Soil water seasonal dynamics and their relation to the record of climate recorded by soil carbonate in the Great Basin of western North America

Abstract:

In arid and semi-arid regions climate seasonality and times of high evaporative demand have a profound influence on the oxygen isotope composition of soil water. Isotopic evaporative enrichment models are well developed for soil waters and indicate that the shape of soil water oxygen isotope depth profiles (and therefore that of soil carbonate forming in equilibrium) is primarily controlled by the evaporation rate of water out of the soil. A climosequence of four sites was established on Holocene soils in Fish Lake Valley, Nevada (western USA) where MAP ranges from ~80 mm/yr to ~220 mm/yr. Continuous measurements of soil and air temperature and humidity were recorded at 0.5 hr intervals, while daily snow cover at the highest elevation site was assessed via time-lapse photography, revealing that the influence of snow on soil hydrology varies along the climosequence and that a majority of winter snow precipitation is lost to sublimation and does not enter the frozen soil. Precipitation samples for δ¹⁸O values and samples of soil atmosphere from 10, 25, 50 and 100cm were collected at various intervals for CO₂ concentration, δ¹³C and δ¹⁸O values (which are a proxy for soil water δ¹⁸O) and indicate that soil water δ¹⁸O values are directly related to precipitation inputs and subsequent soil water evaporation. Soil pit excavations at the beginning of the study yielded detailed depth profiles of soil properties and pedogenic carbonate δ¹⁸O and δ¹³C values, as well as soil water for validation of the soil H₂O-CO₂ δ¹⁸O equilibrium by both vacuum extraction and direct headspace equilibration. Analysis of these data reveals several important features of this semi-arid soil system: 1) precipitation and soil water δ¹⁸O values reflect two distinct seasonal atmospheric circulation patterns, of which only one is reflected in the soil carbonate δ¹⁸O values, and 2) we show that while soil water conditions are variable through the year at these sites, pedogenic carbonate yields information about year-long evaporation rates. We extend this modern understanding of soil water conditions to the interpretation of pedogenic carbonate profiles from modern soils in the Great Basin and demonstrate how to derive similar information about their evaporative environment.