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Neogene Oxygen and Carbon Isotopic Stratigraphy: Correlation and Paleo-Oceanography

James Kennett

The Neogene oxygen-and carbon-isotope stratigraphic record from Pacific DSDP sites provides an important basis for interregional correlations and stratigraphic control, including land-based marine sequences.

The benthic oxygen-isotope record exhibits several major features. During the early Miocene, ^dgr18O values were relatively low, reaching minimum values in the late early Miocene (19.5-16.5 Ma), and recording the climax of Neogene warmth. Following was a major increase in benthic ^dgr18O values between 16.5 and 13.5 Ma, interpreted as representing major, permanent accumulation of the East Antarctic ice sheet and cooler bottom waters. During the remaining middle and late Miocene, benthic ^dgr18O values exhibited distinct and important fluctuations. The latest Miocene and earliest Pliocene (6.2-4.5 Ma) were marked by relatively high ^dgr18O values, indicating increased glaciation. During the middle Pliocene, at about 3.4 Ma, a 0.4 ^pmil increa e in benthic ^dgr18O documented a net increase in average global ice volume and cooling of bottom waters. In the late Pliocene, between 2.6 and 2.4 Ma, a further increase in ^dgr18O occurred, generally interpreted as heralding the onset of glaciation in the Northern Hemisphere.

Intersequence correlation is enhanced by using carbon-isotope stratigraphy. The great similarity of the ^dgr13C time-series records, within and between ocean basins and with water depth, indicates that changes in oceanwide average ^dgr13C of HCO3- in seawater dominated the records, rather than local effects. Broad changes in the Neogene ^dgr13C record were caused largely by transfer of organic carbon between continental and oceanic reservoirs due to sea level changes.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.