Previous research

The development of our SIRIS techniques is strongly application driven. In particular, while at the Center for Isotope Research of the University of Groningen in The Netherlands from 1995 (under the direction of Prof. Wim Mook) until 2009 (by then directed by Prof. Harro Meijer), our initial efforts have focused on applications in paleo-climatology and bio-medicine. To start with the latter, the so-called doubly-labeled-water (DLW) method, enables the quantification of the energy expenditure of humans and free-roaming animals in their natural habitat. The Groningen Center for Isotope Research (in the person of Prof. G.H. Visser of the CIO and RuG Biology) has played a prominent role in the further development and dissemination of this, for biology and medicine, very important method.

The Biomedical Doubly Labeled Water method

In the DLW method a known amount of water, enriched in both ¹⁸O and ²H, is administered to the subject, either orally, intraperitoneally, subcutaneously, or interveneously. After a short equilibration period, an initial blood, salva or urine sample is taken. The enrichment of the body water pool due to administration of DLW enables the calculation of the size of the body water pool. After some time, typically 24 hours, a final sample is taken. The final sample will show lower ¹⁸O and ²H concentrations than the initial. The lower ¹⁸O concentration of the final sample is caused by the CO₂ and H₂O turnover during the measurement interval. The H₂O turnover is independently monitored by the ²H concentration, such that in the end the CO₂ production can be calculated from the difference in ¹⁸O and ²H turnover rates. Thus, at a known diet, the energy expenditure of the animal is known. In the sampling interval, the subject is free to move. This is in stark contrast to the most wide-spread method so-far, dating back to the experiment that Lavoisier (“rien ne se perd, rien ne se cree, tout se transforme”) carried out with his assistant Séguin, in which the subject is confined to a (small) closed chamber and the air is analyzed for O₂ and CO₂ changes. The DLW method has been used extensively, and is especially well suited to study how an individual can adapt to a harsh environment. For example: premature babies, astronauts in space, Emperor penguins in Antarctica.

A major advantage of the laser method is that both the 2H and the 18O isotope ratios are determined on one and the same sample, avoiding the two different (chemical) preparatory paths required for mass spectrometric analyses (and possibly uncorrelated systematic errors). Moreover, it turns out that the incertainty in the SIRIS measurement grows notably slower with increasing enrichment level, such that at the often very high enrichment levels used in the DLW method, the measurement uncertainties of SIRIS and IRMS are comparable or even smaller for SIRIS. Finally, since in the calculation of the energy expenditure the isotope ‘delta-values’ for 18O and 2H are substracted, the final error is dominated by the error in the 2H/1H determination, which is comparable for SIRIS and IRMS over the entire range of enrichment levels.

Ice-core paleothermometry

A second application of our research places very high demands on the measurement precision to resolve changes in the isotopic composition of polar ice, which may be used to reconstruct Earth’s past climate, going several hundreds of thousand years back in time. In the kilometers thick polar ice caps a clear stratification is visible due to annual precipitation events, comparable to the tree-rings in dendrology. Since the central parts of the ice caps have seen practically no melting, the depth-time relation is virtually continuous. The 18O or 2H isotope concentration can be used as a temperature proxy: A graph of either isotope concentration as a function of depth in this glacier ice shows a pattern that corresponds very well to that seen in other climate archives (such as pollen records), but with a much higher temporal resolution.

In a much simplified representation of the actual situation, water evaporates near the equator, the lighter molecules moving more easily into the vapor phase than their isotopically heavier counterparts. Moreover, in precipitation events occurring during transport of the water vapor to higher latitudes, the heavier molecules condense preferentially, and this entire process is temperature dependent. Consequently, during a colder climate, the ice deposited will be lighter than during warmer periods. Cores drilled in this ice thus present a beautiful archive of Earth’s past climate. Our access to this valuable material was guaranteed through collaborations with the group of Sigfus Johnsen in Copenhagen.

One can show that the exact relation between the 2H and 18O concentrations (also known as the deuterium excess) is very sensitive to changes in the temperature and relative humidity in the source area of the moisture, such that can obtain a much more detailed picture of the climate history by measuring both 18O and 2H. In this way, the Copenhagen group already established that the transition from last glacial to the current mild climate (the Younger Dryas – Holocene transition, about 10,700 years ago) occurred much more rapidly (about 15 degrees in 100 years) than assumed until then, and in addition that this change was accompanied by a shift to higher latitudes of the polar front. The immediate cause is believed to be the return of the North Atlantic Current to higher latitudes and an associated northward shift of the polar front (Ruddimand and McIntyre 1981, Broekcer et al. 1985, Bond 1995), creating a vast area of initially cold surface water as an additional source of moisture (Dansgaard and others. 1989). Recently, we have shown that another rapid climate change of about 14,500 years ago (the Bolling-Allerod transition) was very likely caused by a similar mechanism.

