Wetland sulfur isotope signals and dynamic isotope baselines: implications for archaeological research

Sulfur isotopic composition (δ³⁴S) is used in archaeological research to reconstruct past mobility patterns and diet using environmental baselines. Human and faunal collagen δ³⁴S is generally interpreted as reflecting environmental baselines derived from geological sulfur and marine sulfur near the coast. However, recent studies have highlighted that the δ³⁴S of bioavailable sulfur is modified in wetlands and waterlogged environments as a result of microbial sulfate reduction and sulfide oxidation. These processes are driven by fluctuating redox conditions resulting from flooding, and can significantly alter bioavailable δ³⁴S baselines in the biosphere, and subsequently in animal tissues, through the food chain. This challenges conventional approaches that assume local geology and coastal proximity alone determine bioavailable baseline δ³⁴S. This study considers sulfur cycling within terrestrial ecosystems and its implications for archaeological interpretation. We explore how plant δ³⁴S is influenced by anoxic soil conditions, hydrological controls and plant waterlogging adaptations, integrating these factors into a theoretical framework and simple numerical models of plant δ³⁴S in wetland and waterlogged environments. The models simulate the effect of different hydrological regimes on plant δ³⁴S (in flow-through, standing water, drained post-flooding, and simultaneously flooded and drained landscapes). We suggest how δ³⁴S could change for crop plants grown using different past water management techniques, and discuss effects of wetland utilization and resource use on animal and human collagen δ³⁴S. This study demonstrates that different systems can result in multiple “wetland” sulfur isotope signals that can deviate both positively and negatively from local baselines that are driven by geological and marine sulfur. δ³⁴S has the potential to be a useful tool for exploring ancient wetland use and water management strategies. Such strategies have long been argued to be critical to early agricultural development and the establishment of complex societies.

1 Introduction

Sulfur stable isotopic composition has been used in archaeological research to investigate dietary resource use and mobility of people and animals in the past (e.g., Czére et al., 2022; Ebert et al., 2021; Paladin et al., 2020). Recent developments in analytical techniques for δ³⁴S have significantly reduced the amount of sample material needed for analysis (Sayle et al., 2019). This has led to an increasingly large number of publications presenting δ³⁴S of bioarchaeological material, mostly faunal and human bone collagen, and often including complementary information from carbon, nitrogen, oxygen and strontium isotopes. Such research has been built upon the assumption that human and animal bone and dentine collagen δ³⁴S reflects the isotopic composition of bioavailable sulfur at the base of the food chain (plant δ³⁴S) driven by the local geology. This is because the minimal trophic enrichment during plant and animal uptake of sulfur means that the environmental signals are largely conserved (Krajcarz et al., 2019; Nehlich, 2015; Raoult et al., 2024).

In terrestrial environments underlying bedrock provides sulfur-bearing parent material, while lithology exerts physical and chemical controls which affect the movement of sulfur into soil and on to plants, often via interactions with groundwater and microbiology. Proximity to the ocean (due to sea spray) can mix a marine signal with the bioavailable sulfur in soil derived from the geology (Bataille et al., 2020; Guiry and Szpak, 2020). Estimates of geological baselines of bioavailable sulfur δ³⁴S form the basis for archaeological geographic assignment and mobility studies using collagen δ³⁴S (Ebert et al., 2021; Lamb et al., 2023; Paladin et al., 2020; Zavodny et al., 2022). While fish consumption may also influence consumer δ³⁴S (e.g., Nehlich et al., 2010; Privat et al., 2007), here we focus on sulfur isotope variation in plants, and the implications for sulfur isotopic composition of collagen from terrestrial animals that do not consume fish.

Recent studies indicate that the interpretation of δ³⁴S is potentially complex because the δ³⁴S of sulfur available to plants can be influenced by microbial sulfate reduction and sulfide oxidation in soil and sediments (Guiry et al., 2021; Lamb et al., 2023; Nitsch et al., 2019; Stevens et al., 2022, 2025). These important processes in the global sulfur cycle are mediated by soil Bacteria and Archaea, and the soil or sediment iron content. Microbial sulfate reduction is a widespread phenomenon in terrestrial (and marine) environments and is stimulated by waterlogged conditions, for example, in wetlands, on poorly drained soils (Canfield, 2001), and in thermokarst environments (Ren et al., 2023), and is often driven by poor permeability in the underlying lithology. In these environments isotope effects from microbial sulfate reduction followed by sulfide oxidation change the δ³⁴S of soil sulfur available to plants originally derived from the geology. This modified isotopic signal can be incorporated into plants and passed on to animals to be stored in animal proteins. Sulfur in bioarchaeological material produced in these environments can, therefore, reflect a modified isotopic composition due to the effect of coeval microbial sulfur cycling (Lamb et al., 2023). We refer to environmental δ³⁴S baselines of bioavailable sulfur derived from geological and/or marine sources on well-drained land as “dryland” baselines, in order to differentiate them from environmental δ³⁴S baselines that have been altered due to waterlogging and sulfur cycling.

Evidence of early human activity has been found in wetland environments, where access to fresh drinking water and fertile soil would have made these locations attractive for resource use and settlement. These environments include marshes, fens, meres, swamps, lagoons, floodplains, lakesides, riverbanks and riparian zones, saltmarshes, estuaries and deltas, as well as land on the margins of these places. Such locations appear to have been used to provide wild food (e.g., Abu Hureyra, riparian marshland, Hillman et al., 2001), for animal husbandry and to grow crops (e.g., Neolithic Çatalhöyük, Konya plain, Turkey, Ayala et al., 2022; Sherratt, 1980; Must Farm, Cambridgeshire, UK, Ballantyne et al., 2024; Swifterbant, Neolithic wetland creek system, The Netherlands, Huisman and Raemaekers, 2014; Eye, Over, and Langtoft, fenland, Cambridgeshire, UK, Lamb et al., 2023; and Middle Yellow River Valley, China, Wang et al., 2023). Such environments were likely instrumental in the development of farming, with floodplain archaeobotanical assemblages suggesting the presence of crop plants growing alongside wetland plants (e.g., Hillman et al., 2001; Motuzaite-Matuzeviciute et al., 2012).

In archaeological applications, where the sulfur isotopic composition of bioarchaeological material deviates from the local dryland δ³⁴S baseline, it may suggest movement of people or animals in the past. However, it could also indicate a wetland δ³⁴S baseline signal incorporated via the consumption of crops grown on wetlands and wetland derived resources from the local area (and/or beyond) (Guiry et al., 2021, 2022; Lamb et al., 2023; Nitsch et al., 2019). Such signals in human, animal and plant archaeological material could potentially provide unique insights into ancient wetland resource use and early agriculture. Temporal sequences of such samples may also provide information on natural hydrological changes, seasonal resource use, and human manipulation of local hydrology.

