Methane seep carbonates yield clumped isotope signatures out of equilibrium with formation temperatures.

S J Loyd,R Martin,W F Defliese,J Sample,M Torres,J Marlow,M Hovland,K Brooks,T Lyons,R E Tripati,L G Hancock,A E Tripati

doi:10.1038/ncomms12274

Abstract

Methane cold seep systems typically exhibit extensive buildups of authigenic carbonate minerals, resulting from local increases in alkalinity driven by methane oxidation. Here, we demonstrate that modern seep authigenic carbonates exhibit anomalously low clumped isotope values (Δ47), as much as ∼0.2‰ lower than expected values. In modern seeps, this range of disequilibrium translates into apparent temperatures that are always warmer than ambient temperatures, by up to 50 °C. We examine various mechanisms that may induce disequilibrium behaviour in modern seep carbonates, and suggest that the observed values result from several factors including kinetic isotopic effects during methane oxidation, mixing of inorganic carbon pools, pH effects and rapid precipitation. Ancient seep carbonates studied here also exhibit potential disequilibrium signals. Ultimately, these findings indicate the predominance of disequilibrium clumped isotope behaviour in modern cold seep carbonates that must be considered when characterizing environmental conditions in both modern and ancient cold seep settings.

Highlights

Methane cold seep systems typically exhibit extensive buildups of authigenic carbonate minerals, resulting from local increases in alkalinity driven by methane oxidation
Of particular importance is the characterization of ancient cold seep environmental conditions and their relationship to modern seep systems
Modern samples originate from Hydrate Ridge, offshore Costa Rica, the Eel River Basin and the Norwegian Sea

Summary

Introduction

Methane cold seep systems typically exhibit extensive buildups of authigenic carbonate minerals, resulting from local increases in alkalinity driven by methane oxidation. We demonstrate that modern seep authigenic carbonates exhibit anomalously low clumped isotope values (D47), as much as B0.2% lower than expected values. (email: Methane cold seeps host diverse macro and microbiological communities[1,2,3,4,5] These ecosystems are driven by microbially mediated reactions involving methanecontaining fluids advecting from depth[6]. As methane ascends it is primarily oxidized anaerobically by sulfate (termed the anaerobic oxidation of methane (AOM)), or aerobically in the presence of oxic seawater, as follows: CH4 þ SO24À ! We compare modern systems with clumped isotope signatures in ancient seep carbonates, and discuss potential mechanisms to explain the observed disequilibrium values

Methods

Results

Conclusion