Sediment Porewater Sulfide Voltammetry and H2S–HS Speciation

Why a sulfide peak is not one simple chemical

An ocean sediment profile can show dissolved sulfide appearing sharply below an oxygenated surface. What does that plotted peak actually mean? Sulfide is both a chemical family and an operational analytical measurement. Depending on pH and the sensor method, the relevant dissolved forms include molecular hydrogen sulfide, H₂S, and hydrosulfide, HS⁻. A voltammetric microelectrode provides a current response connected to a calibrated electrochemical process. It does not simply count all sulfur atoms in the sediment. The reader first needs the measured fraction, the depth coordinate, the concentration unit and the method’s selectivity.

Sediment porewater is the water occupying spaces between grains. Sulfide can be produced in anoxic zones, consumed by oxidation, bound with metals or transported by diffusion and burrow irrigation. Therefore a high measured concentration is not equivalent to a high instantaneous production rate. A steep vertical gradient does not prove that one particular microbial species produced the sulfide. Our task is to trace the evidence from the sensor response through acid–base chemistry and sample conditions to a defensible description of porewater chemistry, without turning a conditional measurement into an unsupported environmental verdict.

Which dissolved sulfur forms are in the denominator?

Hydrogen sulfide acts as a weak acid in water: H₂S can dissociate into HS⁻ and a hydrogen ion. A further dissociation to S²⁻ is possible in chemical notation, but the free divalent sulfide ion generally contributes little under common marine porewater pH conditions. This distinction is especially important because the phrase total dissolved sulfide may refer to H₂S plus HS⁻ under a particular operational determination, not total reduced sulfur in every solid and dissolved compound. Laboratory preservation and analytical definitions can further alter which fraction was actually quantified.

Read the methodological definition before interpreting the graph’s axis. If the instrument or derivation reports HS⁻, do not silently rename it H₂S. If the reported total was calculated from one species using pH, include the pH measurement and the chosen acid dissociation constant. If a sample contains polysulfides, thiosulfate or dissolved metal–sulfide complexes, these may require separate analytical consideration. Sulfur chemistry is not adequately captured by treating every compound containing sulfur as one interchangeable concentration. This is the first safeguard against a fluent but chemically wrong translation.

An equation for the molecular and ionic fractions

For a simplified two-species equilibrium with dissolved H₂S and HS⁻, the ratio [HS⁻]/[H₂S] is approximately 10 raised to the power pH minus pKa₁ when the activity-based assumptions are appropriately handled. The molecular H₂S fraction is therefore approximately 1 divided by one plus 10^(pH−pKa₁), while the HS⁻ fraction is the remainder. This calculation describes speciation within the defined dissolved-sulfide pool. It does not indicate how rapidly sulfide is being produced or whether the species are in equilibrium at every microscopic location.

The effective acid dissociation value depends on temperature, salinity, pressure and the concentration or activity convention in use. A pKa adopted from a freshwater table cannot automatically be transferred to saline porewater without justification. In a teaching example, we can explicitly assume pKa₁ equals 7.0 to show the mathematics. For real sediment data, use a value and convention validated for the sampled conditions. A difference of a few tenths of a pH unit can materially change the calculated molecular fraction, particularly near the dissociation transition.

Worked speciation calculation at pH 7.5

Assume the simplified apparent pKa₁ of 7.0 and a porewater pH of 7.5. The ratio of HS⁻ to H₂S is 10^0.5, approximately 3.162. The fraction as molecular H₂S becomes 1 divided by 4.162, approximately 0.240, and the HS⁻ fraction approximately 0.760. If the analytically relevant dissolved pool contains 100 micromoles per litre in total, this model gives about 24 micromoles per litre of H₂S and 76 micromoles per litre of HS⁻. The percentages refer to that pool, not to all sulfur in the sediment.

