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A pinboard by
Michael Forster

PhD Student, Macquarie University

PINBOARD SUMMARY

I use an high pressure experimental approach to study the content of nitrogen in Earth's mantle.

In my PhD Studies I use an experimental approach to examine the behavior of nitrogen during subduction and mantle melting. We conducted high-pressure experiments in piston-cylinder- and belt-apparatus at 1-6 GPa. Nitrogen is the main element of Earth’s atmosphere. It is an essential element for life and takes part in complex biochemistry due to its numerous oxidation states spanning from +5 to -3. The various reactions of nitrogen, forming reduced and oxidized species are known as the nitrogen cycle. Besides this well-known surficial cycle, nitrogen is also part of high-pressure fluids. Nitrogen in Earth’s mantle fluids may have two components: the first component is represented by subduction, where organic nitrogen is recycled. The second source of nitrogen is located within Earth’s lower mantle, which has yet to receive more attention. The deep mantle is thought to be a repository for high amounts of nitrogen in yet unknown forms. Nitrogen can form compounds with platinum group elements and hydrogen. However, xenoliths from the Earth’s mantle are degassed and yield low fractions of about 0.1-10 μg g-1 nitrogen. In contrast, chondrites contain up to 100-1000 μg g-1 nitrogen, which indicates that the deep Earth’s interior possesses significant nitrogen-rich reservoirs. Further evidence lies in the solubility of nitrogen in minerals and melt at high pressures, which is also supported by lower oxygen fugacities of the deep mantle. Atmospheric nitrogen is continuously fixed and deposited in marine sediments, which contain approximately 1000 μg g-1 nitrogen. The ongoing recycling of nitrogen to mantle depths is thought to be associated with potassium-bearing silicates in subduction zones, where micas play an integral role, for example through the substitution of ammonium-NH4+ for potassium in phlogopite. During this process, ammonium is likely re-oxidized to N2 and escapes from the mantle via magmatic volatiles in arc settings. Nitrogen behaves highly incompatibly during melting of anhydrous mantle rocks and has low abundances in basalts (<10 μg g-1 N). However, nitrogen-rich ultrapotassic magmas (~400 μg g-1 N) indicate the presence of nitrogen-bearing lithologies that reside within the lithospheric mantle. Melting of these lithologies may lead to ultrapotassic magmas which could be nitrogen-rich and may be different from nitrogen-poor magmas remote to subduction zones.

6 ITEMS PINNED

Earth Without Life: A Systems Model of a Global Abiotic Nitrogen Cycle.

Abstract: Nitrogen is the major component of Earth's atmosphere and plays important roles in biochemistry. Biological systems have evolved a variety of mechanisms for fixing and recycling environmental nitrogen sources, which links them tightly with terrestrial nitrogen reservoirs. However, prior to the emergence of biology, all nitrogen cycling was abiological, and this cycling may have set the stage for the origin of life. It is of interest to understand how nitrogen cycling would proceed on terrestrial planets with comparable geodynamic activity to Earth, but on which life does not arise. We constructed a kinetic mass-flux model of nitrogen cycling in its various major chemical forms (e.g., N, reduced (NH) and oxidized (NO) species) between major planetary reservoirs (the atmosphere, oceans, crust, and mantle) and included inputs from space. The total amount of nitrogen species that can be accommodated in each reservoir, and the ways in which fluxes and reservoir sizes may have changed over time in the absence of biology, are explored. Given a partition of volcanism between arc and hotspot types similar to the modern ones, our global nitrogen cycling model predicts a significant increase in oceanic nitrogen content over time, mostly as NH, while atmospheric N content could be lower than today. The transport timescales between reservoirs are fast compared to the evolution of the environment; thus atmospheric composition is tightly linked to surface and interior processes. Key Words: Nitrogen cycle-Abiotic-Planetology-Astrobiology. Astrobiology 18, xxx-xxx.

Pub.: 11 Apr '18, Pinned: 15 May '18

Microbial nitrogen cycle hotspots in the plant-bed/ditch system of a constructed wetland with N2O mitigation.

Abstract: Artificial microbial nitrogen (N) cycle hotspots in the plant-bed/ditch system were developed and investigated based on intact core and slurry assays measurement using isotopic tracing technology, quantitative PCR and high-throughput sequencing. By increasing hydraulic retention time and periodically fluctuating water level in heterogeneous riparian zones, hotspots of anammox, nitrification, denitrification, ammonium (NH4+) oxidation, nitrite (NO2-) oxidation, nitrate (NO3-) reduction and DNRA were all stimulated at the interface sediments, with the abundance and activity being about 1-3 orders higher than that in non-hotspot zones. Isotopic pairing experiments revealed that in microbial hotspots, both NH4+ oxidation (55.8%) and NO3- reduction (44.2%) provided more NO2- sources than NO2- sinks for anammox, which accounted for 43.0% of N-loss and 44.4% of NH4+ removal in riparian zones but doesn't involve nitrous oxide (N2O) emission risks. High-throughput analysis identified that bacterial quorum sensing mediated this anammox hotspot with B.fulgida dominating the anammox community, but it was B.anammoxidans and Jettenia sp. that contributed more to anammox activity. In the non-hotspot zones, NO3- reduction dominated as the NO2- source but lower than the sink, limiting the effects on anammox. The in-situ N2O flux measurement showed that the microbial hotspot had a 27.1% reduced N2O emission flux compared with the non-hotspot zones.

Pub.: 12 May '18, Pinned: 15 May '18