The aim of this study is to explore how different hydrological conditions can impact terrestrial sulfur cycling and overwrite primary dryland δ³⁴S baselines derived from parent geology. We use conceptual models to explore wetland sulfur cycling effects. We discuss parameters controlling plant-available sulfur δ³⁴S, assess the effect of coeval sulfur cycling on biosphere mapping and palaeo-baselines, and explore factors contributing to wetland isotopic signals in plants. We consider how these factors may have interacted with ancient water management practices to affect the δ³⁴S of crops and wild plants grown in these environments, and explore the influence of the wetland signal on archaeological collagen δ³⁴S.

2 Drivers of bioavailable δ³⁴S baselines

Almost all biologically incorporated sulfur in terrestrial environments is derived from geological reservoirs in rocks, as bedrock provides parent material from which sulfur is released into soil and groundwater. Therefore, the site specific δ³⁴S of geological sulfur is the essential starting point for interpreting δ³⁴S in archaeological collagen and plant material from a particular site. There are two broad categories of geological sulfur deposits, sulfate that reflects the δ³⁴S of coeval ocean water sulfate with a range of 10‰ to 30‰, and sulfide formed from microbial sulfate reduction, usually with significantly lower δ³⁴S values (Bottrell and Newton, 2006; Bottrell and Raiswell, 2000; Claypool et al., 1980; Paytan and Gray, 2012; Pellerin et al., 2019). Together, this results in a very wide range of geological sulfur δ³⁴S values spanning c. 70‰, with offshore pyrite sulfide δ³⁴S values reported down to −55‰ (Pasquier et al., 2021).

Terrestrial geological sulfur is released from rock into soil via a series of physiochemical processes which weather the rock. This occurs most commonly with near-surface sedimentary rock when exposed to changes in temperature and moisture at the surface, or via interactions with groundwater in the subsurface. In this way, sulfur bearing the local geological δ³⁴S reaches soil and groundwater. It is this weathered, dissolved sulfur that is available to be taken up by plants and animals. Geological weathering product δ³⁴S found in these terrestrial reservoirs may reflect a mix of sulfur from different sulfur bearing rocks with varied δ³⁴S values. This mix is affected by preferential weathering of some deposits over others due to chemical and structural differences. For example, during weathering, sulfate in evaporites and carbonates readily dissolves in water, often resulting in high concentrations reaching soil, groundwater and rivers, whereas sulfide in pyrite is insoluble, and requires exposure to oxygen to weather, which usually occurs via microbially mediated reactions.

This geological origin sulfur in soil may mix with sulfur from natural atmospheric sources, including wind blown dust, the biogenic gases dimethyl sulfide and hydrogen sulfide (H₂S), as well as ash from forest fires and sulfur dioxide from volcanic eruptions and passive degassing (Aneja, 1990). When two or more inputs of sulfur with different isotopic compositions are mixed, the relative amounts of sulfur and the isotopic difference between each input determine the resulting δ³⁴S of the mixture. In near-coast sediments and soils the geological signal is likely to be overwritten by the dominant isotopic signal from sulfate-rich ocean water in sea spray (δ³⁴S c. 21‰; Paytan and Gray, 2012), and dimethyl sulfide produced by marine phytoplankton (Aneja, 1990). The distribution of this marine signal is controlled by prevailing weather patterns which affect the continental distribution of sea spray and dimethyl sulfide, and by hydrogeology, which controls the extent of saline intrusion into fresh groundwater in coastal regions.

Archaeological studies may use modern material, (for example from plants), to characterize δ³⁴S baselines. However, in modern environments, soil sulfur of geological origin may mix with atmospheric sulfur dioxide from fossil fuel combustion, and sulfur in agricultural fertilizer. Sulfate originating from fossil fuel combustion has an isotopic composition close to 0‰ (e.g., Chung et al., 2019; δ³⁴S 2.2 ± 1.6‰). Inputs from fossil fuel combustion are likely to be diffuse, dampening the range of soil sulfur derived from geology over the affected area (Stack and Rock, 2011), with a larger effect for geologically derived soil δ³⁴S values that are furthest from the value of fossil fuels. Fertilizer δ³⁴S values are highly variable (Vitòria et al., 2004) and their effect on soil bioavailable sulfur δ³⁴S is likely to be localized and spatially heterogeneous. Sulfur fertilizers may be artificially produced or mined from geological settings far from the locations where they are used. Sulfur from fertilizers can be transported away from the area of application in runoff and groundwater. When anthropogenic sulfur inputs decline or are discontinued, there is likely to be a time lag before their contribution to the bioavailable sulfur pool decreases.

Microbial sulfur cycling can significantly alter δ³⁴S values derived from geological sulfur parent material, marine and atmospheric sources, laying down fresh sulfide deposits in soils and sediments (Figure 1), leading to bioavailable δ³⁴S values which can be lowered or raised. This can occur as a result of poorly draining soils or impervious lithology. Sulfur with δ³⁴S values affected by microbial cycling may be taken up by plants and passed up the food chain. Sulfur cycling can occur at local scales and can change over relatively short time periods, for example with alteration in floodplain extent due to dynamic river braiding and meandering, or at vast spatial and temporal scales, for example during continent-wide permafrost thaw during the Late Pleistocene.

Figure 1

Figure 1. The terrestrial sulfur cycle, showing sources of sulfur and microbial sulfur cycling processes. Sulfate reduction is highlighted in yellow boxes as the main isotopically fractionating process.

Perturbations of geologically derived dryland δ³⁴S baselines due to coeval sulfur cycling are relevant to archaeological studies using sulfur isotopic composition of human, animal or plant material. Such perturbations should be considered in archaeological settings to broaden the scope of interpretation and avoid misinterpretation of archaeological δ³⁴S values. In particular, where archaeological sites were in wetlands, in periodically waterlogged, poorly draining or low-lying environments, or in thermokarst, saltmarshes or estuaries, dynamic sulfur cycling is likely to have affected bioavailable δ³⁴S baselines significantly. However, in order to identify the effects of coeval sulfur cycling, the dryland baseline derived from the local geological parent material, (as well any sea spray influence), should be determined with good spatial resolution and a high degree of confidence. This is particularly important because of the very wide range of possible geological parent material baseline δ³⁴S values. Distinguishing the influence of parent material from coeval sulfur cycling can be challenging, as some wetlands form on poorly permeable lithology that has a sulfur isotopic composition strongly affected by sulfate reduction over geological time scales, for example in lowlands underlain by Jurassic mudstones in central England (Lamb et al., 2023).

