A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane

Abstract

The largest reservoir of the powerful greenhouse gas methane is in marine sediments, and catastrophic release of this methane has been invoked to explain climate perturbations throughout Earth history. Marine methane oxidation is mainly coupled anaerobically to microbial sulfate reduction, which both limits and controls the release of methane from this sedimentary reservoir to the rest of Earth's surface. Methane can be transported within the pore space of marine sediments either via diffusion or as bubbles. When methane travels in bubbles, these bubbles often are not completely oxidized and reach the overlying water where the methane emerges from the sediment in cold seeps. Although paleo-cold seeps can be identified by geological features such as carbonate mounds, a geochemical signature for cold seeps remains elusive. We demonstrate, using the sulfur and oxygen isotope composition of sulfate, that a unique isotopic signature emerges during microbial sulfate reduction coupled to methane oxidation in bubbling cold seeps. This isotope signature differs from that when sulfate is reduced by either organic matter oxidation or by the slower, diffusive flux of methane within marine sediments. We also show, through a comparison with the literature, that this unique isotope fingerprint is preserved in the rock record in authigenic buildups of barite associated with methane cold seeps. δ Figure 3. The d 18 O SO 4 versus d 34 S SO 4 data from cold methane seeps and seeps analogues (gray symbols) and barite deposits associated with cold methane seeps (white symbols) from the Gulf of Mexico (rhombus, Fu and Aharon, 1997; squares, Feng and Roberts, 2011) and from the Sea of Okhotsk (circles, Greinert et al., 2002). VCDT-Vienna Canyon Diablo troilite; VSMOW-Vienna standard mean ocean water.as

High Sulfur Isotope Fractionation Associated with Anaerobic Oxidation of Methane in a Low-Sulfate, Iron-Rich Environment

“…Previous studies reported that sulfur-cycling AOM can induce sulfur isotope fractionation in marine environments and at sulfate concentrations in the millimolar range Antler et al, 2014Antler et al, , 2015Avrahamov et al, 2014;Deusner et al, 2014;Sivan et al, 2014). A few of these studies assigned ε AOM values of 20-30 for AOM in gas seeps (Deusner et al, 2014;Sivan et al, 2014) and suggested that AOM may cause fractionations of up to 60 in marine SMTZs Deusner et al, 2014).…”

“…Previous studies showed that sulfurcycling AOM impacts not only sulfur (δ 34 S SO4 ), but also oxygen isotopes (δ 18 O SO4 ) in the dissolved sulfate pool and apparently creates an unique pattern among them that can help to distinguish between organotrophic and methanotrophic SR to be the main drivers of the footprints (e.g., Antler et al, 2014Antler et al, , 2015Avrahamov et al, 2014;Deusner et al, 2014). Therefore, future studies may probably be best approached by combined investigations of oxygen and sulfur isotopes.…”

“…The potential for a distinct fractionation pattern associated with SR during AOM is further emphasized by the recent hypothesis that AOMdependent SR occurs through a unique S 0 -producing pathway, as hypothesized (Milucka et al, 2012). Antler et al (2015) recently suggested that AOM-dependent SR imparts a unique δ 18 O/δ 34 S ratio to the residual sulfate in estuarine and marine sediments, but this was based on sulfate at millimolar concentrations, and a fractionation factor relative to the reduced products was not determined.…”

Rates and Cycles of Microbial Sulfate Reduction in the Hyper-Saline Dead Sea over the Last 200 kyrs from Sedimentary δ34S and δ18O(SO4)

“…During interglacials, sulfate reduction is confined to the sediments (as it is today), and the rate derived from the δ 18 O (SO4) vs. δ 34 S crossplot suggests the rate of microbial sulfate reduction is faster relative to glacial stages. This could be because of the presence of methane, and that sulfate may be dominantly reduced through anaerobic methane oxidation, associated with a low, unique slope (∼0.39; Antler et al, 2015). This fast rate of microbial sulfate reduction will also contribute to the higher overall sulfur isotope composition of the sedimentary sulfide FIGURE 10 | Conceptual evolution patterns of the Dead Sea lacustrine system across Glacial-Interglacial cycles.…”

“…Note that the Masada M1 section of the Lisan Formation, corresponds to depths of ∼177-90 mblf in the DSDDP core. Bottrell and Newton, 2006;Leavitt et al, 2013;Antler et al, 2015).…”