The Sr/Ba ratio response to salinity in clastic sediments of the Yangtze River Delta

Abstract

The Sr/Ba ratio is a geochemical indicator that distinguishes between terrestrial and marine sedimentary environments in terrigenous clastic sediments. However, conclusions drawn from the Sr/Ba ratio is often inconsistent with those drawn from other sedimentary environment indicators during practical application; therefore, the usability of this indicator is not widely acknowledged. First, to verify this theory and accurately determine the differences between the sedimentary geochemical behaviours of Sr and Ba under different salinity conditions, 14 artificial deltaic sediments were prepared by mixing clean seawater and muddy river water in varying proportions to produce different constant salinities, ranging from 0.0 to 31.2 psu. The Sr and Ba concentrations in the different fractions were then examined via sequential extraction. The experimental results show that, on average, 87% of the total Ba and 65% of the total Sr in the 14 artificial deltaic sediments existed in the residual fraction that is independent of the geochemical conditions of the depositional environment. The experiment also revealed that the decrease in Ba in the sediments was caused by the desorption of adsorbed Ba, owing to the addition of seawater with increasing salinity. Furthermore, the Sr/Ba ratio in the exchangeable fraction yielded a strong linear correlation with salinity, and the Sr/Ba ratio in the carbonate-bound fraction tended to increase slightly with increasing salinity, the Sr/Ba ratios in the FeMn oxide-bound, organic and reductive, and residual fractions, and the total Sr/Ba ratio were not significantly correlated with salinity. Therefore, the Sr/Ba ratio (Sr/Ba-HAc) of exchangeable and carbonate-bound Sr and Ba extracted using dilute acetic acid (HAc) and the Sr/Ba ratio (Sr/Ba-NH4Ac) of exchangeable Sr and Ba extracted using ammonium acetate (NH4Ac) can be used to distinguish between marine and terrestrial sedimentary environments. If the Sr/Ba-HAc or Sr/Ba-NH4Ac ratio in the sediment is >1.0, it can be considered to have been deposited in a sedimentary environment affected by seawater with a salinity >5.0 psu. Second, to verify the aforementioned conclusions, the Sr and Ba in 24 field samples from the Yangtze River Delta were extracted using 10% HAc or 1.0 M NH4Ac. The results indicate that the increase in the amount of Sr in the estuarine terrigenous clastic sediments from the river to the marine environments is caused by the addition of Sr-rich, Ba-poor, marine carbonate shells. The extracted Sr/Ba-HAc ratios were < 1.0 in freshwater (fluvial), 1.0–3.0 in brackish water (delta front), 3.0–8.0 in saltwater (prodelta), and > 8.0 in normal seawater (neritic environments and relict sand); however, in the traditional bulk analysis, total Sr/Ba ratio did not change significantly with increasing salinity. Therefore, it is suggested that the selective extraction of sedimentogenic Sr and Ba is the only effective means of using the Sr/Ba ratio to discriminate between marine and terrestrial sedimentary environments in terrigenous clastic sediments, while the traditional method of using the total Sr/Ba ratio of whole samples is unsuitable.