3 Sulfur cycling affects bioavailable δ³⁴S baselines in wetlands and waterlogged environments

3.1 Sulfur cycling in soil

When considering the effects of waterlogging on plant δ³⁴S, we refer to the partitioning of different amounts of sulfur in organic and inorganic compounds in the soil and their bioavailability to plants (the plant-available fraction), the isotopic composition of these different compounds (δ³⁴S), and microbial sulfur cycling processes that may incur isotopic fractionations affecting the isotopic composition of the compounds. Around 95% of sulfur in soil is in organic form (Kilham, 1994), derived from biomass of plant, microbial, and animal origin. However, plants are not able to utilize this organic sulfur directly. It is broken down slowly and is only very gradually mineralized and oxidized to sulfate by soil microbes (Chaudhary et al., 2023), with negligible change in δ³⁴S (e.g., Norman et al., 2002), to be replenished by organic sulfur in fresh biomass deposits. Breakdown of organic matter is suppressed if the soil is waterlogged (Dušek et al., 2020). Water also limits oxygen diffusion into waterlogged and flooded soils and sediments, which are often formed on poorly permeable lithologies. Under water, there is decreasing oxygen diffusion with depth and distance from the air-water surface interface. As remaining oxygen is used up by aerobic microbial respirers and plant roots, a steep vertical gradient of oxygen depletion develops, stimulating the activity of different anaerobic microbial communities and progressive electron acceptor depletion (Kilham, 1994). In the anoxic zone, microbial sulfate reducing activity is stimulated where sulfate and organic carbon are available. Within these redox gradients the boundary between well-oxidized conditions near the surface and anoxic conditions at depth can move up or down, for example, in response to water level fluctuations and bioturbation. This can result in dynamic switching between reducing and oxidizing conditions, stimulating rapid microbial cycling of sulfate and sulfide from the soil inorganic sulfur pool through sulfate reduction and sulfide oxidation.

Sulfate reduction in anoxic conditions is associated with a varied and often large isotopic fractionation (c. 3‰ to 68‰, Sim et al., 2011), which results in the production of sulfide with lower δ³⁴S values than those of the original sulfate. The isotope effect associated with microbial sulfate reduction occurs via a series of fractionating, reversible enzymatic steps within the sulfate reducing organism. The magnitude of the sulfate reduction isotope effect in the environment is one of the main controls on wetland plant δ³⁴S. Large isotope effects from wetland sulfur cycling have been reported, for example from peat wetland environments (11‰ to 51‰, Price and Casagrande, 1991), brackish marshland (c. 33‰, Stribling et al., 1998), and saltmarshes (25‰, Carlson and Forrest, 1982; 50‰, Peterson et al., 1986), although further studies are needed, especially from freshwater environments. At near neutral pH levels, sulfide produced from sulfate reduction can accumulate in the soil as hydrogen sulfide (H₂S). Persistent large isotopic offsets between sulfate in water and sulfide in sediments are brought about via the formation of sulfide precipitates in the sediment which prevent the upward diffusion of dissolved sulfide to the surface where oxygen would enable its oxidation to sulfate. Most sulfide precipitates are formed with iron (Fe²⁺), as iron sulfide (FeS) and pyrite (FeS₂), thus iron availability is key to the formation of sulfide precipitates and maintaining the large isotopic offsets found between sulfate in water and sulfide in sediments.

Factors causing variation in the isotope effect of sulfate reduction are not fully understood, and include the ratio of sulfate to organic carbon (a larger effect is found when the sulfate to carbon ratio is high), and non-linear effects from differences in sulfate concentration, temperature, microbial strain, the type of organic carbon, the cell specific sulfate reduction rate, and iron and ammonium availability (Sim et al., 2012, 2023). High groundwater levels, river flooding, heavy rainfall, poor soil permeability and impermeable lithology can cause soil saturation, which stimulates sulfate reduction. Soil waterlogging coupled with sulfate reduction may be a temporary or periodic phenomenon, for example in floodplains and estuaries, or a more permanent phenomenon, for example in marshes and fens. A wide diversity of wetlands are prime sulfate reducing environments, with low δ³⁴S sulfide in soil and sediments.

3.2 Pathways for plant sulfur uptake and effects on plant δ³⁴S

Sulfur is an essential nutrient for plants. It is taken up via sulfate transporter chemicals in roots, and used to make different sulfur containing compounds including the amino acids cysteine and methionine, the protective antioxidant glutathione, as well as many essential metabolites such as vitamins, enzymes, and compounds used for plant defense and stress response (Hill et al., 2022). If plants take up excess sulfate, it can be stored in cell vacuoles until it is needed, and if sulfate uptake is restricted due to low availability sulfur may be recycled within the plant from glucosinolates (secondary plant metabolites) (Maruyama-Nakashita, 2017).

Sulfate reduction results in sulfide with lowered δ³⁴S. However, most plants are not sulfide tolerant and cannot take up sulfide directly. In order for a lowered δ³⁴S from sulfide to be incorporated into plant biomass, sulfide must first be oxidized to sulfate. Here we use the term “lowered plant δ³⁴S” as a relative term to refer to plant δ³⁴S values that are lower than those expected from plants grown in non-waterlogged or flooded soil with the same geological parent material. For plants which are not sulfide tolerant, sulfide oxidation to sulfate can occur after waterlogged or flooded soil drains, allowing the ingress of air to oxygenate the soil, for example on seasonally draining floodplains. This enables the oxidizing microbial community to become active resulting in sulfide oxidation to sulfate, utilizing sulfide produced during the preceding period of submergence. Soil sulfide oxidation usually causes negligible isotopic fractionation (Canfield, 2001; Zerkle et al., 2009) so the newly oxidized sulfate retains the lowered δ³⁴S from sulfide. This effect is enhanced by differences in the solubility of sulfate and sulfide. Sulfate is highly soluble and some soil sulfate is removed in water as it drains from temporarily waterlogged soil, as a function of soil water retention on draining, which varies with soil type. Sulfide precipitates such as iron sulfide and pyrite are insoluble and remain in the soil when the water drains away, after which they can be oxidized back to sulfate. The relative proportions of plant-available sulfate from soil oxidized sulfide to remaining sulfate in soil water after draining is referred to as the “soil sulfate source ratio” hereafter.

