Stable Isotope Laboratory

The Stable Isotope Lab has two gas-source isotope ratio mass spectrometers. Isotopic ratios of the elements carbon, nitrogen, oxygen, hydrogen, and sulfur can be routinely measured. Converting a natural sample into gaseous form requires either manual preparation offline or the use of a peripheral device. The lab has glass vacuum lines for manual sample preparation and five peripheral units which automate the preparation of samples and input directly into a mass spectrometer. Types of samples that have been analyzed include water, DIC, snow, soil, soil gas, wood, cellulose, compound specific organic extracts, hair, fish, insects, and carbonates. In addition to the isotope ratio instrumentation, the lab has a DOC/DIC analyzer, a particle size analyzer and hosts glass-blowing facilities.

Lab Team

Principal Investigator: Xiahong Feng

Lab Supervisor: Anthony Faiia

Phone: (603) 646-1642

Resources

Lab Equipment

Mass Spectrometers

  • Delta+ XL
  • Delta+ Advantage

Peripheral Devices

  • H-Device
    • The H-Device allows for the automatic measurement of hydrogen isotopes in a liquid such as water or an organic extract. The liquid is injected and vaporized at 800-1000°C. The vapors react with chromium to produce Cr2O3 and H2 gas.
  • Gas Bench
    • The H-Device allows for the automatic measurement of hydrogen isotopes in a liquid such as water or an organic extract. The liquid is injected and vaporized at 800-1000°C. The vapors react with chromium to produce Cr2O3 and H2 gas.
  • TC/EA
    • The Temperature Conversion Elemental Analyzer is used to analyze oxygen and hydrogen isotopes in wood, cellulose, soil, and liquid samples. The sample is combusted at 400-1500°C in the presence of glassy carbon.
  • Carlo Erba EA
    • The Carlo Erba Elemental Analyzer is primarily used to analyze nitrogen, carbon, and sulfur isotopes in wood, soil, animal tissue, and organic extractions. The sample is combusted at 1200°C in the presence of chemical catalysts to produce CO2, N2, and SO2.
  • HP 5890 + GCC Interface
    • Utilizing the HP 5890 Gas Chromatograph, it is possible to separate individual organic compounds for isotope ratio measurement. The ability to analyze specific compounds is often necessary to understand the complexities of natural systems. The GCC Interface converts each compound into CO2 and N2 through combustion at 940°C.

Research Topics

Effect of Sea Ice on Arctic Precipitation

It has recently been forecasted that Arctic summer sea ice may completely disappear sometime during this century. Sea ice is an indicator of and has an influence on the rest of the climate system. It has a strong, direct and well-documented effect on Arctic albedo and sea surface temperature, and enhances evaporation and the Arctic hydrologic cycle. The sea ice-evaporation/precipitation link is a fundamental component of climate dynamics, and has been cited as an essential element of global warming, abrupt climate change, Dansgaard-Oeschger events, the Pleistocene ice ages, and “snowball earth”. However, there are remarkably few measurements that quantify the fundamental link between the ice-free Arctic Ocean area and evaporation/precipitation. The objective of this project is to quantify how moisture evaporated from the Arctic Ocean and surrounding seas contributes to precipitation in the Arctic regions, and how such moisture supply is controlled by sea ice extent. We do t his by studying oxygen and hydrogen isotopic compositions in precipitation collected storm-by-storm at two Arctic stations, Barrow and Atqasuk, Alaska. More Arctic stations in Canada, Norway, Greenland, and Russia are under development for similar sampling programs. By looking at weather patterns and its evolution that are responsible to a given storm, we are determining what part of the ocean surface contributes to the moisture budget of the storm using D/H and 18 O/ 16 O ratios in the precipitation samples. The results will be interpreted for the individual influences of the ice-free area of the Arctic, of storm tracks, of atmospheric temperatures at both moisture sources and precipitation sites, and of circulation changes associated with the Arctic Oscillation and/or the North Atlantic Oscillation.

