Diagenesis of growth bands in fossil scleractinian corals: identification and modes of preservation

Abstract

Light and dark bands in fully recrystallized fossil hermatypic corals are generally interpreted to represent annual growth increments reflecting a photosymbiotic life style—an interpretation of far reaching significance in palaeoecology. In this paper we describe annual growth bands in the colonial coral Porites in a perfect (aragonite and microstructures retained) and fully recrystallized (sparry calcite mosaic) style of preservation from sediments of Late Miocene age (Crete, Greece). Analysis of a continuous spectrum of transitional preservational stages shows that in Miocene Porites preservation of the growth banding was controlled by preferential dissolution of the high-density band associated with cementation by drusy calcite spar during freshwater diagenesis/shallow burial diagenesis. Marine precipitates (pelletoidal Mg-calcite) preferentially accumulated along tabulate dissepiments producing an additional growth rhythmicity. Massive Porites had annual growth rates of ∼4.0 mm, whereas in ramose branching Porites, a conspicuous banding is formed by concentrations of marine micropelletoidal cement along dissepiments at ∼1.8 mm spacing. If taken as annual growth increments, these bands represent very low extension rates, however, they may rather reflect subannual forcing functions (i.e., lunar cycles). An identical scenario of precipitation and concentration of pelletoidal carbonate along dissepiments and dissolution-controlled documentation of growth bands can be inferred for Late Jurassic microsolenids. Therefore, growth bandings in fossil corals potentially reflect both, monthly and annual cycles. Consequently, care must be taken when using coral growth bands in palaeoecology and palaeoclimatology.