This example illustrates a chemical distinction rather than a reported marine measurement. Real seawater conditions require the appropriate apparent equilibrium convention and may have interacting complexes or nonideal behavior. A researcher should state whether pH is on the total, seawater or another scale, rather than assuming all reported values are numerically interchangeable. A translator who retains the total concentration but drops the pH assumption can make a reported molecular H₂S value appear universal when it is conditional. The remedy is to preserve both the measured quantity and the equilibrium model used for conversion.

A second pH example shows nonlinear sensitivity

Keep the same simplified pKa₁ of 7.0 but change pH to 8.0. The ratio [HS⁻]/[H₂S] becomes 10, so the molecular H₂S fraction falls to 1/11, about 0.091. With a hypothetical total dissolved sulfide of 100 micromoles per litre, the corresponding molecular portion is roughly 9.1 micromoles per litre. The total is unchanged even though the fraction as H₂S has dropped greatly. This is why comparing molecular sulfide across sediment zones requires pH as well as total sulfide concentration.

An analyst must not interpret this calculation as evidence that raising pH necessarily removes sulfide from porewater. It changes the equilibrium partition between forms under the stated model, not the total sulfur inventory by itself. It may also influence adsorption, mineral equilibria and biological processes in actual sediments, which require separate evidence. If a report switches between H₂S and total dissolved sulfide axes, mark the switch and recalculate carefully. Ambiguous graph labels can create an apparent gradient that is really a change in species definition.

The sensor’s electrochemical response comes first

A gold-amalgam or another validated voltammetric microelectrode can be used to examine chemically responsive species in small sediment porewater volumes. The method applies a programmed electrode potential and records current associated with redox reactions or stripping processes. Potential positions and characteristic features can help distinguish electroactive species, while calibration links response magnitude to a specified chemical concentration. The original microelectrode work of Brendel and Luther provided a foundation for resolving dissolved O₂, Fe, Mn and sulfide species in sediment environments. Such instruments are especially useful where sampling a large volume would blur narrow gradients.

Nevertheless, a voltammetric peak is an electrical response rather than a chemical label written by the electrode. It can be influenced by electrode surface state, background current, matrix composition, other electroactive species and the method’s scan sequence. A peak’s position alone need not prove identity if alternative reactions are possible. Before accepting a claim, check the reference electrode, potential scale, concentration standards and the published diagnostic assignments. Translation should preserve the distinction between a measured current, an identified electrochemical feature and a calibrated species concentration.

Why gold amalgam and electrode history matter

The gold–mercury surface used in established gold-amalgam microelectrodes is part of the measurement chemistry. It provides electrochemical response characteristics suitable for several redox species under defined conditions. The film or surface can change during fabrication, conditioning, repeated scanning and ageing. Research on gold-amalgam sulfide sensors in artificial wastewater has documented changes in electrode structure and stripping response over time. Thus the electrode’s sensitivity should be treated as a measured property of its present state, not a permanent manufacturer’s constant.

If two depth profiles were recorded with different probes, their currents cannot be compared directly as sulfide concentrations unless each probe’s calibration and response were established. Even one probe may need checks before and after a series of scans. A shift in the baseline or a changed peak shape can indicate fouling or instrumental drift rather than new porewater chemistry. A reliable report includes relevant conditioning, electrode history and calibration evidence, especially when interpreting small differences between nearby depth horizons. Treating the electrode as invisible removes one of the largest sources of analytical uncertainty.

Calibration converts current into concentration

Suppose a method establishes a linear calibration over an appropriate range: peak current, after baseline correction, equals a slope multiplied by concentration plus an intercept. The calibrated concentration is the observed current minus the intercept, divided by the slope, with units retained. If current is recorded in nanoamperes and slope in nanoamperes per micromole per litre, the resulting concentration is micromoles per litre. The relation is valid only within the tested range and chemical conditions. A nonzero intercept may reflect systematic background and cannot simply be ignored.

An illustrative calibration has current 0.25 nanoamperes plus 0.040 nanoamperes per micromole per litre times concentration. A sample gives 2.25 nanoamperes. Subtract 0.25 to obtain 2.00 nanoamperes; divide by 0.040 to obtain 50 micromoles per litre. This is arithmetic, not a universal sulfide sensitivity. For a real electrode, calibration slope and shape can depend on conditioning, ionic medium, temperature, potential program and concentration. Never apply this teaching slope to field data without the instrument’s own validated calibration.

