Critical evaluation of geochemical indices of palaeosalinity involving boron

Abstract

Despite years of research, palaeosalinity remains one of the most difficult palaeoenvironmental proxies to reconstruct. Numerous approaches have been proposed. Those utilising boron, gallium, strontium, barium, and potassium are among the most commonly used in palaeoenvironmental reconstructions. The comparison of chemical and mineralogical data obtained for hundreds of samples from various sedimentary basins of different ages and burial histories indicated that those proxies are highly sensitive to detrital and authigenic mineralogy changes. It has been shown that boron is preferentially accumulated in the 2:1 clays (illite 1Md and 2M1, illite–smectite), 1:1 clays (kaolinite), and hematite, which all are of non-synsedimentary origin. An increase in the weathering products input, high flux of the detrital mica, and/or enhanced burial diagenesis may increase B/K and B/Ga ratios, suggesting the marine environment of deposition. On the other hand, an increased input of the detrital K-feldspars or plagioclases may decrease those ratios implying freshwater sedimentation. Mineralogy is also responsible for the Sr/Ba ratio variability, resulting in spurious palaeoenvironmental interpretations. Isotope techniques may be more appropriate, especially if the finest clay fractions are used and the diagenetic fluid flow is confined to the analysed basin.