Isotopic analysis of archaeobotanicals to reconstruct past climates: Effects of activities associated with food preparation on carbon,hydrogen and oxygen isotope ratios of plant cellulose

The cellulose fraction of botanicals recovered from archaeologically defined strata may be suitable for stable isotope-based palaeoclimatology in much the same manner as cellulose from tree rings has been utilized. However, manipulation by humans is a distinct possibility with such specimens and constitutes a potential source of isotopic alteration early in the post-mortem history of the samples. We studied the susceptibility of plant cellulose to isotopic alteration caused by manipulations related to food processing to determine if discarded plant foods recovered from archaeological contexts are suitable substrates for isotopic palaeoclimatic studies. The carbon and hydrogen isotope ratios of cellulose nitrate and the oxygen isotope ratios of cellulose were determined for Zea mays cobs, Helianthus annus seeds, Agave americana leaves, Pachyrrhizus erosus tubers and a cellulose standard before and after a variety of treatments, including boiling, roasting, fermentation, liming, molding and carbonization. The oxygen and hydrogen isotope ratios (which are climatically sensitive in all plants) and the carbon isotope ratios (which reflect climate only in certain plants) of all samples, except for Agave, were conserved during various food-processing steps. Thus, it appears that the isotope ratios of cellulose and cellulose nitrate prepared from many archaeological food plants, whether processed or uncooked, will reflect in vivo values and can be used to reconstruct aspects of palaeoclimate. Such isotopic palaeoclimatic studies will extend the utility and value of existing collections of ecofacts by opening a unique window for the study of the relationships among climate, plants and man.