Geochemical investigation of growth in selected Recent articulate brachiopods

Abstract

Buening, N. & Carlson. S. J. 1992 07 15: Geochemical investigation of growth in selected Recent articulate brachiopods. Growth increments have traditionally been used to determine age and growth rates of shelled organisms, particularly bivalves. Brachiopod growth increments and the time span they represent are rather poorly understood, however. Geochemical analyses of trace element concentrations preserved in skeletal calcite may provide an alternative method to determine the age of Recent brachiopods and to interpret their patterns of growth. Magnesium is a particularly important trace element in skeletal calcite because it is thought to vary with temperature, growth rate. and taxonomic affiliation. Electron microprobe analyses of Mg concentrations in the Recent articulate, temperate water brachiopods Terebratulina unguicula and Terebrotalia transversa, have revealed two distinct ontogenetic patterns of Mg concentration. The primary ontogenetic pattern is characterized by elevated and more variable levels of Mg concentration during early growth. followed by lower, more stable Mg concentrations during growth after sexual maturity. This pattern appears to be related to a predictable decrease in growth rate through ontopeny. Secondary peaks of Mg superimposed on this primary pattern may represent growth spurts related to annual cycles of productivity. Preliminary Fourier analyses of patterns of Mg concentration provide additional support for this hypothesis. Thus, a record of productivity characterized by annual peaks of Mg concentration may well allow us to age individual brachiopods by means other than size-frequency histograms. growth lines, and other less precise and accurate methods. Brachiopods, growth. biomineralization, calcification, Terebratali transversa, Terebratulina Unogicui.a, magnesium, Calcium, geochemical. ontogeny.