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

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 mu m) 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 mu m) 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 delta O-18 and delta C-13 of M-thick are on average 0.34 parts per thousand higher and 038 parts per thousand 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 delta O-18 and delta C-13 absolute offsets between M-thin and M-thick are on average -0.15 and 0.25 parts per thousand, whereas average 8180 and delta C-13 absolute offsets between these two morphotypes increase to -0.54 and 0.41 parts per thousand during non-upwelling periods. We attribute the higher delta O-18 and delta C-13 offsets during non-upwelling periods to a difference in final chamber calcification depth and the combined effects of temperature (delta O-18), delta C-13(DIC) (delta C-13), irradiance (delta C-13) and [CO32- (delta O-18 and delta C-13). 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 M-thin and M-thick delta O-18 and delta C-13 offsets. (C) 2015 Elsevier B.V. All rights reserved.