PALEOPROXY FOR VAPOR PRESSURE DEFICIT (VPD) FROM FOSSIL CELLULOSE AND PEDOGENIC CARBONATE

Stable ¹³C and ¹⁸O isotope ratios in modern cellulose and pedogenic carbonate are variable (± 4-10 ‰), and yet are surprisingly highly correlated with each other. This correlation may be due to fractionation, mainly by rubisco, but also by leaf conductance, of light isotopologues of carbonyl, incorporated into plant tissue (for cellulose), and then by decay with little fractionation into soil CO₂ (for carbonate). Our compilation of data from around the world shows that the slope of the least squares regression of ¹³C and ¹⁸O isotope ratios (S) at each site is correlated with vapor pressure deficit (VPD), throughout the growing season in the case of cellulose (VPD=0.2721∙S+0.3028; r²=0.69; n=14; s.e= ± 0.13 kPa; P < 0.001), and at the end of the growing season in the case of pedogenic carbonate (VPD=0.6609∙S+0.1944; r²=0.67; n=9; ; s.e. = ±; 0.12 kPa; P < 0.007). Thus limited atmospheric and soil water enhances the efficiency of discrimination for light isotopologues of carbonyl by rubisco, and conversely abundant soil and atmospheric water dilutes isotopic discrimination by rubisco. Our new proxy for VPD can be used to estimate vapor pressure deficit of 0.43 kPa ±0.19 kPa for cellulose of Eocene Metasequoia from Axel Heiberg Island, and 0.61 kPa ±0.14 kPa for Eocene pedogenic carbonate from Wyoming. Both estimates are in good agreement with other proxies for these sites.