The paleoenvironmental significance of evaporite–dolomite associations in two Spanish saline lakes

Abstract

Dolomite is a mineral which is abundant in the geological record, but its origin is poorly understood. At present, dolomite is mostly formed in saline and hypersaline environments, such as salt lakes, in which it is closely connected to evaporite minerals. This makes the relationship between dolomite formation and evaporite precipitation an important topic for understanding the conditions under which dolomite can form.

In this thesis, the evaporites in the sediment cores of two Spanish saline lakes (Laguna de Salinas (Alicante) and Laguna de Fuente de Piedra) are studied to understand the paleoenvironmental significance of evaporite layers. Petrographic, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence (XRF) and sulphate and oxygen isotope analyses were performed and age models of both cores were constructed.

It was found that evaporites were formed under dry, evaporative conditions, which were formed when the oxygen concentration in the lakes was relatively low. This indicates that the oxygen production by photosynthetic bacteria was reduced. In Laguna de Salinas, these evaporative conditions remained stable for multiple years, during which selenite layers were formed, whereas evaporite laminae in Laguna de Fuente de Piedra alternate with dolomite laminae in annual couplets. A correlation between two sediment cores, which contain sediment from the last glacial period and the Holocene, and paleoclimatic reconstructions show that evaporites were mostly formed during cold periods.