Effects of Secondary Carbonate Precipitation and Dissolution on Changjiang (Yangtze) River Chemistry and Estimates of Silicate Weathering Rates

Abstract

AbstractRiverine solute chemistry has long been used to infer modern weathering fluxes and processes by assuming that river‐dissolved loads are conservative mixtures of weathering products of different lithologies. Secondary carbonate precipitation and re‐dissolution are important processes that can alter the original weathering fluxes. However, it remains less well understood how secondary carbonates affect the solute chemistry of large rivers and the estimates of silicate weathering rates. Here we report the elemental and Sr‐Mg isotopic compositions of the Changjiang (Yangtze River) water collected biweekly from May 2010 to April 2011 and use this data set to explore the influence of secondary carbonate precipitation and re‐dissolution on the Changjiang river chemistry. Variable Ca/Sr and Ca/Mg ratios but stable 87Sr/86Sr and δ26Mg are observed, which can only be explained by the precipitation and dissolution of secondary carbonates. Across a range of possible Ca/Mg ratios of the primary weathering flux, we estimate that up to 30% of the dissolved Ca carried by Changjiang was controlled by secondary carbonate precipitation and re‐dissolution. Using inverse modeling, we show that secondary carbonate precipitation and re‐dissolution have a minor influence on the calculated silicate weathering rates. Our findings provide new insights into how secondary carbonates affect water chemistry in large river systems, highlighting the role of secondary carbonate precipitation and re‐dissolution in modulating riverine dissolved fluxes in the Changjiang basin.