ABSTRACT

Using the method of Anderson (1968), in which carbonate surface areas are computed from the amount of C¹⁴-labeled CO₂ that exchanges with the surface CaCO₃ in a sample, we have determined "exchange surface areas" for about fifty unrecrystallized calcitic and aragonitic molluscs. Measured surface areas for fragments of these fossils range from 0.03 to 1.2 m² gm^-1, with aragonitic forms at the higher end and calcitic forms at the lower end. The variation among different specimens of the same genus is about a factor of three, much less than that between different genera.

Crushing increases surface areas by varying amounts to values ranging from 0.3 to 3 m² gm^-1. The molluscs are composed of CaCO₃ plates or rods having thicknesses of about a micron; thus the total surface area of the ultrastructure is on the order of 1 m² gm^-1. Crushing increases the surface area toward this upper limit. The range in surface areas of crushed samples from one genus is no more than a factor of three, again much less than the range from among different genera.

Uranium uptake during diagenesis increases with increasing fossil surface area, suggesting that the mechanism of U uptake is dependent on processes occurring on the surface of the fossil.