Generally consistent patterns of Eocene-Oligocene oxygen and carbon stable isotopic change are emerging from all ocean basins where well-preserved pelagic carbonates have been studied. The major features of benthic foraminiferal oxygen isotopic change are enrichment of close to 1^pmil in ¹⁸O associated with the middle-late Eocene boundary and the Eocene-Oligocene boundary at locations which differ by more than 3 km (2 mi) paleodepth. The Eocene-Oligocene enrichment occurs mostly in the earliest Oligocene and is clearly isochronous according to planktonic foraminiferal biostratigraphy. Completion of the Eocene-Oligocene event took less than one million years, with maximum earliest Oligocene ^dgr¹⁸O values giving rise to lower values later in the Oligo ene. Surface-dwelling planktonic foraminifera from low latitude locations become enriched in ¹⁸O by only a few tenths ^pmil from the Eocene to the Oligocene, whereas at high latitude locations they show the same enrichment as benthic foraminifera (about 1^pmil).

Down-core oxygen isotopic trends are best interpreted as reflecting changes in the thermal or density structure of the ocean from Eocene to Oligocene time. This reasoning follows from the general lack of covariance of benthic and planktonic ^dgr¹⁸O from tropical locations, which in the Quaternary is the strongest evidence of large glacial-interglacial changes in seawater isotopic composition. Thus, high latitude locations where the planktonic ^dgr¹⁸O increased the most were probably the source of dense water to the deep ocean everywhere. A major increase in the density structure of the ocean should be evident in plots of planktonic foraminiferal ^dgr¹⁸O versus paleolatitude and benthic foraminiferal ^dgr¹⁸O versus paleodepth. Our preliminary data indicate the ^dgr¹⁸O versus latitude gradient increased from Eocene to Oligocene times, although more data from non-upwelling areas is required to establish the significance of this change. Reconstructions of vertical ^dgr¹⁸O structure in the late Eocene suffer from too little data from individual basins, although our synthesis of the early Oligocene North Atlantic reveals about a 1^pmil ^dgr¹⁸O increase between 1 and 2 km (0.6 to 1.2 m) paleodepth.

Carbon isotopic results apparently do not vary systematically in time series, but display differences which probably reflect the hydrographic conditions overlying each site. For example, there are large variations in planktonic foraminiferal ^dgr¹³C, which vary symmetrically about the equator, as does the ^dgr¹³C of total CO₂ today. Some of the lowest ^dgr¹³C values come from sites which backtrack to equatorial latitudes, perhaps reflecting upwelling. Benthic foraminiferal ^dgr¹³C varies by as much as 1^pmil within the North Atlantic during the Oligocene, unlike today when similar variability is only seen between deep basins. This probably reflects dramatically different Paleogene circulation patterns due at least, in part, to the absence of the Panama Isthmus as a barrier between the Atlantic and Pacific.

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