Search

Globigerinoides subquadratus Br��nnimann, 1954 Pl. 7, figs. 11���12 Globigerinoides subquadratus Br��nnimann, 1954, p. 680, pl. 1, fig. 8; Kennett & Srinivasan, 1983, p. 74, pl. 10, fig. 4; pl. 16, figs. 1���3. Description: The wall surface is macroperforate and cancellate-spinose. The test is trochospiral, sub-globular to sub-quadrate in shape and semi-circular in cross section. The chambers are sub-rounded to sub-quadrate in shape, with two basal chambers on the umbilical side, separated by a straight deep incised suture, which is overarched by a large chamber. The final chamber comprises approximately half the size of the test. The primary aperture is large and forms an arch symmetrically above the two basal chambers in apertural view. Two supplementary apertures form on the spiral side of the test along sutures of earlier chambers. Remarks: The relative abundances of Gs. subquadratus are major (Gs. subquadratus are relatively small, measuring 0.3 mm in diameter. This species is very similar to Globigerinoides ruber,but are stratigraphically separate and can be morphologically differentiated. Gs. subquadratus is identified in the Miocene, whereas Gs. ruber is extant and occurs in younger strata. Morphologically, the shape and outline of the test and chambers of Gs. subquadratus is more quadrate in shape compared to Gs. ruber (Bolli et al., 1985). Environmental preferences: Gs. subquadratus is associated with Neogene tropical environments (Br��nnimann et al., 1971; Wade et al., 2011). Global stratigraphic range: This species occurs in Miocene strata from the Aquitanian to Tortonion (Kennett & Srinivasan, 1983; Wade et al., 2011). Regional occurrence: Gs. subquadratus has been identified in middle Miocene strata from the Congo Basin (Kender et al., 2008) and the northern Namibian outer continental shelf, south of the Kunene River mouth (this study)., Published as part of Bergh, Eugene W. & Compton, John S., 2022, Taxonomy of Middle Miocene foraminifera from the northern Namibian continental shelf, pp. 1-55 in Zootaxa 5091 (1) on page 33, DOI: 10.11646/zootaxa.5091.1.1, http://zenodo.org/record/5840434, {"references":["Bronnimann, P. (1954) Probable occurrence of Oligocene on Saipan. American Journal of Science, 252 (11), 673 - 682. https: // doi. org / 10.2475 / ajs. 252.11.673","Kennett, J. P. & Srinivasan, M. S. (1983) Neogene Planktonic Foraminifera. A Phylogenetic Atlas. Hutchinson Ross, Stroudsburg, 265 pp.","Bolli, H. M., Saunders, J. B. & Perch-Nielsen, K. (1985) Plankton Stratigraphy. Cambridge University Press, Cambridge, 1032 pp.","Bronnimann, P., Martini, E., Resig, J., Riedel, W. R., Sanfilippo, A. & Worsley, T. (1971) Biostratigraphic synthesis: Late Oligocene and Neogene of the western tropical Pacific. In: Winterer, E. L., Riedel, W. R., Bronnimann, P., Gealy, E. L., Heath, R., Kroenke, L., Martini, E., Moberly, R., Resig, J. & Worsley, T. (Eds.), Initial Reports of the Deep Sea Drilling Project. Vol. 7. Ocean Drilling Program, College Station, Texas, pp. 1723 - 1745. https: // doi. org / 10.2973 / dsdp. proc. 7.136.1971","Wade, B. S., Pearson, P. N., Berggren, W. A. & Palike, H. (2011) Review and revision of Cenozoic tropical planktonic foraminiferal biostratigraphy and calibration to the geomagnetic polarity and astronomical time scale. Earth-Science Reviews, 104, 111 - 142. https: // doi. org / 10.1016 / j. earscirev. 2010.09.003","Kender, S., Kaminski, M. A. & Jones, R. W. (2008) Early to middle Miocene foraminifera from the deep-sea Congo Fan, offshore Angola. Micropalaeontology, 54, 477 - 568."]}

It has been hypothesized that global temperature trends are tightly linked to tropical thermocline depth, and that thermocline shoaling played a crucial role in the intensification of late Pliocene Northern Hemisphere glaciation. The Pliocene thermocline evolution in the Pacific Ocean is well documented and supports this hypothesis, but thermocline records from the tropical Atlantic Ocean are limited. We present new planktonic foraminiferal Mg/Ca, δ18O, and δ13C records from the late Pliocene interval at Ocean Drilling Program Site 959 in the Eastern Equatorial Atlantic (EEA), which we use to reconstruct ocean temperatures and relative changes in salinity and thermocline depth. Data were generated using surface-dwelling Globigerinoides ruber and subsurface-dwelling Neogloboquadrina dutertrei. Reduced gradients between the surface and subsurface records indicate deepening of the EEA thermocline at the end of the mid-Piacenzian Warm Period (mPWP; ∼ 3.3–3.0 Ma). We connect our late Pliocene records to previously published early Pliocene δ18O data from Site 959 and compare these to the Site 1000 in the Caribbean Sea. Over the course of the Pliocene, thermocline changes in the EEA and Caribbean Sea follow similar patterns, with prominent step-wise thermocline deepening between ∼ 5.5 and 4.0 Ma and gradual shoaling up to the mPWP, followed by minor deepening at the end of the mPWP. The tropical thermocline depth evolution of the tropical Atlantic differs from the Pacific, which is characterized by gradual basin-wide shoaling across the Pliocene. These results potentially challenge the hypothesized link between tropical thermocline depth and global climate. The mechanisms behind the periodically divergent Pacific and Atlantic thermocline movements remain speculative. We suggest that they are related to basin geometry and heterogenous temperature evolutions in regions from where thermocline waters are sourced. A positive feedback loop between source region temperature and tropical cyclone activity may have amplified tropical thermocline adjustments.

Geochemical signals from planktonic foraminiferal tests provide a wide array of tools to investigate past hydrographic changes, but these techniques are limited by our understanding of the habitat at which these organisms form their shell. Quantitative constraints on the variability of the calcification habitat of these organisms can thus lead to better interpretations of these signals and more quantitative constraints on indirect properties related to these signals. We compiled oxygen isotope data from the Tropics to investigate global trends in the apparent calcification depth (ACD) of commonly used planktonic foraminifera. Using Globigerinoides ruber albus, Trilobatus sacculifer, Globorotalia tumida, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, we find that our ACD estimates for the global tropics largely match regional ACD estimates from other studies. G. ruber and T. sacculifer seem to calcify in the surface mixed layer with a mean ACD and 95% confidence interval of 17 [0,86] m and 48 [0,128] m, respectively, and G. tumida, N. dutertrei, and P. obliquiloculata seem to calcify below the mixed layer with a mean ACD of 210 [123,385] m, 114 [42,173] m, and 94 [0,169] m, respectively. The global data set enables the determination of calcification depths across locations with a wide range of thermocline depths. We find that G. tumida exhibits a calcification depth that is more spatially invariant, while N. dutertrei and P. obliquiloculata seem to calcify deeper in the water column in regions where the thermocline is deeper, as has been previously suggested.