Stomatal Index Analysis Reveals Sensitivity of Atmospheric [CO2] to Rapid Climate Changes During the Early Holocene

F Wagner, W M Kuerschner (Both at: Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, NL; e-mail: r.wagner@biol.ruu.nl); S J P Bohncke (Faculty of Earth Sciences,

Free University, De Boelelaan 1085, 1081 HV Amsterdam, NL); B van Geel (Department of Palynology and Paleo/Actuo-ecology, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, NL)

Proxy data from the marine and terrestrial realm provide increasing evidence for climatic variability during the Holocene. The driving force behind high frequency oscillations is still uncertain. Explanations for the variability reach from primary forcing by external processes to ocean - atmosphere interactions. The available high resolution records which provide the base for the paleoclimate reconstructions comprise proxy records from lake sediments, peat deposits, tree rings, glaciers and marine sediments. Until now, the only method to involve the atmospheric [CO2] as part of the earth’s climate system are measurements of gas enclosures in ice-cores. Although this method reveals direct evidence for the history of this greenhouse gas, the resolution that can be reached is often insufficient to cope with the frequency of climate shifts recently postulated for the Holocene. Since the sensitivity of atm. [CO2] is documented for the large scale glacial - interglacial cycles, one can expect that similar changes occurred as a result from perturbations in the global carbon cycle during millenium or century scale climatic oscillations. An alternative method to estimate secular variations in the atm. [CO2] is the analysis of stomatal frequencies on leaves of woody plants. The inverse relationship between stomatal densities and / or stomatal index in leaves of C3 plants and atm. [CO2] has been repeatedly demonstrated by analysis of herbarium material and experimentally by growing seedlings under changing [CO2]. Here we present a paleoatmospheric [CO2] curve based on the stomatal index record of birch leaves covering the first 600 14C years of the Holocene. The analyzed leaves are derived from a palynological and 14C dated peat section from The Netherlands with a resolution of about 30 14C years per analyzed sample. The stomatal index analysis for this interval reveals two distinct phases of change in [CO2] that can be correlated to 1) the termination of the Younger Dryas and 2) the Preboreal oscillation.