Yasuo Miyabuchi

Luminescence dating of volcanogenic outburst flood sediments from Aso volcano and tephric loess deposits, southwest Japan

Luminescence dating has been applied to volcanogenic outburst flood sediments (Takuma gravel bed) from Aso volcano, Japan, and tephric loess deposits overlying the gravel bed. The poly-mineral fine grains (4–11 μm) from loess deposits were measured with pulsed optically stimulated luminescence (pulsed OSL) and post-IR infrared stimulated luminescence (pIRIR) methods, whereas the Takuma gravel bed containing no quartz, was measured with IRSL and pIRIR methods using sand sized (150–200 μm) plagioclase. The loess deposits date back at least to ∼50 ka by consistent IRSL, pIRIR and pulsed OSL ages from the lowermost part of the loess deposits from one section. The ages obtained from the bottom part of the other loess section are not consistent each other. However, we consider that the pIRIR age (72±6 ka) which showed negligible anomalous fading is most reliable, and regard as a preliminary minimum age of the Takuma gravel bed. The equivalent doses (De) for the plagioclase from the Takuma gravel bed have a narrow distribution and the weighted mean of the three samples yield an age of 89±3 ka. This age is in agreement with the last caldera-forming eruption of Aso volcano (∼87 ka) and it is likely that the pIRIR signal has not been bleached before the deposition. IRSL dating without applying pIRIR using small aliquots was also conducted, however, the IRSL signal shows no clear evidence of an additional bleaching during the event of outburst flood from the caldera lake.

Decreasing removal rate of the dissolved CO2 in Lake Nyos, Cameroon, after the installation of additional degassing pipes

Abstract The amount of dissolved CO2 (CO2aq) in Lake Nyos, Cameroon, has been measured annually since January 2011. First, concentration was estimated by a chemical analysis with the assumption of equilibria between CO2aq, and . The estimated was correlated with the observed electrical conductivity of lake water. The profile of [CO2aq] was generated from the electrical conductivity, pH and temperature profiles. The [CO2aq] profile was integrated over the water column to obtain the total amount of CO2aq. The amounts were 8.4, 6.8, 5.5, 4.4 and 3.9 Gmol (=109 mol) in 2011, 2012, 2013, 2014 and 2015, respectively. The fast reduction between 2011 and 2013 was attributed to the installation of two additional degassing pipes in April 2011. However, the rate of reduction decreased between 2014 and 2015 due to the reduced [CO2aq] at the inlet of the degassing pipes.