Ryuji Asami

Two new genera and species of tubular foraminifera ( Shimenawa okuyamae gen. et sp. nov. and Yorishiro morii gen. et sp. nov.) are described from approximately 4000-year-old nuclei of deep-water acervulinid macroids collected from the shelf off Kikai Jima, Ryukyu Islands (Japan). Macroids form extensive beds on mobile sandy and gravelly bioclastic carbonate sediments at water depths of 75–97 m. SEM and Raman analyses show that Shimenawa okuyamae is characterized by a porcelaneous texture with randomly oriented, needle-shaped calcite crystals, whereas Yorishiro morii possesses a three-layered shell composed of rhombohedral, plate-shaped, and rod-shaped calcite crystals. These shell textures have not previously been recognized or described in detail in foraminifera. Shimenawa okuyamae is tentatively assigned to the Squamulinacea based on its porcelaneous, imperforate, unilocular shell. Due to its novel shell texture among tubular foraminifera, Yorishiro morii cannot be assigned with certainty to any family or higher taxonomic rank.

Reefal microbialites are crusts and infills of fine-grained, non-skeletal microbial carbonates developed within primary cavities of coralgal reef frameworks. They have been found in reef margins, slopes and caves with ages ranging from the Last Glacial Maximum to the Holocene. Both global and local factors likely control reefal microbialite development, but their respective contributions remain unknown due to limited data on the formation setting, age, and process. Here we report on lithological and biological successions, mesoscale and microscale fabrics, stable isotopes and radiocarbon ages of reefal microbialites in a core recovered on a reclaimed offshore bank reef in Okinawa Jima, the Ryukyu Islands, Northwest Pacific. A total of 13.1-m-thick sedimentary succession is composed of (1) Pleistocene limestone of the Ryukyu Group (> 1.6 m thick), (2) mixed paleosol and bioclastic sediments (1.5 m thick, ca. 7800 mean calibrated years before Present: cal yr BP), (3) Holocene reef deposits (7 m thick, 7300–6300 mean cal yr BP), and (4) reclaimed deposits (3 m thick) in ascending order. This succession reflects a transition from terrestrial to marine environments associated with the middle Holocene transgression. Reefal microbialites are classified into reefal microbial crusts (RMC), mixed encrusting and micritic crusts (MEMC), and intraskeletal and boring-filling microbialites (IBFM). The RMC developed within a narrow stratigraphic interval (1.85–3.55 m below sea level), while the IBFM and the MEMC were common throughout the middle Holocene reef core. The RMC developed within decades to hundreds of years after the death of coralgal communities in the primary cavities of coralgal frameworks at least 0.5 m beneath the living reef surface. The carbon isotope values (mean: 3.3 ‰) in the RMC are close to those expected for high-magnesium calcite precipitated in equilibrium with seawater, while the narrow range of the oxygen isotope values suggests that seawater temperature and salinity did not change greatly during RMC development. Reefal microbialite developments at ca. 7–6 ka in the study area are likely attributed to an expansion of cryptic primary cavities of reef frameworks associated with rising sea levels and vertical reef accretion, enhanced interstitial water circulation under a high-energy setting, and increasing organic matter, nutrient availability, and alkalinity due to occasional terrigenous sediment input.

Tridacnine oxygen isotope thermometers (TOITs) commonly utilize gridded sea surface temperature (SST) and sea surface salinity (SSS) data without carefully checking whether they were appropriate substitutes for in situ monitoring records at the tridacnine growth site. Many SST and SSS datasets are available, but those best suited for establishing TOITs have yet to be examined sufficiently. In this study, we have established TOITs for shells collected at two islands (Okinoerabu Jima in the north and Ishigaki Jima in the south) in the northwestern Pacific using six combinations of SST (Extended Reconstructed Sea Surface Temperature [ERSST], Sea Surface Temperature data of Integrated Global Ocean Services System [IGOSS SST], and Hadley Centre Sea Ice and Sea Surface Temperature dataset [HadISST]) and SSS (Simple Ocean Data Assimilation ocean/sea ice reanalysis [SODA3.15.2] and Met Office Hadley Centre “EN” series [EN4.2.2]) datasets and evaluated the differences among the TOITs. Consequently, the slopes and intercepts of all TOITs fell within or near the range of TOITs reported in the previous studies, regardless of the combination of datasets. However, a significant difference in the coldest month SST at Okinoerabu Jima (~ 1.0 °C) exists between IGOSS SST and ERSST, caused by that an SST drop by the southward migration of the Kuroshio Front during winter is detected not in IGOSS SST (1° gridded data) but in ERSST (2° gridded data). Thus, the choice of SST dataset can lead to up to 1.0 °C difference in reconstructed winter SST. SSS differences exist between SODA SSS and EN SSS around the two islands up to 0.8 (equivalent to ~ 0.25 ‰ in oxygen isotope composition of seawater), practically leading to insignificant variations of the slopes and intercepts of the TOITs. To establish TOITs using the gridded SST and SSS datasets, we should select datasets with SSTs and SSSs similar to those at the growth sites of the studied tridacnine shells. Even if a gridded SST (SSS) dataset and an in situ monitoring SST (SSS) record show good correspondence, it could be a coincidence. We propose an averaged Ryukyu TOIT: SST = − 3.95 ± 1.02 × (δ¹⁸O_shell [VPDB] − δ¹⁸O_sw [VSMOW]) + 20.50 ± 2.75 (2σ).

Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.

Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.

Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.

Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.

Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.