Holocene lake water-aquifer interactions in La Ballestera Playa-lake (southern Spain) recorded by stable isotopes (δ18O and δD) of gypsum hydration water

Abstract

<p>Oxygen and hydrogen stable isotopes (δ<sup>18</sup>O and δD) of lake water are sensitive to long-term changes in environmental conditions, including relative humidity, temperature and the evaporation/outflow ratio of the lake. Lacustrine gypsum (CaSO<sub>4</sub>·2H<sub>2</sub>O) forms in equilibrium with its parent fluid, so the isotopic composition of its structurally bonded hydration water (GHW) can reflect the δ<sup>18</sup>O and δD of lake water at the time of mineral formation, with insignificant effects of temperature and salinity on the water-GHW isotope fractionation factors. Using the stable isotope content of gypsum-rich sediment cores as a paleoclimatic proxy, the environmental conditions prevailing in the lake setting at the time of gypsum crystallization can be investigated.</p><p>Here we apply this method to reconstruct the δ<sup>18</sup>O and δD of paleo-water in La Ballestera Playa-lake (Seville, southern Spain) throughout the Holocene, from 11.2 cal kyr BP to the present. Gypsum crystallization took place punctually at 11.2 and 4.4 cal kyr BP, and did continuously from 2.9 cal kyr BP to the present. The δ<sup>18</sup>O and δD showed the lowest values at ~11.2 cal kyr BP (2.3‰ and -1.1‰, respectively) and were significantly higher at ~4.4 cal kyr BP (8.8‰ and 29.2‰, respectively). Likewise, relatively higher values (8.2‰ and 29.8‰, respectively) were recorded at ~2.9 cal kyr BP. Thereafter, the isotopic ratios increased until the present (11.4‰ and 37.1‰, respectively), suggesting increasing aridity and/or hydrological closeness of the lake. A relative minimum in δ<sup>18</sup>O and δD occurred at ~2.3 cal kyr BP, during the wetter stage of the Iberian Roman Humid Period, while a relative maximum at ~1.1 cal kyr BP was recorded during the Medieval Warm Period.</p><p>We use a steady-state Isotope Mass Balance to investigate the paleo-hydrological conditions in the lake setting at different stages of the Holocene. Our results suggest that at ~11.2 cal kyr BP La Ballestera Playa-lake was a flow-through lake closely connected to the aquifer with and evaporation/outflow ratio <0.5. At 4.4 cal kyr BP and from ~2.9 cal kyr BP until the present, the system behaved as a terminal lake (evaporation/outflow ratio close to 1), with less connection to the aquifer and the main water output occurred via evaporation. The studied system turned into a playa lake because of a regional water table lowering. This most likely resulted from increasing aridity in southern Iberia during the late Holocene, which has previously been suggested by other lake sediment records in this region. </p><p> </p><p><strong>Acknowledgement</strong></p><p>This study was supported by the Junta del Andalucía PY18-871 to FG, the project<strong> </strong>CGL2017-85415-R of the Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER, the project B-RNM-144-UGR18, Proyectos I+D+i del Programa Operativo FEDER 2018 and the research groups RNM-189 y RNM-190 (Junta de Andalucía). Dr. Antonio García-Alix acknowledges the Ramón y Cajal fellowship, RYC-2015-18966. Fernando Gázquez acknowledges the postdoctoral “HIPATIA” program of University of Almería.</p>