Some plants, primarily those adapted to wetland ecologies (e.g., wetland plants including the crops rice and taro), are sulfide tolerant. They have adapted to root zone (rhizosphere) waterlogging and can oxidize sulfide to sulfate in the root zone (Lamers et al., 2013; Simkin et al., 2013). This occurs through the development of a barrier along the root length to the tip which protects the plant from absorbing toxic sulfide along the root shaft, but allows leakage of oxygen into the rhizosphere from the root tip (Abiko and Miyasaka, 2020; de la Cruz Jiménez et al., 2021; Nishiuchi et al., 2012; Pan et al., 2021). The oxygen seeping from the root tip into the waterlogged rhizosphere (Ando et al., 1983), leads to localized root zone oxygenation and the development of a symbiotic bacterial community of sulfide oxidizers. This supports in situ microbial oxidation of sulfide with lowered δ³⁴S, forming sulfate which can be then taken up and utilized by the plant. Internal sulfide oxidation by symbiotic bacterial colonies within the roots may also occur (Lamers et al., 2013). Wetland plants can also develop adventitious roots which form laterally at the soil/water interface, allowing uptake of oxygen and sulfate from overlying water (Parent et al., 2008; Steffens and Rasmussen, 2016).

A critical factor determining wetland plant δ³⁴S is the relative amounts of sulfur taken up via these two pathways: the uptake ratio of sulfide oxidized in the root zone, to direct uptake of sulfate from overlying water; hereafter referred to as the “plant uptake ratio.” There are limited published data on wetland plant uptake ratios. The few studies that are available show saltmarsh plant uptake ratios ranging from 1.0: 0.0 to 0.6: 0.4 (Carlson and Forrest, 1982; Cornwell et al., 1995; Fry et al., 1982, 2023; Peterson et al., 1986; Price and Casagrande, 1991; Stribling and Cornwell, 1997; Stribling et al., 1998). In these sulfate rich environments sulfate δ³⁴S in overlying water is as high as 21‰, but plant δ³⁴S ranges from −14‰ to 13‰ due to root zone sulfide oxygenation. Although freshwater wetland plant uptake ratio studies are lacking, the range of δ³⁴S from freshwater wetland plants reported recently is similar in magnitude to that of saltmarsh plants, although with lower absolute values (−31‰ to −1‰, Evans et al., 2023; Lamb et al., 2023).

Interestingly, adventitious roots (but not rhizosphere sulfide oxidation), can be induced by waterlogging stress in non-wetland species including the crop plants oats, barley, wheat, maize, and millet (Li et al., 2023; Matsuura et al., 2022; Pan et al., 2021; Yamauchi et al., 2013). This stress response may have enabled early cultivation of crop plants on floodplains and lowlands which would have been at risk of periodic inundation during the growing season.

4 The “wetland” sulfur isotope signal

Archaeologists are rapidly taking on board the concept that lowered δ³⁴S is a marker of wetland habitat use. However, it is necessary to highlight that not all wetland and waterlogged environments will produce a distinctive isotopic signal. Whether such a signal is present depends on many variables, including the magnitude of the isotope effect, plant uptake ratios, the nature of the hydrologic system (flow-through, standing water), the duration of waterlogging and progression of sulfate reduction, the extent of drainage and the interplay between microbial sulfur cycling, isotopic fractionation, and plant adaptation. The hydrology of natural wetlands has traditionally been classified according to hydrogeomorphology, combining information about the dominant source of water, position in the landscape and water movement (Shaffer et al., 1999). Wetlands occur where these factors combine to support a seasonal or permanent high water table, which leads to the development of specific habitat types and wetland soils and sediments, and are often underlain by poorly permeable lithology. Soil waterlogging, while not necessarily leading to wetland development, supports microbial sulfate reduction and changes in δ³⁴S, and is a function of elevation and/or soil type, with low lying areas, riparian zones and clay soil more likely to be temporarily or permanently waterlogged. Water plays two crucial roles in wetland and waterlogged soil sulfur cycling. First, it provides a barrier that limits oxygen diffusion and leads to soil anoxia, stimulating microbial sulfate reducing activity. Second, water carries dissolved sulfate for microbial sulfate reducers to use. Wetlands receive water directly via precipitation, and lose water via evapotranspiration. However, shallow groundwater is often the most important source of water for these systems, bringing in sulfate and other nutrients to support plant life. Here, we use the term shallow groundwater to include groundwater that originates from deeper circulation but becomes shallow once it has risen to the surface to provide baseflow to rivers and wetlands, and near-surface circulating groundwater that is recharged from surface runoff (Hiscock and Bense, 2021).

Natural and engineered wetland systems span a variety of hydrologic regimes which support sulfate reduction and sulfide oxidation alongside plant life (Figure 2). Here we consider sulfur isotope cycling under four different theoretical hydrological regimes which can support a range of wet environments from permanent wetlands to temporarily waterlogged landscapes in natural and engineered systems; flow-through, standing water, drained post-waterlogging, and simultaneously flooded and drained; and model their potential impact on plant δ³⁴S values (models presented in Supplementary Information). Our models represent a first step toward understanding controls on wetland plant δ³⁴S, by presenting four simple scenarios which, in the real world, are expected to be more complex. For example, wetland landscapes are likely to be supported by heterogeneous hydrology that varies with season and rainfall patterns. Sulfate reduction isotope effects may vary with seasonal temperatures and sulfate and organic carbon availability, and sulfate δ³⁴S values may vary with rainfall and hydrology. Furthermore, plant uptake ratios are poorly understood and could vary, for example, with changes in sulfate concentration, temperature, season or phase in the plant's growth cycle, and sulfide adapted and non-adapted plants may grow side by side at wetland margins. These scenarios need to be tested in situ to form the foundation for archaeological studies in wetland environments.

Figure 2

Figure 2. Four hydrological scenarios in natural and engineered wetland systems which support enhanced sulfate reduction and sulfide oxidation, including; (A) flow-through, (B) standing water, (C) drained post-waterlogging, and (D) simultaneously flooded and drained (standing water) hydrologies. Dashed arrows represent precipitation, evaporation, and very slow drainage, solid arrows represent water movement (shallow groundwater and surface water), and dark and light green plant symbols represent plants adapted to sulfide and unadapted plants, respectively. The partitioning of ³⁴S in sulfate and ³²S in sulfide due to isotopic fractionation during sulfate reduction is represented by white or blue font characters (color difference for readability only). These are absent from overlying water in (A) because no partitioning occurs there.

Omitted from these scenarios are ombrotrophic, nutrient poor, acid peat bogs, which are usually dominated by sphagnum. Evidence of early crop cultivation in such bogs is lacking and ombrotrophic bogs are unlikely to have been used for early crop cultivation due to their low level of nutrients and low pH. These bogs rely solely on precipitation for their water supply, and are disconnected from groundwater.

