Sulfidic Pore Waters Research Articles

Rivers and tidal flats as sources of dissolved organic matter and trace metals in the German Bight (North Sea)

The German Bight in the southern North Sea receives nutrients, dissolved organic matter (DOM), and trace metals from rivers, porewater reflux, and porewater outwelling. We studied the marine, riverine, and porewater sources analyzing molecular transformations of solid-phase extracted (SPE) DOM in the German Bight. We applied a combination of ultrahigh-resolution mass spectrometry (FT-ICR-MS) with quantitative data of dissolved organic sulfur (DOS), dissolved black carbon (DBC), dissolved trace metals (Ba, Co, Gd, Mo, Mn, W), and nutrients (nitrite, nitrate, phosphate, silicate). While aromatic DOM and DBC mainly originated from the rivers, nitrogen-containing, more saturated DOM was enriched offshore suggesting greater contributions of marine (algal) sources. Except for dissolved Mo, rivers were the primary source of trace metals and nutrients. However, tidal flats contributed to dissolved nutrient (e.g., dissolved phosphate), trace metal and DOS inventories of the southern North Sea. The input of DOS from intertidal flats was identified by the molecular index derived from sulfidic porewaters (ISuP), non-conservative behavior of elemental sulfur-to-carbon ratio and sulfur content of molecular formulae (from FT-ICR-MS). Dissolved Mn and Si were removed in the German Bight, likely due to precipitation as Mn(hydr)oxides and biological uptake, respectively. Preliminary estimates suggest that DOS from porewater outwelling is approximately four times higher than DOS discharged by the three main rivers in the region. Our study therefore highlights the need to consider porewater discharge in addition to riverine sources to comprehensively assess elemental budgets within the complex interplay and transformations of DOM, nutrients, and trace metals in coastal ecosystems.

Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis

The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13Corg values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis.

No evidence for expansion of global ocean euxinia during the base Stairsian mass extinction event (Tremadocian, Early Ordovician)

A Tremadocian (Early Ordovician, base Stairsian North American Stage) mass extinction event is recorded globally in rocks from several ancient continents and is accompanied by a globally correlated positive carbon isotope excursion (CIE; the largest during the Early Ordovician). In this study, elemental concentrations and uranium isotope compositions (δ238U) were measured for carbonate samples from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin to test the role of ocean anoxia/euxinia on the base Stairsian mass extinction event. Dysoxic/suboxic to oxic water columns and non-sulfidic porewaters are inferred for these sections based on multiple local redox proxies (e.g., Mo/Al, U/Al, Mo and U concentrations, Mo/U ratios).Carbonate δ238U data exhibit different δ238U trends in the three sections. The proximal Ibex area exhibits a small negative δ238U excursion (a magnitude of 0.2–0.3‰) and the Shingle Pass section has one sample with unusually low δ238U, in both cases within the middle of the CIE. Samples with the lowest δ238U in each section also have the highest Mn/Sr ratios, suggesting diagenetic alteration associated with adsorption of isotopically light U onto Mn oxides, followed by dissolution of Mn oxides by reducing porewaters and capture of the isotopically light U by sediments. The distal Shingle Pass and Meiklejohn Peak sections better preserve global ocean signals and the limited δ238U variations in these sections imply no significant changes in global seawater δ238U and thus no significant changes in the extent of global ocean euxinia. Given the non-sulfidic porewaters inferred for these carbonates, we apply a sediment–seawater δ238U offset of 0–0.27‰ (the 0.27 ± 0.14‰ isotopic offset observed for modern Bahamas carbonates with sulfidic porewaters is used as a maximum constraint) to the average carbonate δ238U from the two distal sections (−0.48 ± 0.09‰) to derive a coeval seawater δ238U of between −0.75 ± 0.09‰ and −0.48 ± 0.09‰. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area existed during deposition of these Tremadocian carbonates. Our study implies that a small biotic crisis during a small CIE was not associated with an expansion of global ocean euxinia, in contrast to major Phanerozoic mass extinctions that were commonly associated with significant expansion of ocean euxinia during large carbon perturbations (e.g., the Late Devonian). In addition, we propose that the post-SPICE Cambrian and Early Ordovician oceans were characterized by limited changes in ocean euxinia and only a few episodes of expanded ocean suboxia-anoxia during carbon perturbations. The “biomere” events within this period, including the base Stairsian event, were probably related to these episodes of expanded suboxic-anoxic oceans and/or other ecological stresses.

Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.

