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Potential Project Description for 2013-14

 

Title: Variability in Mixing Models in Paleo-ecology
Domain Expert: Charles Umbanhowar (Biology)

Paleo-ecologists often use stable isotope analysis of charcoal from lake sediment to infer regional changes in the ratio of C3 and C4 plant composition. C3 and C4 plants differ in how they fix CO2; C4 plants are more efficient with water usage and generally survive better in warmer, drier climates. We study the carbon isotope ratios and subsequently infer C3/C4 composition of historical forests from charcoal obtained from lake sediment samples. We are looking at sources of variation that may occur in this process due to burning, heterogeneity of the charcoal mixture and sample size.

Isotopic values for C3 plants range from -25 to -35‰ while C4 plants range from -10 to -18‰. The isotopic values for the fossil charcoal from Sharkey Lake in Big Woods, MN, ranged from -20.9 to -22.2 at ~2000 calBP. Based on a simple mixing model this suggests that 57% of the plants were C3. The error calculated in the standard manner, due to sample heterogeneity of charcoal, was ~+0.18‰. However, natural variation in the isotopic values of the assumed fuels and a preliminary simulation analysis suggest a much larger (~20%) range in inferred plant composition.

I would like to further explore the implications of different levels of variation for the two end members. In particular, we would look at variation due to within species variability vs. among species variability. Most of the information on variance would come from the published literature, but we may generate some original additional data using the mass spec. This is a very viable candidate for a note in a paleoecology journal in as much I've not seen any direct treatment of this issue.