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	The Marijuana Signature Project is exploring isotopes as "signatures" of marijuana growth environments and geographic origins.
	Plants grown in different enviornments have different carbon and nitrogen isotope ratios, in part because of isotopic differences in their carbon and nitrogen sources (atmospheric CO2 and fertilizers). These differences can then be used to infer the growth environments of samples with unknown origins. The Marijuana Signature Project has developed the baseline for interpreting the carbon and nitrogen isotope ratios of marijuana through the analysis of hundreds of marijuana samples grown across the United States and elsewhere.
	The hydrogen isotope ratios of plants, including marijuana, are related to geographic origin primarily because of the spatial variation in precipitation isotopes (see http://waterisotopes.org for downloadable maps). Plants take up soil water that was ultimately sourced from precipitation and they hydrogen atoms of this water are incorporated into organic matter initially during photosynthesis, but also during other biosynthetic reactions. As with carbon and nitrogen, we have established the baseline for interpreting hydrogen isotope ratios for marijuana and the project is moving into new phases to improve on existing models.
	In addition to hydrogen, the isotope ratios of "heavy" elements are being evaluated for their potential to yield geographic information for marijuana. Strontium isotope ratios vary geographically based principally on bedrock/parent material composition and age, and Sr deposition patterns. Although there is no fractionation with uptake, other processes such as erosion and Sr cycling can also affect the Sr isotopic composition of plant-available Sr. Our results show that Sr isotope ratios also yield geographic information for marijuana and we are continuing to explore these tools for geo-sourcing marijuana. (Sr map predictions based on: Beard BL, Johnson CM. 2000. J Foresnic Sci 45:1049-1061).