Determination of Hydroxyl Radical Production from Sulfide Oxidation Relevant to Sulfidic Porewaters

Hydroxyl radical (·OH) production from the reaction between aqueous total sulfide ([H2S]T = [H2S] + [HS–] + [S2–]) and dissolved oxygen is potentially an environmentally important reaction when anoxic, sulfidic water is exposed to oxygen. Using hydroxyterephthalate (hTPA) formation from the reaction of terephthalic acid (TPA) with ·OH as a probe for ·OH production, hydrogen peroxide was verified as an essential intermediate, and the production of free ·OH, versus lower energy hydroxylating agents, was established. The optimal conditions for the quantification of ·OH production kinetics and yield were determined by varying TPA and total sulfide concentrations. An initial total sulfide concentration of 10 μM and TPA concentration of 2 mM was used to find a yield of 15 mmol of ·OH per mole [H2S]T. Additionally, a pseudo-first-order model elucidated a maximum rate of production of 1.04 (±0.05) × 10–4 moles of ·OH per hour per mole of [H2S]T. Experiments with sulfidic wetland porewaters containing up to 294 mg/L of dissolved organic carbon (DOC) revealed that [H2S]T, and not reduced DOC, was the dominant source of ·OH. A simple model considering a water containing [H2S]T, DOC, and methane exposed to a constant concentration of oxygen (∼50 μM) gave steady state values of [·OH] ranging from 5.7 × 10–19 to 3.2 × 10–18 M. The results indicate that [H2S]T should be considered a source of dark formation of ·OH in addition to ferrous iron and reduced DOC.

A new colorimetric DET technique for determining mm-resolution sulfide porewater distributions and allowing improved interpretation of iron(II) co-distributions.

Measurement of sulfide in pore waters is critical for understanding biogeochemical processes, especially within coastal sediments. Here we report the development of a new colorimetric DET (diffusive equilibration in thin films) technique for determining mm-resolution, two-dimensional sulfide distributions in sediment pore waters. This colorimetric sulfide DET method was based on the standard spectrophotometric methylene blue assay, but modified to allow quantitation of sulfide by computer imaging densitometry. The method detection and effective upper measurement limits of the optimised technique were 3.7 and 1000μmolL-1, respectively. The optimised sulfide DET method was combined with the colorimetric iron(II) DET method to obtain co-distributions in coastal seagrass (Zostera muelleri) colonised sediment under light and dark conditions. In the dark, seagrass sediments were more reduced than in the light, with large areas being dominated by high porewater sulfide concentrations. These co-distributions were compared with those obtained using the previously described DET-DGT (diffusive gradients in thin films) method for measuring iron(II) and sulfide co-distributions. There was less overlap of iron(II) and sulfide distributions using the sulfide DET as the two DET methods are influenced most by the later hours of deployment, whereas the sulfide-DGT measurement integrates concentrations over the whole deployment period. Overlap was most apparent in very dynamic sediment zones, such as burrow wall sediments.

Sources and transformations of iron in the sediments of the Gulf of Aqaba (Red Sea)

The Gulf of Aqaba is an oligotrophic marine system with an oxygen-rich water column. Aeolian dust from the Arabian, Sinai, and Sahara deserts is an important source of sedimentary material to the gulf, especially at 700 m water depth. The head of the gulf is affected by sediment transport from the Arava desert during winter flash floods. In this work, we have studied the speciation of iron in the dust, dry creek sediments and sediments of the Gulf of Aqaba at various water depths in order to understand sources and transformations of iron. Two sources of iron, dust and flash floods transported material, were found to possess distinct geochemical signatures: dust was found to be enriched in total and highly reactive iron relative to the sediments in creek beds. Combination of these two sources leads to an increase of highly reactive iron in sediments with water depth. This increase, in turn, results in formation of a lateral redox gradient with sulfidic pore-waters near the shore, and ferruginous-manganous pore-waters and cryptic sulfur cycling at the deeper water sites. Another result of dry aeolian deposition of desert dust to the sediments of the Gulf of Aqaba, overlaid by deep well‑oxygenated waters, is anomalously high ratios of highly reactive to total iron, which have been proposed to be diagnostic for anoxic, iron-rich water columns, when applied as a paleoproxy.

Diagenesis and iron paleo-redox proxies: New perspectives from magnetic and iron speciation analyses in the Santa Barbara Basin

Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O2 < 10 μmol/kg) and sporadically anoxic (no O2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fepy [pyrite Fe]/FeHR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized FeHR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a “false positive” interpretation. In comparison, redox-sensitive metal enrichments (MoEF, UEF, and ReEF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface.

Spatial Dependence of Reduced Sulfur in Everglades Dissolved Organic Matter Controlled by Sulfate Enrichment.

