New Insights into the Evolution of an Intervalley Prehistoric Irrigation Canal System,North Coastal Peru

We present the results of combined AMS‐¹⁴C and single‐grain luminescence dating of pre‐Columbian irrigation canals that are part of the Racarumi Intervalley Canal System (RICS) in northern Peru. Archaeological and archival evidence suggest that the RICS was constructed during Middle Sicán (A.D. 900–1100) rule and continued to operate during Chimú and Inka conquests in the A.D. 1300s and 1400s, respectively, until finally succumbing to Spanish control of the area in the early A.D. 1500s. Detrital charcoal (AMS‐¹⁴C) and sand grains (OSL and IRSL) were collected from active, post‐abandonment, and clean out deposits in six earthen canals located within three separate alignments of the RICS. Resulting ages confirm operation during Middle Sicán, Late Sicán, Chimú, and Inka control, and possibly into the early part of the Spanish period. Placed within their hydroclimatological context, RICS canal deposits document recurrent El Niño flooding ∼A.D. 1300–1600 but indicate that damage to the system was repaired and the canals continued to operate despite periodic disruptions. This study demonstrates that single‐grain luminescence analysis conducted within a framework of geomorphology and formation processes has significant potential for detailed and accurate dating of ancient water‐control systems.     

Influence of the Oyashio Current and Tsugaru Warm Current on the circulation and water properties of Otsuchi Bay,Japan

Mooring and hydrographic observations were conducted from September 2012 to May 2014 at the mouth of Otsuchi Bay, a ria along the Pacific coast of Japan. Our observations quantitatively demonstrated that the circulation and the water properties of Otsuchi Bay are strongly influenced by the Tsugaru Warm Current (TWC) and Oyashio Current (OY) at seasonal and subseasonal time scales. Two bottom-mounted velocity profilers and temperature and salinity measurements beneath the near-surface halocline showed a counterclockwise lateral circulation pattern related to the TWC, which was enhanced from summer to autumn. From winter to early spring, the lateral circulation patterns related to the TWC weakened and the influence of the OY occasionally increased. When the OY was weak, surface flows became an overturning structure, with outflows in the upper layer and inflows in the lower layer. When the OY was strong and passed close to the Sanriku coast, the circulation became highly variable and intermittent. Intrusions of the markedly low-salinity OY water were observed on two occasions and persisted for periods of several weeks to several months. Salinity was sometimes less than 33.7, the lower limit of the typical TWC from late summer to autumn even when the TWC dominates. We suggested that this is the seasonal fluctuations of the TWC itself, as the upstream current of the Tsushima Warm Current is freshened in summer as a result of the influence of the Changjiang River. The surface water was generally fresher in the south of the bay than in the north, suggesting the Coriolis deflection of the river plume.     

A comparative simulation study of coupled THM processes and their effect on fractured rock permeability around nuclear waste repositories

This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings (1) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (2) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal–mechanically induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal–mechanically induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermal–mechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts.     

Variation of the partial pressure of CO2 in surface water from Kuroshio to Oyashio and the relation between environmental factors and the partial pressure at 144°E off Sanriku, northwestern North Pacific in May, 1997

Partial pressure of CO₂ in surface sea water (pCO₂) was measured continuously off Sanriku in May, 1997 by a new pCO₂ measurement system. We have examined the relation of pCO₂ to physical factors such as temperature, salinity and density, chemical and biological factors such as nutrients and carbonate system and chlorophylla. In the Kuroshio region pCO₂ was not correlated to physical, chemical and biological factors in the range of 260 to 290 μatom. In transition water (Tr1) between Kuroshio and the Oyashio second branch, pCO₂ was weakly correlated to physical factors and strongly correlated to nutrients. In transition water (Tr2) between the Oyashio first and second branches, pCO₂ was highly correlated to temperature (SD: 10.9 μatom) and salinity (SD: 8.6 μatom) and also to nutrients. In transition water (Tr1+Tr2), pCO₂ was highly multivariately correlated to temperature (T), salinity (S), chlorophylla (CH) (or nitrate+nitrite (N)) as follows, pCO₂(μatom)= 10.8×T(°C)+27.7×S+2.57CH(μg/1) −769, R²= 0.86, SD = 20.9, or pCO₂(μatom)= 3.9×T(°C)+25.5×S+16.0NO₃(μM) −686, R²= 0.99, SD = 6.4. Moreover, pCO₂ was predicted by only two factors, one physical (S) and the other chemical/biological (N) as follows: pCO₂ (μatom)=32.8×S+19.4N−908, R²=0.97, SD=8.4. The pH measured at 25°C was well correlated with normalized pCO₂ at a fixed temperature. In the Oyashio region pCO₂ was decreased to 160 μatom, probably because of spring bloom, but was not correlated linearly to chlorophylla. The results obtained showed the possibility of estimating pCO₂ of the Oyashio and transition regions in May by satellite remote sensing of SST, but the problem of estimation of pCO₂ in Kuroshio water remains to be solved.     

