Assessing and extending the N2O record in the Vostok ice core

Measurements of trace gas species in ice cores are the primary means for reconstructing the composition of the atmosphere. There are two underlying assumptions implicit in these reconstructions: 1) the bubbles recovered from the ice cores record the composition of the atmosphere at the time the bubbles were formed, and 2) the composition of the bubbles has not been compromised by the extremely long storage periods within the ice sheet. While there is ample evidence that certain trace gas records (e.g. CO2and CH4) have probably not been compromised, anomalous nitrous oxide (N2O) measurements from the penultimate glacial termination at Vostok are thought to be the result of in-situ (N2O) production by nitrifying microorganisms. This 3-pronged pilot project will test the feasibility of applying a combination of molecular techniques, microbial cultivation and physiological assays and isotopic/geochemical measurements in evaluating the potential for nitrifying bacteria to modify nitrogen species in an ice core. First we propose to demonstrate that our molecular and microbiological techniques are capable of identifying nitrifying bacteria in glacial ice. We will demonstrate this by adding live cultures of known psychrophilic/psychrotolerant nitrifiers (e.g. Nitrosomonas cryotolerans) to a suite of melted Vostok ice samples (varying cell concentrations) which will be refrozen. We will then apply our molecular techniques to these 'spiked' samples to establish the detection limit for our molecular probes. We will also start enrichment cultures from these model ice cores to monitor the growth and activity of the nitrifying organisms over time. Finally, we will apply different decontamination techniques to insure we are getting uncompromised samples from the ice cores for the microbial work. We will attempt to measure N2O production and quantify the temperature dependence of the nitrifier activity by adding isotopically labeled substrate (15NH4) and monitoring the incorporation of the label into N2O and NO2. As the metabolic rates are expected to be extremely low, we plan to set up incubations that will extend over a ~3yr period. Finally, we propose to analyze the elemental and isotopic composition of N2O in the Vostok core spanning the last termination (Termination I) to see if the anomalous behavior we documented for Termination II is also apparent during Termination I. We will compare the Vostok N2O record with contemporaneous N2O records from the GISP II, Taylor Dome, and GRIP ice cores.