Geochemical signature of aquitard pore water and its paleo-environment implications in Caofeidian Harbor, China

Abstract

Oxygen-18 (18O), deuterium (D), and chemical patterns were determined in pore water samples extracted from 100-m depth in Caofeidian Harbor, China. The stable isotopic composition indicates that aquitard pore water was of meteoric origin and was basically not influenced by evaporation. Seawater diffusion was identified as the main influence on the isotope signature of pore water, according to the δ18O profile and positive correlation between δ18O and Cl–. The chemistry of aquitard pore water is characterized by high total dissolved solids (TDS) decreasing with depth, ranging from 26.89 to 7.26 g/L. The same trend was observed for Cl–, Na+, and Mg2+, as influenced by seawater. However, the typical ion ratios (i.e., Cl/Br, Sr/Ba) of pore water change significantly in different sedimentary facies along the study profile due to the long-term reaction between pore water and sediment, further supporting the finding that pore water was not replaced by modern water or seawater. Therefore, during long-term aquitard residence, the chemical composition of pore water would be influenced by other processes such as reduction of sulfates and cation exchange. The rare earth elements (REEs) of pore water are mainly affected by water–rock interaction. The shale-normalized REE fractionation patterns display enrichment of heavy REEs (HREE) relative to light REEs (LREE) and even greater fractionation degree in marine sediment pore waters (as larger La/YbNASC). The relative reduction environment in the aquitard and preferential mobilization of Eu2+ in the water–rock interaction lead to the development of positive Eu anomalies (1.13 < (Eu/Eu*)NASC < 1.98) and slightly positive Ce anomalies (–0.13 < (Ce/Ce*)NASC < 0.35).