| The spread of C4 grasslands since the late Miocene has received much attention, but it is unclear how mid-latitude C4 abundace has changed over time. Here we use stable isotope analysis to assess how small mammal diets responded to environmental change. We compare our results with a climate-vegetation model and the delta13C values of sedimentary organic matter, as well as collagen data from Last Canyon Cave, MT. The climate-vegetation model incorporated estimates of past pCO2, mean annual temperature, and summer precipitation for estimating C4 grass abundance. Since carbon isotopic composition (delta113C values) of feces reflects the delta113C values of diet, we reconstructed C4 grass presence/abundance using 67 rodent and rabbit coprolites (fossilized feces) for the late Glacial Period (∼22-12 ka). In addition, we assessed whether modern rodents and recent bushy-tailed woodrats track C4 grass abundance. We compared these results to the delta13C values of mammal collagen from LCC, as well as other sites in Wyoming. Rodent feces and mammal collagen delta 13C values from all time bins, i.e. modern, post-Last Glacial Maximum (LGM), and LGM are similar, with the exception of higher values for large rodents during the post-LGM, modern kangaroo rats (Dipodomys ordii ), and recent bushy-tailed woodrats (N. cinerea). Overall, these delta13C values indicated relatively low C 4 abundance estimates and were consistent with a low C4 diet or an enriched C3 derived diet of chitin, seeds or other non-photosynthetic tissues. In contrast, post-LGM rabbit delta13C values suggest higher C4 grass consumption and greater C4 grass abundance relative to today (∼45+/-6%), in agreement with our modeled results (∼24+/-0%) and sediments (∼65+/-8%). During the LGM, sediment delta 13C values suggest a reduced but significant (∼36+/-11%) seasonal C4 presence, in contrast to our climate-vegetation model and rodent and collagen delta13C values, which reconstructed an absence of C4 cover and lower %C4 (∼10-20%), respectively. Despite expected cold temperatures based on LCC's proximity to the Laurentide Ice Sheet, this result is consistent with the influence of either low pCO2 favoring C4 plants, or warm summer temperatures, or both. Later, warming temperatures and especially warm summer temperatures outpaced the expected offset of increasing atmospheric pCO2 and drove the observed peak of C4 grasses post-LGM. This pulse is undetected by rodent coprolites and large and small mammal collagen, and suggests that as small grazers, rabbits are a better proxy for tracking short term (i.e., sub-seasonal) shifts in vegetation. |
