59th Annual Report on Research 2014

The work associated with the grant this year has centered on expanding the datasets from precipitation in the U.S., completing the plant water study from Mpala Kenya, and initiating a experimental study to determine the triple oxygen isotope fractionation exponent between calcite and water. The main findings from the work this year include 1) the range in Δ¹⁷O of meteoric waters at mid-latitudes (-0.06 to + 0.07‰) is much larger than previously documented (+0.02 to +0.07‰), 2) there are large seasonal differences in Δ¹⁷O in precipitation at a single location (0.02 to 0.11‰), and 3) the relationship between Δ¹⁷O and d-excess is linear when kinetic processes dominate isotopic fractionation. These findings are a part of a greater effort to lay the groundwork for using mass dependent variation in Δ¹⁷O as a tool to study water-rock interactions and paleoclimate. All aspects of this work have involved PhD student Shuning Li who is expected to finish her dissertation in February 2015.

Δ¹⁷O of precipitation in the western U.S.

The triple oxygen isotope analyses of precipitation samples from the United States Network for Isotopes in Precipitation (USNIP) were expanded in the second year of the grant to include winter precipitation. We expanded the dataset to investigate whether the relatively low Δ¹⁷O values of summer precipitation samples were representative of the entire year. Seasonal comparisons of the isotopic composition of rainfall at 20 USNIP stations indicate that winter precipitation is on average 0.041‰ more positive than summer precipitation at the same site. This seasonal offset in Δ¹⁷O is mirrored by δ¹⁸O values, which were on average 13‰ lower in winter precipitation than summer precipitation at the same site. The maximum seasonal range in Δ¹⁷O values was observed in South Dakota where mean Δ¹⁷O from winter precipitation is enriched relative to summer precipitation by 0.110‰. The seasonal Δ¹⁷O difference may be due to evaporative enrichment of water during summer time precipitation (a process that would not affect winter precipitation to the same degree) but this explanation needs to be explored in more detail. The d-excess of these samples will be measured in the upcoming months and will help us evaluate whether evaporation is driving these trends. These results are important for establishing the natural range of variation in Δ¹⁷O among meteoric waters and understanding how this variation is archived in the rock record. The results for this work will be presented at the 2014 AGU Fall meeting by Shuning Li and the manuscript that reports the results is currently in preparation by Naomi Levin.

Plant water study

Studies of the triple oxygen isotopic composition of leaf waters from Mpala, Kenya were pursued with funding from this grant and comprise a portion of Shuning Li’s dissertation research. The results of this study demonstrate how Δ¹⁷O and d-excess linearly covary in leaf waters, following a slope of 0.017‰ Δ¹⁷O per 1‰ d-excess. This relationship is similar to what is predicted for a simple re-evaporation model and the relationship obtained from open-pan evaporation experiments that we conducted in the laboratory. The combination of Δ¹⁷O and d-excess data provides constraints on whether kinetic effects associated with evaporation dominate isotopic fractionation, or whether fractionation is more complicated. The plant water data from this study provides a valuable example of the simple case in which re-evaporation governs the isotopic composition of waters. These data and the data from our evaporation experiments will provide a useful framework for interpreting the influence of evaporation on Δ¹⁷O values in waters (or geologic materials) and the relationship between Δ¹⁷O and d-excessvalues. The results for this work were presented at the 2013 AGU Fall meeting by Ms. Li and the manuscript that reports the results is currently in preparation.

Carbonate precipitation studies:

This grant has also supported carbonate precipitation studies for experimental determination of the triple oxygen isotope fractionation exponent between calcite and water. This effort has been spearheaded by Shuning Li, with the assistance of Dr. Ben Passey (faculty member at Johns Hopkins, not supported on this grant). Calcite was precipitated at five temperatures between 5 and 60˚C; waters used for the precipitations have been measured for Δ¹⁷O and the carbonates have been measured for δ¹³C, δ¹⁸O and Δ₄₇. Analysis of the carbonates for Δ¹⁷O is scheduled for October 2014. The results will be instrumental to interpreting Δ¹⁷O values in carbonates and linking them to Δ¹⁷O variation in the waters from which they form.

Impact of Research

This grant has provided the main support for Shuning Li’s PhD research by providing her with extensive experience in the lab, opportunities to interact with researchers at different universities and disciplines, and training in presentation of data at conferences and in manuscripts. This grant supported the three studies that will comprise her PhD: 1) triple oxygen isotopic survey of tap waters in the U.S. (in review at GCA), 2) Δ¹⁷O variation in leaf waters, and 3) experimental studies of the triple oxygen isotope fractionation factors for the calcite-water system at earth surface temperatures. All of these papers will make significant contributions to understanding the mass-dependent variation of triple oxygen isotopes in the hydrosphere and the geologic record.

This grant has also provided a significant boost for the career of Naomi Levin, the principal investigator, as it has facilitated the establishment of the JHU stable isotope labs as one of the few facilities where high-precision mass-dependent Δ¹⁷O variations can be measured in waters and carbonates. Recognition of Levin’s growing expertise is evident in an invited talk for the 2014 AGU Fall meeting to specifically present on advances in triple oxygen isotopes and applications to isotope hydrology and paleoclimate studies.