£ Hmentology-EheVieT Publishing Company, Amsterdam-Printed in The Netherlands ARAGONITIC OOIDS': EXPERIMENTAL PRECIPITATION FROM SEA WATER IN THE PRESENCE OF HUMIC ERWIN SUESS AND DIETER FOTTERER Geologisch-Patdontologisches Institut und Museum der Universitat Kiel, Kiel (Germany) (Accepted for publication March 20, 1972) .ABSTRACT Suess, E. and Futterer, D., 1972. Aragonitic ooids: experimental precipitation from seawater in the presence of htunic acid. Sedimentohgy, 19:129-139. Humic acid dissolved in artificial seawater influenced the morphology', internal structure, and composition of aragonite when precipitation was induced with dilute NajCOj solution. At sodium humate concentrations of around 20 mg,/l, numerous brownish spherical aragonite bodies developed within one day at 25 °C. The spheres ranged in size from 10-100 microns and resembled natural marine ooids. They formed with gentle agitation of the solution as well as with no water movement at all. The typical structure of natural ooids consisting of concentric alternating aragonite and organic la- minae was experimentally duplicated as layers of aragonite crystals alternating with humate membra- nes; however, in contrast to natural ooids, the individual aragonite crystals here were oriented radially with their c-axes. The aragonite of the spheres contained about 20 wt. % more strontium than the ara- gonite precipitated experimentally without the addition of sodium humate, and organic carbon content of the spherical aragonite was about 7% by weight. INTRODUCTION Theories on the origin of oolites include accretionary (Sorby, 1879), micro- biological (Monaghan and Lytle, 1956; Lalou, 1957) and strictly inorganic phy- sico-chemical processes (e.g., Cayeux, 1935; Newell et al., 1960; Usdowski, 1963; Ball, 1967; Bathurst, 1971). The inorganic precipitation of spherical calcium car- bonate grains from agitated and supersaturated surface seawaters is currently the more favoured theory, but the very presence of organic matter in ooids has been suggested as evidence for its having an active role in their formation (e.g.. Shear- man and Skipwith, 1965; Bathurst, 1968). More specifically, Mitterer (1968), 1971) proposes that a proteinaceous matrix, which is found interlayered with concentric aragonite laminae; might cause the formation of ooids in seawater. General association of organic matter with calcium carbonate in the marine environment has been reported by Chave (1965), and the infiuence of this ^ 1 crminology as proposed and discussed by C. Teichert, 1970. • 'his paper was presented at the International Sedimentological Congress, 8th, Heidelberg, 1971.