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.