15:45 〜 16:00
[MIS14-08] 日本の成層湖を対象とした堆積物中マンガン・鉄の湖沼間・湖沼内分布の支配要因解析
キーワード：湖沼、酸化還元、鉄、マンガン、堆積物
The secondary redistribution of manganese (Mn) and iron (Fe) after sedimentation have been observed from various world lake. Although decoupling of these elements in intrabasin scale are controlled by difference of redox potential between MnO2/Mn2+ and Fe(OH)3/Fe2+, and of kinetics of these redox reactions, effect of lake morphometry should also be considered. In this study, we made comparative limnological approach through monitoring of water, sediment and porewater of Japanese monomictic lakes having different depths, including Lake Kizaki (KZK, 29 m), Lake Nakatsuna (NKT, 13 m), and Lake Nojiri (NJR, 38 m) which have similar total phosphorus level (80-120 nM). Three sediment cores were collected from different depths of each lake to clarify effect of depth. Lake water have been collected in 3 or 4 seasons (March, June, August, October, 2021) at the deepest point of each lake. Temperature, pH were measured on site. Dissolved oxygen, total and dissolved Mn, and Fe in lake water were determined in laboratory by Winkler titration, and inductively coupled plasma mass spectrometer (ICP-MS). The Mn and Fe in sediment was determined by portable X-ray fluorescence spectrometer. Chemical form of Mn, Fe in SPM and sediment were measured by X-ray absorption fine structure (XAFS) measured at BL12C, Photon Factory, Tsukuba, Japan.
In the sampling of October to November 2021, the anoxic water mass was prevailed in the deepest zone of all three lakes. The sediments in three lakes showed different trend of depth-Mn/Fe (mg/g) relationship: decreasing Mn/Fe toward deeper zone in Lake Kizaki (22.0 m : 13.8, 25.0 m :11.3, 28.6 m : 7.4), constant Mn/Fe in Lake Nakatsuna (9.7 m : 7.5, 12.0 m : 6.3, 14.0 m : 6.3), and increasing Mn/Fe toward deeper zone in Lake Nojiri (25.5 m : 26.4, 35.5 m : 46.9, 37.0 m : 187.5). Lowest mean Mn/Fe in Lake Nakatsuna is likely attributed to the preferential removal of Mn, which released from bottom sediment, through river flow due to the small difference of depth between anoxic water mass and epilimnion. Clear accumulation of Mn in the deepest zone of the Lake Nojiri is plausibly due to the transportation of released Mn2+ toward center of the lake followed by the oxidation/deposition as the MnO2, so called geochemical focusing (Schaller et al., 1996). The XAFS measurement indicated that MnO2 is the dominant Mn form in SPM, while it easily dissolves as Mn2+ on the sediment surface. Notable differences between Lake Kizaki and Lake Nojiri was the presence of MnCO3 in sediment, i.e., contribution of MnCO3 in deepest zone of Lake Nojiri was up to 38%. This indicated that fixation of Mn as secondary mineral which is stable under reducing condition is necessary to record Mn accumulation in deep zone of the lake. Porewater chemistry of Lake Nojiri indicated supersaturation of MnCO3, due to higher pH than the other lakes. Overall, lake depth affects interbasin difference of Mn/Fe, while condition which MnCO3 formed is important factor to control intrabasin variability of Mn/Fe.
In the sampling of October to November 2021, the anoxic water mass was prevailed in the deepest zone of all three lakes. The sediments in three lakes showed different trend of depth-Mn/Fe (mg/g) relationship: decreasing Mn/Fe toward deeper zone in Lake Kizaki (22.0 m : 13.8, 25.0 m :11.3, 28.6 m : 7.4), constant Mn/Fe in Lake Nakatsuna (9.7 m : 7.5, 12.0 m : 6.3, 14.0 m : 6.3), and increasing Mn/Fe toward deeper zone in Lake Nojiri (25.5 m : 26.4, 35.5 m : 46.9, 37.0 m : 187.5). Lowest mean Mn/Fe in Lake Nakatsuna is likely attributed to the preferential removal of Mn, which released from bottom sediment, through river flow due to the small difference of depth between anoxic water mass and epilimnion. Clear accumulation of Mn in the deepest zone of the Lake Nojiri is plausibly due to the transportation of released Mn2+ toward center of the lake followed by the oxidation/deposition as the MnO2, so called geochemical focusing (Schaller et al., 1996). The XAFS measurement indicated that MnO2 is the dominant Mn form in SPM, while it easily dissolves as Mn2+ on the sediment surface. Notable differences between Lake Kizaki and Lake Nojiri was the presence of MnCO3 in sediment, i.e., contribution of MnCO3 in deepest zone of Lake Nojiri was up to 38%. This indicated that fixation of Mn as secondary mineral which is stable under reducing condition is necessary to record Mn accumulation in deep zone of the lake. Porewater chemistry of Lake Nojiri indicated supersaturation of MnCO3, due to higher pH than the other lakes. Overall, lake depth affects interbasin difference of Mn/Fe, while condition which MnCO3 formed is important factor to control intrabasin variability of Mn/Fe.