Grass-dominated ecosystems cover about one-third of Earth’s land surface, influence key biogeochemical processes, and serve as major food sources for humans and herbivores. One of the major challenges in studying paleorecords of grasslands is that pollen assemblages are uninformative about C₃ and C₄ grass variations because grass pollen generally cannot be morphologically identified below the family level. We report here (1) a novel technique for analyzing the stable carbon isotope composition of individual grass-pollen grains using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer, and (2) an application of this technique to investigate C₄-grass responses to changes in pCO₂, temperature, and moisture over the last ~25,000 years at Lake Challa, East Africa.

Results/Conclusions

Our results indicate that SPIRAL reliably distinguishes pollen from C₃ and C₄ grasses from herbarium specimens. A strong positive correlation also exists between the estimated proportion of C₄-grass pollen in surface lake sediments and the abundance of C₄ grasses in modern vegetation at ten sites across North America. Preliminary data from sediment core samples from Lake Challa show that C₄-grass abundance fluctuates greatly, having a generally smaller contribution before ~9,000 yr BP and a generally higher abundance since then. Low pCO₂ during the last glacial did not lead to the highest values of C₄-grass abundance. This contrasts with bulk-sediment and leaf-wax δ¹³C-based records from the region. One plausible explanation is that leaf-wax δ¹³C reflects the abundance of C₃ and C₄ grasses plus non-grass C₃ and C₄ plants, whereas the SPIRAL unambiguously indicates grasses. Leaf-wax δ¹³C may overestimate C₄-grass abundance because aquatic sedges (many of which are C₄) were more abundant and trees (all C₃) were less abundant during the last glacial than during the Holocene. Our results also suggest that C₄ grasses were neither favored by severe drought (~20,000-15,000 yr BP) nor by wet conditions (~11,000-9,000 yr BP). C₄-grass abundance was generally low during the last glacial-interglacial transition, which contrasts with the expectation of steadily increasing C₄-grass abundance that should have occurred if temperature were the main driver of C₃- and C₄-grass proportions. We hypothesize that generally greater C₄-grass abundance during the Holocene than during the last glacial resulted from more active fire regimes during the Holocene and that the effects of pCO₂ and climate were manifest indirectly via changes in fuel loads.