TY - JOUR
T1 - Vertical thermal gradient history in the eastern equatorial Pacific during the early to middle Miocene
T2 - Implications for the equatorial thermocline development
AU - Matsui, Hiroki
AU - Nishi, Hiroshi
AU - Kuroyanagi, Azumi
AU - Hayashi, Hiroki
AU - Ikehara, Minoru
AU - Takashima, Reishi
N1 - Funding Information:
We thank H. Takayanagi and Y. Iryu for their support during the stable isotope measurements. We thank S. Kamikuri and T. Yamaguchi for interactive suggestions. We are also grateful to the scientists from the Integrated Ocean Drilling Program (IODP) Expeditions 320 and 321. The manuscript has been improved with the help of constructive comments by the Editor (E. Thomas), the Associate Editor (M. T. Chen), and the reviewers (R. M. Leckie and anonymous one). This work used samples provided by the IODP (request ID = 288IODP). The work was performed under a cooperative research program of the Center for Advanced Marine Core Research at Kochi University (14A033). It was supported partly by Japan Society for the Promotion of Science KAKENHI (Grant-in-Aid for Scientific Research) grant 24244082 (to H.N.) and 25287130 (to R.T.) and by the Global Center of Excellence Program of Earth and Planetary Science, Tohoku University, Japan. Supporting data are included as three figures and three tables in the supporting information; any additional data may be obtained from H.M. (e-mail: hmatsui@dc.tohoku.ac.jp).
PY - 2017/7
Y1 - 2017/7
N2 - 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.
AB - 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.
KW - Miocene
KW - equatorial Pacific
KW - oxygen-carbon isotope ratio
KW - planktic foraminifera
KW - thermocline
KW - vertical gradient
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U2 - 10.1002/2016PA003058
DO - 10.1002/2016PA003058
M3 - Article
AN - SCOPUS:85027709844
VL - 32
SP - 729
EP - 743
JO - Paleoceanography and Paleoclimatology
JF - Paleoceanography and Paleoclimatology
SN - 0883-8305
IS - 7
ER -