Morphometric and stable isotopic differentiation of Orbulina universa morphotypes from the Cariaco Basin, Venezuela

Brittney J. Marshall*, Robert C. Thunell, Howard J. Spero, Michael J. Henehan, Laura Lorenzoni, Yrene Astor

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

35 Citations (Scopus)


Biometric characteristics of Orbulina universa (d'Orbigny) were used to differentiate two morphotypes present in sediment trap samples collected from the Cariaco Basin, Venezuela. Specifically, wall thickness and weight-area relationships were used to separate shells into thin (Mthin) and thick (Mthick) morphotypes. Mthick (mean thickness=19-41μm) comprises 75% of the total O. universa in these samples and has morphometric characteristics similar to that of the previously described Type I Caribbean genotype, whereas Mthin (mean thickness=6-22μm) is comparable to the Type III Mediterranean genotype. The flux of Mthick increases during periods of upwelling, whereas Mthin flux shows no systematic relationship with changing hydrographic regimes in the basin. The δ18O and δ13C of Mthick are on average 0.34% higher and 0.38% lower, respectively, than those of Mthin, suggesting that they calcify their final spherical chamber at different depths in the water column and/or differ in their vital effects on shell geochemistry. Additionally, the absolute offset in the stable isotopic compositions of the two morphotypes varies as a function of surface ocean stratification. During periods of upwelling, the δ18O and δ13C absolute offsets between Mthin and Mthick are on average -0.15 and 0.25%, whereas average δ18O and δ13C absolute offsets between these two morphotypes increase to -0.54 and 0.41% during non-upwelling periods. We attribute the higher δ18O and δ13C offsets during non-upwelling periods to a difference in final chamber calcification depth and the combined effects of temperature (δ18O), δ13CDIC13C), irradiance (δ13C) and [CO32-] (δ18O and δ13C). These data provide field evidence that thin and thick morphotypes of O. universa likely experience different environmental conditions during the formation of their final chamber and, therefore, should not be combined in geochemical analyses for reconstructing past surface ocean conditions. Finally, we introduce a new proxy for reconstructing past surface ocean stratification changes via the use of Mthin and Mthick δ18O and δ13C offsets.

Original languageEnglish
Pages (from-to)46-64
Number of pages19
JournalMarine Micropaleontology
Publication statusPublished - 1 Oct 2015

Bibliographical note

Funding Information:
We thank Eric Tappa for his analytical contributions, particularly with the collection of sediment trap samples and stable isotopic measurements. We thank the Fundación La Salle de Ciencias Naturales, Estación de Investigaciones Marinas Isla Margarita (EDIMAR) and the crew of the R/V Hermano Ginés for their hard work at maintaining the CARIACO Time Series. We thank Pincelli Hull of Yale University and Jay Pickney of University of South Carolina for their advice on concept and statistical assessment. This research was supported by NSF award 1258991 .

Publisher Copyright:
© 2015 Elsevier B.V.


  • Cryptic species
  • Morphotypes
  • Orbulina universa
  • Planktonic foraminifera
  • Stable isotopes


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