A PROPOSED MODEL FOR CARBONATE BANKS CHARACTERIZED BY LARGE, FLAT, BENTHIC FORAMINIFERS

Biostromes and bioherms characterized by exceptionally large, flat specimens of larger benthic foraminifers (LBF) are commonly found in Paleogene and Miocene carbonates of the Neo-Tethys (Mediterranean region to Indo-Pacific). For example, nummulite banks were originally described as low relief, low diversity bioherms characterized by an abundance of very large, flat, microspheric shells of one or two species. Although early descriptions assumed that the banks accumulated in place, sedimentary features often indicated winnowing or transport. By incorporating insights from paleoceanographic research and the biology of living LBF, a new model is proposed that can account for both sedimentological and paleontological features. Sexual reproduction by gamete broadcasting is common in foraminiferal taxa, including extant Nummulitidae. Resultant zygotes can develop into tiny, resistant, easily dispersed propagules. Experiments have revealed that such propagules of shallow-marine species can recruit from sediments collected from deep-ocean sites. The role of algal symbiosis in the biology of LBF is well documented. Paleoceanographically, LBF proliferated during times of reduced ocean stratification. Outer shelf or promontory sites, overlain by exceptionally clear, nutrient-depleted waters, had minimal input of organic matter, limiting recruitment of heterotrophic taxa. At “twilight-zone” depths, light penetration was sufficient, at least intermittently, to support some photosynthesis. The tiny propagules of the deepest-dwelling LBF species could develop, growing very slowly, nourished by feeding on bacteria and photosynthate from their algal symbionts, which were adapted to very low irradiance. Under such conditions, microspheric individuals of one or two LBF taxa could live and grow to very large shell diameters, slowly accumulating resources required for asexual reproduction. Resulting carbonate accumulation rates would have been very slow, such that even rare disturbances by currents, storms or internal waves could produce sedimentary structures. The fossil evidence of such habitats could be biostromes or low-relief bioherms of low diversity assemblages characterized by abundant, exceptionally large, flat, microspheric LBF.