The North Atlantic Oscillation (NAO) has a major effect on the modern precipitation patterns in the southern Iberian Peninsula and also controlled the hydroclimate of this region in the past [1,2]. The oxygen and hydrogen stable isotopes (δ18O and δD) of lake waters are sensitive to long-term changes in hydrological conditions (e.g. relative humidity, evaporation/outflow ratio, etc.). Here we reconstruct the δ18O and δD values of Laguna Grande de Archidona Lake (Málaga Province, southern Spain) from the stable isotopes of hydration water of gypsum (CaSO4·2H2O)  that precipitated and deposited in the lake from 2.7 to 1.4 ka BP.
The maximum δ18O and δD of values of the lake water (7.8‰ and 23.9‰, respectively) were recorded at 2.7 ka BP (~700 BCE), suggesting that relatively dry conditions prevailed during the Early Iron Age in southern Iberia. Subsequently, the δ18O and δD of the lake decreased to minimum values (3.0 and -2.2‰, respectively) at 2.4 ka BP, during the early stages of the Iberian Roman Humid Period (~600 BCE to ~400 CE). A relatively arid phase was also recorded at 2.2 ka BP, which preceded a wetter stage at 2.0 ka BP. A longer drier phase occurred later on, from 1.6 ka to 1.4 ka BP, coinciding with the decline of the Western Roman Empire (395-476 CE).
The comparison of our paleo-humidity proxy with long-term NAO index  shows that the lake water was isotopically enriched during periods of persistent NAO+ mode, suggesting drier and more evaporative conditions and probably lake level lowstands. In contrast, lower δ18O and δD of values of the lake water occurred during periods of NAO- configuration, resulting in wetter and less evaporative conditions and lake level highstands. In summary, our results indicate that the hydrologic balance of Laguna Grande de Archidona during the Iberian Roman Humid Period was controlled by the strength of the long-term NAO and that stable isotopes of gypsum hydration water are a powerful tool for paleo-hydrologic reconstructions.