Test-size evolution of the planktonic foraminifer <i>Globorotalia menardii</i> in the eastern tropical Atlantic since the Late Miocene

Abstract

The mean test size of planktonic foraminifera (PF) is known to have increased especially during the last 12 Myr, probably in terms of an adaptive response to an intensification of the surface-water stratification. On geologically short timescales, the test size in PF is related to environmental conditions. In an optimal species-specific environment, individuals exhibit a greater maximum and average test size, while the size decreases the more unfavourable the environment becomes. An interesting case was observed in the late Neogene and Quaternary size evolution of Globorotalia menardii, which seems to be too extreme to be only explained by changes in environmental conditions. In the western tropical Atlantic Ocean (WTAO) and the Caribbean Sea, the test size more than doubles from 2.6 to 1.95 and 1.7 Ma, respectively, following an almost uninterrupted and successive phase of test-size decrease from 4 Ma. Two hypotheses have been suggested to explain the sudden occurrence of a giant G. menardii form: it was triggered by either (1) a punctuated, regional evolutionary event or (2) the immigration of specimens from the Indian Ocean via the Agulhas leakage. Morphometric measurements of tests from sediment samples of the Ocean Drilling Program (ODP) Leg 108 Hole 667A in the eastern tropical Atlantic Ocean (ETAO) show that the giant type already appears 0.1 Myr earlier at this location than in the WTAO, which indicates that the extreme size increase in the early Pleistocene was a tropical-Atlantic-Ocean-wide event. A coinciding change in the predominant coiling direction likely suggests that a new morphotype occurred. If the giant size and the uniform change in the predominant coiling direction are an indicator for this new type, the form already occurred in the eastern tropical Pacific Ocean at the Pliocene–Pleistocene boundary at 2.58 Ma. This finding supports the Agulhas leakage hypothesis. However, the hypothesis of a regional, punctuated evolutionary event cannot be dismissed due to missing data from the Indian Ocean. This paper presents the Atlantic Meridional Overturning Circulation (AMOC) and thermocline hypothesis in the ETAO, which possibly can be extrapolated for explaining the test-size evolution of the whole tropical Atlantic Ocean and the Caribbean Sea for the time interval between 2 and 8 Ma. The test-size evolution shows a similar trend with indicators for changes in the AMOC strength. The mechanism behind this might be that changes in the AMOC strength have a major influence on the thermal stratification of the upper water column and hence the thermocline, which is known to be the habitat of G. menardii.