3:45 PM - 4:00 PM
[MIS14-08] Limnological factors controlling inter and intrabasin variations of sedimentary manganese and iron distribution in Japanese stratified lakes
Keywords:lake, redox, iron, manganese, sediment
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.