Akio Omura

Reconciliation of late Quaternary sea levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records

A major discrepancy between the Late Quaternary sea level changes derived from raised coral reef terraces at the Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores is resolved. The two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka (ka = 1000 yr before present), but between 70 and 30 ka the isotopic sea levels are 20–40 m lower than the Huon Peninsula sea levels derived in earlier studies. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic δ18O data. Using the sea level and deep sea isotopic data, oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from Greenland ice cores. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.

Paleoenvironmental Changes in the Japan Sea During the Last 85,000 Years

Five distinct changes in the paleoenvironment of the Japan Sea within the last 85,000 years are revealed from the sedimentary record of a piston core recovered from the Oki Ridge. Changes in both surface and deepwater conditions are registered by changes in lithology, calcium carbonate content, organic carbon content, oxygen and carbon isotope ratios, and microfossil assemblages including calcareous nannoplankton, diatoms, radiolaria, and foraminifera. Between 85 and 27 ka the warm Tsushima Current did not flow into the Japan Sea, and cold surface water conditions prevailed. Environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions. Between 27 and 20 ka, freshwater input to the Japan Sea, probably from the Huang Ho River in China, stratified the water column, and the severe anoxic conditions eliminated most benthic fauna. Between 20 and 10 ka the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Shallow water benthic assemblages of the North Pacific Ocean subsequently colonized the Japan Sea and occupied the vacant niches of the deep basins. Between 10 and 8 ka the foraminifer compensation level (FCL) gradually rose to a depth shallower than 1000 m, and bottom conditions changed from dysaerobic to oxic. At 10 ka the warm Tsushima Current started to flow into the Japan Sea through the Tsushima Strait to establish the modern oceanographic regime which has existed since 8 ka. The eustatic sea level during the last glacial maximum was above the sill depths (130 m) of the Tsushima and Tsugaru straits, assuming that tectonic movements at these straits were negligible for the last 20 ka.

An Early Wisconsin Reef Terrace at Barbados, West Indies, and Its Climatic Implications

A discontinuous reef terrace, 0 to 4.5 m above sea level dated at approximately 60,000 yrs B.P. by thorium-230 and protactinium-231, occurs along the northwest coast of Barbados. This terrace developed as a narrow fringing reef during the last relatively warm period (St. Pierre interstadial), prior to sea-level lowering and extensive glaciation of the “Early” Wisconsin. Interstadial terraces like this one were formed about 28,000, 45,000, and 65,000 yrs ago and are exposed today only in areas such as Barbados that have undergone subsequent uplift.

Contrasting styles and rates of tectonic uplift of coral reef terraces in the Ryukyu and Daito Islands, southwestern Japan

Abstract Coral reef terraces on Kikai, Hateruma and Yonaguni in the Ryukyu Islands on the Eurasian lithospheric Plate and those of Kita-Daito and Minami-Daito Islands on the Philippine Sea Plate indicate three distinct patterns of coral reef terrace formation and preservation, primarily reflecting different uplift rates. The Kikai-type, which includes many terraces from Oxygen Isotope Stages 5e to 1, is a result of high uplift rate of ca. 1.8 m/kyr on the fore-arc rise of a Chilean-type subduction zone. Arcward (westward) tilting and faulting are also characteristic of the Kikai-type. The Hateruma-type is characterized by two coral reef terraces younger than the Stage 5e terrace and they are arranged in a concentric annular pattern. Slow uplift rate of ca. 0.3 m/kyr, adjacent to a Mariana-type subduction margin is typical of the Hateruma-type. The emergence of Kikai and Hateruma Islands is recent, younger than Isotope Stage 5e and 7, respectively, reflecting the active deformations of the leading edge at the overriding Eurasian Plate. In the Daito-type, atolls surrounded by fringing reefs have been uplifted very slowly (less than 0.05 m/kyr), but over a time period of at least several million years. Slow uplift and long history of Daito-type islands is the result of up-bulging of the Philippine Sea Plate as it migrates toward the Ryukyu Trench.

Neotectonic rates in the central Ryukyu Islands derived from 230Th coral ages

Kikai, Amami, Toku, Okierabu and Yoron Islands in the Ryukyu island arc north of Okinawa show differing rates of vertical displacement during the past 130,000 years. Kikai has been rising at a rate of 1–2 m/103 years over this period whereas the other islands show either recent submergence at a rate of 0.7 m/103 years or little movement during the last 6,000 sidereal years. A structural axis probably exists between Kikai, the outermost island in the central segment of the Ryukyu Arc, and the four inner islands. Both the coralliferous nature of the limestone units on Kikai and 230Th growth method ages of individual corals indicate that the limestone units were deposited during interglacial stages. Dated limestones from Kikai are the: 1. (1) Raised Coral Reef Limestone; average age of 3 samples is 5,600 years. 2. (2) Araki Limestone; average age of 5 samples is 42,000 years. 3. (3) Riukiu Limestone (younger member); average age of 3 samples is 67,000 years. The age of the older member of the Riukiu Limestone is geologically inferred to be approximately 130,000 years, which correlates with the Barbados III terrace of Broecker et al. (1968). The rates of vertical displacement of Kikai compare closely to other rates determined for post-Miocene movement in the main islands of Japan.

