Tungsten isotopes (δ186/184W) are a promising proxy for reconstructing the evolution of marine redox conditions over geological time. Conventional understanding posits that under sulfidic conditions – in contrast to oxic environments – tungsten predominantly remains in dissolved form within seawater. Consequently, marine sulfidic sediments are thought to lack authigenic W enrichment and to typically display isotopic signatures comparable to upper continental crustal values. Here, we present new δ186/184W data from sulfidic methane-seep sediments at the Haima Cold Seep (South China Sea) that reveal unexpectedly elevated isotopic compositions (to +0.43‰). These markedly heavy W isotopic signatures contrast with the near-homogeneous values documented in other sulfidic marine environments, including Black Sea sapropels (+0.05‰ to +0.07‰) and globally distributed analogues. The positive correlations between carbonate content, δ186/184W, and W enrichment factor
are indicative of authigenic carbonate precipitation as the principal control on the W isotopic compositions of sediments in these systems. Methane-derived carbonates exhibit particularly heavy δ186/184W values (+0.40‰ to+0.48‰), supporting this hypothesis and unequivocally identifying such carbonates as a significant sink for heavy W isotopes. These findings demonstrate that some sulfidic sediments host authigenic W burial, and highlight the role of carbonate phases as an archive for reconstructing marine redox evolution. Our results further extend the utility of W isotopes as a proxy for paleoenvironmental reconstruction.
