Anaerobic oxidation of methane alters sediment records of sulfur, iron and phosphorus in the Black Sea

Abstract

<p><strong>Abstract.</strong> The surface sediments in the Black Sea are underlain by extensive deposits of iron (Fe) oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The subsequent downward diffusion of marine sulfate into the methane-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ), including anaerobic oxidation of methane (AOM) with sulfate. While the sedimentary cycles of sulfur (S), methane and Fe in the SMTZ have been extensively studied, relatively little is known about the diagenetic alterations of the sediment record occurring below the SMTZ. Here we combine detailed geochemical analyses of the sediment and pore water with multicomponent diagenetic modeling to study the diagenetic alterations below the SMTZ at two sites in the western Black Sea. We focus on the dynamics of Fe, S and phosphorus (P) and demonstrate that diagenesis has strongly overprinted the sedimentary burial records of these elements. Our results show that sulfate-mediated AOM substantially enhances the downward diffusive flux of sulfide into the deep limnic deposits. During this downward sulfidization, Fe oxides, Fe carbonates and Fe phosphates (e.g. vivianite) are converted to sulfide phases, leading to an enrichment in solid phase S and the release of phosphate to the pore water. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe<sup>2+</sup>) lead to sequestration of downward diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front. <br><br> Our model results further demonstrate that downward migrating sulfide becomes partly re-oxidized to sulfate due to reactions with oxidized Fe minerals, fueling a cryptic S cycle and thus stimulating slow rates of sulfate-driven AOM (~ 1–100 pmol cm<sup>−3</sup> d<sup>−1</sup>) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe<sup>2+</sup> below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe<sup>2+</sup> in the pore water at depth. Our results reveal that methane plays a key role in the diagenetic alterations of Fe, S and P records in Black Sea sediments. The downward sulfidization into the limnic deposits is enhanced through sulfate-driven AOM with sulfate and AOM with Fe oxides may provide a deep source of dissolved Fe<sup>2+</sup> that drives the sequestration of P in vivianite below the sulfidization front.</p>

Rapid Sediment Accumulation Results in High Methane Effluxes from Coastal Sediments

“…This difference between the estimated influx of SO 4 2- into the sediment and measured areal SRR is likely due to the fact that diffusive fluxes represent net SO 4 2- consumption, while SRR are a measure of total SO 4 2- turnover [ 69 ]. Estimates based on pore water profiles of SO 4 2- may thus underestimate the actual rate of SO 4 2- reduction in marine sediments, as shown previously for example for Black Sea sediments [ 22 , 53 ].…”

“…In this mechanism, Fe oxides enhance the recycling of dissolved sulfide to SO 4 2- in a cryptic S cycle by formation and subsequent disproportionation of elemental S (S 0 ) [ 85 ]. This SO 4 2- production from re-oxidation of dissolved sulfide with oxidized Fe minerals may thus partly explain the significant SRR (~ 3–20 nmol cm -3 d -1 ) measured below the main zone of SO 4 2- reduction at our study site ( Fig 2 ), by stimulating slow rates of organoclastic SO 4 2- reduction and/or SO 4 2- -driven AOM [ 16 , 25 , 53 , 86 – 90 ].…”

“…In theory, the preservation of Fe oxides in the methanogenic sediments as a result of rapid sediment accumulation and thus reduced exposure of solid phase Fe oxides to sulfidic pore waters would allow for such a coupling between CH 4 oxidation and Fe oxide reduction in the sediments of the Scharendijke basin. Although an increasing body of geochemical evidence indicates that Fe-AOM might be occurring in a variety of different aquatic environments [ 36 , 53 , 92 – 97 ], the microbes facilitating these reactions have yet to be identified. The large multi-haem cytochromes (proteins mediating electron transport) in the genomes of one type of methanotrophic archaea known as ANME-2, however, indicate that these organisms should also be able to respire solid Fe oxides through extracellular electron transfer [ 13 – 15 , 98 , 99 ].…”

“…To gain a better quantitative understanding of the CH 4 cycling at our study site, a simplified version of a previously developed 1-dimensional reactive transport model was applied [ 35 , 53 ]. The model describes the cycling of 14 particulate and dissolved chemical species (Table B in S1 File ) in the upper 100 cm of sediment (0.5 mm vertical resolution) via a set of mass conservation equations, which include transport processes as well as biogeochemical transformations [ 43 , 54 , 55 ]: where ϕ is the sediment porosity (volume of pore water per volume of total sediment), C S the concentration of the solid species (mol L -1 ; mass per unit volume of solids), C aq the concentration of the dissolved species (mol L -1 ; mass per unit volume of pore water), t is time (yr), x the distance from the sediment-water interface (cm), D ′ the diffusion coefficients of dissolved species in the sediment (cm 2 yr -1 ) at in situ conditions and corrected for the tortuosity in the porous medium [ 56 ] (Table C in S1 File ).…”

Evidence for microbial iron reduction in the methanogenic sediments of the oligotrophic SE Mediterranean continental shelf

“…The observed intensive iron reduction in the methanogenic sediments is the first in the Southeastern Mediterranean shelf. The phenomenon of iron reduction in the methanogenic depth has been observed before in other marine provinces (Egger et al, 2016;Jorgensen et al, 2004;März et al, 2008;Riedinger et al, 2014;Slomp et al, 2013;Treude et al, 2014), however, the type of link to the methane cycle is complex. Usually, iron reduction is coupled to oxidation of organic matter (Lovley and Phillips, 1988) and is performed by iron reducing bacteria, which is probably the case in zone 1.…”