Significant Human Impact on the Flux and δ~(34)S of Sulfate from the Largest River in North America

摘要

Riverine dissolved sulfate (SO_4~(2-)) flux and sulfur stable isotope composition (δ~(34)S) yield information on the sources and processes affecting sulfur cycling on different spatial and temporal scales. However, because pristine preindustrial natural baselines of riverine SO_4~(2-) flux and δ~(34)S cannot be directly measured, anthropogenic impact remains largely unconstrained. Here we quantify natural and anthropogenic SO_4~(2-) flux and δ~(34)S for North America's largest river, the Mississippi, by means of an exhaustive source compilation and multiyear monitoring. Our data and analysis show that, since before industrialization to the present, Mississippi River SO_4~(2-) has increased in flux from 7.0 to 27.8 Tg SO_4~(2-) yr~(-1) and in mean δ~(34)S from -5.0‰, within 95% confidence limits of -14.8‰ to 4.1‰ (assuming normal distribution for mixing model input parameters), to -2.7 ± 1.6%©, reflecting an impressive footprint of bedrocks particular to this river basin and human activities. Our first-order modern Mississippi River sulfate partition is 25 ± 6% natural and 75% ± 6% anthropogenic sources. Furthermore, anthropogenic coal usage is implicated as the dominant source of modern Mississippi River sulfate, with an estimated 47 ± 5% and 13% of total Mississippi River sulfate due to coal mining and burning, respectively.