Spatial and temporal variation in tree-ring α-cellulose oxygen and hydrogen isotope values as a record of water availability in the Atacama Desert

Abstract:

Previous studies have documented that tree ring oxygen and hydrogen isotopes primarily reflect source water; however, biosynthetic fractionation processes modify this signal and can have a varied response to environmental conditions. The degree to which source water contributes to δ²H and δ¹⁸O values of plant α-cellulose is species-specific and modern calibration studies are necessary. Here we present a calibration data set of P. tamarugo α-cellulose δ²H and δ¹⁸O values from the Atacama Desert in Northern Chile. P. tamarugo trees are endemic to the region and have adapted to the extremely arid environment where average annual precipitation is < 5mm/yr. This modern isotope chronology has been constructed from living P. tamarugo trees (n=12) from the Pampa del Tamarugal Basin in the northern Atacama. Generally, the tree-ring α-cellulose δ¹⁸O values are poorly correlated with meteorological data from coastal stations (i.e. Iquique); however, there is good agreement between regional groundwater depth and α-cellulose δ¹⁸O values. Most notably, average α-cellulose δ¹⁸O values increase by >2 ‰ over the past 20 years associated with a ~1.1 m lowering of the local groundwater table throughout the area. The correlation between a-cellulose isotope values and hydrologic conditions in modern times provides a baseline for interpretation of tree-ring isotope chronologies from the past 9.5 kya. A high-resolution Holocene (1.8-9.1 kya) age record of Prosopis sp. tree ring α-cellulose δ¹⁸O values provides a proxy for climatic and hydrologic conditions. During the early Holocene δ¹⁸O values range from 31 to 35‰ (2σ=0.58‰), while during the late Holocene values are much more variable (27.4 to 41‰; 2σ=2.64‰). Anthropogenic demand on local water sources is the most significant environmental factor affecting the variation in modern α-cellulose δ¹⁸O values; however, climate induced changes in regional water availability are the dominant driver of variability in the paleo-record. Increased variability in α-cellulose δ¹⁸O values in the late Holocene most likely indicates a reduction in annual recharge and an increase in episodic flood events driven by ENSO and other modes of atmospheric variability.