Rhenium Concentration Variations in the Non-flood Period of the Yangtze River Water and Estimation of the Oxidation Rate of Organic Carbon

In geological history, the uplift of the Tibet plateau has accelerated the silicate weathering and organic carbon burial at the same time, which made great influence on the global carbon cycle by increasing the carbon sink. Because of the vital connection between tectonic uplift and carbon cycle, more and more attention was casted on rivers originating from orogens. The Yangtze River, as an important large river in the world, is one of them. However, although silicate weathering has been studied thoroughly, researches on organic carbon cycle are much less, and oxidation of fossil organic carbon remained poorly constrained. In this study, we try to use rhenium(Re) as a proxy to estimate the oxidation rate of fossil organic carbon and thus proceed our understanding towards the carbon cycle, the silicate weathering. This is because Re has a close relationship with organic carbon in the sediments and will be released into hydrological network in the mountain river catchments by being oxidized and exist as soluble ReO4-, so that we can use Re concentration in river water to estimate the oxidation rate of organic carbon. We collected water samples from the Yangtze River fortnightly at Banqiao Ferry and the sampling date cover the non-flood period. In this way, we are able to have a rough estimate of the amount of carbon dioxide that released to the atmosphere by the oxidation of organic carbon, using the data of non-flood period we got. We found that Re concentration in Yangtze River ranges approximately from 45 to 85 pmol/L. The rate of organic carbon weathering is estimated using the expression, ΦCO2,fossil=[Re]×runoff×[OC/Re]rock, and according to researches on the black shale of Yangtze River, the value 2.86×106 is chosen as the ratio OC(organic carbon) to Re in the black shale. The result is a really high flux, up to 152×109mol/y, just a little less than of the CO2 consumption rates from silicate weathering which is 191×109mol/y and about 166×109mol/y in non-flood period. Our result indicates that in the Yangtze Basin, oxidation of fossil organic carbon can very likely offset the carbon dioxide that removed by silicate weathering.