Millennial‐scale stratigraphy of a tide‐dominated incised valley during the last 14 kyr: Spatial and quantitative reconstruction in the Tokyo Lowland,central Japan

Spatial and quantitative analysis of infilling processes of the tide‐dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post‐Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea‐level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet‐like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea‐level rise of 4 to 7 mm yr^?1. Before migration of the Tone River at 5 ka, the tide‐dominated bay in the Nakagawa Valley was filled by upward‐fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide‐dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward‐coarsening deltaic sediments.     

Growth and Survival of Pacific Saury Cololabis saira in the Kuroshio-Oyashio Transitional Waters

Growth and mortality rates of larval and early juvenile Pacific saury Cololabis saira were estimated for spring and autumn spawning seasons in the Kuroshio-Oyashio transitional waters and for winter spawning season in the Kuroshio waters in 9 years from 1990–1998, based on quantitative fish sampling and otolith daily ring readings. Growth and mortality rates were more variable in the Kuroshio-Oyashio transitional waters than in the Kuroshio waters. The estimated production of 40-mm preschooling juveniles was a positive function of larval production in the hatching length class (5.9–9.9 mm) in the Kuroshio waters. In the Kuroshio-Oyashio transitional waters, rather than larval production in the hatching length class, cumulative survival through the larval and early juvenile stages determined the juvenile production. Variable growth and survival rates of saury observed in the transitional waters seem to be associated with large environmental variability in the waters, including shifts of the Kuroshio and Oyashio fronts and development of streamers and eddies between the fronts. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stock Structure Analysis of the Japanese Spanish Mackerel Scomberomorus niphonius (Cuvier, 1832) Along the China Coast Based on Truss Network

The Japanese Spanish mackerel Scomberomorus niphonius(Cuvier,1832)is widely distributed in the subtropical and temperate waters of the northwestern Pacific Ocean,supporting one of the most important commercial fisheries in China.However,ignoring the potential population structure changes induced by fishing pressure and climate change may undermine the population stability under the current management strategy.In this study,the population structure of the Japanese Spanish mackerel was investigated based on a morphometric truss network system.A total of 534 individuals were randomly collected from commercial gill nets spanning eight major spawning grounds in the Bohai,Yellow,and East China Seas during the peak spawning seasons respectively.A total of 17 measurements(including eye diameter)were conducted in each specimen and subjected to principal component analysis(PCA)and discriminant function analysis(DFA).The results of PCA indicated that the first two factors cumulatively caused 78.38%of the total morphometric variation and observable differences,primarily fin the caudal and trunk areas.The results of DFA revealed that the eight spawning groups can be divided into three stocks,i.e.,southern,middle,and northern stocks,with 68.7%of total accuracy.In contrast to previous studies,the spawning groups of the Japanese Spanish mackerel demonstrated a tendency to disperse to northern regions.In conclusion,this study found that to maintain the stability of the population structure and the total production of Japanese Spanish mackerel S.niphonius(Cuvier,1832),a newly revised management method should be developed and implemented.     

Comparison of reductive accumulation of Re and Os in seawater-sediment systems

In order to understand the fractionation of Re and Os in marine environments, their removal from artificial seawater to Tokyo Bay sediments is studied using a multitracer technique. The chemical processes of the removal of Re and Os are also estimated based on their speciation analyses by X-ray absorption fine structure (XAFS) spectroscopy. The partitioning experiments, which use the multitracer technique, provide information on Re and Os regarding (i) their distributions between artificial seawater-sediment systems, (ii) their complexation with humic acid, and (iii) their carriers in sediments. In addition, XAFS spectroscopy provides direct information on the chemical states of Re and Os in the sediments.In an artificial seawater-sediment system containing a multitracer, Re is removed from the artificial seawater only under a reducing environment. The speciation of Re by X-ray absorption near-edge structure (XANES) suggests that the majority of Re remains as in the artificial seawater even under highly reducing conditions, during laboratory time scale (about 2 weeks). Moreover, XANES simulation shows that some Re exists at a lower oxidation state, such as ReO₂, in the reducing sediment. These results can be explained by the slow kinetics of the reaction which is similar to those suggested by previous geochemical studies.In contrast, Os is readily removed from the artificial seawater into sediments under various redox conditions. Even under oxic conditions, a large fraction of Os is removed from the artificial seawater to sediments without organic matter. Based on the Os XANES study, it is confirmed that the oxidation states of Os incorporated in the reducing sediment and oxic sediment are trivalent and tetravalent, respectively. Sequential extraction suggests that the main carrier of Os in the organic-rich sediment is either ferromanganese oxides or organic matter, and that the Os in these two fractions may correspond to hydrolyzed insoluble Os species and Os species interacting with organic matter, at lower valence, respectively. The results of distribution study of Os in the absence and presence of humic acid (HA) also imply that Os assumes more than one chemical species, and a small fraction of Os may interact with HA in the experimental system. Meanwhile, extended X-ray absorption fine structure (EXAFS) confirms that the first neighboring atom of Os in the reducing sediment is oxygen. If Os(VIII) is the main dissolved species in seawater, as is expected thermodynamically, reductive removal may control the enrichment of Os in the sediment. Osmium, which is removed as Os(IV), is reduced further to Os(III) by a diagenetic process and may be complexed with organic matter in the reducing sediment.The results of the removal behaviors of Re and Os obtained in the current study show that Re can be removed from the artificial seawater only under highly reducing conditions within 2 weeks, but Os removal from the artificial seawater can be found under various redox conditions. Thus, a high ¹⁸⁷Re/¹⁸⁸Os ratio can occur only in reducing sediments, such as black shales. The high¹⁸⁷Re/¹⁸⁸Os ratio, in turn, makes black shales suitable for Re-Os dating. In contrast, authigenic sediments (and minerals) under oxic environments can enrich Os, but since Re is not distributed to the sediments under oxic conditions, this will cause a much lower ¹⁸⁷Re/¹⁸⁸Os ratio than that of seawater. The Os isotope system of these materials can be used as a paleo-marine environmental tracer since the ¹⁸⁷Os/¹⁸⁸Os ratio cannot grow significantly due to its extremely low ¹⁸⁷Re/¹⁸⁸Os ratio.