Sulfate inputs to the Florida Everglades stimulate sulfidic conditions in freshwater wetland sediments that affect ecological and biogeochemical processes. An unexplored implication of sulfate enrichment is alteration of the content and speciation of sulfur in dissolved organic matter (DOM), which influences the reactivity of DOM with trace metals. Here, we describe the vertical and lateral spatial dependence of sulfur chemistry in the hydrophobic organic acid fraction of DOM from unimpacted and sulfate-impacted Everglades wetlands using X-ray absorption spectroscopy and ultrahigh-resolution mass spectrometry. Spatial variation in DOM sulfur content and speciation reflects the degree of sulfate enrichment and resulting sulfide concentrations in sediment pore waters. Sulfur is incorporated into DOM predominantly as highly reduced species in sulfidic pore waters. Sulfur-enriched DOM in sediment pore waters exchanges with overlying surface waters and the sulfur likely undergoes oxidative transformations in the water column. Across all wetland sites and depths, the total sulfur content of DOM correlated with the relative abundance of highly reduced sulfur functionality. The results identify sulfate input as a primary determinant on DOM sulfur chemistry to be considered in the context of wetland restoration and sulfur and trace metal cycling.

Tungsten–molybdenum fractionation in estuarine environments

Dissolved tungsten (W) and molybdenum (Mo) concentrations were measured in surface waters and sediment pore waters of Terrebonne Bay, a shallow estuary in the Mississippi River delta, to investigate the biogeochemical processes that fractionate these Group 6 elements relative to one another during transit from weathering to sedimentary environments. Although many of the chemical properties of W and Mo are similar, the two elements behave autonomously, and the fractionation mechanisms are only partly understood. In sulfidic pore waters, dissolved Mo is depleted relative to river water–seawater mixtures, whereas dissolved W is >10-fold enriched. Reductive dissolution of poorly crystalline phases like ferrihydrite, which is a preferential host of W relative to Mo in grain coatings on river-borne particles, can explain the dissolved W enrichment. Dissolved W becomes increasingly enriched as H2S(aq) rises above about 60μM due to transformation of WO42− to thiotungstates as well as to additional reductive dissolution of phases that host W. In contrast, as rising sulfide transforms MoO42− to thiomolybdates in pore waters, dissolved Mo is suppressed, probably owing to equilibration with an Fe–Mo–S phase. This putative phase appears to control the aqueous ion product, Q=[Fe2+][MoS42−]0.6 [H2S0]0.4/[H+]0.8, at a value of 10−7.78. Concentrations of dissolved W and Mo in pore waters bear no relation to concentrations in surface waters of the same salinity. In surface waters, dissolved Mo is nearly conserved in the estuarine mixing zone. Dissolved W appears also to be conserved except for several cases where W may have been enhanced by exchange with underlying, W-rich pore waters. With increasing salinity, the molar Mo/W ratio rises from about 10 to about 1000 in surface waters whereas it is mostly <10 in underlying pore waters and in highly sulfidic pore waters is mostly near 1. Differences in two chemical properties may account for this fractionation of Mo with respect to W; MoVI is more susceptible to reduction than WVI, but the latter is more prone to adopting octahedral coordination. We propose that the first difference facilitates Mo but not W precipitation in sulfidic sediments, whereas the second explains tungsten’s proportionately greater sequestration on river-borne particles and its subsequent release to sulfidic pore waters after the particles are deposited in the delta and become subject to reductive dissolution during diagenesis.

Reassessing the role of sulfur geochemistry on arsenic speciation in reducing environments

Recent evidence suggests that the oxidation of arsenite by zero-valent sulfur (S(0)) may produce stable aqueous arsenate species under highly reducing conditions. The speciation of arsenic (As) in reducing soils, sediments and aquifers may therefore be far more complex than previously thought. We illustrate this by presenting updated E h –pH diagrams of As speciation in sulfidic waters that include the most recently reported formation constants for sulfide complexes of As(III) and As(V). The results show that the stability fields of As(III) and As(V) (oxy)thioanions cover a large pH range, from pH 5 to 10. In particular, As(V)–S(–II) complexes significantly enhance the predicted solubility of As under reducing conditions. Equilibrium calculations further show that, under conditions representative of sulfidic pore waters and in the presence of solid-phase elemental sulfur, the S 0 (aq)/HS − couple yields a redox potential ( E h ) ∼ 0.1 V higher than the SO 4 2−/HS − couple. S(0) may thus help stabilize aqueous As(V) not only by providing an electron acceptor for As(III) but also by contributing to a more oxidizing redox state.