4.1 Flow-through hydrology

Plants living in wetland systems with flow-through hydrology (including natural wetland plants and the crops rice and taro) are sulfide adapted. The primary source of water and additional sulfate for many natural wetland environments is continuous lateral and/or vertical shallow groundwater flow (Winter, 1999) which supplies an ongoing input of sulfate derived from geological weathering and atmospheric sources for use by microbial sulfate reducers (Figure 2A). In such flow-through systems, sulfate which is reduced to sulfide in anoxic sediments is replenished with sulfate in the continuously flowing shallow groundwater. Such natural groundwater fed wetlands can be permanently waterlogged if shallow groundwater levels are stable, or temporarily waterlogged if shallow groundwater levels recede, and include riparian wetlands, marshes and fens. A similar hydrology existed where engineered flow-through systems were built, supplied with flowing water channeled from nearby sources such as springs and streams.

Referring to the partitioning of ³²S and ³⁴S within different reservoirs of sulfur during isotopic fractionation, flow-through wetlands maintain a near constant sulfate δ³⁴S and sulfate concentration in overlying water because the continuous replenishment of sulfate from flowing water erases the effect of preferential removal of ³²S by sulfate reduction (Figure 2A). ³²S accumulates in sedimentary sulfide resulting in a lowered sulfide δ³⁴S, which is passed on to plants via rhizosphere oxygenation and sulfide oxidation, resulting in plants with lowered δ³⁴S. The difference between sulfate δ³⁴S in the overlying water and sulfide δ³⁴S in the sediment is stable if the magnitude of the isotope effect of sulfate reduction is stable (Högberg, 1997; Kendall and Caldwell, 1998; Mariotti et al., 1981), and is the first control on the lowered δ³⁴S of the wetland plants. The magnitude of the isotope effect has been characterized for a limited number of wetlands (e.g., 11‰ to 51‰ Carlson and Forrest, 1982; Peterson et al., 1986; Price and Casagrande, 1991; Stribling et al., 1998). If minor fluctuations in groundwater sulfate δ³⁴S occur as a result of changes in the relative proportions of sulfate source contributions, such as after recharge from rainfall or during drought, this could result in fluctuations in sulfate and sulfide δ³⁴S values. As plant uptake ratios are not well known (see Section 2) and the sulfate reduction isotope effect is dependent on many factors (see Section 3.1), we use ranges from the literature, modeling plant uptake ratios of 0.6:0.4 to 0.8:0.2 and isotope effects of −10‰ to −40‰ (Figure 3A and Supplementary Information). Our models illustrate the effect of sulfate reduction as changes in plant δ³⁴S (denoted by Δ³⁴S) in different systems relative to baseline “initial sulfate,” driven by the geology, and are based on the simplifying assumption that there is no pre-existing sulfate or sulfide in the system. In the real world initial sulfate δ³⁴S values could vary significantly depending on the underlying geology, which can encompass values from −20‰ to 30‰ (Seal, 2006).

Figure 3

Figure 3. Representations of four wetland environments used for crop growing with different hydrologies; (A) flow-through, (B) standing water, (C) drained post flow-through waterlogging, and (D) simultaneously flooded with standing water and drained, with charts presenting ranges of plant δ³⁴S. The plant changes in sulfur isotopic composition (Δ³⁴S, where 0‰ refers to no change) relative to initial values were derived by combining modeled effects of sulfur fractionation associated with sulfate reduction (Högberg, 1997; Kendall and Caldwell, 1998; Mariotti et al., 1981), with four isotope effects for sulfate reduction; 10‰, 20‰, 30‰, 40‰, based on reported values from the literature (Carlson and Forrest, 1982; Peterson et al., 1986; Price and Casagrande, 1991; Stribling et al., 1998). These were combined with plant uptake ratios for emergent plants informed from the literature for (A) and (B) (Carlson and Forrest, 1982; Cornwell et al., 1995; Fry et al., 1982, 2023; Peterson et al., 1986; Price and Casagrande, 1991; Stribling and Cornwell, 1997; Stribling et al., 1998), and for soil sulfate source ratios for (C) and (D) using the proportion of sulfate in water remaining on draining based on soil porosity and the maximum available soil moisture on draining across three soil types (Kumar Rai et al., 2017; Marshall and Holmes, 1996). Initial values are not specified as they are controlled by the geological parent material and can span a range of at least 50‰ (e.g., Seal, 2006) (please refer to Supplementary Information for further details).

4.2 Standing water hydrology

Most plants growing in persistent standing water hydrology (including natural wetland plants and the crops rice and taro) are also sulfide adapted. Standing water systems receive water containing sulfate from rainfall and include seasonally and ephemerally waterlogged landscapes in low lying areas, and on poorly draining soil. In engineered standing water systems, pulses of water may have been diverted from local streams, lakes, or seasonal floodwaters, and retained by field edge structures, while poor soil permeability may have been enhanced by soil puddling (tillage under flooded conditions to reduce permeability), to prevent vertical drainage (Figure 2B). In these systems sulfate in overlying water slowly percolates into anoxic sediments to support microbial sulfate reduction. In the overlying water, sulfate δ³⁴S and sulfate concentration undergo dynamic changes as sulfate reduction progresses, which in turn affect the δ³⁴S of sulfide produced. In standing water systems which receive a single input of water, sulfate is progressively transferred from the overlying water to sulfide in the sediments where it reacts with iron to form insoluble precipitates. This results in an increase in ³⁴S in the remaining sulfate in overlying water and in ³²S in reduced sulfide in sediments as sulfate reduction proceeds. In this system, sulfate concentration in the overlying water decreases over time and the δ³⁴S of sulfate rises well above the δ³⁴S of the initial sulfate, while the δ³⁴S of sulfide in the sediment rises gradually as more sulfide is sequestered, reaching a maximum δ³⁴S sulfide value the same as that of initial sulfate. This point is reached when all sulfate from the single input has been reduced to sulfide (Högberg, 1997; Kendall and Caldwell, 1998; Mariotti et al., 1981) (Supplementary Information). Due to the fact that sulfide δ³⁴S rises as sulfate reduction proceeds, there is a smaller difference between the initial sulfate δ³⁴S and that of sulfide than the difference seen from flow-through systems (Figure 3B).

In some standing water systems, periodically repeated inputs of water supply fresh sulfate. Although the δ³⁴S of sulfate and sulfide respond dynamically during sulfate reduction (Högberg, 1997; Kendall and Caldwell, 1998; Mariotti et al., 1981), the periodic inputs of sulfate buffer these effects, resulting in a similar effect to that seen from flow through systems. We also modeled plant δ³⁴S with standing water hydrology and periodic inputs of “initial” sulfate in water, using sulfate and sulfide δ³⁴S over time. The same plant uptake ratios and isotope effects are used as in the flow-through system. However, the range of resulting plant δ³⁴S values is slightly smaller (Supplementary Information). Note that the plant uptake ratio is particularly sensitive in the standing water scenario as, if a large proportion of sulfate is taken up from overlying water after significant sulfate reduction, plant δ³⁴S values could be raised.

