CALCITE ACCUMULATION RATES ESTIMATED FROM 14C AGES IN IRRIGATED DRYLAND AGRICULTURAL SOILS IN WEST TEXAS, USA

Conversion of natural dryland to irrigated agricultural land is an ongoing practice to support a growing global population. Consequently, use of irrigation water accelerates the natural accumulation of salts, especially calcite (CaCO₃). CaCO₃ precipitates readily from available Ca²⁺ and HCO₃^- in irrigation water. There has been managed irrigated agriculture along the Rio Grande in western Texas and southern New Mexico for over 100 years. Prior research suggests that these managed soils accumulated up to 10 wt% CaCO₃ in just this century of cultivation, with rates likely to accelerate as climate change necessitates use of poor-quality water for irrigation. The goal of this research is to determine CaCO₃ accumulation rates, as supported by the ¹⁴C ages of the C in soils on a pecan orchard near Tornillo, TX, which has been cultivated for about 100 years. Of particular interest was comparing CaCO₃ accumulation in two soil texture end members, Pecan_Fine and Pecan_Coarse. Pecan_Fine has high silt and clay content, and Pecan_Coarse contains more sand-sized particles. Both sites have a clay layer at ~150cm, which is may be a location for CaCO₃ precipitation in both textures, more significantly in the Pecan_Fine. If CaCO₃ accumulation due to irrigation is greater in the Pecan_Fine, C at the 150cm depth should be younger in the Pecan_Fine. To calculate C and CaCO₃accumulation rates, ¹⁴C dating was done on samples from Pecan_Fine and Pecan_Coarse, and a natural site which has never been farmed. The natural site returned the oldest ¹⁴C ages, 9770 ± 30 years at 20-40cm, and 27220 ± 110 years at 90-99cm depth. Pecan_Fine ages were 6300 ± 30 years at 41cm, 5110 ± 30 years at 122cm, and 9380 ± 30 years at 163cm. Pecan_Coarse ages were 8570 ± 30 years at 40cm, 5420 ± 30 years at 120cm, and 12980 ± 40 years at 160cm. At these ages, CaCO₃ accumulation rates are 8.54 gCaCO₃/m²/year for Pecan_Coarse, and 33.82 gCaCO₃/m²/year for Pecan_Fine. Study in this area of the Rio Grande flood plain suggests that deposition is likely to have occurred ~2ka. This implies mixing of C with variable ages, from differing sources, and that ¹⁴C ages alone cannot inform actual CaCO₃ accumulation rates. Depositional ages, from pending optically-stimulated luminescence, will clarify C and CaCO₃accumulation rates in the orchard, and give insight into textural controls.