Oxygen isotope values of tree ring α-cellulose as a proxy of hydroclimate variability in arid regions

Abstract:

One of the main goals of tree ring isotope studies is to reconstruct climate-driven variations in the source water and antecedent precipitation; however, evaporation in the soil and leaves can significantly modify the isotope values of the source water. This is particularly the case in arid environments where evaporative effects are perhaps the most significant unknown variable when attempting to reconstruct regional-scale hydroclimate variations from tree ring isotope proxies. To quantify the effects of extreme aridity on α-cellulose δ¹⁸O values, we measured the oxygen isotope values of groundwater, xylem water, leaf water, and tree ring α-cellulose in an endemic species of drought-resistant trees (Prosopis tamarugo) from different microenvironments throughout the Atacama Desert of Northern Chile. Average annual precipitation is <5 mm/yr, and groundwater is the primary water source for P. tamarugo trees in the region. Groundwater δ¹⁸O values at the sample locations range from -6.7 to -9.7‰, and xylem water δ¹⁸O values record a systematic increase (ave. Δ¹⁸O_x-gw =+1.3‰; 2σ =1.0‰). Leaf waters are significantly affected by evaporative enrichment with a range of δ¹⁸O values from 7 to 23‰. This range most likely reflects a number of physiological and environmental conditions including tree size, canopy development, and sample time (i.e. morning vs. evening). However, despite the large variation in leaf water δ¹⁸O values, the average difference between the α-cellulose and groundwater is very consistent (Δ¹⁸O_c-gw = +39.7‰; 2σ =1.3‰). P. tamarugo samples were collected in austral spring, when tree growth was at its maximum; therefore, any seasonal variations in plant physiology not captured with this dataset will have a limited impact on cellulose production. These data demonstrate that despite the variable evaporative enrichment of ¹⁸O in the leaf water, the α-cellulose δ¹⁸O values provide a remarkably consistent record of variations in groundwater δ¹⁸O values in this extremely arid environment.