Methane Index: Towards a quantitative archaeal lipid biomarker proxy for reconstructing marine sedimentary methane fluxes

In marine sediments, anaerobic methane oxidation (AOM) carried out by consortia of methanotrophic archaea (ANME groups) and sulfate reducing bacteria (SRB) represents a major sink for methane (40-290 Tg of C yr- 1). The variability of seafloor methane seepage and AOM is crucial to sedimentary biogeochemistry and global carbon/sulfur cycling, yet poorly constrained for Earth's history. Based on the rationale that a major group of ANME preferentially synthesizes certain tetraether lipids, Methane Index (MI) measured from sedimentary archaeal lipid biomarkers called GDGTs (glycerol dialkyl glycerol tetraethers) has been widely used as a qualitative indicator for methane-impact on sediments. However, it was unclear whether small amounts of diagenetic methane in porewater could cause high MI values or that high methane flux associated with, for example cold seeps, is required. Since not all ANME groups synthesize GDGTs, another contested issue is whether this index is broadly representative of AOM. Here, we use modern porewater and sedimentary lipid profiles from the world's ocean to further explore MI's potential to address these questions. The new data show that MI is quantitatively linked to sedimentary methane diffusive flux and the depth of sulfate-methane transition zone (SMTZ) which is the diagenetic front where AOM activity is mostly concentrated. In particular, high MI values (>0.3-0.5) are strongly associated with high methane fluxes (>-0.1 mmol m- 2 d-1) and shallow SMTZ depths (<1-3 m) commonly linked to gas hydrate dissociations. Sensitivity analyses suggest that when the variability of several key parameters (e.g., seawater sulfate concentration, geothermal gradient, and sediment porosity) is small, modern MI - methane flux and MI - SMTZ depth relationships can be applied to reconstruct methane history and diagenetic zonation of the geological past. Although only Group I of ANME archaea synthesizes GDGTs, the cooccurrence of AOM biomarkers (e.g., crocetane, PMI, and hydroxyarchaeol) that are attributed to ANME-2 or ANME-3 and high MI values suggest that MI is broadly representative of AOM. Lastly, we applied MI to probe the methane venting history by using new data from a Cascadia Margin site (Hydrate Ridge, IODP U1328) since the penultimate interglacial, and reinterpreted recently reported MI data from three Southern Ocean sites (ODP 1168, 1170, and IODP U1356) during the late Oligocene - early Miocene. Overall, these applications demonstrate that MI can serve as a sensitive and quantitative proxy to help broadly establish Cenozoic and Mesozoic history of marine methane biogeochemical cycling.

Automated summary

In marine sediments, anaerobic methane oxidation (AOM) carried out by consortia of methanotrophic archaea (ANME groups) and sulfate reducing bacteria (SRB) is a major carbon sink for methane, and the Methane Index (MI) measured from sedimentary archaeal lipid biomarkers has been widely used as an indicator for methane impact on sediments. This study shows that MI is linked to sedimentary methane flux and the depth of sulfate-methane transition zone (SMTZ). Furthermore, the study suggests that MI can be used to reconstruct methane history and diagenetic zonation in the geological past.