Rapid and significant perturbations on the global geochemical cycle of rhenium by human activitiesA case study in Yangtze River basin

Rhenium, mostly found in sedimentary organic matter, is used to estimate petrogenic organic carbon (POC) weathering rates. Understanding its cycle on the Earth's surface is key to using riverine dissolved rhenium (Re) as a marker for the weathering of POC. Yet, human interference, especially from the mining, industrial and chemical sectors that commonly use Re, can affect use of this marker. In this study, Yangtze River water samples and industrial wastewater samples from 2018 to 2019 were collected and Re and major ion concentrations were analyzed to investigate the major sources and anthropogenic effects on riverine dissolved Re flux. The concentration of dissolved Re in the Yangtze River ranged from 67.7 to 167.6 pmol/L, much higher than the world average of 2.3 pmol/L and the calculated annual Re flux is 16.98 tons. Based on the assumption that denudation materials in the Yangtze River basin are entirely weathered, the calculated upper limit of the Re flux provided by the natural weathering process is approximately 1.48 t/yr, accounting for only 8% of the measured total Re flux. Therefore, we estimate that human activities contributed at least 92% of the dissolved Re flux in the Yangtze Basin. Analyses of wastewater from factories closely related to Re use in the Yangtze River basin showed little influence on the dissolved Re flux, so other human activities along the Yangtze River may contribute a significant amount of Re, such as mining or coal utilization. With the dominant control of human activities on riverine dissolved Re flux in the Yangtze River, the annual Re flux may have increased from 9.44 t/yr to 16.98 t/yr from 2007 to 2019. Our study suggests that human activities may rapidly and significantly change the geological elemental cycle of Re on decadal timescales.