An atmospheric ¹³C/¹²C reconstruction generated through removal of climate effects from tree-ring ¹³C¹²C measurements

Abstract

An accurate reconstruction of past ¹³C/¹²C ratios of atmospheric CO₂ may provide key constraints on the historical activity of the biosphere as CO₂ source or sink. Tree rings appear to be a repository of this information but there is much noise in the collection of previous reconstructions, presumably associated with site selection, radial variability. choice of representative wood chemical constituent, and subtle effects of climate on fractionation. This study attempts to avoid these pitfalls and develop a 50-yr δ¹³C_ATM, record from juniper trees (genus Juniperus), in fact, by taking advantage of the influence of climate on fractionation. Trees were harvested from suitable sites in close proximity to weather stations with monthly records of temperature and precipitation, and ring material was separated from the sections in 5-year intervals around their full circumference. From each interval, relationships of δ¹³C of the cellulose fraction with mean December temperature or precipitation of the respective sites emerged, showing nearly constant slopes from one interval to the next (averaging-0.27‰ C-1 for temperature and-0.04‰ mm^-1 for precipitation) but with different intercepts. The separation of these regression lines of different intervals is interpreted as the response of the trees to the changing δ¹³C of atmospheric CO₂ so that δ¹³C_ATM curves are constructed from this spacing.

The shape of the best-fit reconstruction suggests the biosphere has acted as a CO₂ source to about 1965 but has become a sink afterward. Although there are several recent studies whose results are compatible with this interpretation, the assumptions and statistical considerations in this study would still allow an exponentially decreasing curve through the data points from 1930-1979 and therefore, other interpretations are not excluded.