Stable hydrogen and carbon isotopic compositions of long-chain (C _(21)-C _(33)) n-alkanes and n-alkenes in insects

摘要

We report the molecular and stable isotopic (δD and δ ~(13)C) compositions of long-chain n-alkanes in common insects including the cabbage butterfly, swallowtail, wasp, hornet, grasshopper, and ladybug. Insect n-alkanes are potential candidates of the contamination of soil and sedimentary n-alkanes that are believed to be derived from vascular plant waxes. Long-chain n-alkanes (range C _(21-33); maximum C _(23)-C _(29)) are found to be abundant in the insects (31-781μg/dry g), with a carbon preference index (CPI) of 5.1-31.5 and an average chain length (ACL) of 24.9-29.3. The isotopic compositions (mean±1σ, n=33) of the n-alkanes are -195±16‰ for hydrogen and -30.6±2.4‰ for carbon. The insect n-alkanes are depleted in D by approximately 30-40‰ compared with wax n-alkanes from C3 (-155±25‰) and C4 vascular plants (-167±13‰), whereas their δ ~(13)C values fall between those of C3 (-36.2±2.4‰) and C4 plants (-20.3±2.4‰). Thus, the contribution of insect-derived n-alkanes to soil and sediment could potentially shift δD records of n-alkanes toward more negative values and potentially muddle the assumed original C3/C4 balance in the δ ~(13)C records of the soil and sedimentary n-alkanes. n-Alkenes are also found in three insects (swallowtail, wasp and hornet). They are more depleted in D relative to the same carbon numbered n-alkanes (δD _(n-alkene)-δD _(n-alkane)=-17±16‰), but the δ ~(13)C values are almost identical to those of the n-alkanes (δ ~(13)C _(n-alkene)-δ ~(13)C _(n-alkane)=0.1±0.2‰). These results suggest that these n-alkenes are desaturated products of the same carbon numbered n-alkanes.