Vertical thermal structure evolution of Miocene surface waters: Western equatorial Pacific DSDP Site 289

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Time series analyses (from ∼ 18.7 to 5.8 Ma) of the oxygen isotopic composition of four shallow-dwelling planktic foraminifera ( Globorotalia mayeri, Dentoglobigerina altispira, Globoquadrina baroemoenensis and Globigerinoides immaturus), one deep-dwelling planktic foraminifer ( Globoquadrina venezuelana) and a benthic foraminifer ( Cibicidoides spp.) from Deep Sea Drilling Project Site 289 (2206 m present day depth) are used to determine the evolution of vertical thermal structure in the western equatorial Pacific. Comparisons of composite oxygen isotopic curves, representing the surface mixed-layer, the thermocline, and deep waters reveal three distinct intervals of stability in the position and strength of the thermocline during the Miocene (∼ 18.7 to 16.1 Ma; ∼ 11.1 to 9.9 Ma; and ∼ 7.5 to 5.8 Ma) separated by two intervals of instability of the thermocline (∼ 16.1 to 11.1 Ma; and ∼ 9.9 to 7.5 Ma). From ∼ 18.7 to 16.1 Ma, vertical thermal gradients were the weakest of the Miocene interval studied and the water column was highly stable. Vertical thermal gradients strengthened between ∼ 16.1 and 11.1 Ma as equatorial surface waters warmed and deep waters cooled during an inferred major growth phase of the East Antarctic ice sheet. As a consequence, the strength and position of the thermocline became highly variable. This was followed, from ∼ 11.1 to 9.9 Ma, by relative climatic stability, well-defined vertical thermal gradients and a slight warming of the water column. By ∼ 9.9 Ma, increasing restriction of surface water flow through the Indonesian Seaway led to the piling-up of warm surface waters in the western equatorial Pacific and a deepening of the thermocline. Deep waters continued to cool and vertical thermal gradients reached a Miocene maximum by ∼ 7.5 Ma. From ∼ 7.5 to 5.8 Ma, vertical thermal gradients were stable and the water column became highly stratified and marked by a thick, warm surface layer and a deep, well-defined thermocline. Based upon a previous assumption that ice volume growth during the middle Miocene (∼ 16.1 to 11.1 Ma) contributed to an increase inδ 18O values of 0.5 to 0.7‰ we have estimated water column temperature changes at Site 289 between ∼ 18.7 and 5.8 Ma. Surface waters are inferred to have warmed 2° to 4°C between ∼ 18.7 and 5.8 Ma, with about half of the warming occurring from ∼ 16.1 to 11.1 Ma. Between ∼ 11.1 and 9.9 Ma the entire water column warmed and from ∼ 9.9 to 7.5 Ma the mixed-layer expanded and the thermocline deepened as warm waters piled-up in the western equatorial Pacific in response to the closure of the Indonesian Seaway. The net decrease of thermoclinalδ 18O values between ∼ 11.1 and 7.5 Ma reflects an upward change in position of the deep-dwelling planktic foraminifera, Globoquadrina venezuelana, relative to the position of the thermocline as the thermocline deepened. After 7.5 Ma, relativeδ 18O values indicate that Globoquadrina venezuelana was a deep-dwelling, mixed-layer species. Deep waters (∼2400 to 2200 m) at Site 289 cooled 2° to 4°C between ∼ 18.7 and 5.8 Ma, with about half of the cooling occurring from ∼ 16.1 to 11.1 Ma and the other half from ∼ 9.9 to 7.5 Ma.