Abrupt lowstands of Laguna de Zoñar (southern Spain) during the Iberian Roman Humid Period recorded by stable isotopes of gypsum hydration water

Centimeter-thick layers of gypsum (CaSO₄·2H₂O) were deposited at the bottom of Laguna de Zoñar (Córdoba Province, southern Spain) from ~2120 to ~1900 cal yr BP [1], coinciding with the apogee of the Roman Empire in the Iberian Peninsula. The presence of gypsum deposits in lake sediments is generally interpreted as evidence of dry climatic periods in the past [2]; however, gypsum in Laguna de Zoñar formed during the so-called Iberian Roman Humid Period (IRHP, 2600-1600 cal yr BP), the wettest episode of the last 4000 years in the southern Iberian Peninsula. At present, the lake is fed by two springs (~3.5 and ~1.1 l/s on average) and direct rainfall, extends over 37 ha and is up to 15 m deep, being the deepest natural water body in this region. Water salinity is relatively low (~1 g/l), dominated by Cl^-, SO₄^2- and Na⁺ and it is undersaturated in gypsum (SI_gyp<-1.9).

The oxygen and hydrogen stable isotopes (δ¹⁸O and δ²H) of lake waters are sensitive to long-term changes in hydrological conditions (e.g. relative humidity, evaporation/outflow ratio, etc.). Here we determine the isotopic composition of Laguna de Zoñar at the time of gypsum precipitation from stable isotopes of hydration water in this mineral. The δ¹⁸O and δ²H values of the paleo-lake at 2122, 2051 and 1897 cal yr BP were significantly higher (mean values of 9.7±0.2‰ and 31.2±0.5‰, respectively) than those of the modern lake waters (δ¹⁸O<4.5‰ and δ²H<4.1‰), suggesting that during some stages of the IRHP the evaporation/outflow ratio of the lake was much higher than at present.

From a paleoclimatic perspective, the gypsum layers in Laguna de Zoñar formed during the transition from the wettest stage of the IRHP (2400 to 2000 cal yr BP) to the subsequent relatively drier phase (2000 to 1600 cal yr BP) [1]. Our results suggest that the second half of the IRHP was drier than the previous and later stages of the late-Holocene in this region. Also, it is possible that, at least during some periods of the Roman occupation, the hydrological regime of the lake was artificially modified by diverting the feeding creeks for urban supply. This could explain the significant increase in water salinity that led to gypsum precipitation. In the southern Iberian Peninsula, where few permanent lakes are present, Laguna de Zoñar may have been an essential water source for the Roman development in the surrounding areas, including the nearby Roman city of Ipagrum (3 km away). Combined detailed archaeological surveys of the area and analyses of anthropogenic proxies at higher resolution in the core may help to solve the relative role of water diversion and short-term aridification phases during the last centuries of the Roman Age in Hispania.

 [1] Martín-Puertas, C. et al. (2008), The Holocene, 18, 907–921; [2] Evans, N.P. et al. (2018), Science, 361, 6401, 498-501.

 

Acknowledgement

This study was supported by project PY18-871 of the Junta de Andalucía. Dr. Fernando Gázquez acknowledges the Ramón y Cajal fellowship, RYC2020-029811-I.