Geochemical signature of incremental growth; rugose corals from the Middle Devonian Traverse Group, Michigan

Incremental growth has long been known in living and fossil scleractinian corals, and also has been recognized in Paleozoic rugose and tabulate corals. The usefulness of growth studies and geochemistry has been demonstrated in the study of environmental aspects and population structure of modern and fossil corals. Until recently it has not been possible to evaluate variations in the geochemistry of fossil skeletal carbonate. Use of a modern electron microprobe (Superprobe) allows mapping of trace element distribution within skeleton, here of Tabulophyllum traversensis from the Middle Devonian (Givetian) Traverse Group of Michigan. Detailed microprobe mapping of magnesium distribution clearly illustrates incremental skeletal growth in this species, with growth marked by alternating bands of magnesium-free and magnesium-rich carbonate (relatively speaking, as maximum concentrations approximate only 0.5 weight percent MgO). Such banding allows evaluation of diagenetic change in this type of carbonate skeleton, here almost lacking. Understanding of incremental uariation of chemical composition (trace elements) can add to our knowledge of coral skeletal structure and its diagenesis, chemistry of ancient sea waters, and understanding of paleobiology of extinct Paleozoic corals, the Rugosa and Tabulata.