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While major explosive eruptions of anhydrite-bearing silicic magma have been known to influence earth's climate through the release of sulfate aerosols, quantifying the atmospheric loading of these eruptions remains an elusive goal. The eruptions that have had the most impact on climate typically erupted oxidized and anhydrite-bearing magma. The > 9 km3 of dacite magma erupted during the VEI 6 eruption of Huaynaputina (Peru) in 1600 is oxidized (ƒO2 NNO + 1.4) but does not contain magmatic anhydrite. Yet it is thought to have a significant impact on climate. Herein we follow the approach that S contents in magmatic apatite grains can be used to provide an independent estimate of the magnitude and source of the atmospheric sulfur loading. Apatite SO3 concentrations of 0.09 to 0.17 wt.% yield an estimate of ~0.8 Mt of total erupted sulfur derived from degassing of the melt. This is considerably less than the stratospheric sulfur input of 16–55 Mt of S estimated from ice core data and requires that much of the stratospheric sulfur load from Huaynaputina was from other sources than the melt. While a fluid phase in equilibrium with the melt is a likely source, a significant contribution from combustion of sulfur from a fossil hydrothermal system excavated during eruption is evidenced by comminuted hydrothermal debris and lithics that make up to 2.5 to 10 wt.% of erupted material. We calculate that up to 22% of the total S budget of the eruption could reasonably be attributed to combusted hydrothermal sulfur. Thus, three sources of sulfur: the melt, a coexisting fluid phase, and external hydrothermal sulfur, appear to explain the atmospheric input from this eruption. Our work confirms that unraveling the S budget of large silicic eruptions is extremely complex and requires a multi-faceted approach based on a detailed understanding of the eruption.