Equal abundance of odd and even n-alkanes from cycad leaves: can the carbon preference index (CPI) faithfully record terrestrial organic matter input at low latitudes?

Long chain n-alkanes from the leaves of thirteen extant cycad species within the Cycadacaea, Stangeriaceae, Zamiaceae families were measured by gas chromatographymass spectrometry (GC-MS). Cycad n-alkane patterns ranged from nC10-nC37, were unimodal in distribution, maximised at nC27-nC33 and gave average chain length (ACL) values of 23.9 to 31.0. Low carbon preference indices (CPI) in the range of 0.88 to 2.70 were observed which is atypical of lipids from the leaves of terrestrial plants. Analysis of variance between the three families of true cycads showed that there were no significant differences between CPI values. The unusually low CPI values (<1.5) in nine of the thirteen cycad species analysed suggests that caution needs to be exercised in the use of n-alkanes distibutions as a chemical marker of terrestrial plant input in sediments from tropical and sub-tropical regions.