Vertical thermal gradient history in the eastern equatorial Pacific during the early to middle Miocene: Implications for the equatorial thermocline development

• Published in: Paleoceanography 2017-07, Vol.32 (7), p.729-743
• Main Authors: Matsui, Hiroki, Nishi, Hiroshi, Kuroyanagi, Azumi, Hayashi, Hiroki, Ikehara, Minoru, Takashima, Reishi
• Format: Article
• Language: English
• Subjects: Antarctic ice; Antarctic ice sheet; Climate; Climate change; Distribution; Drilling; equatorial Pacific; Glaciation; Gradients; History; Ice; Ice sheets; Isotherms; Miocene; Mixed layer depth; Ocean currents; oxygen‐carbon isotope ratio; planktic foraminifera; Plankton; Pliocene; Sea surface; Sea surface temperature; Surface temperature; Temperature distribution; Temperature effects; Temperature gradients; Thermocline; Thermocline depth; Tropical climate; Unevenness; vertical gradient; Water column
• ISSN: 0883-8305; 2572-4517; 1944-9186; 2572-4525
• DOI: 10.1002/2016PA003058

Knowledge of the equatorial thermocline is essential for understanding climate changes in the tropical Pacific. Multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline. Although the secular thermocline development has been documented back to the late Miocene, the early to middle Miocene interval has rarely been examined. In addition, relationships with the dynamic Antarctic ice sheets remain unclear. Here we investigate the vertical thermal gradient in the upper water column at Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific (EEP) throughout the early to middle Miocene (23.1 to 11.7 Ma). The gradient increased over the Miocene Climatic Optimum, whereas it decreased during the East Antarctic Ice Sheet Expansion (EAIE). Comparison of the EEP record with its western equatorial Pacific (WEP) counterpart suggests that sea surface temperature was more stable in the WEP than in the EEP. We further estimated equatorial thermocline from two diagonal gradients between the EEP and the WEP: thermocline shoaled from 16.7 to 15.7 Ma and tilt weakened between 16.5 and 13.8 Ma. The onset of the “Monterey Excursion” and the reduced Antarctic ice sheet volume would have affected thermocline depth and tilt, respectively. Thermocline depth was likely much deeper compared to Pliocene‐to‐modern conditions. Furthermore, a 4‐point‐based distribution of isotherms (4DI index) was used as a metric of the evenness or unevenness of the isotherm distributions. The 4DI index considerably reduced at around the EAIE and other Mi‐events, reflecting the evenly distributed isotherms under a more glaciated Antarctica. Key Points Changes in the vertical thermal gradient in the eastern equatorial Pacific (23–12 Ma) was revealed Equatorial thermocline shoaled and tilt weakened in relation to the “Monterey Excursion” and the ice sheet retreats A 4‐point‐based index (4DI) was proposed to characterize distribution of isotherms in the upper water column