Standing water hydrology and flow-through hydrology can occur on a continuum within the same wetland, often controlled by the seasonality of water inputs. For example, flow-through wetland systems may behave like standing water systems at their margins when water flow is reduced (e.g., Peterson et al., 1986), if they have poorly permeable sediments which restrict water movement, and with sediment depth (e.g., Carlson and Forrest, 1982; Price and Casagrande, 1991). In addition, many standing water systems receive sporadic inputs of water and sulfate from precipitation, flood events, high tides, or temporarily high groundwater levels (Supplementary Information).

4.3 Temporarily flooded and drained post-waterlogging

Plants grown after drainage of flooded soil conditions, (such plants include wild grasses and the crops barley, wheat, sorghum, and barnyard millet), are not able to oxidize sulfide but can tolerate transient flooded conditions. These plants have to take up their sulfur directly as sulfate from the environment. Many flow-through and standing water wetlands are temporarily waterlogged rather than permanently inundated. When flooded soil drains, the water which submerged the soil is mostly lost. This can occur via gradual drainage when flood waters recede and shallow groundwater levels drop, or, in engineered systems, as a result of intentional release (Figure 2C). Due to the high solubility of sulfate, when overlying water drains, much of the sulfate is lost via transport in drainage water. When drainage occurs, oxygen enters the soil triggering sulfide oxidation, creating low δ³⁴S sulfate from the sulfide produced during waterlogging and stored in precipitates formed with iron in anoxic sediments (Figure 2C). Plants growing immediately after soils are drained incorporate sulfate primarily from sulfide which has been oxidized in the freshly aerobic soil. Where temporary wetlands are repeatedly flooded and drained, a cumulative effect can occur if conditions promote substantial sulfide oxidation each time the wetland is drained (for example if soils have high porosity, readily allowing oxygen to enter previously waterlogged pore space). Some wetlands, once drained, remain so permanently, whether as a result of natural processes (e.g., alterations in river courses), environmental change (e.g., lowered shallow groundwater levels as a results of decreases in recharging precipitation), or alteration in land use (e.g., marshland drained to create pasture or rice paddy fields drained to grow different crops). Permanent drainage enables oxygen to remain in the soil, which, as the soil is no longer waterlogged, no longer supports sulfate reduction. Within this drained soil, low δ³⁴S sulfide (produced and stored in the soil before it was drained), is gradually oxidized to sulfate and is not replenished. This results in gradually decreasing inputs of low δ³⁴S into the plant available sulfate pool from sulfide reserves near the soil surface; a process which may take many years (e.g., Chung et al., 2017). We have modeled δ³⁴S for plants grown on drained soils after a flow-through hydrology (Figure 3C). We have estimated the proportion of sulfate lost in water on draining by considering soil water content under saturation and plant-available water on draining, for three soil types with different drainage; sandy, loam and clay soils. The different porosity of these soil types means they hold different proportions of water when saturated, and on draining, these soil properties affect how much of this water is available to plants (Kumar Rai et al., 2017; Marshall and Holmes, 1996) (Supplementary Information). Based on these estimates, we use soil sulfate source ratios (the relative proportions of plant-available sulfate from soil oxidized sulfide to remaining sulfate in soil water after draining), of 0.7:0.3, 0.8:0.2 to 0.9:0.1 for loam, clay and sandy soils, respectively. We use the same isotope effects as in the previous models. This system results in lowered plant δ³⁴S. If before draining, the system had a standing water hydrology (rather than flow-through modeled here), we expect δ³⁴S values of plants grown in drained soil to be lowered, but not lowered to the extent of plants growing in a drained flow-through system, reflecting the smaller signal expected from standing water systems in general (5.2). In landscapes that are repeatedly flooded and drained, we expect δ³⁴S of plants grown in drained soil to be lowered by a similar amount as that expected from flow-through systems after draining.

4.4 Simultaneously flooded and drained hydrology

Plants grown in simultaneously flooded and drained systems (including the crops barley, wheat, squash, maize, beans) are not able to oxidize sulfide, but can tolerate partially saturated soil conditions. Such conditions occur through minor variations in elevation in waterlogged and flooded natural landscapes, and in some engineered wetlands (e.g., ridge and furrow, raised fields, floating fields) (Figure 2D). The hydrology in these systems may be standing water or flow-through. Such systems simultaneously support sulfate reduction in the waterlogged soil at depth, producing low δ³⁴S sulfide values, and sulfate oxidation in the soil above the water, producing low δ³⁴S sulfate (Figure 2D). However, plants grown on unsaturated soil above a high-water table are expected to incorporate sulfate primarily from roots reaching the upper level of saturated soil and the overlying water directly. The sulfate δ³⁴S of these sources is controlled by the flow-through or standing water status of the water. We have modeled the plant δ³⁴S with standing water hydrology over a range of soil and water sulfate source ratios of 0.4:0.6 to 0.2:0.8, at 50% reduction of sulfate to sulfide (but as discussed above the extent of reduction is dynamic). In simultaneously flooded and drained systems with standing water hydrology, plant δ³⁴S could be raised above the initial sulfate δ³⁴S, as illustrated in Figure 3D. However, if such systems have flow-through hydrology we would expect plant δ³⁴S to be slightly below the initial sulfate δ³⁴S of the water.

5 Sulfur wetland isotopic signals in archaeology

5.1 Ancient water management practices

These model results suggest that through analysis of archaeological crop δ³⁴S we may be able to differentiate different water management practices, many of which played a critical role in subsistence agriculture and its development and intensification. In ancient societies, the ability to access, control and distribute water effectively often determined the success or failure of agricultural production, and increasingly sophisticated techniques often accompanied agricultural intensification (Fuller, 2020). Some of the earliest arable farmers harnessed the natural environment through “least-effort” cultivation on floodplains, as these areas offered fertile soils enriched by seasonal flooding, along with a reliable supply of water (Ma et al., 2016). Later, arable farmers developed infrastructure to manage water supply either to enable cultivation on land that required irrigation, or to utilize waterlogged or frequently flooded land through drainage (Langlie, 2018; Zhuang et al., 2025). This utilization or management of water is argued to have enabled early farmers not only to continue producing food in unpredictable climates but also to create agricultural surpluses that are thought to have underpinned the development of complex societies (Zhuang et al., 2014). While broad patterns of natural and managed water use in prehistoric agriculture are well-documented, the study of past water management regimes primarily relies on indirect evidence such as traces of irrigation infrastructure and weed ecology (Petrie and Bates, 2017; Zhuang et al., 2025). These methods often fall short in accurately reconstructing past water conditions, restricting interpretations to a coarse resolution. Analysis of δ³⁴S values of archaeological crop remains may have the potential to elucidate past regimes of water management for cultivation and to determine the hydrological conditions under which agricultural production was taking place (Stevens et al., 2022), particularly if dryland bioavailable sulfur δ³⁴S baselines derived from local geological and marine sulfur can be well-characterized.

