Oceanographic significance of Pacific late miocene calcareous nannoplankton

Abstract

An analysis of the variability in the composition and distribution of Pacific Late Miocene calcareous nannoplankton about their average biogeography shows that there are primarily two environmental factors causing that variability, climate and dissolution. Climate produces a latitudinal, biogeographic differentiation of the Late Miocene nannoflora, while selective dissolution superimposes a bathymetric differentiation of the nannoflora on that due to climate. Together, these two factors produce three distinct Late Miocene nannofloral assemblages, a high-latitude, temperate assemblage characterized by Reticulofenestra pseudoumbilica and Coccolithus pelagicus, and two tropical assemblages, their differences in composition depending on water depth and surface-water productivity: (1) in shallower water and beneath areas of higher organic production and sedimentation of calcite there is an undissolved assemblage characterized by sphenoliths, small elliptical placoliths and Coccolithus pataecus; (2) in deeper water and areas of lower productivity there is a dissolved assemblage dominated by discoasters. Selective dissolution produces most of the apparent biogeographic variation in Pacific Late Miocene nannoplankton compositions, the variation in compositions observed between the seventeen sites studied. Dissolution preferentially removes the more soluble constituents of the tropical nannoflora so that increasing dissolution tends to give tropical nannoflora a cooler, more temperate aspect. At the same time, selective dissolution shifts the composition of the warmer, tropical component towards its more resistant taxa. Nannoplankton records show a period of greatly decreased calcite dissolution in deep tropical and temperate South Pacific sites between about 8 and 10 m.y. ago. This decrease is strongly correlated with a temporary increase in the 13C composition of Pacific deep waters. Calcite dissolution increased during this same period in the deep North Pacific. Nannoplankton records of Late Miocene climate in the tropics are distinctly different from those at higher, south temperate latitudes. Tropical records show a sharp warming in the earliest Late Miocene after a generally cool late Middle Miocene. This was followed by a temporary cooling, nearly to Middle Miocene levels, about 7 m.y. ago. Toward the end of the Late Miocene, the tropical Pacific warmed again and remained warm into the Pliocene. Warming of temperate climates occurred much later. Not until latest Miocene did the southern the Pliocene. Warming of temperate climates occurred much later. Not until latest Miocene did the southern temperate latitudes warm appreciably. Southern subpolar climate cooled continuously through the Late Miocene. We attribute the resulting increases in the latitudinal climatic contrast across the southern Pacific Ocean to the development and migration of a strong subtropical convergence. On the basis of the nannoplankton oceanographic records we postulate that beginning about 10.5 m.y. ago Pacific surface circulation became primarily zonal and the production of deep and bottom waters in the Southern Ocean increased sharply. This produced a northward decrease in calcite preservation, an increase in benthic 13C, and a strong climatic gradient across southern latitudes. The period of most vigorous deep Pacific circulation ended 7 m.y. ago in response, we speculate, to the reduced ocean salinities during the Messinian.