Triple Oxygen Isotopes Reveal Anthropogenic Overprinting of Riverine Sulfate Sources in the Yellow River Basin, China

Riverine sulfate, primarily derived from the weathering of continental sulfur-bearing minerals, plays a key role in shaping the isotopic composition of marine sulfate. In arid regions, gypsum dissolution is typically assumed to be the dominant sulfate source, but conventional sulfur and oxygen isotope systems (δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-) struggle to distinguish it from overlapping atmospheric and anthropogenic sources. Here, we apply the triple oxygen value (Δ'¹⁷O-SO₄^2-) in the Yellow River Basin─characterized by widespread agriculture, abundant gypsum, and minimal atmospheric deposition─to resolve these ambiguities. We observe strongly negative Δ'¹⁷O-SO₄^2- values (as low as -0.3‰) in upper basin waters during high-discharge periods, which we attribute to fertilizer-derived sulfate flushing from agricultural soils. These negative anomalies diminish downstream, where gypsum contributions become dominant. Isotopic modeling reveals that traditional approaches may overestimate gypsum inputs by up to 40%. Our results demonstrate that Δ'¹⁷O-SO₄^2- is a powerful tracer of anthropogenic sulfate inputs in modern rivers. Moreover, its nonconservative behavior during transport challenges the assumption that negative Δ'¹⁷O values in paleo-sulfate archives strictly reflect contributions of atmospheric oxygen to sulfate.