Quantifying the sources of dissolved inorganic car

Authors: ZiJun Wu HuaiYang Zhou DeZhang Ren Hang Gao JiangTao Li

Publish Date: 2016/08/15

Volume: 59, Issue: 10, Pages: 1959-1970

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Abstract

The significance of the various biogeochemical pathways that drive carbon cycling and the relative fractions of dissolved inorganic carbon DIC produced by these reactions within the sulfatemethane transition zone SMTZ are still being debated Unraveling these processes is important to our understanding of the benthic DIC sources and their contributions to the global carbon cycle Here we measure pore water geochemistry chlorine sulfate methane Ca2+ Mg2+ DIC and δ 13CDIC as well as solid geochemistry sedimentary organic carbon SOC and δ 13C of SOC in nearshore sediments from Qi’ao Island in the Pearl River Estuary of the Southern China Sea Our analysis indicates that SOC originates from the mixing of carbon from terrestrial and marine sources and that terrestrial materials dominate the net loss of SOC during the degradation of organic matter especially at sites located near the river outlets Sulfate reduction via SOC degradation is not appreciable in the upper sediment layer due to conservative mixingdilution by freshwater However below this layer the anaerobic oxidation of methane AOM and methanogenesis occur Within the SMTZ the δ 13C mass balance shows that the proportions of DIC derived from organoclastic SO4 2 reduction OSR and AOM are 503 to 667 and 01 to 179 respectively whereas methanogenesis contributes 170 to 439 This study reveals that the upward diffusion of DIC from ongoing methanogenesis significantly influences carbon cycling within the SMTZ in these estuarine sediments As a result we suggest that the plots of the ratio of change in sulfate to change in DIC in pore water should be used with caution when discriminating between sulfate reduction pathways in methanerich sediments

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