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 (M thin ) and thick (M thick ) morphotypes. M thick (mean thickness = 19-41 μm) 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 M thin (mean thickness = 6-22 μm) is comparable to the Type III Mediterranean genotype. The flux of M thick increases during periods of upwelling, whereas M thin flux shows no systematic relationship with changing hydrographic regimes in the basin. The δ
18O and δ 13 C of M thick are on average 0.34‰ higher and 0.38‰ lower, respectively, than those of M thin , 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 δ