Stratospheric water

Water in the lower layer of the atmosphere (the upper troposphere, extending from about 5 to 12 km, and the lower stratosphere from approximately 12 to 30 km) is the most important greenhouse gas and plays a crucial role in the climate system by its effect on the radiation balance and the energy transport associated with the many phase changes occurring in the hydrological cycle. Water is also a key component of the atmospheric chemistry. For example, there are indications that an increasing stratospheric water concentration in the Arctic vortex is linked to an increasing ozone destruction. Still, our knowledge of the origin and the fate of stratospheric water in particular is quite rudimentary. The stratosphere is extremely arid, and the common explanation is that the tropospheric air is freeze-dried during transport to stratospheric heights. However, the stratospheric water concentration is actually so low, that one would have to assume that the troposphere to stratosphere transport predominantly occurs near the tropical tropopauze, where the average temperature is the lowest (~ -80 °C), and even then, only during the coldest periods of the year. Recent measurements show that this explanation is not sufficient. While the tropical tropopauze and the lower stratosphere have become colder over the last decades, measurements indicate an increase in the stratospheric water concentration. In addition to the direct injection of water into the stratosphere by deep convection, water is also formed in-situ by methane oxidation. The latter process involves oxygen 1D atoms originating from ozone. One may therefore expect to see some of the 17O-18O isotope anomaly observed in ozone, also in stratospheric water.

Within the framework of the Dutch FOM program Molecular Atmospheric Processes we obtained funding already in 1999 to start developing the tools to study the above phenomena. Determining the isotopic signature of water vapor and ice particles in the upper troposphere and lower stratosphere, exchange processes can be followed in great detail and the relative contribution of methane oxidation to the overall water budget may be determined quantitatively. A major obstacle is the very low water concentration at these altitudes. Although we initially considered whole air sampling in combination with laboratory isotope analysis, such an approach was rejected because of the practical problems envisioned and the low temporal and spatial resolution. Instead we developed an ultra-sensitive infrared spectrometer, in collaboration with the group of Daniele Romanini at the University of Grenoble (France). The spectrometer uses an optical cavity of very high finesse to create an effective optical path length of the order of 10 km. Injection of the laser light into the consequently very narrow cavity modes is done using a patented scheme involving self-locking of the diode laser as a result of optical feed-back. For the atmospheric science part of this project we have a close collaboration with the group of Hans-Jurg Jost of NASA Ames. In May 2004 the instrument was flown on NASA’s DC-8 of the Dryden Flight Research Center (at Edwards Air Force Base, California) during an engineering test flight. The flight was very successful in that it provided a wealth of engineering data, and even some science quality data as to the performance of the instrument in the harsh environment of an airplane platform. The instrument was also flown on NASA's WB-57, and more recently in 2006, during a EUFAR funded campaign in Italy and Burkina Faso, on the Geophysica high altitude airplane, as part of the larger African Monsoon Multidisciplinary Analyses (AMMA) campaign.

A very similar device was adapted to continuously monitor the isotopic signature of troposheric air moisture.

Volcano research

Through a close collaboration with the the group of Livio Gianfrani at the Second University of Naples we got involved in investigations of fumarolic emissions by spectroscopic means, and in particular the in-situ measurement of the isotopic composition of carbon dioxide. The diagnostic value of such measurements promises to be very high, as previous research indicates that changes in the isotopic composition of the fumarolic emissions are a precursor of volcanic (seismic) activity. In additions, isotopes can help in elucidating the nature and origin of the emissions, as well as the structure of the volcano.

Plant leaf water evaporation

We have studied the temporal behavior of the deuterium isotope ratio of water vapor emerging from a freshly cut plant leaf placed in a dry nitrogen atmosphere. The leaf material was placed directly inside the sample gas cell of the stable isotope ratio infrared spectrometer. At the reduced pressure (∼40 mbar) inside the cell, the appearance of water evaporating from the leaf is easily probed by the spectrometer, as well as the evolving isotope ratios, with a precision of about 1‰. The demonstration experiment we describe measures the 2H/1H isotope ratio only, but the experiment can be easily extended to include the 18O/16O and 17O/16O isotope ratios. Plant leaf water isotope ratios provide important information towards quantification of the different components in the ecosystem water and carbon dioxide exchange.