17:15 〜 19:15
[BPT04-P06] Paleoenvironmental reconstruction of the Equatorial Pacific Ocean based on calcareous nannofossils during the early Pliocene
キーワード：石灰質ナノ化石、鮮新世、古環境、温暖化
The Pliocene time experienced an extraordinary warm period called the mid-Pliocene Warm Period (mPWP) around 3.3–3.0 Ma when global temperatures were 2-3°C higher than today (Prescott et al., 2014). The mPWP event has been a major subject of climate and ocean research during the Pliocene (e.g., Wara et al., 2005; Zubakov and Borzenkova, 1988). In addition, since environmental changes in the equatorial Pacific Ocean have strongly influenced global climate, for example, the El Niño-Southern Oscillation (ENSO), it is very important to understand the past marine environments in the equatorial Pacific region.
Because distributions and combinations of calcareous nannofossil species, a kind of phytoplankton fossils, differ depending on marine environments, nannofossil assemblages have been used to infer past sea-surface environments (e.g., Molfino and McIntyre, 1990). Generally, calcareous nannofossil species are abundantly observed in low-latitude waters. Thus, calcareous nannofossils are useful for examining low-latitude waters. Especially, depths of thermocline/nutricline in upper waters can be inferred by calcareous nannofossils, because the habitat segregation of nannofossil species in the lower and upper photic zones are known in calcareous nannofossils (Molfino and McIntyre, 1990; Flores et al., 2000).
The equatorial Pacific Ocean is one of the most important areas with high biological productivity and its sea-surface condition has changed related to global climatic changes. The purpose of this study was to investigate sea-surface changes in the western equatorial Pacific Ocean. We used lower Pliocene samples from Hole 806B, Ocean Drilling Program (ODP) Leg 130, drilled at the Ontong Java Plateau.
Forty-three samples corresponding to 5.5–3.5 Ma were examined and they contain 18 genera and 30 species of calcareous nannofossils throughout the studied section. Both the upper photic taxon, Reticulofenestra spp., and a lower photic species, F. profunda, dominated and F. profunda abundantly occurred upward, especially after 4.6 Ma. In the western equatorial Pacific Ocean, the high-temperature, low-salinity water masses, called the western Pacific warm pool (WPWP) were present and the thermocline /nutricline level exists deeper than the lower photic zone. Because relative abundances of F. profunda increased after 4.6 Ma, the thermocline/nutricline seemed to become shallower, even in the lower photic zone. In the eastern equatorial Pacific, the upwelling in the surface waters gradually weakened after 5.3 Ma (Imanishi, 2019) and the depth of the thermocline/nutrient level deepened, the east-west gradient of the thermocline/nutricline became smaller after 4.5 Ma, suggesting the establishment of the present-day El Niño-like environment.
References
Prescott et al. (2014) Earth and Planetary Science Letters, 400, 261-271.
Wara et al. (2005) Science, 309 (5735), 758-761.
Zubakov and Borzenkova (1988) Palaeogeography, Palaeoclimatology, Palaeoecology, 65(1-2), 35-49.
Molfino and McIntyre (1990) Science, 249 (4970), 766-769.
Flores et al. (2000) Palaeoecology, 161 (3-4), 459-478.
Imanishi et al. (2018) The geological society of Japan, p.594
Because distributions and combinations of calcareous nannofossil species, a kind of phytoplankton fossils, differ depending on marine environments, nannofossil assemblages have been used to infer past sea-surface environments (e.g., Molfino and McIntyre, 1990). Generally, calcareous nannofossil species are abundantly observed in low-latitude waters. Thus, calcareous nannofossils are useful for examining low-latitude waters. Especially, depths of thermocline/nutricline in upper waters can be inferred by calcareous nannofossils, because the habitat segregation of nannofossil species in the lower and upper photic zones are known in calcareous nannofossils (Molfino and McIntyre, 1990; Flores et al., 2000).
The equatorial Pacific Ocean is one of the most important areas with high biological productivity and its sea-surface condition has changed related to global climatic changes. The purpose of this study was to investigate sea-surface changes in the western equatorial Pacific Ocean. We used lower Pliocene samples from Hole 806B, Ocean Drilling Program (ODP) Leg 130, drilled at the Ontong Java Plateau.
Forty-three samples corresponding to 5.5–3.5 Ma were examined and they contain 18 genera and 30 species of calcareous nannofossils throughout the studied section. Both the upper photic taxon, Reticulofenestra spp., and a lower photic species, F. profunda, dominated and F. profunda abundantly occurred upward, especially after 4.6 Ma. In the western equatorial Pacific Ocean, the high-temperature, low-salinity water masses, called the western Pacific warm pool (WPWP) were present and the thermocline /nutricline level exists deeper than the lower photic zone. Because relative abundances of F. profunda increased after 4.6 Ma, the thermocline/nutricline seemed to become shallower, even in the lower photic zone. In the eastern equatorial Pacific, the upwelling in the surface waters gradually weakened after 5.3 Ma (Imanishi, 2019) and the depth of the thermocline/nutrient level deepened, the east-west gradient of the thermocline/nutricline became smaller after 4.5 Ma, suggesting the establishment of the present-day El Niño-like environment.
References
Prescott et al. (2014) Earth and Planetary Science Letters, 400, 261-271.
Wara et al. (2005) Science, 309 (5735), 758-761.
Zubakov and Borzenkova (1988) Palaeogeography, Palaeoclimatology, Palaeoecology, 65(1-2), 35-49.
Molfino and McIntyre (1990) Science, 249 (4970), 766-769.
Flores et al. (2000) Palaeoecology, 161 (3-4), 459-478.
Imanishi et al. (2018) The geological society of Japan, p.594