Oceanographic significance of Pacific late Miocene clacareous nannoplankton

Abstract

An analysis of the variability in the composition and distribution of Pgcific 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 productivity 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 17 sites studied. Dissolution preferentially removes the more-soluble constituents of the tropical nannoflora so that increasing dissolution tends to give tropical nannoflora a cooler water aspect. At the same time, selective dissolution shifts the composition of the warmer water component towards its more resistant taxa. This produces an assemblage typical of deeper tropical waters and areas of lower productivity. Nannoplankton records show a period of greatly decreased calcite dissolution in deep tropical and temperate South Pacific sites between 8 and 10 m.y. ago. This decrease is strongly correlated with an increase in the δC-13 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 was much more gradual. Not until latest Miocene did the southern temperate latitudes warm appreciably; they continued warming into the Pliocene. This rapid warming of the tropics and gradual warming farther south produced a temporary increase in the latitudinal climatic gradient across the southern Pacific Ocean 8 to 10 m.y. ago. On the basis of the nannoplankton oceanographic records we postulate that beginning 10 m.y. ago the production of deep and bottom waters in the Southern Ocean increased. This produced the northward decrease in calcite preservation, the increase in benthic δC-13, and the strong climatic gradient across southern latitudes. This period of increase deep Pacific circulation ended 7.5 rn.y. ago.