Reports: DNI252642-DNI2: Variations in 17O/16O and 18O/16O of Meteoric Waters from the Conterminous USA

Naomi E. Levin, PhD, Johns Hopkins University

The first year funding from this grant advanced the study of triple oxygen isotopes in earth surface materials with activities that included the refinement of methods for determining Δ17O in water, calibration with other laboratories, analysis of natural waters from the U.S. and Kenya, and expansion of analytical capabilities in Δ17O to carbon dioxide and carbonates.

Analytical Refinements and Inter-Lab Calibration

      Refinement of the fluorination method resulted in analysis of waters that routinely yielded precise results regardless of the analyst. Repeated analyses of four USGS reference waters yielded an external precision (1σ) of <0.008‰ for Δ17O. Comparison of these results to measurements of the same waters in another IRMS laboratory and in a laboratory using a laser absorption spectrometer show excellent agreement between labs and methods. This inter-lab calibration study has just been accepted for publication in Analytical Chemistry (Berman et al., 2013,

Δ17O in waters

      The main focus of this work has been to generate Δ17O datasets on samples from the United States Network for Isotopes in Precipitation (USNIP), rivers and precipitation from the Willamette Valley in Oregon, and plant and surface waters from Kenya. In addition, this work supported the collection of precipitation samples from Baltimore, Maryland that are slated for isotopic analysis in the upcoming year. 

      Triple oxygen isotope and hydrogen isotope data were generated from 55 weekly USNIP precipitation samples, 23 weekly precipitation samples from Corvallis Oregon, and 18 river samples from the Willamette Basin. The results show that 1) there is significant spatial and seasonal variation in Δ17O of precipitation in the U.S., 2) Δ17O and d-excess of precipitation and rivers do not uniformly co-vary, and 3) the average Δ17O of precipitation in the U.S., 0.015±0.017‰, is lower than the previously reported global average, 0.037±0.004‰. The results of this work also show that Δ17O values of rivers in the Willamette Basin are similar to Δ17O values of rainy season precipitation. Collectively these results indicate that Δ17O values of precipitation at low- to mid-latitudes capture spatial and seasonal climate differences and that rivers waters are faithful recorders of the Δ17O composition of rainy season precipitation. The results were reported in an abstract presented at the Goldschmidt conference in Florence, Italy, August, 2013 (Levin et al., 2013). In the second year of this grant, the spatial variation in Δ17O of precipitation will be explored further and the river-precipitation Δ17O comparisons will be expanded to other regions in the U.S. The work is in collaboration with Jeff Welker at the University of Alaska Anchorage and J. Renée Brooks at the Environmental Protection Agency (EPA).

      Δ17O measurements of leaf water, stem water and surface waters from Kenya were an unintended product of this first year of this grant but they were pursued because samples that met the goals of this project became available for analysis. Fifty waters were analyzed from a well-monitored environmental research station in Mpala, Kenya. Water samples, δ18O and δD data, and environmental data were provided by collaborators Keir Soderberg, Kate Dennis and Kelly Caylor at Princeton University. The results show that the relationship between δ18O and δ17O values of stem and leaf water (λtranspiration) varies diurnally and is a function of both relative humidity and leaf type. This study demonstrates the need to consider plant species and time of day when using λtranspiration values for paleoclimate studies or in models of the global triple oxygen isotope budget. Graduate student Shuning Li will present these results at the Fall Meeting of the American Geophysical Union (Li et al., 2013). This study will result in a chapter of Ms. Li’s PhD dissertation.

Δ17O in CO2 and Carbonates

      The streamlined water fluorination line at JHU enabled the development of a method to measure Δ17O in CO2 and carbonate, led by JHU faculty member Benjamin Passey and graduate student Huanting Hu. The new method involves the reduction of CO2 to water using hydrogen gas and an iron catalyst. The resultant water is then analyzed for Δ17O using the water fluorination line. Δ17O values of carbonates are measured by first reacting carbonates in phosphoric acid to produce CO2 which is then processed through the reduction and fluorination systems. This method results in external precision (1σ) of Δ17O in CO2  of < 0.01‰ for relatively small COsample sizes (50-80 μmol) and represents an improvement over existing methods. A manuscript describing this new method will soon be submitted to Analytical Chemistry by Passey and others. Abstracts with the first reports of data using this new method include one presented at the Goldschmidt Conference, August 2013 (Passey et al., 2013) and another that is slated for presentation at the 2013 Fall American Geophysical Union Meeting (Hu et al., 2013).

Impact of Research

      The studies described above have made a significant impact on the career of Levin. They have established the JHU isotope lab as one of the few labs capable of producing high-precision Δ17O data in waters. The data from this first year of work associated with this grant also establishes Levin as an emerging leader in studying Δ17O variation in waters from mid- to low-latitudes with a focus on building baseline datasets that can be used to interpret Δ17O variation in the geologic record. The additional capability of measuring Δ17O in carbonates, developed by Passey with partial support from this grant, furthers these goals, situating the JHU stable isotope lab to make significant advances in Δ17O studies of waters, CO2and carbonates.

      This grant has provided full support to Ms. Li’s PhD dissertation. It has also supported her involvement in projects that will not be part of her dissertation but on which she will be a co-author, facilitating her interactions with a wide range of researchers, teaching her skills in generating and handling datasets, and providing her first-hand exposure to the process of publishing a manuscript.  This grant has also supported the training of other graduate students as they have benefited from the analytical advances in the lab, even though they are not directly supported by the grant.