Ancient Antarctic ice cycles

 Ocean productivity Vs Antarctica’s Ancient Ice Cycles 

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. UW–Madison study links Antarctic ice sheet growth and decay to a 40,000-year rhythm in subtropical marine productivity. Research suggests that ancient shifts in Antarctica’s ice sheets may have influenced ocean life far beyond the polar regions. Scientists found that a subtle astronomical cycle tied to Earth’s tilt unexpectedly shaped biological productivity in distant subtropical waters millions of years ago. 

Ancient Antarctic ice sheet cycles affected subtropical ocean productivity by altering nutrient circulation. The 40,000-year obliquity cycle played an unexpectedly strong role, revealing deep global climate connections. Fluctuations in Antarctica’s ice sheets once influenced marine life far beyond the polar regions, shaping biological productivity in subtropical oceans thousands of miles away. This conclusion comes from new research. 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. This result surprised researchers because the 40,000-year cycle, although important at the poles, usually has a weaker effect on climate and ocean conditions closer to the equator. However, the team found a clear and dominant impact from the 40,000-year cycle on subtropical marine productivity over a span of about 1 million years, a period tied to the early growth of the Antarctic ice sheets around 34 million years ago.

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 which 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 study with Meyers as a PhD student at UW–Madison and participated in the expedition. She is now a postdoctoral researcher at MARUM in Bremen, Germany, where she continues working with ocean drilling data. Oscar Cavazos (Marine Laboratory Specialist, IODP JRSO) also joined other marine techs in preparing the core new to be sectioned on the catwalk. 

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 which showed how strongly the 40,000-year obliquity cycle affects marine-based ice sheets. The sediment cores allowed scientists to reconstruct how life in subtropical oceans responded to changes in the Antarctic ice sheet occurring thousands of miles away. To understand this connection, “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.” 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.

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 work builds on earlier UW–Madison research showing the strong influence of the 40,000-year obliquity cycle on marine-based ice sheets. Scientists can now link this cycle to broader ocean circulation patterns with effects which extend across the globe, underscoring the tight connections within Earth’s climate system in the universe.