This project will yield a quantitative understanding of the link between sea ice and moisture sources of Arctic precipitation, which is important in the study of climate dynamics on a wide range of time scales. It will also provide new data and insight for interpretations of climate information recorded in ice cores, which is relevant for testing global warming projections, abrupt global change scenarios, and ice age theories, as well as for further verification of climate models.

Research Topics

Hydrogen, oxygen and carbon isotope studies of tree rings

The main objective of this project is to extract climatic information from tree rings using hydrogen, oxygen isotopes. Tree cellulose inherits hydrogen and oxygen isotopic signatures of meteoric water that can be related to climatic variables, such as temperature, source of moisture and precipitation amount. Therefore, isotopic compositions of tree rings are potential tools for paleoclimatic studies over time scales of hundreds to thousands of years. Using radiocarbon-dated wood, one can study paleoclimate deeper in geological time.

In collaboration with Eric Posmentier, our most recent work has been focused on a reconstruction of prevailing wind directions and moisture transport pathways in mid latitudes of North America using wood samples from Holocene and the last glacial period. We showed that between 40 and 50˚N in North America, which is within the zone of the northern hemisphere westerlies today, experienced prevailing easterly winds during the last glaciation. The change was caused by intensification and enlargement of the northern circumpolar vortex under the powerful influence of the Laurentide Ice Sheet.

Research Topics

Oxygen and Hydrogen isotopic compositions in leaf water

Understanding the isotopic variations in plants’ leaf water is important for a number of climatological and biogeochemical studies. Leaf water isotopic composition is affected by the isotopic composition of the source water, and the relative humidity of the air, both of which are related to climate. This dependency is the basis for climate reconstruction using isotopic compositions of tree-ring cellulose and leaf waxes from lake sediments. The isotopic composition of leaf water has also been used to estimate terrestrial biological productivity either by modeling d 18O of the atmospheric CO 2 or through calculating the Dole Effect.

In addition to the isotopic composition of source water and the relative humidity of the air, we have found that the oxygen and hydrogen isotopic compositions in leaf water also varie with the age and shape of the leaf. Along single pine needles, we have observed a 20‰ change in d 18O from the base to the tip. We have also observed a ~20‰ difference in d 18O between young and old leaves. In collaboration with Eric Posmentier and Leslie Sonder, we are constructing models to simulate isotopic variations along single pine needles, and using the model we are studying the pattern of isotopic enrichment of leaf water as a function of environmental and anatomical conditions.

Research Topics

Study of plants’ water used efficiency using C isotopes of tree rings

Carbon isotopes in tree rings record the isotopic composition of the atmosphere and, under certain conditions, the atmospheric CO 2 concentrations. Using carbon isotopes of tree rings we can examine whether natural trees have experienced any physiological changes as a result of increasing atmospheric CO 2 concentration. One physiological character of plants that are important for the global water cycles and carbon cycle, and can be quantified by carbon isotopes is plants’ water use efficiency, which is defined as the mass water transpired per unit mass carbon fixed by photosynthesis. I have shown that many natural trees increase their water use efficiency with an increasing atmospheric CO 2 concentration. This research may be linked with that of C and N dynamics in plant-soil systems (see Carbon and nitrogen cycles in plant-soil systems). We hope to quantify changes of both water and nutrient use efficiency as a function of space (under different climate) and time.

Research Topics

Carbon and nitrogen cycles in plant-soil systems

In collaboration with a number of ecologists both inside and outside Dartmouth College, I have been studying carbon and nitrogen isotopic variations of plants, litter and soil organic matter. The central focus of these projects is to understand C and N cycles in plant-soil ecosystems, and to use this understanding to predict future changes of carbon pools of terrestrial ecosystems as one of the carbon source or sink of the atmospheric CO 2 in response to climate and environmental changes. The following are two on-going projects under this theme.

