The paleoceanography of the Arctic Ocean has been studied for the past 60 years. The 2004 Arctic Coring Expedition reconstructed the Cenozoic history of the Arctic for the first time and recognized the important role of the region in global climate change. Recent global warming has changed Arctic environments drastically, and the prediction of the future Arctic climate is imperative. The main scientific questions associated with the Arctic climate are concerned with the progress of Cenozoic cooling, the sea-ice distribution under warmer climatic conditions, the climatic significance of the Bering Strait, and the impact of a changing climate on Arctic human societies. Arctic drilling, coring, and bathymetric surveys using icebreakers are necessary to further our understanding of the paleoceanography of the Arctic.
Our understanding of the paleoceanography of the Bering Sea has been considerably advanced by IODP Expedition 323. The expedition aimed to create a high-resolution record of changes in paleoceanography since the Pliocene in a relatively high-latitude region of the North Pacific, subject to polar amplification. The expedition recovered 660 cores, mainly high-quality Advanced Piston Cores (APC), with a total length of 5741 m of continuous cores from seven sites distributed around the perimeter of the Aleutian Basin, including the Bowers Ridge, the Bering Slope edge, and the Umnak Plateau. These cores are crucial to our understanding of sea-ice distribution, productivity, laminated sediments, input of detrital materials, the formation of the North Pacific Intermediate Water mass, the Pacific water mass entry, the history of the Arctic gateway, and the enigma of the intensification of the Northern Hemisphere Glaciation and the mid-Pleistocene Transition. The maximum ages of the cores are ~5 Ma at the Bowers Ridge sites and 2.5 Ma at the Bering Slope sites. Meltwater from the Alaskan Ice Sheet has influenced the Bering Sea since 4.3 Ma, increasing in influence at 3.3 and 2.8-2.5 Ma. The significant development of sea-ice formation was identified at two sites on the Bowers Ridge at 2.7 and 2.2-2.0 Ma, based on analysis of sea-ice related diatoms and silicoflagellates. Such sea-ice formation affected the extent of the North Pacific Intermediate Water in the Bering Sea, which was strengthened during cold intervals such as when the Bering Strait closed due to falling sea level.
Integrated Ocean Drilling Program (IODP) drilled into coral reefs, deep-water coral mounds and sediments to understand the change of climate, and biogeochemical cycle. IODP Expedition 310 around Tahiti enabled to delineate the course of last deglacial sea level rise and its impact on reef growth and geometry at this island. Amplitude of the sea-level jump around melt water pulse-1A (MWP-1A) was estimated at 12-22 m (most likely 14-18 m). Sea-surface temperature variations in 20-10 kyr indicated a temperature drop of 1.5 °C at the Younger Dryas. IODP Expedition 325 recovered Pleistocene reef materials around Great Barrier Reef. More than 1,000 radiometric dates revealed detailed sea level pictures before and after the last glacial maximum (LGM: 20,000 years ago). We successfully reported more than 5℃ lowering of SST during the LGM. IODP Expedition 307 revealed the interior of a deep-water coral mound at ~800 m deep in Northern East Atlantic. Our age model based on Sr isotope recognized two growth stages; the depositionally continuous lower reef (2.6-1.7 Ma) accumulated under the low-amplitude relative sea-level change, and the discontinuous upper reef (1.0 Ma to mid-Holocene) developed under the high-amplitude relative sea-level change. The reef initiation was temporally correlated to the global cooling at the beginning of Pleistocene, when modern circulation was established in Atlantic. IODP Expedition 320/321 recovered a series of equatorial Pacific sediments covering the past 53 million years. Cenozoic evolution of carbonate compensation depth in the equatorial Pacific was reconstructed. It tracks a long-term deepening from 3.0-3.8 km during the Eocene to 4.6 km at present, which is superimposed by large fluctuations during the middle to late Eocene, and ended with a sharp >500 m deepening during the Eocene-Oligocene transition. Those variations are closely linked to changes in global climate and carbon cycle.
The NanTroSEIZE project has been one of the most complex and challenging scientific ocean drilling projects in history, representing a milestone for the Integrated Ocean Drilling Program (2005–2013) and the current International Ocean Discovery Program (2013–013) and the current International Ocean Discovery Program (2013fic ocseismogenesis of the Nankai Trough is now approaching the final stage; i.e., directly sampling, analyzing, and monitoring the plate boundary fault system responsible for historically recurring mega-earthquakes and associated tsunamis. The study area is located southeast of Kii Peninsula and comprises a transect of drill sites extending from the Kumano Basin across the Nankai Trough to the incoming Philippine Sea Plate.
The drilling of the Nankai seismogenic subduction zone, initiated in 2007, has resulted in the re-evaluation of previously accepted geological models. The main findings are as follows:
1) The Nankai forearc grew intermittently between ~6 and ~2 Ma due to rapid terrestrial sediment supply, resulting in the formation of a hanging wall wedge as a result of the occurrence of great earthquakes.
2) Slip along the plate boundary megathrust and along the associated splay fault has previously reached as far as the Nankai trough and ocean floor.
3) The fault, composed of clay-rich gouge, is weak in both static and dynamic cases.
4) The in situ stress conditions of the accretionary wedge and incoming Philippine Sea Plate are well constrained, and the horizontal compressional stress, parallel to the direction of plate convergence, suggests tectonic loading of accretionary sediments, implying a possible stress buildup that could result in the next great Nankai earthquake.
5) Borehole observatories and an ocean floor network recorded the earthquake, tsunami, and slow slips along the megathrust on 1 April 2016, and represent a new and innovative technology for application in the field of ocean floor science.
The first offshore ‘rapid response drilling,’ a drilling immediately after an earthquake for a scientific purpose, was conducted at the Japan Trench after the 2011 off the Pacific coast of Tohoku Earthquake as IODP Expedition 343. It brought various insights into the mechanism of huge tsunami induced by the mega earthquake. Major findings include 1) the fault zone is thin (5 meters) and weak (0.2-0.26 and 0.08-0.1 as its friction coefficient under low and high velocity slip conditions respectively), 2) low friction of the fault zone at seismic slip rates is due to abundance of smectite (a clay mineral) and thermal pressurization effects that can explain the large co-seismic slip, 3) the stress field dramatically changed from compressional (pre-earthquake) to extensional (post-earthquake). Remaining issues are to understand 1) the complete profile of physical property from the seafloor to the basement, 2) the factors that determine the slip heterogeneity along strike. Another scientific drilling is expected to solve these in near future.
Scientific ocean drilling over the past half-century has significantly expanded our knowledge of life, ocean, and Earth. The discovery of the spatially vast deep-subseafloor biosphere, a milestone scientific achievement, has extended the planetary habitability of life on Earth. To date, multiple lines of evidence form the core samples have demonstrated that a remarkable amount of physiologically unknown microbial life is present deep beneath the ocean floor where water supply and energy substrates are severely limited. Although subseafloor microbial ecosystems generally function extremely slowly, the consequence of long-term activity on geological timescales may play significant ecological roles in global biogeochemical carbon and other elemental cycles.