Fossil findings reveal the surprising impact of volcanic activity on Earth’s early life

New research into fossilized rock formations in Zimbabwe is shedding light on Earth's nitrogen cycle before the rise of atmospheric oxygen.

Scientists analyzing ancient stromatolites—layered rock structures formed by microbial activity—have uncovered evidence suggesting that nitrogen cycling was shaped by volcanic activity and hydrothermal processes long before oxygen became abundant.

Nitrogen is an essential element for life, but in its atmospheric form, it is largely unusable. It must first be converted into bioavailable compounds like ammonium or nitrate. The unusual nitrogen isotope values found in sedimentary rocks dating back 2.75 billion years provide a new perspective on how nitrogen was cycled in Earth’s early oceans.

Dr. Ashley Martin, a researcher at Northumbria University, led an international team to investigate these nitrogen cycling patterns in ancient stromatolites preserved in Zimbabwe.

Their findings, published in Nature Communications, suggest that ammonium accumulated in deep waters before being brought to the surface by oceanic upwelling. This process could have played a crucial role in sustaining microbial life long before oxygen was widely available.

Scientists have debated how nitrogen moved through the environment before the Great Oxidation Event, which occurred between 2.5 and 2.3 billion years ago. This event, driven by the evolution of photosynthetic organisms, led to the first significant rise in atmospheric oxygen. However, the conditions that shaped life prior to this transformation remain a mystery.

Dr. Martin explained the significance of nitrogen in early ocean productivity, “There are two key nutrients that control productivity in the oceans on geological timescales—nitrogen and phosphorus. Together they ultimately control the productivity of marine life.”

The research team analyzed nitrogen isotope ratios in different sediment layers. Stromatolites formed in shallow waters contained high nitrogen isotope values, while deeper marine sediments had lower values. These findings suggest that ammonium, a reduced form of nitrogen, accumulated in deep waters and was periodically brought to the surface through upwelling.

This deep-water ammonium reservoir would have provided a stable nitrogen source for early microbial life, which was likely thriving in an ocean environment with minimal dissolved oxygen. The study supports the idea that volcanic and hydrothermal activity played a major role in supplying bioavailable nitrogen, creating conditions that may have driven key biological innovations.

The study focused on rock formations from the Manjeri and Cheshire Formations in Zimbabwe’s Belingwe greenstone belt. These formations date back to around 2.75 billion years ago and record a period of increased volcanic activity associated with convective mantle overturning.

The Manjeri Formation consists of a thin, rapidly deposited sedimentary layer that formed as the region experienced a marine transgression due to crustal subsidence. This was followed by extensive mafic and ultramafic volcanism. The overlying Cheshire Formation, deposited after this volcanic episode, provides further evidence of the environmental conditions at the time.

Analysis of the rocks revealed positive europium anomalies, which suggest an excess of europium under reducing seawater conditions. These findings indicate that volcanic and hydrothermal activity influenced the ocean’s chemistry, supporting the idea that nitrogen cycling in this period was closely linked to geological processes.

Dr. Eva Stüeken of the University of St Andrews emphasized the connection between nitrogen isotope values and hydrothermal nutrient recycling, “We have long been puzzled by the unusual nitrogen isotope values in these rocks. Our new findings suggest a strong linkage to hydrothermal nutrient recycling, meaning that early life may in part have been fueled by volcanic activity.”

This hypothesis aligns with previous research that suggests bioavailable nitrogen may have accumulated in ancient oceans due to volcanic emissions, fueling the development of early microbial ecosystems.

Dr. Axel Hofmann from the University of Johannesburg reinforced this view, “Volcanism was exceptionally active 2.75 billion years ago and left a lasting impact on the evolution of life at that time. Rocks in Zimbabwe preserve a remarkable record of this interval.”

The research adds to growing evidence that Earth's nitrogen cycle was complex long before the widespread presence of oxygen. While early biological nitrogen fixation has been well-documented, this study highlights the role of geological processes in shaping nitrogen availability for life.

The findings contribute to a broader understanding of how Earth’s biogeochemical cycles evolved before the oxygenation of the atmosphere. Dr. Martin and colleagues propose that the nitrogen cycle was already dynamic, with deep-sea ammonium reservoirs supporting microbial life in an environment influenced by volcanic and hydrothermal activity.

These conditions may have set the stage for the Great Oxidation Event, as microbial communities adapted to fluctuating nutrient availability and oxygen levels. Understanding the pre-oxygen nitrogen cycle provides a crucial piece of the puzzle in reconstructing the early Earth’s environment and the conditions that led to the evolution of complex life.

The research team includes experts from Northumbria University, the University of St Andrews, the University of Kaiserslautern-Landau, Leibniz University Hannover, the Max Planck Institute for Chemistry, and the University of Johannesburg. Their work highlights how interdisciplinary collaboration continues to uncover new details about Earth’s ancient past.

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