Last interglacial coral record of enhanced insolation seasonality and seawater 18O enrichment in the Ryukyu Islands, northwest Pacific

We present a calibrated, high-resolution 18O/16O and 13C/12C record for a well-preserved Last Interglacial Porites sp. coral (U-Th age of 127±6 ka) from the sea-level high-stand terrace of Yonaguni Island, Japan. Seasonal variations in the δ18O and δ13C values for the fossil coral are greater than those observed in modern coral records from the same reef setting and appear to be driven by the enhanced insolation seasonality in the northern hemisphere during the Last Interglacial maximum. The 18O enrichment of 1.1‰ in the fossil coral compared to the modern analogue cannot be due entirely to a reduction in sea-surface temperature because corals in this region are already growing at their lower thermal limit. Instead, most of the 18O enrichment must be due to a change in the δ18O of the surface seawater, probably in response to enhanced evaporation of the ocean or a higher volume flux of the Kuroshio Current.

230Th-238U age of Rotoehu Ash and its implications for marine terrace chronology of eastern Bay of Plenty, New Zealand

Abstract Rotoehu Ash, from the Wairoa River mouth, is dated at 71 000 ± 6000 years by the 230Th-238U radioactive disequilibrium system. The age is significantly older than the commonly quoted 14C age of 41700 ± 3500 years B.P. Marine deposits of Te Papa Terrace in the Bay of Plenty area, younger than Otamaroa Terrace of the last interglacial maximum (stage 5e), are usually conformably overlain by Rotoehu Ash. Thus, Te Papa Terrace, which emerged prior to the deposition of Rotoehu Ash, was probably formed during a global high sea-level stand at c. 85 ka. Some locations of Te Papa Terrace, however, may have been cut during an older high sea-level stand at c. 107 ka, because a peat layer occurs between marine deposits and Rotoehu Ash. Te Papa Terrace should be subdivided into two terraces which represent two successive relative high sea levels following the major high sea level of the last interglacial.

ESR and U-series analyses on corals from Huon Peninsula, New Guinea

Ten coral samples from Huon Peninsula were selected for an ESR and alpha spectrometric U-series dating comparison. Th and U isotopes of seven ESR samples were also analyzed by mass spectrometry. It was found that the dose response of the paramagnetic centre at g = 2.0007 is not strictly exponential, but defect creation can be observed. The comparison of the results shows considerable discrepancies between all three techniques but also inconsistencies between the results of the radiometric dating study with the assumed geological position of the samples. Problems of ESR seem to lie in AD estimation and problems of U-series dating in open system behaviour of the samples. The study implies that samples have to be very carefully selected in order to perform a meaningful dating comparison.

Penultimate Interglacial emerged reef around Kadavu Island, Southwest Pacific: Implications for late Quaternary island‐arc tectonics and sea‐level history

Abstract The Kadavu island group is a Pliocene‐Quaternary volcanic island arc in the Southwest Pacific, along which the last eruptions occurred during the late Quaternary. Recent investigations have focused on late Quaternary tectonic history with a view to illuminating regional plate interactions. The main island (Kadavu) is divisible into three structural blocks, the westernmost of which is the youngest and the least submergent. This block is surrounded by fragments of emerged reef limestone reaching 7.1 m above the modern reef surface (minimum emergence magnitude), cut by a prominent erosional (wave‐cut) bench at 2.6–3.4 m above the modern shore platform. Th/U dating of this limestone at the largest, highest outcrop—offshore Nagigia Island—revealed it to be of wholly Penultimate Interglacial age (207.2–223.2 ka). This result is explainable—plausibly not exclusively—by the following scenario. It is assumed that Penultimate Interglacial sea level reached a maximum of c. 10 m (relative to the present) aro...

The quaternary coral reef tracts of hateruma, Ryukyu Islands, Japan

The island of Hateruma is located at the southwestern end of the Ryukyu Island chain and covers an area of about 15 km2 with its highest point at 59.5 m.a.s.l. The morphology of Hateruma is characterized by an eight-step staircase (I to VIII after Ota et al., 1982) most of which eroded into the previously formed coral reefs (II and IV to VIII). This study presents 100 age estimations on 43 coral samples using electron spin resonance (ESR), alpha- and mass spectrometric U-series methods. The dating results imply that the Ryukyu Limestone was predominantly formed during oxygen isotope stages 7 and 5e. Two age estimates of around 300 ka suggest an earlier reef-building period. The main problem for establishing the Quaternary history of Hateruma lies in the fact that the coral ages and the staircase morphology do not correlate. Additionally, it is difficult to distinguish autochthonous from allochthonous samples, the latter having possibly been relocated by tsunamis. Corals relating to substages 5a and 5c (around 75 and 105 ka, respectively) are found in the immediate vicinity of much older samples. The results of this study prove the disappointing fact that Hateruma cannot yet be used as a tool for reconstructing palaeo sea levels.