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–SO42– and δ18O–SO42–) struggle to distinguish it from overlapping
atmospheric and anthropogenic sources. Here, we apply the triple oxygen
value (Δ′17O–SO42–) 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–SO42– 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–SO42– 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.