The detection threshold is a property of the method

When a sensor gives no reliable sulfide peak, the result may be below its limit of detection rather than chemically absent. A detection threshold depends on baseline noise, calibration slope, sample matrix and the decision rule adopted by the method. A stronger peak at one depth and a nondetect above it may establish a resolved onset within the sampled depth support, but it does not prove the upper layer contains zero sulfide molecules. Porewater can also contain very small or transient concentrations that a sampling sequence cannot resolve.

A report may express the detection threshold in molar concentration, mass per litre or raw current. Convert to a common chemical and unit basis before comparing instruments. Detection limits from an artificial wastewater validation are not automatically transferable to marine sediment, which can contain different ionic matrices and electrode-fouling compounds. If the reported concentration is near the threshold, do not display many decimal places simply because the spreadsheet produced them. Quantitative precision should reflect calibration, blank variability and response stability, not the convenience of a numerical output format.

Why depth resolution can be misleading

Microelectrodes can interrogate porewater at small spatial increments without collecting a large bottle sample, but the effective measurement volume is still finite. Probe tip size, insertion path, sediment structure and the time needed for a stable response can all affect the position and meaning of a reading. A profile measured every millimetre does not imply the chemistry is known at every microscopic position between observations. Smooth interpolation may help visualise the trend but cannot establish submillimetre peaks without supporting data.

An animal burrow can carry oxygenated water below the nominally anoxic interface, while sulfide-rich water may persist only a short distance away. A single vertical profile can therefore differ markedly from another profile separated by a few centimetres laterally. This is not necessarily analytical failure. The seafloor is chemically heterogeneous. State the number of independent profiles, lateral spacing and relation to visible burrows when drawing regional conclusions. More depth steps along one probe do not constitute more independent sediment cores.

Oxygen and sulfide are related but not exact mirror images

At an oxygenated sediment surface, molecular oxygen may be consumed while reduced sulfur is oxidised, but the exact location of their measured profiles can differ. Sulfide can react with iron, manganese oxides and other oxidants, or migrate along microchannels. Oxygen detection and sulfide nondetection at the same depth do not prove that those two substances reacted directly with one another there. Likewise, their apparent separation can reflect other redox intermediates and sampling support. Thermodynamic plausibility is valuable but is not a substitute for kinetic and transport evidence.

Some studies measure oxygen, manganese, iron, iodide and reduced sulfur with related microelectrode approaches, as in the marine porewater work of Luther and colleagues. Combined profiles strengthen interpretation because a redox sequence can be examined rather than guessed. However, a vertical ordering of species does not uniquely establish rates of individual microbial processes. When describing a sediment zone, distinguish observed concentrations from hypothesised oxidation pathways and from measured rates. This is especially important when translating technical terms such as sulfide oxidation, sulfate reduction and iron sulfide precipitation.

A steep gradient is not automatically a flux

Concentration changes with depth can support a diffusive-flux estimate if the relevant diffusion coefficient, coordinate direction, porosity or tortuosity treatment and boundary conditions are established. The simplest water-side Fick’s-law relation is flux equals minus the diffusion coefficient times the concentration gradient. In porous sediment, an effective sediment diffusion treatment and the appropriate bulk-area convention may be needed. A steep plotted profile does not, by itself, supply the diffusivity or demonstrate that steady diffusion is the only transport process.

Animals can irrigate burrows, tides can drive porewater advection, and chemical reactions can consume sulfide as it diffuses. These processes complicate the interpretation of a gradient. If the paper reports an upward flux, check the sign convention for depth: some plots use depth increasing downward and others height above bed increasing upward. An apparent negative value may indicate direction, not negative chemistry. Use units such as millimoles per square metre per day for an areal flux, and retain the distinction from micromoles per litre for a concentration.

