Antarctica’s Ice Cycles Linked to Subtropical Ocean Productivity Millions of Years Ago

Cycles in the growth and decay of Antarctica’s ice sheets appear to have influenced marine biological productivity in subtropical regions thousands of miles away, according to new research from the University of Wisconsin–Madison. The study, published in the Proceedings of the National Academy of Sciences, reveals a surprising connection between a 40,000-year astronomical cycle – tied to Earth’s axial tilt – and ocean productivity levels dating back approximately 34 million years, when the Antarctic ice sheet began to expand. This finding challenges conventional understanding of how astronomical cycles impact climate and ocean conditions, particularly in equatorial latitudes.

Traditionally, scientists have focused on other astronomical cycles, like the Milankovitch cycles, as primary drivers of climate change. These cycles relate to variations in Earth’s orbit and are thought to have a more direct influence on equatorial regions. However, this new research highlights the significant, and previously underestimated, role of the obliquity cycle in shaping ocean dynamics and biological activity in distant subtropical zones.

Ancient Ice, Distant Waters: The Mechanism

The research team’s conclusions stem from an analysis of chemical signals preserved in sediment cores collected from the ocean floor. These cores, retrieved during ocean drilling expeditions between 2020 and 2022 aboard the now-retired scientific drilling vessel JOIDES Resolution, act as a historical record of past biological productivity. By examining the composition of these sediments, scientists can reconstruct how life in the oceans changed over millions of years.

The key lies in understanding how Antarctic ice sheet dynamics influence ocean circulation. As Alexandra Villa, a co-lead author of the study and now a postdoctoral researcher at MARUM in Bremen, Germany, explains, “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.” The Southern Ocean is a critical source of nutrients, and its circulation patterns play a vital role in distributing these nutrients to lower latitudes. When the Antarctic ice sheet expanded around 34 million years ago, it altered these circulation patterns, impacting the delivery of nutrients to subtropical regions.

Specifically, the researchers found that the 40,000-year obliquity rhythm of the marine-based ice sheets directly impacted nutrient delivery to the subtropical site they studied. This suggests that changes in the size and extent of the Antarctic ice sheet, driven by variations in Earth’s axial tilt, triggered shifts in ocean circulation and, in biological productivity.

The JOIDES Resolution: A Legacy of Discovery

The JOIDES Resolution, funded by the US National Science Foundation and 23 collaborating countries, has been instrumental in uncovering these deep-time connections. For decades, the vessel recovered ocean sediment cores, providing scientists with invaluable archives to study Earth’s oceans and geological history. As Meyers notes, “The vessel has provided archives that ground huge scientific discoveries related to global climate events, evolution of life and plate tectonics.” Oscar Cavazos, a Marine Laboratory Specialist with the IODP JRSO, was among the team preparing sediment cores for analysis during expeditions.

Implications for Understanding Earth’s Climate System

This research has significant implications for our understanding of Earth’s interconnected climate system. It demonstrates that changes in one part of the planet – the polar regions – can have far-reaching effects on ecosystems thousands of miles away. This is particularly relevant in the context of modern climate change, as the Antarctic ice sheet is once again undergoing significant changes due to rising global temperatures.

Previous research from UW–Madison has already established a strong link between the 40,000-year obliquity cycle and the behavior of marine-based ice sheets (see here and here). This new study builds upon those findings by connecting this cycle to global ocean dynamics and highlighting its impact on marine ecosystems.

Evidence and Limitations

The study’s methodology relied on analyzing chemical signals in sediment cores, specifically focusing on biomarkers that indicate past biological productivity. Although this approach provides valuable insights into ancient ocean conditions, it’s important to acknowledge its limitations. Reconstructing past environments from sediment records is inherently complex, and interpretations are based on inferences drawn from proxy data. The researchers acknowledge that other factors could have too contributed to the observed changes in subtropical ocean productivity.

the study focused on a specific time interval – around 34 million years ago – and a single subtropical location. While the findings suggest a broader connection between Antarctic ice sheet dynamics and global ocean productivity, further research is needed to determine the extent to which these patterns apply to other regions and time periods.

What Comes Next: Continued Exploration of Ocean Archives

The research team plans to continue exploring ocean sediment archives to investigate the long-term relationship between Antarctic ice sheet dynamics and global ocean conditions. They are particularly interested in examining how these connections have evolved over millions of years and how they might be affected by ongoing climate change. Villa, now at MARUM, is continuing her research using these valuable science ocean drilling archives. Future studies will likely involve more sophisticated modeling techniques to simulate ocean circulation patterns and assess the impact of different climate scenarios on marine ecosystems. The legacy of the JOIDES Resolution continues to fuel new discoveries, providing critical insights into the complex workings of our planet.