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

Abstract

Abstract. 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 porewater 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. In line with previous studies in the Black Sea, we 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 porewater. Below the sulfidization front, high concentrations of dissolved ferrous Fe (Fe2+) lead to sequestration of downward-diffusing phosphate as authigenic vivianite, resulting in a transient accumulation of total P directly below the sulfidization front.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−3 d−1) in the sulfate-depleted limnic deposits. However, this process is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that besides organoclastic Fe oxide reduction and reactivation of less reactive Fe oxides by methanogens, AOM coupled to the reduction of Fe oxides may also provide a possible mechanism for the high concentrations of Fe2+ in the porewater 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 Fe2+ that drives the sequestration of P in vivianite below the sulfidization front.

Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

“…The deep basin of the Black Sea is characterized by lower sedimentation rates and higher bottom-water sulfide concentrations than the Bornholm Basin and, as a consequence, sulfide is diffusing into the lake sediments, promoting a downwardmigrating iron(II)-phosphate front. As has already been discussed in detail by Egger et al (2016), the vivianite front in the Black Sea has already migrated more than 2 m below its initial location at the lake-marine transition.…”

“…The formation of vivianite acts as a major sink for PO 4 in the lake sediments, whereas the trend in porewater Fe 2+ is affected mostly by upward diffusion and, for the Black Sea, also by removal with sulfide. The vivianite diagenesis in deep basin sediments of the Black Sea is discussed in detail by Egger et al (2016). Note the difference in sediment depth between the Bornholm Basin and the Black Sea sediment record.…”

“…More details on the porewater procedures can be found in Andrén et al (2015). The methods used to analyze porewaters from the multicore are described in Egger et al (2016).…”

Rapid Sediment Accumulation Results in High Methane Effluxes from Coastal Sediments

“…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).…”

Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

“…In contrast, Black Sea lake sediments that are located just below the Holocene marine deposits are high in porewater SO 4 2− and lack Fe 2+ . As a consequence, vivianite precipitates only in the deeper sediments far (∼2 m) below the lake-marine transition where downward diffusing SO 4 2− is fully consumed by anaerobic oxidation of CH 4 and organoclastic SO 4 2− reduction (Egger et al 2016). This illustrates that Fe, S, and CH 4 dynamics in non-steadystate regimes act as an important control on postdepositional P diagenesis.…”