Pollen fraction sample in a core at a depth of 513 cm (Sluyter and Dominguez 2006:1149).
Sluyter, A. and G. Dominguez 2006 Early maize (Zea mays L.) cultivation in Mexico: dating sedimentary pollen records and its implications. Proceedings of the National Academy of Sciences, USA 103(4):1147-1151.