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

Abstract

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 NO3- profiles 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 CO32-, 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 CO32- 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.