Carbon-isotopic analysis of individual pollen grains from C3 and C4 grasses using a spooling-wire microcombustion interface

Pollen grains from grasses using the C-3 and C-4 photosynthetic pathways have distinct ranges of delta C-13 values that may be used to estimate their relative abundance in paleorecords. We evaluated a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer (SWiM-IRMS) for delta C-13 analysis of individual grass-pollen grains. Pollen from four C3 and four C4 grass species was isolated through micromanipulation and analyzed as single grains suspended in water. A carbon yield greater than the 2 sigma range of the carbon content of blanks containing only water was used to distinguish samples containing pollen ("pollen present") from those not containing pollen. This criterion resulted in the exclusion of similar to 45% of the 946 samples applied to the wire. The average delta C-13 values (+/- 1 sigma) of the remaining samples were -26.9% (+/- 6.3 parts per thousand) and -11.5 parts per thousand ( +/- 9.6 parts per thousand) for C-3 grasses and C-4 grasses, respectively, after blank-correcting the delta C-13 data. These results suggest that the SWiM-IRMS system can be used to distinguish C-3 from C-4 grass pollen. The high variability in measured delta C-13 values is likely caused by a combination of factors. These include natural isotopic variability among individual pollen grains; the relatively poor precision that can be obtained when determining delta C-13 values of such small samples; and the uncertainty in the magnitude, isotopic composition, and stability of the analytical blank. Nonetheless, high percentages of individual pollen grains were correctly classified as being of either C-3 or C-4 origin. On average, 90% (range = 78-100%) of pollen grains from C-3 grasses had delta C-13 values more negative than the cutoff threshold of -19.2 parts per thousand while 84% (range = 77-90%) of pollen grains from C-4 grasses had delta C-13 values more positive than -19.2 parts per thousand. Compared with analysis using an elemental analyzer interfaced with an IRMS (EA-IRMS), the number of pollen grains required for delta C-13-based evaluation Of C-3/C-4 grass composition is many times lower with the SWiM-IRMS. Additionally, delta C-13 data from the SWiM-IRMS does not need to be incorporated into a mixing model to derive estimates of the abundance Of C-3 and C-4 grass pollen. Carbon-isotopic analysis of individual grass-pollen grains using the SWiM-IRMS system may help improve our understanding of the evolutionary and ecological significance of grass taxa in the paleorecord. (c) 2007 Elsevier Ltd. All rights reserved.