Calculation of Paleosalinities from Boron and Clay Mineral Data

Abstract

The validity of as a salinity index has been established by numerous studies over the past 2 decades. It is known that all the major clay minerals may contain which can be correlated with paleosalinity. However, under given conditions, illites will contain the most and kaolinites the least. Montmorillonites and probably chlorites are intermediate. Published data indicate that a detrital clay will adsorb from solution, fix it at the surface, and later incorporate it within the structure. For seawater systems, this uptake can be expressed as a Freundlich adsorption isotherm, as log B = C1 log S + C2, where B = uptake (ppm), S = salinity of the water (^pmil), and C1, C2 = constants, dependent on the particular clay. Factors other than salinity that affect content are inherited clay mineralogy, grain size, and crystallinity of the clay minerals. Sufficient independent data were available to use an adsorption isotherm to calculate paleosalinities for a kaolinitic Tertiary shale in Nigeria. The constants necessary for calibration of the isotherm were evaluated as: Inherited kaolinite boron, O; Kaolinite boron uptake at 1 ^pmil salinity 1.3 ppm (C2 = 0.11); and Kaolinite boron uptake at 35 ^pmil salinity 65 ppm (C1 = 1.28). Boron concentrations, related to kaolinite, were converted graphically to paleosalinities from a log-log calibration curve with the use of these values. Paleosalinities determined in this manner are in good agreement with independent faunal interpretations.