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

[摘要] The mean test size of planktonic foraminifera (PF) isknown to have increased especially during the last 12 Myr, probably in termsof an adaptive response to an intensification of the surface-waterstratification. On geologically short timescales, the test size in PF isrelated to environmental conditions. In an optimal species-specificenvironment, 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 sizeevolution of Globorotalia menardii , which seems to be too extreme to be only explained by changesin environmental conditions. In the western tropical Atlantic Ocean (WTAO)and the Caribbean Sea, the test size more than doubles from 2.6 to 1.95and 1.7 Ma, respectively, following an almost uninterrupted andsuccessive phase of test-size decrease from 4 Ma. Two hypotheses have beensuggested to explain the sudden occurrence of a giant G. menardii form: it wastriggered by either (1) a punctuated, regional evolutionary event or (2) theimmigration of specimens from the Indian Ocean via the Agulhas leakage. Morphometric measurements of tests from sediment samples of the OceanDrilling Program (ODP) Leg 108 Hole 667A in the eastern tropical AtlanticOcean (ETAO) show that the giant type already appears 0.1 Myr earlier at thislocation than in the WTAO, which indicates that the extreme size increase inthe early Pleistocene was a tropical-Atlantic-Ocean-wide event. A coincidingchange in the predominant coiling direction likely suggests that a newmorphotype occurred. If the giant size and the uniform change in thepredominant coiling direction are an indicator for this new type, the formalready occurred in the eastern tropical Pacific Ocean at thePliocene–Pleistocene boundary at 2.58 Ma. This finding supports the Agulhas leakage hypothesis. However, the hypothesis of a regional, punctuatedevolutionary event cannot be dismissed due to missing data from the IndianOcean. This paper presents the Atlantic Meridional Overturning Circulation(AMOC) and thermocline hypothesis in the ETAO, which possibly can beextrapolated for explaining the test-size evolution of the whole tropicalAtlantic Ocean and the Caribbean Sea for the time interval between 2 and 8Ma. The test-size evolution shows a similar trend with indicators forchanges in the AMOC strength. The mechanism behind this might be thatchanges in the AMOC strength have a major influence on the thermalstratification of the upper water column and hence the thermocline, which isknown to be the habitat of G. menardii .

[学科分类] 大气科学