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A Proposed Microfossil Reliability Index and a Test Using Mid-Cretaceous Calcareous Nannofossil Data


A problem often encountered while using microfossil marker taxa for biostratigraphy is determining their reliability, particularly during the Mesozoic Era due to the patchy abundance and distribution of taxa. The last appearance datum (LAD) of these marker species are of particular interest because their abundances decrease towards the end of their stratigraphic range. Current reliability index methods exist; however, they produce low values for Mesozoic taxa because these methods were designed for high-resolution sampling of expanded sections, and do not work well for condensed intervals. A new reliability index method, based on binomial theory, is proposed and tested using upper Albian-lower Cenomanian calcareous nannofossil distributions from Deep Sea Drilling Project (DSDP) Sites 137 and 547. These sites were chosen due to the presence of well-preserved nannofossil assemblages. The proposed method of reliability testing is based on binomial probabilities (Bernoulli trials), where independent events are classed as “successes” or “failures”. This method determines the probability of success of finding the marker species over its entire stratigraphic range. The binomial probability equation is used to determine if these bioevents, especially LADs, are an accurate representation of the stratigraphic range. Bioevents were chosen for indexing in two ways. Specimens were counted for approximately 300 fields of view (FOVs) per slide from the upper Albian-lower Cenomanian calcareous pelagic sediments from DSDP Sites 137 and 547. These data were combined with existing data from other sites and analyzed using RAnking and SCaling (RASC), a probabilistic biostratigraphic method, in order to identify the most consistent first appearances (FADs) and LADs in these sections. This method produced a total of 16 bioevents based on FADs and LADs. In addition to these events, bioevents from the literature were selected for reliability testing. This method produced a range of values reflecting substantial variation in bioevent reliability. Several bioevents used in published zonations were ranked as unreliable, while three very reliable bioevents that are not used in the current biostratigraphic schemes were identified: FAD of Lithraphidites eccentricum, LAD of Staurolithites mutterlosei, and LAD of Broinsonia stenostaurion. The reliability index method used in this current study is a viable option for determining the true stratigraphic range of marker species.