About 27 per cent of nitrogen in the open ocean is because of the influence of humans compared to the previously estimated 80 per cent, the study says.
“We expected to find that as clean air blew in, there’d be lower concentrations of nitrogen, and with the dirty air, higher concentrations, but we didn’t find that.”
Katye Altieri, Energy Research Centre, University of Cape Town
“Ours is the first long-term dataset looking at all characteristics of the nitrogen cycle, showing organic compounds that look like they come from surface ocean activities, and don’t look like pollutants,” says Katye E. Altieri, a co-author of the study and a senior research officer at the Energy Research Centre, University of Cape Town, South Africa.
In the study published in the journal Proceedings of the National Academy of Sciences last month (26 January), researchers from Bermuda, South Africa and the United States analysed rainwater and aerosol organic nitrogen collected on the island of Bermuda in the North Atlantic Ocean over 18 months.
They found that as marine biological activity increased, so did the concentration of organic nitrogen in aerosol samples. Supported by detailed chemical composition, the team concluded that organic nitrogen in marine aerosols derived mainly from biological production in the surface ocean.
Bermuda is unique in that it has two meteorological cycles, according to the researchers. For much of the year, air blowing northward from the open ocean dominates. However, a shift in atmospheric pressure during winter pulls in air from the United States carrying industrial and agricultural pollutants.
“We expected to find that as clean air blew in, there’d be lower concentrations of nitrogen, and with the dirty air, higher concentrations,” explains Altieri. “But we didn’t find that, because there appears to be a natural cycle between surface ocean and the atmosphere that we didn’t know was there.”
Nitrogen is found naturally in the atmosphere, but human activities such as burning fossil fuels and biomass can contribute to its concentration. Some of that nitrogen is carried by wind and deposited in the oceans, but it’s unclear which one is the predominant source.
According to Warren Joubert, a senior scientist at the Southern Ocean Carbon and Climate Observatory (SOCCO) in South Africa, this study is unique in how the origin of nitrogen was identified.
“Previous estimates of the ocean's total nitrogen uptake quantified only the anthropogenic [human-induced] contribution,” says Joubert, who was not involved in the study. “The techniques used in this study improve the previous estimate, and what is appealing is the use of isotopic signatures of molecules to identify the origins.”
However, he notes that the findings are valid for the North Atlantic Ocean region only, and could be different for other ocean regions.
Currently, Altieri and fellow researchers are working to overcome that constraint by launching a study based on samples from Cape Point in South Africa. “The scale of pollution is much smaller in the air from the Southern Ocean, and it has a larger ocean basin than the northern Atlantic,” Altieri notes. Such a study would be advantageous for the South African scientist, says Joubert. “We have access to dedicated research platforms and high performance computing capabilities to develop climate models as well as the technical capabilities required to ask and answer difficult climate questions,” Joubert adds.
This piece was produced by SciDev.Net’s Sub-Saharan Africa English desk.