Oxygen decline in the ocean: Implications for oceanic N2O production and the atmosphere

点击翻译按钮获取中文摘要

N2O is an important greenhouse gas in the troposphere and has a per mol global warming potential ~300 times that of CO2. The atmospheric lifetime of N2O is also sufficiently long enough for it to survive transport to the stratosphere where it undergoes ozone depleting photochemical reactions (de Bie et al. 2002). The ocean is a significant source of N2O to the atmosphere, accounting for ~35% of all natural sources. The microbial production of N2O, (via NH4+ oxidation, nitrifier-denitrification, denitrification), is largely regulated by dissolved oxygen (DO) concentrations. Below a certain but poorly defined DO threshold, N2O production will increase substantially. There is increasing evidence that DO concentrations in the ocean are decreasing; given the atmospheric importance of N2O, this raises cause for concern as it will undoubtedly lead to an increase in oceanic N2O production and emissions to the atmosphere. Before this happens it is critical that we 1) identify the current N2O conditions (i.e. N2O distributions, N2O production and the production pathways) in the ocean, and 2) determine how N2O conditions will change under different future DO scenarios. To address the first point, work will be conducted in the north eastern tropical Atlantic where DO concentrations are more characteristic of much of the contemporary ocean. To address the second point, we will conduct work in two extreme oxygen minimum zones (the eastern tropical South Pacific and a low DO eddy in the north eastern tropical Atlantic) where DO concentrations are much lower than in much of the present day ocean. In each of our study regions we will measure 1) N2O concentrations, 2) del15N, del18O and 15N site preference signatures of N2O to determine the relative importance of the different N2O producing pathways to bulk N2O pools, and 3) N2O production rates via each of the different production pathways using 15N tracer techniques. Molecular analyses will also be conducted to characterize and quantify the different N2O producing organisms. Of course, if we are to accurately assess the impact that increasing N2O production in the ocean, as a result of decreasing DO concentrations, will have for the atmosphere, it is critical that we develop a more comprehensive understanding of the factors that influence sea-to-air fluxes of N2O. To address this, we will use the eddy covariance technique to directly measure N2O fluxes from the ocean and compare these to a range of physical, chemical and biological parameters.