Worked flux arithmetic with declared assumptions

Consider a hypothetical water-side boundary layer with a concentration increase of 100 micromoles per litre over 0.10 centimetres in the downward direction. Express 100 micromoles per litre as 0.10 micromoles per cubic centimetre. The downward gradient is then 1.0 micromole per cubic centimetre per centimetre. If the appropriate molecular diffusion coefficient is assumed to be 1.0 × 10⁻⁵ square centimetres per second, Fick’s relation gives a diffusive flux of minus 1.0 × 10⁻⁵ micromoles per square centimetre per second, meaning upward relative to the downward coordinate.

Convert the magnitude by multiplying by 86,400 seconds per day and 10,000 square centimetres per square metre. The result is 8,640 micromoles per square metre per day, or 8.64 millimoles per square metre per day. This is a deliberately idealised numerical example, not a measured seabed flux. Real interpretation requires temperature- and medium-appropriate diffusion, consideration of multiple sulfide species, defined boundary location and possible reaction during transport. The worked example shows exactly where units and sign enter so the result can be independently checked.

Porewater sulfide can disappear through metal reactions

Reduced sulfur can precipitate with dissolved iron or react with solid iron minerals, altering both the free dissolved fraction and the form detectable by a particular electrode. Such reactions can create sulfide minima or sharp fronts even where microbial sulfate reduction continues. A low dissolved sulfide concentration therefore does not necessarily mean no sulfate is being reduced. The chemical balance includes production, transport, oxidation and mineral formation. Interpreting one concentration profile as a direct production-rate profile omits most of the relevant terms.

A useful paired approach is to examine sulfide alongside dissolved iron, manganese, pH and sulfate, while considering the presence of reactive solid phases. A ferruginous porewater horizon may have rapid sulfate reduction but little free sulfide because newly formed sulfur is sequestered. Conversely, a strong sulfide signal may indicate that dissolved production exceeds local sinks or that sulfide arrives from elsewhere. These are testable mechanisms, not observations supplied automatically by the sulfide curve. Every causal conclusion should identify the measurements that make it more than a guess.

Calibration solutions must not silently change species

A sulfide calibration standard has a total chemical inventory and a pH-dependent distribution of dissolved forms. If the sensor response is described as HS⁻ under one pH condition, a standard prepared at another pH may not have the same relation between current and the requested species. A careful protocol controls oxygen exposure and the relevant ionic medium and reports whether calibration is assigned to total dissolved S(−II), a specific form, or a method-defined sulfide response. Without that definition, converting a calibration slope into a field concentration risks a category error.

Stock sulfide solutions are chemically reactive and must be handled according to an established laboratory method, not by a casual recipe borrowed from a plot caption. The reader needs the standard preparation and verification record, rather than instructions to generate a toxic gas. For interpretation, check preparation date, standard concentration verification, pH, dilution chain and blank response. If the authors used calibration in a saline matrix, preserve that fact. A response validated in an artificial wastewater matrix does not automatically prove identical accuracy in oxygen-poor marine porewater.

Interference is not solved by a plausible peak label

Other electroactive constituents may affect a sulfide-associated current by overlapping features, shifting electrochemical behaviour or modifying the electrode surface. Iron species, iodine chemistry and partially oxidised sulfur compounds occur in some marine systems and can complicate assignment. Voltammetric studies have used multiple peak features and standards to distinguish targets, but interpretation remains conditional on the specific potential program and reference scale. A result should say which peak was used, how it was assigned and what evidence rules out important alternatives.

Consider a depth horizon with a larger apparent sulfide signal than its neighbours, but also a striking change in dissolved iron. The signal may be real; it may also require a check of calibration in that matrix and possible iron-sulfide interactions. Do not erase the feature because it is inconvenient, and do not declare it biological without supporting tests. Examine peak shape, scan repeatability and related species, then retain a quality-control flag if interpretation is uncertain. The measured curve can be valuable even when one chemical attribution remains provisional.

