Sensing plant community and climate change by charcoal-carbon isotope analysis

Mass spectrometric carbon isotope (delta C-13) analyses of charcoal-carbon found in lake sediments provide evidence of paleoclimate and paleovegetation independent of pollen. By chemically separating the microscopic charcoal-carbon, which originates from upland fires, from total carbon in lake, ocean, or ice cores, changes in C-3/C-4 biomass of upland plant communities can be measured, wet or dry climates identified, and the evolution of specific plant assemblages traced. In the North American Midwest, delta C-13 values of lake sediment charcoal-carbon from Lake Mendota, Wisconsin, U. S. A., document two shifts from C-3 (forests) to C-4 (prairie grasses) plants since deglaciation: an abrupt change at about 8.5 ka (8 500 years BP) and a longer duration change between 6 and 3 ka. A pine-savannah is interpreted for the earlier date while oak-savannah dominated the landscape between 6 and 3 ka. C-4 biomass, which was almost non-existent in the region before 8.5 ka, increased to more than 50% between 6 and 3 ka. Spruce-tundra parkland evident in the Midwest before about 12 ka contained no C-4 grasses and was probably similar in composition to modern North American tundra-parkland which has no C-4 biomass. The climate inferred from the delta C-13 data compares well with climate inferred from pollen and sediment data from the same core and supports climate model simulations of decreased effective moisture during the middle Holocene in the North American Midwest. The delta C-13 results also suggest future landscape changes should greenhouse-gas warming occurs. Savannah, with increased C-4 biomass, could again become prevalent in the North American Midwest.