CRYSTALLITE SIZE DISTRIBUTIONS OF MARINE GAS HYDRATES Stephan A. Klapp 1* , Susanne Hemes 2 , Helmut Klein 2 , Gerhard Bohrmann 1 , Werner F. Kuhs 2 , Fritz Abegg 1 1 Marum / Research Center Ocean Margins, University of Bremen, P.O. 330440, D-28334 Bremen, Germany 2 Abteilung Kristallographie, Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany ABSTRACT Due to experimental difficulties, size distributions of gas hydrate crystallites are largely unknown in natural samples. For the first time, we were able to determine crystallite size distributions of several natural gas hydrates for samples retrieved from the Gulf of Mexico, the Black Sea and from Hydrate Ridge offshore Oregon from varying depth below the sea floor. High-energy syn- chrotron radiation provides high photon fluxes as well as high penetration depth and thus allows the investigation of bulk sediment samples. The gas hydrate crystallite sizes measured with a newly developed diffraction technique, utilizing the excellent beam collimation, appear to be (log-) normally distributed in the natural samples and to be of roughly globular shape. The mean crystallite sizes are typically in the range from 200-400 µm for hydrates recovered from the sea floor while a tendency for bigger grains was noticed in greater depth for the Hydrate Ridge sam- ples, indicating a difference in the formation age or formation process. Laboratory produced methane hydrate, starting from ice and aged for 3 weeks, shows half a lognormal curve with a mean value in the order of 40µm. This one order-of-magnitude smaller grain sizes suggests that care must be taken when transposing crystallite-size sensitive (petro-) physical data from labora- tory-made gas hydrates to natural settings. Keywords: crystallite size distribution, synchrotron radiation, Black Sea, Gulf of Mexico * Corresponding author: Phone: +49 421 218 8656 Fax +49 421 218 8664 E-mail: sklapp@marum.de INTRODUCTION Gas hydrates are materials which crystallize under inclusion of gas molecules into rigid cages of water molecules. On Earth, gas hydrates form in polar environments, particularly in onshore and offshore sediments, as well as on continental margins [1]. Hydrates need elevated pressure and cold tempera- ture to form; in marine environments gas concen- trations must exceed solubility in interstitial waters [2]. Ambient gas composition governs the crystal- lizing gas hydrate structures. Methane and small fractions of ethane form structure I gas hydrate. Methane and hydrocarbon molecules up to the size of butane (C 4 ) form structure II gas hydrate. C 5 molecules within the gas mixture of hydrates would lead to the formation of a hexagonal gas hydrate (structure H). Gas hydrates are considered a large hydrocarbon reservoir on Earth [3-5]. The absolute quantity of carbon stored in gas hydrates is uncertain, yet Buf- fett and Archer [6] figure 3000 Gt carbon being stored in gas hydrates. Klauda and Sandler [7] report 74,400 Gt of methane being stored as hydrates; their model is regarded state-of-the-art as it enables pre- diction of almost all known hydrate occurrences [8]. The significance of hydrates as reservoirs for meth- ane and other hydrocarbons lies in their potential role as a source for greenhouse gases and conse- quently as a probable driving force for global warm- ing [9-11]. Additionally, hydrates might turn into Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.