The University of Arizona

Variations in the Isotopic Composition of Dissolved Inorganic Carbon in the Unsaturated Zone of a Semi-Arid Region

I Carmi, D Yakir, Y Yechieli, J Kronfeld, M Stiller

Abstract


A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ18O and tritium content, dissolved inorganic carbon (DIC) and its δ13C (‰) and Δ14C (‰) content, and δ13C (‰) and Δ14C (‰) in the solid sediment. The profiles of moisture and δ18O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr–1). The source of carbon in the DIC is CO2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ13C (‰) profiles show clearly the chemical transition from dissolved CO213C = –22‰) to bicarbonate (δ13C = –14‰). At greater depth (–11.3‰), the δ13C becomes similar to the δ13C in the aquifer below (–12.5‰). The effect of secondary processes is evident in the profile of Δ14C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10‰ yr–1. Precipitation of carbon from the DIC on the sediment is 1.1 mg C Lsed–1yr–1, negligible compared to the 28 g C in 1 Lsed.  In the solid sediment, there is a gradient in Δ14Ccarb at the top meter. The net precipitation of 14C from the DIC on the sediment (0.25 to 1.1‰ yr–1), corrected for decay, cannot be observed in the deeper sediment. The presence of 14C in the top 1 m of the sediment is explained by two possible processes: accumulation of 14C-tagged dust (~0.05 mm yr–1) and/or long-term cumulative precipitation from the DIC.

DOI: 10.2458/azu_rc.57.18356


Keywords


unsaturated zone, carbon isotopes

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