Sequestration mechanisms and anthropogenic inputs of rhenium in sediments from Eastern Canada lakes

The concentrations of Re, as well as those of several other geochemical variables, were measured in dated sediment cores and in porewater samples from four lacustrine basins in Eastern Canada: one, perennially oxic, located 40 km from Québec City and three, seasonally anoxic, located within 25 km of non-ferrous metal smelters. The drainage basins of these lakes are uninhabited and have not been affected by human activity or wildfires. All of the depth profiles of dissolved Re indicate: higher Re concentrations in the water overlying the sediment than in the porewater; diffusion of Re across the sediment–water interface; a progressive decrease in porewater Re concentrations to reach minimum values of ∼0.5 pM within a 10-cm sediment depth interval. Modeling of these Re porewater profiles with a one-dimensional transport-reaction equation indicates that Re is removed from porewater within this depth interval. Based on thermodynamic predictions of Re speciation and of saturation states and on comparison of these predictions with sulfide porewater profiles, we infer that Re is removed from porewater by precipitation of rheniite (ReS 2(s)). The rate constant for the formation of ReS 2(s) in sediments is estimated from the modeling exercise to be 0.51 ± 0.64 × 10 −21 mol cm −3 s −1. Accumulation of sedimentary Re shows a strong authigenic component, as in anoxic marine sediments. Sharp increases in solid-phase Re during the last century are attributed to atmospheric deposition of anthropogenic Re deriving from coal burning and nearby smelter emissions.

Bottom water anoxia, inoceramid colonization, and benthopelagic coupling during black shale deposition on Demerara Rise (Late Cretaceous western tropical North Atlantic)

The bulk rock geochemistry and inoceramid isotopic composition from Cenomanian to Santonian, finely laminated, organic‐rich black shales, recovered during Ocean Drilling Program Leg 207 on Demerara Rise (western tropical North Atlantic), suggest persistent anoxic (free H2S) conditions within the sediments and short‐term variations within a narrow range of anoxic to episodically dysoxic bottom waters over a ∼15 Ma time interval. In addition to being organic‐rich, the 50–90 m thick sections examined exhibit substantial bulk rock enrichments of Si, P, Ba, Cu, Mo, Ni, and Zn relative to World Average Shale. These observations point to high organic burial fluxes, likely driven by high primary production rates, which led to the establishment of intensely sulfidic pore waters and possibly bottom waters, as well as to the enrichments of Cr, Mo, U, and V in the sediments. At the same time, the irregular presence of benthic inoceramids and foraminifera in this facies demonstrates that the benthic environment could not have been continuously anoxic. The δ13C and δ15N values of the inoceramid shell organics provide no evidence of chemosymbiosis and are consistent with pelagic rain as being a significant food source. Demerara Rise inoceramids also exhibit well‐defined, regularly spaced growth lines that are tracked by δ13C and δ18O variations in shell carbonate that cannot be simply explained by diagenesis. Instead, productivity variations in surface waters may have paced the growth of the shells during brief oxygenation events suitable for benthic inoceramid settlement. These inferences imply tight benthopelagic coupling and more dynamic benthic conditions than generally portrayed during black shale deposition. By invoking different temporal scales for geochemical and paleontological data, this study resolves recent contradictory conclusions (e.g., sulfidic sedimentary conditions versus dysoxic to suboxic benthic waters) drawn from studies of either sediment geochemistry or fossil distributions alone on Demerara Rise. This variability may be relevant for discussions of black shales in general.

The relationship between seagrass ( Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments

In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H 2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6 μM) to those growing over higher sulfide conditions (33.5 μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10 μmols H 2S L −1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea.

Sulfide Controls on Mercury Speciation and Bioavailability to Methylating Bacteria in Sediment Pore Waters

A chemical equilibrium model for Hg complexation in sediments with sulfidic pore waters is presented. The purpose of the model was to explain observed relationships between pore water sulfide, dissolved inorganic Hg (HgD), and bulk methylmercury (MeHg) in surficial sediments of two biogeochemically different ecosystems, the Florida Everglades and Patuxent River, MD. The model was constructed to test the hypothesis that the availability of Hg for methylation in sediments is a function of the concentration of neutral dissolved Hg complexes rather than Hg2+ or total HgD. The model included interaction of mercury with solids containing one or two sulfide groups, and it was able to reproduce observed HgD and bulk MeHg trends in the two ecosystems. The model is consistent with HgS0 as the dominant neutral Hg complex and the form of Hg accumulated by methylating bacteria in sulfidic pore waters. The model-estimated decline in HgS0 with increasing sulfide was consistent with the observed decline in bulk sediments MeHg. Since bacterial Hg uptake rate is one of the factors affecting methylation rate, Hg complexation models such as the one presented are helpful in understanding the factors that control MeHg production and accumulation in aquatic ecosystems.