October 2013 Issue - Volume 9, Number 5

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Nitrogen and Its (Biogeocosmo) Chemical Cycling

Gray E. Bebout, Marilyn L. Fogel, and Pierre Cartigny – Guest Editors

Overview

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2013 Topics

Overview

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2013 Topics

Thematic Articles

Nitrogen: Highly Volatile yet Surprisingly Compatible

By Gray E. Bebout, Marilyn L. Fogel, Pierre Cartigny

Nitrogen exhibits an intriguing combination of highly volatile behavior (particularly as N2), appreciable reactivity, and surprising compatibility in the deep Earth. Nitrogen is incorporated into the biosphere and then, through diagenesis and low-grade metamorphism, is conveyed into the lithosphere and the deeper Earth. The investigation of N behavior in the biosphere, hydrosphere, and atmosphere has led to many important discoveries regarding biogeochemical pathways, including in areas such as trophic interactions and anthropogenic impacts on terrestrial and marine environments (e.g. nutrient pollution, eutrophication). Nitrogen can act as an excellent tracer of the transfer of sedimentary/organic materials into and within deep-Earth reservoirs and shows great potential as a tracer of life on early Earth and elsewhere in the Solar System.

Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts

By Meredith G. Hastings, Karen L. Casciotti, Emily M. Elliott

The global nitrogen cycle has been perturbed by human activities, including agriculture, land-use change, and fossil fuel burning. This perturbation ranges from the local to global scale, as anthropogenic reactive nitrogen can be transported over long distances in the atmosphere, in groundwater, and in stream networks and can even impact the open ocean. Stable isotope signatures characteristic of reactive nitrogen can be used to trace its deposition in the present day, as well as in the past. Here we focus on the use of stable isotopes to trace the sources, transport, and impacts of anthropogenic nitrogen in the modern nitrogen cycle.

Biogeochemical Cycling of Nitrogen on the Early Earth

By Christophe Thomazo, Dominic Papineau

Variations in the nitrogen isotope composition of ancient organic matter and associated sediments provide clues for the early evolution of Earth’s atmosphere–ocean–biosphere system. In particular, large isotopic variations have been linked to the protracted oxygenation of Earth’s atmosphere during the Precambrian. Important problems being investigated include the nature of the variations observed at specifi c times in Earth’s history and the degree of preservation of ancient nitrogen biogeochemical signatures during diagenesis and metamorphism. Interpreting these records in Archean sedimentary environments and their possible implications for the evolution of Earth’s early atmosphere, ocean, and life is challenging.

Nitrogen in the Silicate Earth: Speciation and Isotopic Behavior during Mineral–Fluid Interactions

By Vincent Busigny, Gray E. Bebout

Nitrogen is the main constituent of Earth’s atmosphere and a key component of the biosphere, but it is a trace element in the major silicate reservoirs. The relatively low concentrations (parts per million level) complicate efforts to constrain the nitrogen speciation and abundance in the mantle and crust. In most silicates, nitrogen occurs as NH4 + (substituting for K+), whereas its speciation in hydrous fluids and silicate melts can vary widely depending in large part on redox conditions. Current knowledge of nitrogen isotope fractionation among relevant mineral and fluid/melt phases is limited by the lack of experimental data to confirm theoretical predictions of these fractionations. Modeling of modern and long-term nitrogen cycling on Earth will be advanced by better constraints on the sizes and isotopic compositions of the major crust and mantle nitrogen reservoirs.

Nitrogen Isotopes and Mantle Geodynamics: The Emergence of Life and the Atmosphere– Crust–Mantle Connection

By Pierre Cartigny, Bernard Marty

Nitrogen shows unique features among the volatile elements. To be cycled, atmospheric di-nitrogen (N2) needs to be reduced, which is efficiently done by bacterial processes. Crustal uptake of nitrogen and its eventual recycling into the mantle is thus primarily mediated by the biosphere. There is also a marked isotopic contrast between the mantle (15N depleted) and the Earth’s surface (15N enriched). Although the cause of such disequilibrium is not fully understood, it provides insights into mantle–surface interactions over geological time, including recycling of surface sediments into the deep mantle.

Nitrogen in Extraterrestrial Environments: Clues to the Possible Presence of Life

By Marilyn L. Fogel, Andrew Steele

Nitrogen is a critical element for living organisms on Earth. While atmospheric N2 is plentiful, organisms find it difficult to metabolize, requiring chemical modifications that are rare or absent in abiotic chemistry. Living organisms reduce N2 to NH3 with elaborate, energy-intensive, biochemical processing to create nitrogen-bearing carbon compounds essential for life. Astrobiologists have long discussed what role nitrogen could play in shaping life on other planets. Work on Martian meteorites has provided new insights into nitrogen cycling on Mars. Research on meteorites ties into investigations by NASA’s Mars Science Laboratory that are providing on-theground information to piece together a more cohesive picture of the importance of nitrogen for establishing a habitable environment.