Identification of dissolved sulfate sources and the role of sulfuric acid in carbonate weathering using dual-isotopic data from the Jialing River, Southwest China

Abstract

Rock weathering by carbonic acid has played a substantial role in both the global carbon cycle and related climate change. Carbonic acid as the major weathering agent has been accepted, whereas the importance of other acid (sulfuric, nitric or organic acids) as an agent is gradually recognized. Here, we examine sulfate dual-isotopic evidence ( δ 34 S SO 4 and δ 18 O SO 4 ) and water chemistry from the Jialing River (Sichuan Basin, Southwest China) to identify dissolved sulfate sources and the role of sulfuric acid in carbonate weathering. A survey was carried out at 29 sites where surface water was sampled during the rainy (July, 2008) and dry (February, 2009) seasons in the Jialing River. The chemical composition of river water was characterized by a dominance of Ca 2+, Mg 2+ and HCO 3 - , and SO 4 2 - , which accounted for more than 90% of the total ion concentrations. Water chemistry varied greatly in time and space, particularly for Na +, Cl −, and SO 4 2 - . This variation was a result of anthropogenic influences, such as acid deposition and domestic sewage inputs. Co-variation of the equivalent ratios of [Ca 2+ + Mg 2+] and [ SO 4 2 - + HCO 3 - ] indicate that it required significant additional SO 4 2 - to achieve ionic balance, which implied that sulfuric acid might play a relatively important role in carbonate weathering of this river basin. Water samples from the Jialing River were significantly rich in SO 4 2 - , and increased almost two times from 274 μM in the period of 1958–1990 to 499 μM in this study. The use of co-variations of δ 34 S SO 4 vs. δ 18 O SO 4 and of δ 18 O SO 4 vs. δ 18 O H 2 O allowed us to demonstrate that most of the sulfate in the waters of the Jialing River was derived from sulfide oxidation and atmospheric inputs by high sulfur-content coal combustion while the contribution of sulfate from domestic and industrial wastewater could be important in the dry season. Thus, the contribution of sulfuric acid, produced by such sulfide oxidation and the oxidation of atmospheric SO 2 emitted from coal combustion, as a weathering agent is also confirmed.