Reconciling Taxonomic Discrepancies in the Globigerinoides Ruber-Elongatus Plexus, and Isotopic Implications in the Gulf of Mexico and Caribbean Basins

Abstract

The reliability of foraminifera as stratigraphic index fossils, and as isotopic proxies of marine environments, is based on the assumption that fossil concepts represent uniform species, responding consistently to their ambient environments. Understanding sources of uncertainty is, therefore, critical. We explore a potential bias in the application of the Globigerinoides ruber-elongatus plexus, planktonic foraminifera utilized extensively for Cenozoic biostatigraphy and paleo-reconstruction. Taxonomic revisions since 1826 have resulted in the merging of multiple species under one general name, the implications of which are now under scrutiny. These “morphotypes” of G. ruber have been shown to incorporate isotopes and trace elements dissimilarly, and correspond to multiple genetic species (e.g., Globigerinoides elongatus), occupying different environments. Various criteria exist to distinguish these types, but their efficacy has not been tested quantitatively. Most rely on the distinctive traits of end-members, while specimens in the morphological “transitional zone” are left to an observer’s subjective interpretation. This prevents quantification in census counts, and may lead to erroneous geochemical analyses. Furthermore, molecular clock estimates suggest that the G. elongatus species evolved significantly later than G. ruber, affecting its potential as a biomarker.

Here, we present a minimal-criteria system for classifying Globigerinoides-type morphologies using only three conditions: final chamber compression, final chamber asymmetry, and aperture compression. Morphometric analyses on 200 specimens, grouped according to this new system allow us to assess to what degree visual classification reflects morphospace discontinuity, and how reliable it is. The consistency is promising, and supports the potential of this relatively simple diagnostic scheme in future paleoceanographic and biostratigraphic studies. Armed with this information, we explore the use of isotopic offsets within the Globigerinoides plexus, in reconstructing regional differences in climatic influences, carbonate preservation potential, and microfossil burial in the Gulf of Mexico and Caribbean basins.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018