Naoto Koiwa

Three-dimensional structure of involutions formed in a late Pleistocene tephra layer, northeastern Japan

Abstract The involutions in the middle Kitakami River basin in northeastern Japan are formed between the Murasakino Pumice (MP) layer, which fell on the area during the Last Glacial age, and overlying brown volcanic ash soil. These involutions are the festoon and/or injection type, in which the MP moved upward and brown volcanic ash soil sunk down. This paper attempts to reconstruct the three-dimensional structure of the involutions by examination of several horizontal and vertical forms at the lower, middle, and upper parts of the involutions. In the horizontal sections at the lower part, numerous circular and elliptic forms filled with a brown volcanic ash soil in the MP are observed. At the middle part, the circular forms are generally larger than those of the lower part. In addition, the MP exhibits circular or elliptic forms in the brown volcanic ash soil. It is also observed that the MP decreases upward in the vertical sections. The reconstruction shows that the MP shows an upturned structure vertically (more than 30 cm in height), and an irregular horizontal network circumscribing circular and elliptical forms, each of which is 0.5–3 m in the long-axis diameter. This structure is interpreted as load structures that result from density inversion during thawing of seasonally frozen ground (periglacial loading).

Fluvial transport of carbon along the river-to-ocean continuum and its potential impacts on a brackish water food web in the Iwaki River watershed, northern Japan

Riverine transport of dissolved inorganic carbon (DIC) from land to the ocean is an important carbon flux that influences the carbon budget at the watershed scale. However, the dynamics of DIC in an entire river network has remained unknown, especially in mountainous Japanese watersheds. We examined the effects of watershed land use and geology on the transports of inorganic carbon as well as weathered silica (Si) and calcium (Ca) in the Iwaki River system where agricultural and residential areas have developed in the middle and lower parts of the watershed. The concentration and stable carbon isotope ratios (δ13C) of DIC showed the longitudinal increase of 13C-depleted inorganic carbon along the river. As a result, most streams and rivers were supersaturated in dissolved CO2 that will eventually be emitted to the atmosphere. The possible origin of 13C-depleted carbon is CO2 derived from the decomposition of organic matter in agricultural and urban landscapes, as well as from in-stream respiration. In addition, agricultural and urban areas, respectively, exported the large amount of dissolved Si and Ca to the rivers, suggesting that CO2 increased by respiration accelerates the chemical weathering of silicate and carbonate materials in soils, river sediments, and/or urban infrastructure. Furthermore, riverine bicarbonate flux is likely to enter shell carbonates of Corbicula japonica, an aragonitic bivalve, in the downstream brackish lake (Lake Jusan). These results revealed that the flux of DIC from the human-dominated watersheds is a key to understanding the carbon dynamics and food-web structure along the land-to-river-to-ocean continuum.