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Paul Dennis
Head of the Stable Isotope and Noble Gas Laboratories

My main research interests are in the application of natural stable isotope chemistry to environmental and palaeoclimate studies. I am also very active in instrument design, developing new, high sensitivity isotope ratio mass spectrometers in order to both analyse small samples with a high degree of precision and to measure the natural variation of oxygen in the atmosphere.

The stable isotopes of many elements are fractionated during changes in state of a compound and by chemical reactions between compounds in which the elements occur. For example, in the water cycle both oxygen and hydrogen stable isotopes are strongly fractionated during evaporation from the liquid to the vapour phase and vice versa as the vapour condenses back to a liquid. Similarly there is fractionation of carbon and oxygen isotopes between dissolved inorganic carbon and carbonate minerals that are precipitated from solution. The elements whose isotopes are susceptible to fractionation include H, O, C, N and S. These are all towards the light end of the periodic table with large relative mass differences between their stable isotopes. The extent of fractionation is dependent on the difference in mass between two isotopes and the temperature of the phase change or reaction. Whilst the degree of fractionation, that is the enrichment or depletion of an isotope between two phases or compounds is small it can be measured to a high degree of precision by mass spectrometry. These measured variations in the isotopic composition of the light elements conveys information about the physical, geochemical and biological processes that have acted on natural samples.

In my laboratory we use stable isotope geochemistry to help us understand aspects of past and present climate and environment change. The isotopic composition of fossil rainwater trapped in stalactites and stalagmites collected from caves helps us to unravel details of the climate in western Europe over the past 11,000 years. An analysis of magnetic dust and the isotopic composition of tiny marine creatures known as foraminifera in deep sea marine cores gives us clues to the processes that occur at the end of an ice age 130 thousand years ago when the climate changed rapidly from a cold glacial to a warm interglacial world. Even further back in geological time, some 65 million years ago, the dinosaurs became extinct at the end of the Cretaceous period. There is speculation as to the cause of the extinction: meteorite impact or volcanic activity with the huge eruption of the Deccan Traps in India. An analysis of dinosaur egg shells, collected from sediments that intermingle with the Deccan Trap volcanic lavas has helped us to understand the climate and atmospheric carbon dioxide levels at the time the dinosaurs became extinct.

On the modern time scale measurement of the tiny variations in the oxygen isotope composition of ocean water allows us to characterise the role of evaporation and precipitation, the formation of sea ice and the melting of glacial ice in the formation of deep and bottom waters of the World's oceans. The sinking of surface water to form deep and bottom water in the North Atlantic and Antarctic oceans is the main driving force for the ocean thermo-haline circulation and has a major impact on our climate. Our drinking water, much of it extracted from underground reservoirs, is a valuable resource. Measurements of the stable isotope composition of water in these underground aquifers shows that many were recharged at the time of the last glaciation. Information on periods and rates of recharge is an important tool in the management of these resources. In a similar way, stable isotopes can be used to understand the pollution chemistry of our water supplies. Nitrate pollution and high nitrate concentrations in drinking water, long associated with blue baby syndrome, is an important issue in agricultural areas. Studies of the nitrogen isotope chemistry of groundwater helps us to understand the source of nitrate pollution and the rate at which nature recovers through natural denitrification process. In the atmosphere stable isotopes can be used to study the role of carbon dioxide as a greenhouse gas and also sources of marine and terrestrial pollution.

Some Recent Publications

Andrews J. E., Tandon S. K., and Dennis P. F. (1995) Concentration of carbon dioxide in the Late Cretaceous atmosphere. Journal of the Geological Society, London 152, 1-3.

Dennis P. F., Rowe P. J., and Atkinson T. C. (2001) The recovery and isotopic measurement of water from fluid inclusions in speleothems. Geochimica Cosmochimica Acta 65, 871-884.

Feast N. A., Hiscock, K.M., Dennis, P.F. (1999) Nitrogen isotope hydrochemistry and denitrification within the chalk aquifer system of north Norfolk, UK. Journal of Hydrology 211, 233-252.

Frew R. D., Heywood K. J., and Dennis P. F. (1995) Oxygen isotope study of water masses in the Princess Elizabeth Trough, Antarctica. Marine Chemistry 49, 141-153.

Frew R. D., Dennis P. F., Heywood K. J., Meredith M. P., and Boswell S. M. (2000) The oxygen isotope composition of water masses in the northern North Atlantic. Deep Sea Research, 47, 2265-228

Fukada, T., Hiscock, K.M. and Dennis, P.F. (2004) A dual-isotope approach to the nitrogen hydrochemistry of an urban aquifer. Applied Geochemistry 19, 709-719

Hiscock K. M., Dennis P. F., Saynor P. R., and Thomas M. O. (1996) Hydrochemical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK. Journal of Hydrology 180, 79-107.

McArdle N., Liss P., and Dennis P. F. (1998) An isotopic study of atmospheric sulphur at three sites in Wales and mace Head, Eire. Journal of Geophysical Research 103(D23), 31,079-31,094.

Maher B. A. and Dennis P. F. (2000) Northern hemisphere dust fluxes: Significance for Fe fertilisation in the Southern Ocean. Nature 411, 176-180.

Tandon S. K., Sood A., Andrews J. E., and Dennis P. F. (1995) Palaeoenvironments of the dinosaur-bearing Lameta Beds (Maastrichtian), Narmada Valley, Central India. Palaeogeography, Palaeoclimatology, Palaeoecology 117, 153-184

Yeatman, S.G., Spokes, L.J., Dennis, P.F. and Jickells, T.D. (2001) Can the study of nitrogen isotopic composition in size-segregated aerosol nitrate and ammonium be used to investigate atmospheric processing mechanisms. Atmospheric Environment 35, 1337-1345