Geochemical evolution of clay pore water as an indicator for palaeoenvironmental variability in the Hebei Plain, northern China

Abstract

In order to elaborate geochemical evolution of clay pore water (CPW) and associated palaeoenvironment in Hengshui, where there is shallow saline water in the area of Hebei Plain, a borehole with a depth of 130 m was drilled in August 2013 in Hengshui city. Q₄ sediments occurred above 35 m and Q₃ mainly at depth of 130–35 m. Sediment particle size, geochemistry and deuterium and oxygen isotopes of CPW were analyzed. The CPW was generally of meteoric origin and was influenced by evaporation. Total dissolved solids (TDS) in pore water were high, which was mainly controlled by Na⁺, Mg²⁺, SO₄ ²⁻ and Cl⁻. Ratios of rNa/rCa of CPW were mostly larger than 1, indicating the missing of Ca²⁺, as a result of evaporation and cation exchange. At depths of 20–10 m, ratios of rNa/rCl were larger than 1, reflecting the accumulation of Na⁺ during the continent salinization process. The TDS and δ ¹⁸O values of CPW above 6 m demonstrated that CPW was primarily affected by human irrigation, as well as by atmospheric precipitation. The mean δ ¹⁸O values of CPW were −11.5, −12.1 and −10.6 ‰ at depths of 130–90, 65–29 and 23–0 m, respectively, which were lower than the δ ¹⁸O values of groundwater at depths of 90–65 m (−9.8 ‰) and 29–23 m (−8.2 ‰). The depletion in ¹⁸O of CPW suggested that the CPW saved the palaeoenvironment media at depths below 6 m. The δ ¹⁸O of CPW indicated that the temperature decreased at the depths from 130 to 90 m, and the climate became colder at depths of 65–29 m but evolved to be warm in Holocene at depths of 23–0 m.