Tree-ring isotope and satellite records of vegetation changes reveal transition from cold to hot degassing in the Yellowstone Plateau volcanic field

Magmatic degassing often provides the earliest clues to changes in magmatic systems, yet detection and characterization of the escaping gases is challenging. Recent work suggests that satellite-based normalized difference vegetation index (NDVI) calculations may provide an indirect method for detecting changes in degassing. Here, we use tree ring δ¹³⁢C, δ¹⁸⁢O, and radiocarbon (¹⁴C) data to assess NDVI as a proxy for diffuse magmatic degassing and to better understand the development of the Tern Lake thermal area (TLTA) in the Yellowstone Plateau volcanic field, USA. We measured δ¹³⁢C, δ^18⁢O, and ¹⁴C in wood from four TLTA trees and two control trees. The ¹⁴C data indicate that TLTA trees were exposed to significant magmatic CO₂ degassing during the 1990s, corroborating satellite-based NDVI changes observed at a time when no other indications of degassing existed. Magmatic degassing also increased water availability for trees closest to the degassing center, as indicated by decreasing δ¹³⁢C and δ^18⁢O values relative to background throughout the 1980s and 1990s. Increased H₂S degassing in the early 2000s impaired stomatal functioning, increasing δ¹³⁢C and δ^18⁢O values. These isotope values were strongly correlated with NDVI, demonstrating that ground- and satellite-based approaches yield similar conclusions. Together, the satellite and isotope datasets suggest that TLTA degassing was characterized by cold CO₂ emissions with minimal H₂S until the early 2000s, when it transitioned to a high-temperature system with significant CO₂ and H₂S emissions. These results add to a growing body of evidence that vegetation responses can be an effective proxy for magmatic degassing.