An international team of researchers has uncovered a striking chemical pattern preserved in ancient seafloor sediments: during major ice ages, the oceans near Earth’s poles appear to have remained substantially richer in oxygen than tropical waters. The conclusion comes from an analysis of more than 27,000 geological samples collected across six continents and spanning over two billion years of Earth history. The study suggests that glacial climates did not produce globally uniform oceans. Instead, they may have created sharply divided marine environments in which oxygen availability varied dramatically with latitude, reshaping the chemistry of the sea and influencing where life could thrive.
The research, led by first author Xubin Wang, a postdoctoral researcher at Syracuse University, examined manganese concentrations in marine sediments deposited during six major glaciations. The record begins with the Huronian Ice Age approximately 2.4 billion years ago and extends to the Late Paleozoic Ice Age, which ended about 260 million years ago. By comparing samples from ancient high-latitude and tropical environments, the scientists identified a repeated pattern: manganese was consistently more abundant in sediments formed closer to the poles. That pattern appeared across widely separated geological periods, suggesting that it reflects a fundamental response of the ocean system to extreme global cooling rather than a local geological anomaly.
Manganese is particularly valuable to geochemists because it behaves differently depending on the amount of oxygen present in seawater. Under oxygen-poor conditions, manganese can remain dissolved and mobile in the ocean. When oxygen becomes available, dissolved manganese is oxidized and transformed into solid manganese oxides, which settle to the seafloor and become incorporated into marine sediments. These deposits can survive for hundreds of millions or even billions of years, preserving a chemical signal of the conditions under which they formed. High manganese concentrations therefore indicate that oxygen was present at the sediment-water interface or in the overlying water column at the time of deposition.
The researchers’ central innovation was to investigate manganese geographically rather than treating ancient sediment samples as isolated points. They analyzed the distribution of manganese from the poles toward the equator and compared glacial intervals with non-glacial background periods. During ordinary climatic conditions, the latitudinal pattern was nearly flat, meaning manganese levels did not differ dramatically between high and low latitudes. During ice ages, however, the gradient became steep. Polar sediments contained significantly more manganese, while sediments deposited in tropical settings showed lower concentrations. According to the researchers, this contrast indicates that oxygen-rich conditions persisted in high-latitude oceans even as tropical upper waters became comparatively depleted in oxygen.
The result was independently supported by other trace elements with contrasting chemical behavior. Molybdenum and uranium, for example, are often more readily removed from seawater under oxygen-poor or reducing conditions. Their concentrations displayed inverse geographic patterns relative to manganese, strengthening the interpretation that oxygen availability was driving the differences. The team also used numerical simulations developed with the cGENIE Earth system model by Alexandre Pohl of Université de Bourgogne in France. The model reproduced the broad chemical pattern recorded in the rocks, indicating that oxygen-poor upper oceans in the tropics could have generated the observed distribution of manganese and related elements.
The most dramatic gradients occurred during the Proterozoic Snowball Earth episodes, when ice may have covered nearly the entire planet. At that time, atmospheric oxygen levels were far below modern concentrations, yet the sediments still reveal a strong contrast between polar and tropical environments. The finding suggests that even under extreme ice-covered conditions, ocean circulation and gas exchange could have maintained relatively oxygenated waters at high latitudes while oxygen was consumed more rapidly in tropical regions. The tropics may have been especially vulnerable to oxygen loss because biological respiration and the decomposition of organic matter could have consumed available oxygen faster than circulation replaced it.
The manganese record also documents a long-term change in Earth’s atmosphere and oceans. During the Paleozoic, atmospheric oxygen gradually moved closer to modern levels, and the manganese gradients became weaker, although they remained detectable during major glaciations. This progressive flattening is consistent with a planet in which oxygen became more abundant and more evenly distributed through the ocean-atmosphere system. The sediments therefore provide more than a record of individual ice ages. They also trace the evolving relationship between atmospheric oxygen, ocean circulation and the habitability of marine environments over geological time.
The findings challenge overly simple assumptions about the connection between climate and ocean oxygen. It is often argued that a warmer planet will experience expanding marine dead zones because warm water holds less dissolved oxygen and because increased stratification can restrict the delivery of oxygen to the deep ocean. That mechanism is important, but the geological record shows that cooling can also be associated with widespread deoxygenation under particular circumstances. Changes in wind patterns, sea-ice cover, ocean circulation, nutrient delivery and the supply of organic matter can alter the oxygen balance in ways that are not captured by temperature alone. Ancient glaciations demonstrate that the relationship between climate and habitability is controlled by a complex network of physical and chemical processes.
For Zunli Lu, professor of Earth and environmental sciences at Syracuse University, the study illustrates how the history of metals can reveal the evolution of the atmosphere, oceans and life. The scale of the investigation was unusual for geochemical research: datasets containing 500 samples are often considered substantial, whereas this project examined tens of thousands of measurements from North America, South America, Europe, Asia, Africa and Oceania. The work was supported by the National Science Foundation and the Thonis Family Postdoctoral Research Fellowship, with climate modeling conducted using high-performance computing facilities in France. It also contributes to UNESCO’s IGCP 735 initiative, which investigates early Earth environments and the emergence of ancient life. By combining global geology, chemical tracers and climate simulations, the researchers have transformed scattered fragments of seafloor into a planetary record of how oxygen moved through Earth’s oceans during its most extreme climate states.
Subject of Research: Ancient ocean oxygenation, manganese geochemistry, glacial climates and the evolution of marine habitability.
Web References: https://artsandsciences.syracuse.edu/people/faculty/lu-zunli/
References: Nature Communications.
Keywords: Ancient oceans, manganese, ocean oxygen, Snowball Earth, glaciation, marine sediments, geochemistry, Earth history, oceanography, climate change, habitability, molybdenum, uranium, atmospheric oxygen.