The electrode tip changes the sediment being measured

Microelectrode insertion is less disruptive than extracting a bottle of porewater, yet it is not noninvasive in the literal sense. A probe displaces a finite volume of sediment and may leave a track that temporarily changes local porewater exchange. Soft mud can compress or crack around an instrument, and a small burrow can be intersected accidentally. The position of the tip relative to a thin chemical boundary should therefore be recorded with a realistic uncertainty. A nominal half-millimetre step does not guarantee an undisturbed half-millimetre chemical sample.

Repeated penetration along nearly the same track may alter the local profile. That makes the acquisition order and any waiting period important. When profiles are compared, an observer should ask whether the same probe was reinserted after withdrawing, whether the core was allowed to stabilise and whether probe response was tested between sequences. A changing trace may be the environment evolving, the electrode changing, or both. One cannot distinguish those explanations by plotting depth and current without a protocol for drift and disturbance.

A steep front can move in time

Sediment redox fronts are dynamic. Small changes in overlying-water oxygen, temperature, biological pumping or organic matter supply may move the zone where sulfide first becomes detectable. If a profile is taken at noon and another after a tidal reversal, their apparent difference need not be a permanent vertical migration of a contamination plume. The useful report includes sampling time, core history, interface reference and environmental conditions. Where gradients change rapidly, simultaneous or temporally matched measurements provide a stronger basis for mechanistic comparison.

An apparent movement of two millimetres may also result from choosing a different threshold for first detection or from uncertainty in the interface location. A careful comparison can plot the actual concentration curves, analytical thresholds and coordinate uncertainty rather than just one transition-depth number. If the change is comparable to the resolution or detection limit, phrase it as uncertain rather than claiming a statistically resolved shift. This keeps a compelling environmental interpretation from outrunning the data it is supposed to explain.

What an oxygen–sulfide gap can and cannot show

Some sediment profiles show oxygen declining to near its detection threshold before dissolved sulfide rises above its own detection threshold. That interval may contain oxidation by other electron acceptors, metal reactions, bioirrigation or sulfur intermediates. Its width also depends on the thresholds and the exact locations and times sampled by each probe. Describing it as an oxygen-free zone is stronger than the sensors necessarily justify unless oxygen nondetection is interpreted with the detection limit. Describing it as a zone with no sulfur cycling is still less defensible.

A better method describes the measured oxygen and sulfide boundaries independently, including the rule used to identify each. Iron and manganese redox profiles or microscopy can then constrain possible reactions. The 1998 marine porewater microelectrode research demonstrated the usefulness of measuring several redox-sensitive species together. That does not turn every observed sequence into proof of a complete thermodynamic equilibrium. Organic substrates, reaction kinetics and localized biological activity can create deviations from a simple textbook ordering.

Why sulfide oxidation can hide sulfide production

When sulfate-reducing microbes generate sulfide below the surface, upward diffusion may carry some toward oxidants. At a narrow reaction zone sulfide can be consumed before it reaches oxygenated bottom water. A concentration maximum in porewater reflects the balance of several processes, not merely the local production term. A low concentration immediately above an active sediment layer may therefore coexist with substantial gross sulfate reduction below. This is why ³⁵S rate measurements and dissolved sulfide microprofiles can complement one another without being identical.

If a paper reports strong sulfate reduction alongside little measurable dissolved sulfide, the first question is where the newly reduced sulfur goes. Iron sulfide formation, oxidative recycling and transport may be important. The required investigation uses additional chemical evidence, not a rhetorical assertion that one measurement invalidates another. In a useful teaching explanation, write a balance: change in dissolved sulfide equals sources minus sinks plus net transport. Every term can vary with depth and time, and the electrode directly observes only a part of that system.

Dissolved sulfide is not all reduced inorganic sulfur

A common sample report separates free dissolved sulfide from acid-volatile sulfide, chromium-reducible sulfur or other solid-phase fractions. Those operational fractions have different extraction chemistries and do not measure the same material. A microelectrode cannot be said to quantify all sediment sulfur simply because it senses an S(−II)-related response. Some reduced sulfur is trapped in particles or present as intermediates that may require different techniques. Combining fractions needs a clear chemical definition and compatible units to avoid double counting.