1) C and N Isotopic Variations During Organic Matter Decomposition: We have been studying C and N isotopic variation during organic matter decomposition in partially decomposed litter materials. We are using the isotopic signature to infer decomposition rates and mechanisms of various components of organic matter. One specific project, in collaboration with Ross Virginia at Dartmouth and Diana Wall at Colorado State University, involves analyzing litter samples that has been partially decayed at over 20 locations of the world with wide distributions of temperature and precipitation.

2) C and N dynamics and Isotopic Systematics in Plant-Soil Systems Along Climate Gradients: In order to predict ecosystem responses to future climate change and its feedbacks to climate systems, it is important that we study C and N dynamics along climate gradients. Understanding from these studies helps us answer questions such as how water use and nutrient use efficiency of various ecosystems change when climate becomes warmer. Both water use and nutrient use efficiency are important variables for assessing changes of the terrestrial carbon pool and have strong implications to the atmospheric CO 2 budget. In collaboration with Weiguo Liu, Guoan Wang and Liping Zhuo, we are studying soils along several climate gradients in China, including the Northeast forests, the Loess Plateau and the Gangga Mountains in the Tibetan Plateau.

Research Topics

Isotope hydrology and hydrochemistry

There has been an increasing recognition of a variety of hydrochemical problems at the watershed scale, including long-term acidification of surface and subsurface waters, sensitivity of aquatic biota to episodic pulse of pH depression associated with large runoff events, impact of land use on aquatic lives, and transport of pollutants from nonpoint sources. One of the most important approaches to studying these problems is to identify the pathways of water and water-soluble components by linking hydrological models to chemical transport mechanisms. This requires specifying the water sources that contribute to stream discharge, and understanding the nature and rates of the chemical reactions (e.g., adsorption, reduction, dissolution and cation exchange) that occur in each of these source reservoirs and along each pathway to the stream. In collaboration with a number of investigators, I am involved in several projects related to watershed hydrology and hydrochemistry.

  1. Studies of Stable Isotopes and Chemistry of Meltwater from Seasonal Snowpacks: Snowmelt is one of the major water sources in drainage basins and may dominate hydrology in many areas. Seasonal snowpacks may also be sources of surface and groundwater pollution because they accumulate atmospherically derived contaminates through the winter and release them to catchments often in more concentrated pulses. This project involves both cold room experiments and field investigations in a well-instrumented snow laboratory, the Central Sierra Snow Laboratory in California. The ultimate objective of this project is to predict the isotopic and chemical compositions of snowmelt given a set of input compositions of snowstorms and climatic conditions at watershed scale.
  2. Studies of time series of precipitation and stream chemistry: The travel time of water through a catchment -- that is, the time it takes for rainfall to reach the stream -- is a fundamental hydraulic parameter controlling the retention of soluble contaminants, and thus the downstream consequences of pollution episodes. In collaboration with Dr. James Kirchner at the University of California at Berkeley, U.S.A. and Colin Neal at the Institute of Hydrology in England, we are studying travel time (or residence time) distributions in a number of watersheds using the relationship between time series of precipitation and stream chemistry. This study has important implications for transport of soluble contaminants through watersheds.
  3. Multi-Tracer Approach to Hydrochemistry: Hydrochemistry studies chemical interactions of water with the soil and rocks of the earth’s crust when water moves through them, which determines the chemical character of groundwater or surface water. Chemical compositions of groundwater water are essential measures of water quality. These compositions are shaped by many intricate connections among hydrological, chemical and biological processes in catchments. Where water enters a catchment, what pathway it takes, how it recharges, how long it stays in the subsurface, what materials and organisms it encounters on its journey – all interact to determine its chemical properties. Different pathways have different chemical interactions between water and the solid medium. For example, within soil layers, the interactions are typically cation exchange, while in the deep bedrock, chemical weathering reactions dominant the water chemistry. Our approach to understanding the chemistry of groundwater and surface water is to use different type of tracers to study water having different flow history. One on-going project, in collaboration with Bill McDowell at UNH and Mukul Sharma and Eric Posmentier at Dartmouth, is to study water chemistry in a granite catchment in Puerto Rico.