Silicon as a permanent-carbon sedimentation tracer

Abstract

A procedure to quantify permanent carbon (C) sedimentation rates was required to compare these rates to methane (CH4) and carbon dioxide (CO2) water–air emission rates measured during reservoir C flux studies. Therefore, a new method to estimate C burial rates using silicon (Si) as a tracer was devised and applied. Burial rates in 8 tropical reservoirs were measured. Ages of these 8 reservoirs varied between 3.7 and 49 years. Each reservoir was surveyed 3 times during 1 year. Median burial rate was 78 (min 12, max 516; n = 66) mg C m−2 d−1. Trapped C (Ct) rates were also measured; the resulting median was 845 mg C m−2 d−1 (min 179, max 19 064; n = 40). Burial efficiency (comparison between C burial rate and Ct rate) was ~10%. Carbon burial efficiency of the 8 reservoirs showed strong dependence on bottom water temperature, efficiency being halved for each 3.4 °C increase in annual average temperature of reservoir bottom water. This finding strongly supported the adequacy of the Si-tracer method for rate measurements of carbon burial in sediments. Simultaneous with our new Si-tracer method we conducted traditional lead 210 isotope (210Pb) dating. The resulting median was 133 (min 11, max 441; n = 15) mg C m−2 d−1. Compared to the Si-tracer median, the 210Pb-dating technique resulted in a higher C median burial rate because the sampling sites that lacked sediment (and therefore contributed a null burial rate) were, in retrospect, erroneously disregarded.