Reconstruction of oceanic circulation patterns in the tropical Pacific across the early/middle Miocene boundary as inferred from radiolarian assemblages

作者：Kamikuri Shin ichi^*; Moore Theodore C

来源：Palaeogeography, Palaeoclimatology, Palaeoecology, 2017, 487: 136-148.

DOI：10.1016/j.palaeo.2017.08.028

• Mid-Miocene climatic optimum
• East-west thermocline gradient
• Upwelling
• Equatorial undercurrent
• Indo-Pacific seaway

Although there have been attempts to infer Neogene oceanic circulation patterns in the tropical Pacific on the basis of multiple marine proxies, oceanic circulation patterns across the early/middle Miocene boundary are still poorly understood despite paleoclimate having significantly changed during this interval. In this study, we reconstruct the changes in tropical oceanic circulation patterns in the Pacific across the early/middle Miocene boundary based on radiolarian assemblages obtained at Integrated Ocean Drilling Program Site U1335 in the eastern tropical Pacific.
The radiolarian upwelling taxa increased during four intervals (18.4-18.1 Ma, 16.9-15.7 Ma, 15.2-14.6 Ma, 14.2-13.4 Ma). Radiolarian temperature index (RTI) was calculated using the relative abundances of the warm water (cluster B1) and cool water taxa (cluster B2) in order to evaluate relative temperature fluctuations of the surface water. The sea surface temperature index was relatively high from 16.8 to 16.0 Ma and gradually decreased from 16.0 to 14.6 Ma. Subsequently, the temperature index decreased stepwise at 13.9 and 13.4 Ma and became relatively low from 13.4 to 12.7 Ma. Comparison of the RTI record in the eastern tropical Pacific to the climatic index based on the benthic foraminiferal delta O-18 record shows that variability in the two records-follows a roughly similar trend from the early to the middle Miocene. However, the sea surface temperature record does not indicate a sharp increase in warmth in the eastern tropical Pacific during the mid-Miocene climatic Optimum-1 (MMC0-1), as suggested by the benthic isotopic record. This is likely because of increased upwelling of cool, nutrient-rich water at this time.
Beginning in the latest early Miocene (similar to 17 Ma) radiolarian assemblages were dominated by different taxa in the eastern and western tropical Pacific. This pattern is interpreted as indicating a shallower thermocline in the east and a deeper thermocline in the west, based on the relative abundance of the upwelling taxa. There is an overall increasing trend in this difference since the latest early Miocene. We tie these events to the effective closure of the Indo-Pacific seaway and the development of a substantial western Pacific warm pool along with the development of a strong Equatorial Undercurrent.

• 出版日期2017-12-1