The same distinction matters when comparing two nearby articles. A ³⁵S radiotracer experiment may estimate sulfate-reduction turnover from labelled reduced inorganic sulfur products, whereas a voltammetric microelectrode estimates concentrations of selected redox species. The two numbers can be connected through chemical kinetics and transport, but not by changing the axis label. Preserving the difference between a rate and a concentration is a basic discipline in translating environmental science. It protects readers who might otherwise mistake a large concentration for fast metabolism.

Molar units, sulfur mass units and molecules

A concentration of 100 micromoles per litre of dissolved sulfide corresponds to 0.100 millimoles per litre. If expressed as elemental sulfur mass, use the approximate sulfur molar mass 32.06 grams per mole. Multiplying 0.000100 mole per litre by 32.06 gives 0.003206 gram per litre, or 3.206 milligrams of sulfur per litre. This mass-as-sulfur value is not numerically identical to a mass stated as molecular H₂S, whose molar mass includes hydrogen. The reporting basis must accompany any converted mass value.

Some environmental documents use mg S/L, while others use mg H₂S/L or micromoles per litre. They can refer to related inventories yet appear numerically different due to the chosen mass basis. Do not convert a graph to milligrams per litre without checking the substance represented by the numerator. Likewise, a sulfide concentration per porewater volume should not be treated as mass per gram dry sediment without an explicit porewater-volume and dry-density conversion. The cleanest comparison retains the original units and adds a checked equivalence when genuinely useful.

A worked instrument-calibration check

Imagine a validated linear electrode response within a certain concentration interval, with a slope of 0.050 nanoampere per micromole per litre and intercept 0.20 nanoampere. A porewater scan yields 3.20 nanoamperes on the specified feature. After subtracting the intercept, 3.00 nanoamperes remain; dividing by 0.050 gives a modelled concentration of 60 micromoles per litre. The result is conditional on the calibration representing the same operational species and not requiring another matrix or drift correction.

Now suppose an instrument’s zero-check shows the intercept shifted to 0.50 nanoampere while the slope remains supported. The same measured 3.20 nanoamperes would imply 54 micromoles per litre, not 60. That illustrates why a small electrical baseline change can affect the chemical answer. It does not establish whether the environmental concentration moved. A good field report states when the calibration was checked and whether a retrospective drift correction was allowed by protocol. Never choose whichever intercept makes a preferred trend look smoother.

An uncertainty budget without invented precision

Several uncertainty components enter a calculated sulfide concentration: the calibration fit, blank and baseline variability, electrode reproducibility, temperature effects, potential interference and the sample’s actual pH if a speciation transformation is required. Some components can be propagated numerically, while others represent model choices or potential bias. For a ratio of ion and molecule calculated from pH, uncertainty in pKa and pH can become particularly important near the transition region. A precise current measurement does not guarantee precise chemical speciation.

A useful report may give a central concentration with an interval and state the confidence or coverage convention. If the electrode result is 55 micromoles per litre with an estimated uncertainty of several micromoles, reporting 55.000 suggests more certainty than the method supports. A plot with many decimal places is not an uncertainty analysis. Compare values by their credible separation relative to uncertainty and spatial heterogeneity. Where matrix interference has not been tested, acknowledge that an exact numerical uncertainty calculation may understate the real model uncertainty.

Independent problem: identify the measured chemical

Here is a reader test. A caption says the electrode measured HS⁻ at pH 7.7, but the accompanying discussion calls that number the total mass of reduced sulfur in sediment. Is the summary faithful? No. An HS⁻-related response is one chemical measurement under a defined method; total sediment sulfur includes other dissolved and particulate forms. Even a calculated H₂S-plus-HS⁻ pool is not total sulfur in the core. The correct repair is to retain the operational electrode quantity and add any explicitly justified equilibrium conversion.

