Isotopy, micromorphology and composition of pedogenic gypsum in loess-palaeosol sequences in the Ebro Valley as a combined paleoenvironmental proxy

Abstract

The loess palaeosol sequences in the Ebro valley are associated with arid and semi-arid climates in the past, where the main pedogenic process is the mobilisation of carbonate and gypsum through the soil, resulting in secondary accumulations. Due to the fact that, in this context, the conservation of palaeoenvironmental indicators is very limited, the objective of this research is to explore the potential of pedogenic gypsum to provide with information about the palaeoenvironmental conditions affecting its development and preservation. This study has consisted in the sampling and analysis of pedogenic gypsum accumulations from five Loess-Palaeosol Sequences (LPS) applying several approaches and analytical techniques. Traditional methods based on the morphological study of gypsum and supported by micromorphological research have been applied, allowing us the observation of crystalline shapes, pedofeatures and mineral arrangements. We also performed isotopic analyses of pedogenic gypsum, including the isotopic signature of the stable sulphate (δ34S, δ18O) and gypsum hydration water (δ2H, δ18O). Besides, the minor and trace elements composition (Ba, Sr, Na, K, and Mg) was also analysed. Despite the difficulties when analysing pedogenic gypsum, due in part to its high solubility, our results show that the intensity of the dissolution/reprecipitation processes combined with the loess sedimentation rates determine gypsum characteristics and distribution in the studied loess profiles. Besides helping to determine the gypsum primary source areas, isotopic and micromorphological analyses suggest possible regional and temporal paleoclimatic differences (more/less arid) from the close or more open systems of gypsum in relation to meteoric water. In addition, the minor and trace elements composition suggests the presence of fluid and solid inclusions in the gypsum crystals of some horizons. This corresponds to gypsum formation in a medium with water rich in sulphates and other salts, and an increased proportion of primary gypsum in the sediment, in contrast to the precipitation of gypsum in unsaturated water. The combination of different methods has confirmed that pedogenic gypsum is a potential tool for improving palaeoenvironmental knowledge in arid and semi-arid regions. However, due to its limitations, it is recommended to combine additional techniques and indicators to use pedogenic gypsum as an effective palaeoenvironmental proxy.

This publication is part of the I + D + i RTI2018-094927-B-I00 project, funded by the Spanish MCIN/AEI/10.13039/501100011033/and FEDER “A way to make Europe”.