Antarctica's Ancient Ice Sheets Shaped Subtropical Oceans Thousands of Miles Away
New research, led by scientists at the University of Wisconsin–Madison, indicates a surprising ancient link: cycles in the growth and decay of Antarctica's ice sheets once influenced marine biological productivity in subtropical oceans thousands of miles away.
Published in the Proceedings of the National Academy of Sciences, the study reveals that the obliquity cycle—a 40,000-year astronomical cycle tied to Earth's axial tilt—affected ocean productivity in subtropical latitudes approximately 34 million years ago. This period coincides precisely with the initial expansion of the Antarctic ice sheet.
The Unexpected 40,000-Year Cycle Influence
Researchers expressed surprise at this discovery. The 40,000-year obliquity cycle typically exerts a more limited influence on climate and ocean conditions near the equator, despite its significant importance at the poles. Stephen Meyers, a professor of geoscience at UW–Madison and a lead author, noted that other astronomical cycles are generally expected to have a greater impact in these regions.
"A significant influence of the 40,000-year cycle on the ancient subtropical ocean's bioproductivity was observed across a 1-million-year period associated with the Antarctic ice sheet's first expansion."
This finding suggests that bioproductivity in lower latitudes was affected by a distant high-latitude process via nutrient delivery, according to Meyers.
Unearthing Past Productivity
The team's conclusions are based on detailed chemical signals preserved in ocean sediment, which serve as a historical record of past biological productivity. These crucial sediments were collected during ocean drilling expeditions between 2020 and 2022 aboard the JOIDES Resolution scientific drilling vessel. Funded by the US National Science Foundation and 23 collaborating countries, this vessel has been instrumental in numerous significant scientific discoveries.
Ocean Circulation: The Global Conveyor
Alexandra Villa, who co-led the research with Meyers and served as a shipboard scientist, elaborated on the critical role of ocean circulation in driving bioproductivity.
"Today, about three-quarters of marine bioproductivity north of 30 degrees south of the equator relies on nutrients from Southern Ocean circulation."
She explained that this nutrient-rich water sinks, travels to lower latitudes, and then mixes upward to the surface, profoundly impacting bioproductivity in those regions.
The emergence of the Antarctic ice sheet approximately 34 million years ago profoundly altered these global ocean circulation patterns and nutrient movement. As the ice sheet expanded sufficiently to reach the Southern Ocean, its distinct 40,000-year obliquity rhythm directly influenced the delivery of vital nutrients to the subtropical site, Villa noted.
Underscoring Earth's Interconnectedness
This new research builds upon prior UW–Madison studies that had already demonstrated the strong effect of the 40,000-year obliquity cycle on marine-based ice sheets. Now, scientists can directly link this polar cycle to global ocean dynamics, revealing widespread effects and underscoring the deeply interconnected nature of Earth's climate system.
Meyers emphasized that the Earth system is highly interconnected, where changes in one region can trigger surprising ripple effects across the globe.
"Polar ice sheets and global ocean circulation are key mechanisms for this, affecting marine food webs far from the ice sheets."
The study vividly demonstrates the dynamic, variable, and sometimes unexpected nature of these crucial 'global teleconnections'.