Microscale mineralization pathways in surface sediments: A chemical sensor study in Lake Baikal

We used an array of ion-selective electrodes (oxygen [O2], hydrogen [H+], carbonate [CO32-], calcium [Ca2+], ammonium [NH4+], and nitrate [NO3-]) with a micromanipulator to study mineralization processes in the surface sediments in Lake Baikal. Concentration profiles at submillimeter resolution were measured in sediment cores from four depths (160–1,400 m) in the South Basin. Oxidation rates of organic carbon (C) estimated from O2 and NO3profiles measured in March and July 2001 ranged between 2.2 and 4.9 mmol C m^-2 d^-1. The characteristic shape of the O2 profiles allowed separation of oxidation of organic carbon from reoxidation of reduced compounds at the oxic–anoxic boundary. Of the benthic carbon turnover, 60–75% was metabolized through oxic respiration and 11– 28% through anoxic mineralization. The remainder (12–14%) was due to denitrification. Carbon dioxide (CO2) profiles calculated from O2 agreed well with those from pH and CO3^2-, supporting the concept that oxic respiration was the prevailing mineralization pathway. Alkalinity balance calculated from flux rates of reduced compounds and bicarbonate (HCO3-) calculated from pH and CO3^2- profiles showed that the sediment was a sink for alkalinity. The flux rates in the range of 0.13–1.0 mmol m^-2 d^-1 were caused by buffering the hydrogen ions (H+) generated from reoxidation processes of reduced compounds. Potential dark CO2 assimilation by chemoautotrophic bacteria in the sediment was 0.03–0.1 mmol C m^-2 d^-1. Because of the long O2 exposure time of 25–2,500 yr, however, only 3–14% of the initially settled organic carbon was finally buried in the sediments, forming the paleolimnological record of Lake Baikal.