--> Abstract: A High Resolution Isotopic Record of C4 Plant Abundance in Soils: Implications for Early Holocene Climate Conditions in Southwest Montana, by William M. Simmons and Lisa M. Pratt; #90181 (2013)

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A High Resolution Isotopic Record of C4 Plant Abundance in Soils: Implications for Early Holocene Climate Conditions in Southwest Montana

William M. Simmons and Lisa M. Pratt
Indiana University, Bloomington,IN

Late Quaternary climate and ecological reconstructions in the Cenozoic basins of the central Rocky Mountains are hindered by complicated basin-fill stratigraphy and a paucity of easily obtainable, spatially and temporally extensive proxy data at lower elevations. Stable-carbon isotopic signatures of C3 and C4 plant organic matter in soils and paleosols provide a reliable yet underutilized record of vegetational responses to climate change throughout the semi-arid lowlands of this region. 45 soil sediment samples were collected in situ from two vertical trenches in the tectonically active North Boulder Basin (southwest Montana) for stable-carbon isotopic and mineralogic analysis. Isotopic results from soil organic matter reflect 5-25% greater C4 plant productivity relative to modern at roughly 11,000 cal yr BP and 7000 cal yr BP. These periods of increased relative C4 plant productivity indicate warmer and drier climate conditions than currently prevail in the area, conditions which are also observed in pollen sequences from nearby sub-alpine lakes. Radiocarbon dead paleosols that contain evidence of C4 plants are inferred to be Miocene in age based on regional ages of major soil formation and the fact that the C4 photosynthetic pathway did not evolove in this region until early Miocene. Observed sub-centennial fluctuations in relative C4 plant productivity indicate responses to ecosystem-scale variables such as proximity to active stream channels and local microclimates. Millenial scale increases in C4 plants relative to C3 plants are contemporaneous with documented increases in solar insolation.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013