A new study published in Nature Communications provides fresh insights into the nitrogen cycling processes that existed prior to the Great Oxidation Event. The research was conducted by an international team of experts in geology, microbiology, and geochemistry, led by Dr. Ashley Martin of Northumbria University.
The team examined ancient stromatolites preserved in southern Zimbabwe, which date back more than 2.5 billion years. Stromatolites are layered biochemical structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by microorganisms. By analyzing these fossilized formations, the researchers aimed to understand how nitrogen cycles operated in early Earth's ecosystems.
Their findings reveal that ammonium reservoirs in early Earth's oceans, likely influenced by volcanic activity, may have supported microbial life before the Great Oxidation Event. The event, which occurred between 2.5 to 2.3 billion years ago, is considered a major milestone in Earth's history as it saw the first rise of oxygen concentration in the atmosphere, likely caused by the evolution of photosynthesis.
"A large ammonium reservoir would have been very beneficial for early life, providing the nitrogen source needed for biological processes to occur," said Dr. Ashley Martin from Northumbria University, according to SciTechDaily. "These conditions, likely in an ocean depleted of dissolved oxygen with a strong volcanic or hydrothermal influence, would have helped to support microbial growth, potentially spurring biological innovations and paving the way for the Great Oxidation Event."
Nitrogen is essential for life, but it must first be converted into bioavailable forms as it moves through the atmosphere, soil, plants, and animals in the nitrogen cycle. Prior to this study, little was known about nitrogen cycles before the Great Oxidation Event. The research team's analysis suggests that in some areas of the ancient oceans, large quantities of bioavailable nitrogen in the form of ammonium may have accumulated due to volcanic activity, fueling the development of life.
"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," explained Dr. Eva Stüeken from the University of St Andrews, as reported by Phys.org.
The team discovered high nitrogen isotope values in 2.75-billion-year-old shallow water stromatolites and lower values in deeper marine sediments. "This suggests that ammonium, which is nitrogen in its reduced form, accumulated in the deep waters and was brought into shallow waters by upwelling—the movement of deep nutrient-rich water towards the surface of the ocean," Dr. Martin said. "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."
At the time before the Great Oxidation Event, early Earth looked very different from today, with most continents still submerged beneath a vast ocean. Volcanic activity was exceptionally active 2.75 billion years ago. "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 time interval," noted Professor Axel Hofmann from the University of Johannesburg.
The research team believes that the unusual nitrogen isotope patterns found in Zimbabwe can offer new understandings of the mechanisms at play in Earth's early marine environment before the Great Oxidation Event. Scientists have long debated the biological and chemical conditions that led to this pivotal period, and this study contributes information to the discussion.
An earlier study by Dr. Martin, Dr. Stüeken, and Dr. Michelle Gehringer from the University of Kaiserslautern-Landau supports the proposal that volcanic activity led to the accumulation of large quantities of bioavailable nitrogen in the ancient oceans. The results of that study were published in the journal Geology.
Dr. Ashley Martin and Dr. Monika Markowska are members of Northumbria University's Environmental Monitoring and Reconstruction (EnMaR) research group. The EnMaR group studies modern and ancient environments, from the tropics to the polar regions, seeking to answer fundamental global questions about climate and the environment.
The article was written with the assistance of a news analysis system.