The Deleterious Effects of Phosphates in the Preparation of Ostracode Shells

M. Kontrovitz
University of Louisiana, Monroe, LA 71209

Ostracodes are the most useful group of Crustaceans in geological practice, but little attention has been directed to the consequences of preparation techniques and related implications for use in paleoenvironmental reconstruction and stratigraphy. Common practice in preparation includes the use of sodium hexametaphosphate [(NaPO₃)n] and/or sodium tripolyphosphate (Na₅P₃O₁₀), marketed in “Calgon Water Softener.” Here, waters from several sources were used with Calgon solutions of different concentrations for different periods of time. Experiments and comparisons were applied to modern and fossils ostracodes.

These dissolution experiments simulate some diagenetic processes in a predictable, sequential removal of the epicuticle and exposure of the inner layers of organic mesh. In exterior view, the epicuticle undergoing dissolution first developed pits about 700 um across, then spines appeared worn, pores were enlarged by 20-50%, and patches of epicuticle were lost. Exposed then was the exocuticle with pits (1000 um) and solution tracks, 1000 um across; the exocuticle too was lost in patches. The pits formed between the mesh of the relatively insoluble organic net work. Then margins of the central muscle scars were etched and the scars were then dissolved, forming holes. The endocuticle was exposed with its coarser mesh and larger pits between fibers. This sequence was compatible with accepted carapace models.

Information from these solution experiments was compared with alteration features of fossils, including those from metamorphic rocks which provide real taphonomic sequences. Experimental results were compatible with natural sequences and provide a concomitant appreciation of information loss.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana