Techniques and methodology BGC

The hydrogen isotope ratio [= the relative deuterium content (δD) expressed in ‰ relative to standard mean ocean water (VSMOW, Vienna Standard Mean Ocean Water)] of water is depending on evaporation and precipitation. Water vapor contains relatively little deuterium (= low/more negative δD value) and the remaining water becomes enriched in deuterium (= high/more positive δD value). Seawater evaporates mostly at lower latitudes, close to the equator, there the seawater becomes enriched in deuterium, the water vapor is transported to higher latitudes, the poles, and rains out. The first rain close to the water vapor source is relatively enriched in deuterium, leaving water vapor that becomes more and more depleted in deuterium. Therefore, later rain, further away from the vapor source, will contain less and less deuterium. Freshwater from different locations on Earth will contain different relative amounts of deuterium, different δD values, depending on the distance from the water vapor source. These differences are used to determine the authenticity of different products, for instance, does Spanish wine originate from Spain or Greek olive oil from Greece etc.

At the NIOZ hydrogen isotopes are being used to reconstruct past climate and ocean currents. By measuring the hydrogen isotopic composition of plant material we can get an idea of the evaporation/ precipitation balance in the area where the plants are or were growing. By analyzing the specific plant derived chemical fossils extracted from sediment cores for their hydrogen isotopic composition we can reconstruct changes in precipitation versus evaporation in the past (for an extensive review see Sachse et al., 2012).

Since seawater not only becomes more enriched in deuterium when water evaporates, but also becomes more saline and freshwater contains relatively little deuterium, salinity and the δD value of seawater are correlated. Unfortunately we cannot measure the hydrogen isotopic composition of seawater from the past, but we can measure the δD value of chemical fossils specific for certain algae that lived in that water. From cultivation experiments it is clear that the hydrogen isotopic composition of algae is strongly correlated with the salinity of the water in which they were grown (Schouten et al. 2006). First because the δD value of the water is correlated with salinity, but also because algae build in more deuterium at higher salinities or in other words, algae fractionate less against deuterium when growing at higher salinities. The correlations between δD and salinity and biological fractionation and salinity can now be used to reconstruct salinities in the past (van der Meer et al., 2007) and therefore changes in ocean circulation.

Besides reconstructing climate and ocean circulation in the past, hydrogen isotope ratios of organic matter can also provide information on the metabolism of the organisms producing that organic material. For instance, organisms that use light as energy and CO₂ as carbon source have a very different hydrogen isotope ration than organisms that use the oxidation of sulfide as energy and CO₂ as carbon source and that is again very different from organisms using the oxidation of organic matter as energy and carbon source. Hydrogen isotope analysis could therefore potentially provide a metabolic fingerprint for different ecosystems, also those from the past.

In order to measure the hydrogen isotopic composition of organic matter it has to be converted into hydrogen gas (H₂ or HD) and graphite at very high temperature (approximately 1450 °C). The graphite formed is also the catalyst for this reaction. Individual compounds first have to be extracted, cleaned and separated on a gas chromatography column before they can be converted to hydrogen gas and graphite in a high temperature conversion reactor.

Literature:  

Sachse, D., I. Billault, G.J. Bowen, Y. Chikaraishi, T. Dawson, S. Feakins, K.H. Freeman, C. Magill, F.A. McInerney, M.T.J. van der Meer, P. Polissar, R. Robins, J.P. Sachs, H.-L. Schmidt, A. Sessions, J. White, J.B. West & A. Kahmen (2012). Molecular paleohydrology: interpreting the hydrogen-isotopic composition of lipid biomarkers from photosynthesizing organisms. Annual Review of Earth and Planetary Sciences 40: 221-249.

Schouten, S., J. Ossebaar, K. Schreiber, M.V.M. Kienhuis, G. Langer, A. Benthien & J. Bijma (2006). The effect of temperature, salinity and growth rate on the stable hydrogen isotopic composition of long chain alkenones produced by Emiliania huxleyi and Gephyrocapsa oceanica. Biogeosciences 3: 113-119.

Van der Meer, M.T.J., M. Baas, W.I.C. Rijpstra, G. Marino, E.J. Rohling, J.S. Sinninghe Damsté & S. Schouten (2007) Hydrogen isotopic compositions of long-chain alkenones record freshwater flooding of the Eastern Mediterranean at the onset of sapropel deposition, Earth Planet. Sci. Lett. 262: 594-600.

Gaschromatograph-Thermal Conversion isotope ratio Mass Spectrometer

Increasing numbers and amounts of polar organic contaminants (e.g., pesticides, pharmaceuticals, personal care products) are released into the environment. Chemical monitoring is essential for protecting the environment against possible adverse effects of these compounds. Passive sampling devices (PSDs) are powerful tools for monitoring the chemical status of water bodies because they yield estimates of time-integrated aqueous concentrations at low detection limits, and at relatively low costs, compared with batch water sampling. PSDs consist of a central sorption phase that is covered by a membrane. They are deployed for a period of several weeks to months.

Sampling device for polar organic contaminants

Contaminant uptake by PSDs is linearly proportional to their aqueous concentrations, but also depends on the environmental conditions during the deployment (e.g., water flow, temperature, salinity, pH). Our research focusses on understanding and quantifying the effect of these exposure conditions, and on developing methods for the in-situ calibration of PSD uptake rates, aiming to improve the accuracy and the precision of time-integrated aqueous concentrations of nonpolar and polar organic contaminants in marine monitoring programs.

Working principle of a Polar Sampling Device (PSD)