Paleoenvironmental Application of Calcareous Nannofossils
Alicia C. Kahn
Chevron Energy Technology Company, Houston, TX
Biostratigraphers primarily use benthic foraminifera and palynological assemblages for paleoenvironmental reconstruction; calcareous nannofossils and planktonic foraminifera for detailed age dates. Preliminary research comparing modern to Miocene coccoliths indicates that broader application of calcareous nannofossils may be possible. Ocean depth preferences of the modern calcareous nannoplankton can be extrapolated to the fossil record, if we assume that morphological variability can imply habitat.
Intraspecific variability in the modern calcareous nannoplankton contrasts markedly from the shallow to the deep photic zone. Considerable intra- and interspecific variability in coccolith and coccosphere morphology characterizes the shallow photic zone, whereas low intraspecific variability distinguishes the deep photic zone. It is this juxtaposition in variability that we can use in examining the fossil record.
I present a quantitative comparison of the morphological variability between coccoliths of extant calcareous nannoplankton species and the Miocene species, Minylitha convallis. The morphology of the coccoliths of M. convallis remains constant through time, with overall low intraspecific variability in all samples, indicating that it may have lived in the deep photic zone. Following this assumption, greater relative abundance of M. convallis, like that of the modern deep photic zone species, should indicate a highly stratified paleo-ocean, deep nutricline, and warmer climate, whereas reduced abundance relative to other calcareous nannoplankton species should indicate a cooler climate and shallow nutricline.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery