Gully systems under undisturbed indigenous forest, East Coast Region, New Zealand

Gully erosion is commonly associated with agricultural landscapes where vegetation clearance and farming practices increase runoff, leading to fluvial incision. However, gully erosion can also occur in forests that have undergone some form of disturbance, either natural or resulting from human impacts. This paper reports on recent gully development within areas of undisturbed indigenous forest as a result of a high magnitude rainfall event on the East Coast of New Zealand's North Island. This region, through a combination of crushed and sheared rock types, steep topography, and tectonic and climatic setting, has high natural rates of erosion, exacerbated by European deforestation in the late 19th and early 20th centuries.Sequential air photographs, spanning a 58 year period between 1939 and 1997 were used to classify and document the growth and recovery of gully systems in the 14.1 km² headwaters of the Mangaoporo catchment. Following a severe cyclone in 1988, with a rainfall of 535 mm, there were 21 active gully systems within the indigenous forest. On photography prior to 1988 only four gully systems were present. During this period there were 8 major rainfall events (150–250 mm). Despite further 5 rainfall events of 150–250 mm between 1988 and 1997 all gully systems showed signs of recovery, with a combined reduction in active area of 37%. The nature and location of these features is strongly influenced by lithology (orientation of jointing and bedding), and to a topographic threshold defined by catchment slope and catchment area.     

Influence of submarine fumaroles on the distribution of mercury in the sediment of Kagoshima Bay,Japan

To estimate the influence of mercury emitted from submarine fumaroles, the horizontal and vertical distribution of mercury in sediment of Kagoshima Bay was studied. The fumaroles are located in the northern bay head area, and the sediment samples had been taken from 52 points throughout the bay with a gravity core sampler. The core samples obtained were cut at a thickness of 1–2 cm and used for measurements. The total concentration of mercury in surface sediment in the northern and central areas of the bay was 51–679 μg kg^− 1 (average 199 μg kg^− 1, n = 22) and 23–100 μg kg^− 1 (average 55 μg kg^− 1, n = 30), respectively. The highest value was obtained in the vicinity of the fumaroles. The mercury concentration in sediment near the fumaroles varied with depth, which may reflect the variation in fumarolic activity. A successive extraction method was applied to the speciation of mercury in the sediment. The results showed that sediment taken in the vicinity of submarine fumaroles contained a higher percentage of mercury bound with organic matter.     

Isotopic geochemistry of Hualalai shield-stage tholeiitic basalts from submarine North Kona region,Hawaii

The geochemical data of Hualalai tholeiitic basalts allow extension of the temporal variations established at Mauna Loa back in time, and provide important information for the long-term temporal variation of the Hawaiian lavas. We report new Hf, Pb, Nd, and Sr isotope compositions for 32 Hualalai tholeiitic basalts collected from deep submarine portions of the North Kona region. The samples were collected from the lower section of the North Kona bench (dives K218 and K219), a submarine stratigraphic section at Hualalai volcano's northwest rift zone (dive S690), and an elongate ridge outboard of the central section of the bench (dive S692), during two JAMSTEC Hawaii cruises in 2001 and 2002. The Hualalai shield-stage tholeiitic basalts have magma source isotopic signatures similar to Mauna Loa. The new data shows temporal Pb and Sr isotope trends that correspond to the long-term temporal variations in Loa-trend lavas, and the Hualalai–Mauna Loa lavas seem to show inter-shield geochemical excursions. Variation in Pb and Sr isotopes at Hualalai appears to take place over a longer time scale than at Mauna Loa. The merged Hualalai–Mauna Loa isotopic trends support models where heterogeneous material in the plume conduit is distributed chaotically, with variable cross-sectional density and length scale.     

Two modes of climatic control in the Holocene stalagmite record from the Japan Sea side of the Japanese islands

The Holocene stalagmite FG01 collected at the Fukugaguchi Cave in Itoigawa, central Japan provides a unique high‐resolution record of the East Asian winter monsoon. Because of the climate conditions on the Japan Sea side of the Japanese islands, the volume of precipitation during the winter is strongly reflected in the stalagmite δ¹⁸O signal. Examination of the carbon isotopes and the Mg/Ca ratio of FG01 provided additional information on the Holocene climate in Itoigawa, which is characterized by two different modes separated at 6.4 ka. Dripwater composition and the correlation between the δ¹³C and Mg/Ca data of FG01 indicate the importance of prior calcite precipitation (PCP), a process that selectively eliminated ¹²C and calcium ions from infiltrating water from CO₂ degassing and calcite precipitation. In an earlier period (10.0–6.4 ka), an increase in soil pCO₂ associated with warming and wetting climate trends was a critical factor that enhanced PCP, and resulted in an increasing trend in the Mg/Ca and δ¹³C data and a negative correlation between the δ¹³C and δ¹⁸O profiles. A distinct peak in the δ¹³C age profile at 6.8 ka could be a response to an increase of approximately 10% in C4 plants in the recharge area. At 6.4 ka, the climate mode changed to another, and correlation between δ¹⁸O and δ¹³C became positive. In addition, a millennial‐scale variation in δ¹⁸O and pulsed changes in δ¹³C and Mg/Ca became distinct. Assuming that δ¹⁸O and PCP were controlled by moisture in the later period, the volume of precipitation was high during 6.0–5.2, 4.4–4.0, and 3.0–2.0 ka. In contrast, the driest interval in Itoigawa was during 0.2–0.4 ka, and broadly corresponds to the Little Ice Age.     

Using sediment travel distance to estimate medium‐term erosion rates: a 16‐year record

Fine grained (80 µm) magnetite was introduced onto a semi‐arid grassland hillslope in 1992, as part of a set of rainfall‐simulation experiments. Using measurements of magnetic susceptibility, the median distance travelled by these magnetite grains during subsequent natural runoff events in the 16‐year period up to 2008 was estimated. Coupling this estimate to direct measurements of sediment flux obtained during the rainfall‐simulation experiments has enabled estimation of the erosion rate over this period. The estimated average erosion rate of between 2·61 × 10^?2 and 4·36 × 10^?2 kg m^?1 year^?1, is equivalent to a rate of ground lowering between 0·020 and 0·033 mm year^?1. This estimate is consistent with (in the sense of being less than) an estimate of total sediment detachment over the same period. The rate of erosion measured using this travel‐distance approach is an order of magnitude less that obtained from a study based on ¹³⁷Cs in a nearby catchment, and compatible with the longevity of continents. Copyright © 2010 John Wiley & Sons, Ltd.     

Sedimentary effects on the expansion of a Himalayan supraglacial lake

Supraglacial Tsho Rolpa Lake in the Nepal Himalaya has been increasing rapidly in size since the 1950s, corresponding to the mountain-glacier shrinkage after the Little Ice Age. The lake basin expansion results from the subsidence by dead-ice melt below the bottom of the lake, and the retreat of the glacier terminus. Field observations of Tsho Rolpa in 1996 revealed that the retreat of glacier terminus is connected to a wind-induced vertical circulation of surface water heated by solar radiation. In order to clarify the mechanism of the lake expansion associated with sedimentary processes, we measured bottom sedimentation rate with some sediment traps, and vertical suspended sediment concentration (SSC) and water temperature, and analyzed the grain size of suspended and trapped sediments. The sediments, mostly composed of clay-sized grains, are dominantly supplied by glacier-melt water inflow at the glacier terminus. Sedimentary processes of such fine sediment comprise: (1) suspended-sediment fallout from intrusion of horizontal currents; (2) sediment sorting by sediment-laden underflows; and (3) the debris supply from the ice collapse at the glacier terminus. The (1) and (2) processes produce the density stratification of the lake, accompanied by a pycnocline at a depth of about 27 m. The existence of the pycnocline builds up the vertical water circulation in the surface layer to enhance the glacier-melt at the terminus. With respect to the subsidence of the lake bottom, nearly molecular thermal diffusion is probably dominant near the bottom of the deepest point, which results from the kinetic-energy dissipation of sediment-laden underflows. The stable existence of the bottom turbid water throughout the year could cause continuous dead-ice melt below the lake bottom.