Thursday, 6 November 2014

Nitrogen and the Anthropocene… by Melanie Leng

A 2013 art installation at Edge Hill University near Liverpool, 
by Robyn Woolston included this mock sign
Last week BGS hosted a workshop entitled “The Nitrogen Cycle and the Anthropocene”. The aim of the workshop was to bring together nitrogen experts from around the UK to discuss the modern day increase in the amount of nitrogen currently being deposited (through atmospheric fall out and as a result of mans’ activities on the Earth’s surface). The main culprits of this increasing nitrogen are industrial processes, increasing use of fertilisers and combustion of fossil fuels. Here Melanie Leng tells us about the workshop….

The first talk of the day was an overview of the Anthropocene (of which the increasing nitrogen deposition is one of hundreds of man made changes currently affecting the Earth system). Dr Jan Zalasiewicz (University of Leicester) gave a thought provoking synthesis of human-driven changes to the Earth. Of note is the spread of an urban stratum (refashioning of sediments into buildings), human bioturbation (digging and remodelling) of the surface of the earth, the ongoing mass extinctions of plants and animals, greenhouse warming through increasing CO2 etc. One main question that Jan tackled was should this period of human driven change be defined as a new geological epoch of geological time, the Anthropocene Epoch. We await the “Sub-Commission on Quaternary Stratigraphy” decision in 2016…

The delegates at the Workshop

Dr Tim Heaton (BGS) described the doubling of nitrogen available to the Earth’s biosphere, and how the timing and impact of this might be recorded in terms of changes in the isotope composition of nitrogen deposited in lake sediments. He showed how such changes might be different in lakes in inhabited versus remote locations; how they might be modified by sediment diagenesis; and how isotope data for plants revealed the strong coupling of the nitrogen cycle with the carbon cycle. He concluded that the changes observed in the isotope composition of nitrogen deposited in lake sediments in remote regions during the 20th century were almost certainly caused by anthropogenic activity, but that the exact mechanisms responsible were not yet established.

Prof Jan Kaiser (University of East Anglia) described changing nitrogen in the atmosphere. Most of the nitrogen in our atmosphere is inert nitrogen gas (N2) which stays within the atmosphere for millions of years. However other forms (i.e. nitrous oxide, N2O) are increasing. N2O is stable in the troposphere but breaks down in the stratosphere and changes its isotope composition. These features enable tracing of N2O. Jan explained that the nitrogen trapped in ice cores is showing a changing nitrogen cycling over the last 100 years including the addition in nitrogen from soils. Species of ammonia (NH3) only survive for hours/days but through isotopes have been sourced to fossil fuels and biomass burning.

The keynote speakers from left to right:
Tim Heaton, Jan Kaiser, Eric Wolff, Melanie Leng (organiser) and Jan Zalasiewicz.

Prof Eric Wolff (University of Cambridge) presented data for nitrogen compounds extracted from ice cores and bubbles trapped in the ice. Recent increases in concentrations could be found in the Arctic, and could be attributed to atmospheric pollutants arising from anthropogenic activities in the heavily industrialised northern hemisphere. Changes were less evident in the ice core records of the Antarctic.

Overall it is evident that the increasing amount of nitrogen being added to the environment by human activity is not only having a marked effect on the terrestrial nitrogen cycle, but is also causing significant changes to the chemistry of the atmosphere. By this means the impact of human activity is being exported to the remotest parts of the planet. Whether nitrogen changes can contribute to the debate on whether to formally declare the Anthropocene a new epoch is still under debate. However, the conclusions from all 3 talks suggest that changes in nitrogen are likely man made and therefore the nitrogen debate can play a role in the decision.

By Melanie Leng, Director of the Centre of Environmental Geochemistry


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