In addition to supporting early agricultural development and intensification, the human manipulation of natural wet and waterlogged environments and the creation of engineered wetlands for crop production are likely to have resulted in environmental harms to the immediate environment and beyond. Such detrimental effects could have included biodiversity loss, the disruption of sediment cycling, and increases in greenhouse gas emissions (Erickson, 1992; Fuller et al., 2011; Marston and Scott Branting, 2016; Robinson and Lambrick, 1984). By studying archaeological crop δ³⁴S we may be able to further our understanding of the timing and extent of these practices to clarify the likely extent of the environmental damage associated with managing water and wetlands for crop production.

The effects of past water management techniques on archaeological crop δ³⁴S depends on the hydrology of the technique involved, and any waterlogging adaptation of the crop grown (Table 1). Here we suggest which model scenario (Section 4.) would best represent a particular past water management technique for specific crops. Rice and taro are probably the only truly flood adapted crops able to oxidize sulfide in the rhizosphere and to thrive when partially submerged (Abiko and Miyasaka, 2020; Nishiuchi et al., 2012). In the natural environment some wild rice and taro species would have grown in permanent wetlands which were likely to have included flow-through and standing water hydrology (Fullagar et al., 2006; Müller et al., 2010; Zheng et al., 2009; Zong et al., 2007) (Figures 3A, B). Under paddy cultivation, flow-through systems and standing water systems were likely used to grow wet rice and taro (Kay et al., 2019; Lee et al., 2014), depending on water source availability, with paddy drainage before harvest. Where such seasonal drainage occurred this would have removed much of the sulfate in overlying water and allowed oxidation of sulfide in the soil, which would then have become available for uptake by catch crops, as in the case of the rice-wheat system (Bates et al., 2017) (Figures 3A, B, followed by Figure 3C). Irrigation techniques which involved temporarily submerging soil, for example by engineered trapping of seasonal flood waters, would have suited cultivation of crops with some waterlogging tolerance such as dry rice, sorghum, and barnyard millet (Bates et al., 2017; Bhinda et al., 2023; Kay et al., 2019; Khalifa and Eltahir, 2023; Dal Corso et al., 2022; Neumann, 2018 (Figure 3C).

Table 1

Table 1. Past wetland utilization and water management techniques for crop cultivation, showing example crops grown using these techniques, the hydrological type they correspond to (Figures 3A–D), and examples of geographical regions and periods associated with techniques were found (examples included in italics with references).

Similarly, crops grown after natural drainage of flood waters would have been cultivated in environments with seasonally high groundwater levels such as riverbanks, floodplains, lake margins, river valley lowlands, and ridge and furrow on low lying, poorly draining soil, and were likely to be able to tolerate temporary flooding and high groundwater levels in the root zone (Brechbühl et al., 2024; Hamerow et al., 2020; Kay et al., 2019; Kingwell-Banham, 2019; Motuzaite-Matuzeviciute et al., 2012; Ortloff, 2009). Where simultaneous flooding and drainage was engineered, cultivation may also have taken place at elevations above the reach of high groundwater levels, and crop plants may not have required flood tolerance (Figure 3D). Raised fields, floating fields, and sunken gardens, found in the Americas and parts of Africa (Blatrix et al., 2022; Comptour et al., 2018; Erickson, 1987; Frederick, 2007; Ortloff, 2009), would have enabled a steady supply of water to reach plant roots while protecting crops from waterlogging. It is possible, therefore, that in cases where dryland environmental δ³⁴S baselines can be established, and in combination with other contextual information, analysis of δ³⁴S in archaeological crop remains could be useful to differentiate between different wetland utilization strategies, as represented in our four modeled scenarios (Figures 3A–D).

5.2 Signals from past climate adaptation

Where changes in δ³⁴S values are found in charred archaeological grains from the same site across different phases, this could suggest different water management practices or environmental change through time, especially if this can be supported by information from weed ecology or weed phytoliths. Since its domestication, rice has been cultivated using wet (flooded) and rainfed (dryland) methods. Switches between cultivation methods, and expansion into higher yielding wet rice cultivation have been linked to changes in climate in Iron Age North-eastern Thailand and the Lower Yangtze, China between 7000 and 6000 BP (Castillo et al., 2018; Patalano et al., 2015). These studies could be further elucidated by investigating variations in δ³⁴S of charred archaeological rice from these sites which may be associated with changes in cultivation method, and to ascertain whether signals correspond in time to changes seen in climate proxies. We would expect significantly lower δ³⁴S values from rice cultivated using wet techniques with flow-through hydrology than from rice grown using the rainfed method.

5.3 Sulfur isotopes in human and animal collagen and wetland resource use

Sulfur in human and animal bone and dentine collagen is derived through diet. At the base of the food chain, sulfur, an essential nutrient for plants, is taken up as sulfate from soil. Once within the plant, sulfur is used to make amino acids cysteine and methionine, and many other compounds (Hill et al., 2022; Li et al., 2022). Sulfur is passed up the food chain with minimal trophic enrichment when plants are eaten, so plant δ³⁴S is broadly conserved during the transfer of sulfur from plants to animals (Cavallaro et al., 2022; Krajcarz et al., 2019; Nehlich, 2015; Raoult et al., 2024; Tcherkez and Tea, 2013). Collagen in bones and dentine primarily consists of type 1 collagen, and its sulfur content arises exclusively from methionine (Anné et al., 2019). Therefore, most sulfur preserved in archaeological collagen is derived from the essential amino acid methionine, which is only synthesized by plants. With respect to the transfer of δ³⁴S from plants to animals, it is not known whether the overall (bulk) plant δ³⁴S is close to, or is significantly different from plant methionine δ³⁴S. It has been suggested that plant methionine δ³⁴S could differ significantly from the δ³⁴S of bulk plant material. This is because methionine is produced via a series of reactions which may cause isotopic fractionations in both directions, possibly resulting in an overall isotopic offset (Tcherkez and Tea, 2013). Clarification would require studies using compound-specific measurements of plant methionine δ³⁴S in comparison to bulk plant δ³⁴S from the same samples. However, until such studies are published, useful information can be found in two controlled animal feeding studies, which compared faunal bone collagen δ³⁴S against plant material feed with characterized bulk plant δ³⁴S (Tanz and Schmidt, 2010; Webb et al., 2017). These studies found a very small isotopic offset in animal bone collagen δ³⁴S (which is primarily from methionine) relative to the bulk δ³⁴S of plant-based feed (−0.5‰, Tanz and Schmidt, 2010; −1.5± 0.8‰; Webb et al., 2017). This suggests bulk plant δ³⁴S is similar to plant methionine δ³⁴S, and that bulk plant δ³⁴S can be used to represent “base of the food chain” δ³⁴S values in archaeological studies of bone and tooth collagen within a small margin of error. In this way, herbivore bone collagen δ³⁴S can be seen as an integrator of environmental signals carried in plant δ³⁴S as plant δ³⁴S reflects the environment in which the plant grew. See Tanz and Schmidt (2010), and Tcherkez and Tea (2013), for further details on plant sulfur metabolism and isotopic fractionation, Anné et al. (2019) for discussion of collagen synthesis and sulfur preservation in archaeological bones and Devignes et al. (2022), for detailed information on the role of amino acids including methionine in bones.