Holocene palaeomagnetic secular variation at Lake Biwa, central Japan

We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and Kikai-Akahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV.     

A computerized geologic mapping system based on logical models of geologic structures

In this article we describe the basic framework of the computerized geologic mapping system cigma. The system, whic is based on a mathematical formulation of geologic concepts, consists of the following six subsystems: (1) input of geologic data set; (2) inference of stratigraphic sequence; (3) construction of logical models of geologic structures; (4) determination of three-dimensional geologic boundary surfaces; (5) construction of three-dimensional solid model of geologic structures; and (6) graphical presentation. Geologic structures are summarized in several tables called logical models of geologic structures. Each model is constructed automatically from input data on structural relations between geologic bodies. The model interprets the data automatically to create data files necessary to determine the shapes of geologic boundaries; it also provides a threedimensional solid model of geologic structures referring to the shapes of boundaries. As a prototype, we introduce two types of contacts corresponding to conformity and unconformity into the logical model and show that it is possible to draw a geologic map automatically. More complex geologic structures can be introduced into the geologic mapping system through further formulation of geologic structures.     

Estimation of codaQ using the maximum likelihood method

A seismic coda wave from local earthquakes is an indication of heterogeneity in the crust and upper mantle, and codaQ (Q _c ) is the parameter defining the temporal decay of the coda wave. ButQ _c estimates obtained with the conventional least-square method are not based on any solid statistical background, assuming the Gaussian distributiona priori. In this study, we propose a statistically reliable estimation method of estimatingQ _c using the maximum likelihood method, and show its validity and usefulness with the data from the 1986 Joint Seismological Research in the western Nagano Prefecture. We found first that theQ _c estimation with the maximum likelihood method is statistically valid and its reliability can be checked with the -square test. Next,Q _c around Ohtaki village, within the studied area, is estimated using the maximum likelihood method. The averageQ _c value at low frequencies (up to 2 Hz) is one of the lowest in the world, which is in harmony with the geological setting of the site condition directly above an active fault.     

Propagation of Rossby Waves over Ridges Excited by Interannual Wind Forcing in a Western North Pacific Model

Numerical experiments with a multi-level general circulation model have been performed to investigate basic processes of westward propagation of Rossby waves excited by interannual wind stress forcing in an idealized western North Pacific model with ocean ridges. When the wind forcing with an oscillation period of 3 years is imposed around 180°E and 30°N, far from Japan, barotropic waves excited by the wind can hardly cross the ridges, such as the Izu-Ogasawara Ridge. On the other hand, a large part of the first-mode baroclinic waves are transmitted across the ridges, having net mass transport. The propagation speed of the first-mode baroclinic wave is accelerated (decelerated) when an anticyclonic (cyclonic) circulation is formed at the sea surface, due to a deeper (shallower) upper layer, and to southward (slightly northward) drift of the circulation. Thus, when the anticyclonic circulation is formed on the northern side of the cyclonic one, they propagate almost together. The second-mode baroclinic waves converted from the first-mode ones on the ridges arrive south of Japan, although their effects are small. The resulting volume transport variation of the western boundary current (the Kuroshio) reaches about 60% of the Sverdrup transport variability estimated from the wind stress. These characteristics are common for the interannual forcing case with a longer oscillation period. In the intraseasonal and seasonal forcing cases, on the other hand, the transport variation is much smaller than those in the interannual forcing cases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sensitivity of the Interannual Kuroshio Transport Variation South of Japan to Wind Dataset in OGCM Calculation

Numerical experiments were carried out using OGCM (Ocean General Circulation Model), MOM2.2 (Modular Ocean Model Ver. 2.2), over realistic topography data, ETOPO5 (Earth Topography - 5 Minute), to investigate the interannual variability of the Kuroshio transport in 1960–2000 south of Japan; 1) the PN line located off the East China Sea, and 2) the ASUKA (Affiliated Surveys of the Kuroshio off Cape Ashizuri) line located off Cape Ashizuri. We adopted two wind datasets as driving forces of the OGCM: 1) the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis monthly mean wind stress data, and 2) the ECMWF (European Centre for Medium-range Weather Forecasts) daily wind data. In the ECMWF experiments we replaced the NCEP/NCAR data only in 1979–1993 because of the availability of the data. The OGCMs and observation basically agree on the temporal variation patterns of the transports until 1986 on the PN line with correlation coefficients of about 0.6. During the 1990s, when data were collected on the ASUKA line, the NCEP/NCAR experiments give lower correlation coefficients (less than 0.3), on both PN and ASUKA lines, while the ECMWF experiments have a higher value on the ASUKA line (0.5). One of the reasons for the disagreement between the observations and OGCMs during the 1990s might arise from the NCEP/NCAR data. An additional analysis of a wind-driven circulation was performed to examine the sensitivity of integrated Sverdrup transport along the western boundary to the propagation speed of a baroclinic Rossby wave, which is varied by stratification. A variation of the stratification, which might be induced by variability of air-sea heat and freshwater fluxes, cannot be a main cause of the disagreement. This revised version was published online in July 2006 with corrections to the Cover Date.