes its way to the lower latitudes, where it is mixed upward to the surface, influencing bioproductivity."

When the Antarctic ice sheet emerged about 34 million years ago, it altered circulation patterns and the movement of nutrients through the oceans.

"And when the ice sheet became large enough to extend to the Southern Ocean, the 40,000-year obliquity rhythm of the marine-based ice sheets impacted the delivery of nutrients to our subtropical site," Villa says.

The new research builds on previous UW-Madison studies that showed how strongly the 40,000-year obliquity cycle affects marine-based ice sheets.

Now, scientists are able to connect this cycle to global ocean dynamics with far-ranging effects. Indeed, the new findings highlight how tightly connected Earth's climate system is.

"The Earth System is so interconnected, and changes in one part of the planet can ripple out in surprising ways," Meyers says. "The polar ice sheets and global ocean circulation are important ways this manifests, impacting marine food webs far from the ice sheet. Our study shows how dynamic, variable and sometimes surprising, these 'global teleconnections' can be."

This research received support from the National Science Foundation (OCE-1450528), the Heising-Simons Foundation (2021-2797), the John Simon Guggenheim Memorial Foundation and UW-Madison.