Some studies also use δ³⁴S of archaeological keratin from antlers, horns, hooves and hair. However, sulfur in keratin is mainly in the form of the non-essential amino acid cysteine which can be synthesized by mammals from various dietary sources of sulfur, such that keratin may be a less direct tracer of wetland environments than collagen δ³⁴S. Despite this, evidence for a relatively minor effect from isotopic fractionation during sulfur uptake into keratin is also suggested by controlled feeding studies and wild animal tracking (Kabalika et al., 2023; Richards et al., 2003).

The incorporation of a wetland δ³⁴S signal into bone and dentine collagen has the potential both to complicate and enhance the interpretation of archaeological human and faunal isotope data. In addition, wetland δ³⁴S signals may overlap geologically driven δ³⁴S baselines, particularly where geological δ³⁴S reflects past large-scale microbial sulfate reduction. The incorporation of a wetland δ³⁴S signal by mammals via diet is relevant for studies of hunter gathers, mobile pastoralists, and sedentary agricultural communities. For example, significant wild wetland resources use by hunter gatherers could potentially be seen in human collagen δ³⁴S (Rand et al., 2021). Similarly, where a significant proportion of human dietary methionine intake originated from crops cultivated on wetlands and from livestock which grazed on or were foddered from the wetlands, a wetland δ³⁴S signal from plants will be reflected in the consumer's collagen δ³⁴S. This could be used to indicate the cultivation of wetlands by early agricultural communities.

Variations in the magnitude of a wetland signal in human and animal collagen δ³⁴S could originate from differences in the proportion of dietary protein from a waterlogged or recently drained environment over time. For example, this could occur with different farming strategies for floodplain use including floodplain grazing, seasonal fodder using floodplain grown plants, and flood recession arable farming (Figures 4A–C).

Figure 4

Figure 4. Illustrations of three possible floodplain utilization scenarios leading to different incorporation of sulfate reduction signals into cattle dentine collagen δ³⁴S [left-hand panels (A–C)], with corresponding patterns of expected changes in sulfur isotopic composition (Δ³⁴S) where 0‰ on the left hand axis refers to no change, with δ¹⁸O in millimeter increments of dentine and enamel [right-hand panels (A–C)], which may be revealed by serial analysis.

With well-characterized dryland baseline δ³⁴S, serial analysis of δ³⁴S in dentine collagen from archaeological herbivore teeth could provide high-resolution insights into ancient farming practices, including the movement of animals across regions with different geologies and the use of resources from wetlands or recently drained areas within landscapes. Dentine protein, which comprises c. 90% collagen (Goldberg et al., 2004), records a time series of isotopic inputs from diet which is unchanged after deposition. Dentine growth bands form in hypsodont herbivore teeth every few months (Zazzo et al., 2006), potentially recording seasonal signals in dentine collagen. Thus, incremental δ³⁴S analysis of hypsodont tooth dentine collagen, as has been conducted for δ¹³C and δ¹⁵N (Díez-Canseco and Tornero, 2024), could be used to explore different floodplain utilization scenarios (e.g., Figures 4A–C), perhaps with seasonal climate signals constrained by incremental analysis of δ¹⁸O in enamel bioapatite adjusted for differences in timing and mineralization processes between dentine and enamel (Balasse, 2002).

6 Conclusions

This work highlights the dynamic nature of sulfur isotope baselines affected by sulfur cycling in wet and waterlogged environments, while acknowledging the complexity of establishing local dryland δ³⁴S baselines derived from geological and marine sulfur against which to assess potential wetland signals for archaeological applications. Wetland effects on bioavailable sulfur δ³⁴S can both confound and enrich the interpretation past human and animal movement and crop production, and provide exciting new avenues for archaeological investigation.

This study provides a step toward integrating palaeo sulfur cycling into archaeological isotope studies, but there is much further work to be done. The complex interactions between physiochemical conditions that support isotopically fractionating microbial sulfate reduction and subsequent sulfide oxidation, with varied isotope effects, hydrology and differences in plant sulfate and oxidized sulfide uptake, lead to potentially wide ranges of plant δ³⁴S. Wetland signals may overlap geological δ³⁴S and mimic the effects of movement and diet in faunal and human archaeological collagen δ³⁴S. Factors affecting the isotope effect of sulfate reduction, plant uptake ratios and soil sulfur source ratios are not well-characterized, yet are key to our understanding of δ³⁴S in bioarchaeological material. Further studies are needed to understand these interactions so they can be better resolved for use in archaeological studies in relation to natural and manipulated waterlogged environments. We emphasize the necessity of acknowledging possible effects from coeval sulfur cycling when interpreting δ³⁴S values in bioarchaeological material. Where it is possible to resolve a wetland δ³⁴S signal in collagen or archaeological crop remains, this could provide important new information about past resource use, early farming techniques, and animal feeding patterns. Wetlands, both natural and engineered, have long been integral to human subsistence and agricultural innovation, from the fertile floodplains of early farmers to the sophisticated irrigation and drainage systems that sustained complex societies. With further knowledge, δ³⁴S analysis of bioarchaeological material could become a powerful tool for directly tracing past wetland use and water management strategies.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

SKW: Writing – review & editing, Conceptualization, Writing – original draft, Methodology. RES: Writing – review & editing, Writing – original draft, Methodology, Conceptualization.

Funding

The authors declare that financial support was received for the research and/or publication of this article. NERC Pushing the Frontiers grant (NE/W000792/1).

Acknowledgments

We would like to thank Simon Bottrell, Delphine Frémondeau, and Daniel H. James for their comments on a draft of this article, and Brian Fry and Clement Bataille for helpful discussions on sulfur cycling and plant sulfur uptake and the implications for archaeology. This research was undertaken for the Iso-Wetlands project funded by NERC Pushing the Frontiers grant (NE/W000792/1).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

Generative AI statement

The author(s) declare that no Gen AI was used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fearc.2025.1599779/full#supplementary-material

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