The next part asks whether the pH alone identifies the total amount of sulfide. No. The pH helps determine the relative distribution of acid–base forms under a chosen equilibrium model, but at least one concentration or total inventory is needed to set the scale. An assumption such as pKa₁ equals 7.0 must be declared and matched to the intended chemical environment. This simple distinction prevents a common explanatory leap: interpreting chemical proportions as if they were absolute concentrations.

Independent problem: interpret a nondetect above a strong peak

Suppose an upper sediment interval is below a sulfide sensor’s reporting threshold of 2 micromoles per litre and a lower interval measures 80 micromoles per litre. Can the upper value be written as exactly zero and the vertical change as precisely 80? Not without qualification. The upper concentration is constrained by a limit, not known to be zero. Under the stated threshold, the contrast may be at least approximately 78 micromoles per litre if other uncertainties are negligible, but exact difference and gradient require more information.

A responsible answer reports the lower concentration with its uncertainty and upper interval as below the reporting limit, while stating the depth spacing and species basis. If the lower value was calibrated as total dissolved S(−II) and the upper limit is for HS⁻, the comparison first needs a compatible speciation convention. A difference between mismatched species is not a physical concentration gradient. When flux is later estimated, the consequences of censoring and pH-dependent composition must be considered.

Independent problem: direction and units of a gradient

A hypothetical sediment profile has 20 micromoles per litre sulfide at depth one centimetre and 120 micromoles per litre at depth two centimetres. The concentration increases by 100 micromoles per litre over one centimetre downward, a positive downward gradient. Under a simple constant-diffusivity convention with depth positive downward, Fickian diffusion is directed upward. This direction can be identified before assigning any diffusion coefficient. The measured gradient alone still cannot establish the magnitude of an actual sediment-water interface flux.

Why not? Because the chemical is reactive, the porewater geometry affects effective transport, and the relevant gradient at the interface may differ from that between two interior depth points. Animals or currents may add non-diffusive exchange. A correct answer states the mathematical direction under the declared model but does not present the interior difference as a directly observed net emission to seawater. The sign convention must be consistent with the depth axis and with the reported areal-flux units.

Frequently asked questions about sediment sulfide sensors

Does a sulfide peak prove that a sediment layer is polluted? Not by itself; natural sulfate reduction can create sulfide, and pollution assessment needs context. Is H₂S the same as HS⁻? No; they interconvert through acid–base equilibrium. Can a gold-amalgam microelectrode detect every sulfur compound? No; its response depends on the species and voltammetric method. Is a nondetect zero? No. Can a sulfide concentration be substituted for a sulfate-reduction rate? No. Does a sharp gradient directly measure flux? Only with a justified transport model and appropriate supporting parameters.

Why may two nearby profiles differ? Sediment heterogeneity, animal burrows, redox fronts, timing and analytical perturbation can all matter. Does a high current always mean high sulfide? Not before chemical assignment and calibration. Can pH alone reveal sulfide toxicity? No; exposure, speciation, temperature and biological context are needed. Why mention calibration drift? Because a moving electrical baseline can masquerade as changing chemistry. Keeping these answers separate makes a technically dense graph easier to read without compromising what the instrument actually established.

A practical reading sequence for the whole figure

First identify the voltammetric feature and calibrated species, not just the label sulfide. Next check the electrolyte medium, electrode conditioning, reference scale, calibration span and reporting threshold. Establish the pH value and equilibrium convention if the report converts between H₂S, HS⁻ and total dissolved sulfide. Locate the sediment-water interface and the depth interval represented by each measurement. Examine other redox species and independent profiles to assess spatial variability. Only then attempt a mechanistic interpretation or use a defined diffusion model to estimate transport.

A strong plain-language conclusion might say that a calibrated microelectrode resolved increasing operational dissolved sulfide within a specified porewater depth range, with speciation dependent on measured pH and with possible chemical consumption near the oxidised boundary. Whether the signal reflects local microbial production, metal reactions or transport requires additional evidence. That wording is not evasive: it identifies the observation, the mathematical transformation and the remaining causal alternatives. The goal is to preserve exactly what the science has measured so a new reader can check its meaning.