Jun'ichi Okuno

Ice-sheet collapse and sea-level rise at the Bølling warming 14,600 years ago

Past sea-level records provide invaluable information about the response of ice sheets to climate forcing. Some such records suggest that the last deglaciation was punctuated by a dramatic period of sea-level rise, of about 20 metres, in less than 500 years. Controversy about the amplitude and timing of this meltwater pulse (MWP-1A) has, however, led to uncertainty about the source of the melt water and its temporal and causal relationships with the abrupt climate changes of the deglaciation. Here we show that MWP-1A started no earlier than 14,650 years ago and ended before 14,310 years ago, making it coeval with the Bølling warming. Our results, based on corals drilled offshore from Tahiti during Integrated Ocean Drilling Project Expedition 310, reveal that the increase in sea level at Tahiti was between 12 and 22 metres, with a most probable value between 14 and 18 metres, establishing a significant meltwater contribution from the Southern Hemisphere. This implies that the rate of eustatic sea-level rise exceeded 40 millimetres per year during MWP-1A.

Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume

The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’, but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

Effects of water load on geophysical signals due to glacial rebound and implications for mantle viscosity

We investigate the effects of the ocean function on predictions of the sea-level changes and other geophysical signals due to glacial rebound. To precisely predict these signals, a realistic ocean function including the effects of the palaeotopography, the distribution of ice sheet and meltwater influx is required. The adoption of a precise ocean function is very important in simulating the observables in Hudson Bay for an earth model with a low lower mantle viscosity of ∼1021 Pa s. In this case, the contribution from water loads can be comparable to that from ice loads. In the Fennoscandian region, however, the predictions are less sensitive to the details of the ocean function, because the width of the Gulf of Bothnia is very small compared with that of Hudson Bay. With an assumption that the ice model is represented by ARC3+ANT4b, we have examined the viscosity structure using relative sea-levels, gravity anomaly and solid surface gravity changes in North America and northern Europe. This study suggests a lower mantle viscosity of greater than 1022 Pa s and a upper mantle viscosity of (4 ∼ 10) × 1020 Pa s.

Evaluation of Holocene crustal movement in the Ako Plain, western Japan

Holocene sea-level observations have been obtained from the Ako Plain in western Japan. In this coastal area, the Holocene crustal movements have been evaluated, except the crustal response due to the last deglaciation, by comparing observations and theoretical predictions of relative sea-level (RSL) variations. Analyses of diatom assemblages and sedimentary sulphur in core sediments were used along with radiocarbon dates to derive the RSL variations. A crustal movement rate between +0.2 mm and-0.2 mm per year, corrected for the prediction, fits well with the observed RSL index points. Owing to the constraint of the reconstructed palaeo-mean sea level (PMSL) at 7300 cal. BP, the coast of the Ako Plain may have the best estimate of a tectonic subsidence rate of 0-0.2 mm/yr. The tectonic uplift rates along the tectonically active coast of western Kobe were derived to be 0.3-0.7 mm/yr and 0.11-0.45 mm/yr for Tarumi and Tamatsu, respectively, relative to Ako over the period concerned. The relative uplift along the traverse from Ako to western Kobe is primarily the result of the crustal movement resulting from the active faulting of the Rokko-Awaji fault system (RFS).

Species-area curve for land snails on Kikai Island in geological time

Abstract Historical changes in the coastline of Kikai Island of the Ryukyu Islands in the southeast part of Japan were estimated by using a numerical simulation based on a glacio-hydro-isostasy model. Temporal changes in the area of the island during the last 40 Kyr were compared with temporal changes in species diversity in fossil land snails of the island. The species number in the past was theoretically estimated by the area of Kikai Island in the past and a species-area relationship among the modern land snail fauna of the Ryukyu Islands. The theoretical species numbers are very close to the actual ones. This suggests that the change in island area is the main cause of the change in species diversity in Kikai Island. In addition, we discuss causes other than the area, such as island elevation, distance to the nearest large island, climate change, human activity, and imperfection of fossil data. We also discuss the change in Fishers alpha and body size against the change in the area.

A Well-Preserved Beach Landform and Sedimentary Structure on the East Antarctic Coast Affected by Glacial Isostatic Rebound

A well-marked stepped topography is observed on a beach that faces an inner cove of the East Antarctic coast. The beach area is subject to mild wave action and glacial isostatic upheaval. The sedimentary structure suggests that the landform was formed by gradual migration of progradational processes of the upper shore-face slope in the offshore direction during the upheaval after Antarctic ice retreat.