Problems in interpreting tree-ring δ ¹³C records

Abstract

THE stable carbon isotopic (¹³C/¹²C) record of twentieth-century tree rings has been examined^1–3 for evidence of the effects of the input of isotopically lighter fossil fuel CO₂ (δ ¹³C∼−25‰ relative to the primary PDB standard⁴), since the onset of major fossil fuel combustion during the mid-nineteenth century, on the ¹³C/¹²C ratio of atmospheric CO₂(δ ¹³C∼−7‰), which is assimilated by trees by photosynthesis. The decline in δ¹³C up to 1930 observed in several series of tree-ring measurements has exceeded that anticipated from the input of fossil fuel CO₂ to the atmosphere, leading to suggestions of an additional input ‰) during the late nineteenth/early twentieth century. Stuiver³ has suggested that a lowering of atmospheric δ ¹³C of 0.7‰, from 1860 to 1930 over and above that due to fossil fuel CO₂ can be attributed to a net biospheric CO₂ (δ ¹³C∼−25‰) release comparable, in fact, to the total fossil fuel CO₂ flux from 1850 to 1970. If information about the role of the biosphere as a source of or a sink for CO₂ in the recent past can be derived from tree-ring ¹³C/¹²C data it could prove useful in evaluating the response of the whole dynamic carbon cycle to increasing input of fossil fuel CO₂ and thus in predicting potential climatic change through the greenhouse effect of resultant atmospheric CO₂ concentrations. I report here the trend (Fig. 1a) in whole wood δ ¹³C from 1883 to 1968 for tree rings of an American elm, grown in a non-forest environment at sea level in Falmouth, Cape Cod, Massachusetts (41°34′N, 70°38′W) on the northeastern coast of the US. Examination of the δ ¹³C trends in the light of various potential influences demonstrates the difficulty of attributing fluctuations in ¹³C/¹²C ratios to a unique cause and suggests that comparison of pre-1850 ratios with temperature records could aid resolution of perturbatory parameters in the twentieth century.