Miocene equatorial and southwest Pacific paleoceanography from stable isotope evidence

Stable isotope results from seven Miocene Deep Sea Drilling Projects in the equatorial and southwest Pacific Ocean, previously correlated using carbon isotope stratigraphy, have been examined, discussed, and interpreted in terms of the development of the Miocene Pacific Ocean. The most obvious features of the benthonic foraminiferal stable isotopic records are a major increase inδ¹⁸O(～1.0‰) during the Middle Miocene, a series of long-term oscillations (2–3 My) of amplitude 0.5–0.75‰ and a decrease inδ¹³C values (0.5–;1.0‰) during the latest Miocene. Planktonic foraminiferalδ¹⁸O records show different trends for high and low latitude regions. In the equatorial Pacific, planktonicδ¹⁸O values actually decrease during the Miocene whereas in the higher southern latitudes planktonicδ¹⁸O values become more positive in response to cooling surface waters.Planktonicδ¹³C records show opposite trends toδ¹⁸O with the high latitude values becoming more negative relative to the tropical regions. The development of the Miocene Pacific Ocean in terms of its vertical and horizontal thermal structure and isotopic composition is well illustrated by examining changes in the isotopic difference between planktonic and benthonic foraminifera.Δδ¹⁸O_B-P (Benthonic-Planktonic) is a measure of the thermal structure of the water column.Δδ¹⁸O_PH-PL (high latitude-low latitude) planktonic values is a measure of the latitudinal temperature gradient.Δδ¹³C_B-P is an indirect measure of nutrient concentrations in the water column, andΔδ¹³C_PH-PL measures differences in surface-water nutrient concentrations between high and low latitude.Δδ¹⁸O_B-P increases during the Miocene with the greatest increase occurring in the Middel Miocene at about 14 Ma. By the latest Miocene the isotopic gradient at Site 289 in the equatorial Pacific approaches the present-day isotopic gradient (about 4–5‰). An increase inΔδ¹⁸O_PH-PL during the Miocene suggests that the latitudinal temperature gradient increased by about 6°C to a value of 12°C in the latest Miocene between Sites 289 (Equator) and 281 (subantarctic).Δδ¹³C_B-P and Δδ¹³C_PH-PL values are relatively constant through the Early Miocene but begin to increase during the Middle Miocene. Bottom-waterδ¹³C values respond similarly at all sites, but surface-waterδ¹³C values exhibit different trends because higher latitude values begin to decrease. This decrease perhaps suggests that phosphate concentrations may have increased due to increased upwelling as the circum-Antarctic circulation system evolved its present day characteristics.The isotopic data compiled in this paper suggest that the southwest Pacific was responding uniformly to some global or at least Pacific-wide control during the Early Miocene. In the Middle Miocene the response became more complex as the low and high latitudes began to show independent trends. The changes in the thermal (vertical and latitudinal) structure probably occurred in respons to the build-up of the East Antarctic ice-sheet, intensification of bottom-water circulation and an increase in zonal circulation in surface waters in the southern hemisphere.The changes inδ¹³C (vertical and latitudinal) gradients are due to some complex interaction of sea-level, continental hypsometry, climate, and biological processes coupled with oceanic circulation changes. A strong correlation between estimated sea-level changes andδ¹³C values suggests that transgressions and regressions play a critical role in controlling the flux of oxidized organic carbon enriched in¹²C, from the continental shelves and epicontinental seas to the open ocean.