TITLE: Cosmogenic 14C in shallow firn at Summit, Greenland PI: Jeffrey Severinghaus Scripps Institution of Oceanography Mail Code 0244 Univ. California San Diego 9500 Gilman Dr. La Jolla, CA 92093, USA +1 858 822 2483 jseveringhaus@ucsd.edu Contact for data questions: Vasilii Petrenko Department of Earth and Environmental Sciences University of Rochester Rochester, NY 14627 USA +1 585 276 6094 vpetrenk@z.rochester.edu FUNDING: NSF OPP Award 0806450 DATA SET OVERVIEW This project successfully examined the question of whether or not cosmic ray-produced 14C forms radiomethane (14CH4) in polar firn. Firn samples from ~4.5 m depth were taken at Summit, Greenland. Around 1000 kg of firn per sample was melted in the presence of a 14C-free carrier gas to extract gases from the firn matrix. The findings indicate a small but clearly present cosmogenic 14CH4 component. A larger cosmogenic 14C – carbon monoxide (14CO) component was also found. The results further indicated that almost all cosmogenic 14C is being lost rapidly from the firn matrix at this depth level. These results confirm a surprising earlier finding (Petrenko et al., 2009) that cosmogenic 14CH4 may be present in glacial ice. Work using paleoatmospheric 14CH4 to study the fossil source contribution to the methane budget therefore needs to take the cosmogenic component into account. The results are currently in preparation for publication. The firn samples were collected between Jul 29 and Aug 18, 2009. The sampling site coordinates are: N 72.57963˚,W 38.49253˚ INSTRUMENT AND METHOD DESCRIPTION The field and analytical system for determinations of in-situ cosmogenic 14CO and 14CH4 in glacial ice and firn have been described in (Petrenko et al., 2008a; Petrenko et al., 2008b; Petrenko et al., 2009). This system involves the melt-extraction of occluded air from very large volumes of glacial ice or at the sampling / ice coring site. Briefly, the present field system consists of a large chemically polished aluminum vacuum melting tank (~670 L internal volume) and a series vacuum and transfer pumps. The ice is loaded into the tank, and the headspace is evacuated and flushed 3x with either ultra-high purity (UHP) air, nitrogen or argon. The ice is then melted, releasing the ancient air into the headspace. This air is then recirculated through the tank via a bubbler manifold at the bottom, equilibrating all the gases between the water and the headspace. The air is then extracted from the tank by clean diaphragm transfer pumps and stored in electropolished stainless steel canisters for further laboratory handling and analyses. In the laboratory, the air is first processed through a system that converts either CH4 or CO to CO2, and captures this CO2 for further handling. In the case of CH4 processing, H2O, CO2, N2O and other condensibles are first removed by a series of traps at liquid nitrogen temperature. CO is then quantitatively oxidized to CO2 by the Sofonocat reagent and subsequently removed by further cryotraps. CH4 is then combusted to CO2 by passing the air through a 800˚C furnace containing platinized quartz wool. The CH4-derived CO2 is then captured. This CO2 is then converted to graphite over ultra-high-purity iron powder and subsequently measured for 14C by AMS. The combined procedural 14CH4 blank for all steps of sampling handling was determined to be 0.75 ± 0.38 pMC, on the basis of 72 processed blank and standard samples (Petrenko et al., 2008b). For CO analyses, the sample handling is very similar except that the air bypasses Sofnocat, and the furnace temperature is reduced from 800 to 150 ˚C. This allows for complete combustion of CO while CH4 passes through unaffected. DATA COLLECTION AND PROCESSING The 14CH4 data are corrected for ambient air added to the samples from air bubbles in the firn as well as for the procedural blank. For 14CO, the ambient air correction is insignificant, but the samples are corrected for the procedural blank. DATA FORMAT Excel file. Column Descriptions: Sample Name Top Depth: Depth below snow surface for the shallowest firn included in sample Bottom Depth: Depth below snow surface for the deepest firn included in sample Measured 14CH4: average cosmogenic 14CH4 content in sampled firn Measured 14CO: average cosmogenic 14CO content in sampled firn Predicted 14CO: Predicted average cosmogenic 14CO content in sampled firn based on cosmic ray scaling of Lifton et al (2005) and production rates of Heisinger et al (2002) DATA REMARKS None REFERENCES: Heisinger, B., Lal, D., Jull, A. J. T., Kubik, P., Ivy-Ochs, S., Knie, K., and Nolte, E. (2002a). Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons. Earth and Planetary Science Letters 200, 357-369. Lifton, N. A., Bieber, J. W., Clem, J. M., Duldig, M. L., Evenson, P., Humble, J. E., and Pyle, R. (2005). Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications. Earth and Planetary Science Letters 239, 140-161. Petrenko, V. V., Smith, A. M., Brook, E. J., Lowe, D., Riedel, K., Brailsford, G., Hua, Q., Schaefer, H., Reeh, N., Weiss, R. F., Etheridge, D., and Severinghaus, J. P. (2009). (CH4)-C-14 Measurements in Greenland Ice: Investigating Last Glacial Termination CH4 Sources. Science 324, 506-508. Petrenko, V. V., Severinghaus, J. P., Brook, E. J., Mühle, J., Headly, M., Harth, C., Schaefer, H., Reeh, N., Weiss, R., Lowe, D. C., and Smith, A. M. (2008a). A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon. Journal of Glaciology 54, 233-44. Petrenko, V. V., Smith, A. M., Brailsford, G., Riedel, K., Hua, Q., Lowe, D., Severinghaus, J. P., Levchenko, V., Bromley, T., Moss, R., Muhle, J., and Brook, E. J. (2008b). A new method for analyzing C-14 of methane in ancient air extracted from glacial ice. Radiocarbon 50, 53-73.