Distribution of Paleogene planktonic foraminifera — analogies with the Recent?

Abstract

Distribution patterns and morphologies of modern and Paleogene planktonic foraminifera in the Atlantic Ocean were compared. Although many Paleogene and modern planktonic foraminifera are homeomorphs, they are not ecomorphs. Morozovellid homeomorphs of the modern higher spired globorotaliids inhabited warmest layers of the mixed layer. The planorotalitids, homeomorphic with modern low trochospiral globorotaliids, have different geographic ranges; they predominated at temperature and higher latitudes where they grew to largest sizes. The cool water low-spired subbotinids little resemble the non-spinose, modern neogloboquadrinids which dominate polar faunas today. Spinosity of Paleogene polar forms indicates that they could have been carnivorous or omnivorous, rather than herbivorous like the modern neogloboquadrinids. Early Paleogene polar seas may have been relatively oligotrophic for at least part of the year. Some Paleogene genera may have been ecomorphic with their modern analogues. The dentoglobigerinids and globoquadrinids occupied equatorial and offshore upwellings much as Neogloboquadrina dutertrei does today. Because the globorotaloids and catapsydracids deposited calcitic crusts and register most enriched oxygen isotope values, they may have lived within and below the thermocline like some modern globorotaliids. Globorotaloids and catapsydracids may have been strictly herbivorous as are most modern, non-spinose species. Their predominance in later Paleogene lower latitude upwellings, but their near exclusion from polar regions indicates that subeuphotic equatorial waters may have been more eutrophic. Paleogene-type warming (in Zones P1d, P3a, P5, P6–7, late P12, late P16, late P19, and early P22) involves proliferation, evolutionary diversification, and poleward migration of thermophilic forms, but decreased species abundance at low latitudes, and diminished numbers of fertility indices (the guembelitrids, biserial heterohelicids, globigerinathekids, globoquadrinids, or dentoglobigerinids). Neogene-type warmings (in Zones P4 and probably P alpha and P21a) involved diversification of cooler water taxa, retreat of thermophilic forms from higher latitudes and their concentration in lower latitudes, and proliferation of fertility indices. Under these conditions oxygen isotope records include lower latitude warming, but cooling of temperature and higher latitudes and tropospheric waters. Planktonic foraminiferal diversity and population structure reflect changes in oceanic fertility and the depth of the thermocline. Cooling and eutrophication resulting in a contraction of the TRC (Trophic Resource Continuum) in Atlantic surface waters from late middle Eocene through mid Oligocene time explain the low diversities, extinctions of morphologically complex thermophilic species, increased dominance of opportunistic, subeuphotic, and temperature species, proliferation of euphotic and subeuphotic fertility indices, and geographic expansion of siliceous primary producers.