The boron concentration in the clay fraction of argillaceous sediments has been used for several years as an index to paleosalinity. Although experimental studies have shown that adsorption is the initial uptake reaction between clay and solution, basic questions have remained concerning the permanent fixation of boron in a clay mineral, and other possible diagenetic effects. The present study considered that question in the light of data obtained from sediment and expressed porewater from shallow cores taken from various parts of the Mississippi Delta system.

Clays entering the Gulf of Mexico from the Mississippi River adsorb boron from seawater while the clays are still in suspension. After burial, this initially adsorbed boron is fixed by the clay, and additional boron is adsorbed from the interstitial pore-water. Very soon after burial, considerable boron may be released from organic matter in the sediment (perhaps due to microbiological decomposition), causing an early enrichment of boron in the porewater. This boron, however, is also adsorbed by the clay minerals in time.

Contrary to the conclusions reached by others, these data indicate that late diagenetic enrichment is not required to explain the boron content of deeply buried Gulf Coast shales.

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