Preservation of 34S-enriched sulfides in fossil sulfate-methane transition zones: new evidence from Miocene outcrops of the northern Apennines (Italy)

Abstract

We provide new evidence of the preservation of 34S-enriched signals in methane seep-impacted sediments from two onshore Miocene outcrops located in the northern Apennines (Italy). Selected outcrops include methane-derived authigenic carbonates (MDAC) with δ13C composition between − 42.3 and − 18.2‰. MDACs contain chemosynthetic clams and abundant pyrite indicative of formation close to or within a shallow sulfate-methane transition zone (SMTZ). This study aims to evaluate the relative contributions of background organic matter mineralization and anaerobic oxidation of methane (AOM) to sulfate consumption and to gain insight into the transport process (i.e., diffusion, advection) controlling the depth of the SMTZ. Host sediments were investigated by CHN elemental analysis coupled with total sulfur (TS) and sulfur isotopic measurements on bulk samples. Total organic carbon (TOC) measurements reveal a consistent and low amount of organic carbon (TOC 0‰ and up to + 17.1‰), which is characteristic of sulfides precipitated in association with AOM. We propose that advection of methane-rich fluids was responsible for maintaining the shallow depth of the studied paleo-SMTZs. AOM at paleo-SMTZ positions in the investigated seep-impacted sediments resulted in excess bicarbonate and sulfide production, favoring solid-phase MDAC and iron sulfide precipitation.