Varudi bog (59°26^′19^′′ N, 26°35^′13^′′ E) is located in Lääne-Virumaa, northeastern Estonia, consisting of fen-bog habitats. The site is approximately 10 km south of the coast of the Gulf of Finland and Baltic Klint (Fig. 1) and spans ca. 12.6 km². The site is primarily a forested Sphagnum bog interspersed by numerous bog pools and hummocks, with an overstory of Pinus sylvestris. Varudi peatland receives 478 mm of rainfall per year, has a mean annual temperature of 7.3 °C, and prevailing winds are from the southwest and south. The underlying bedrock is composed of Cambrian and Lower Ordovician siliciclastic sedimentary rocks and Middle Ordovician limestones (Sibul et al., 2017) covered with a relatively thin layer of glacial and post-glacial sediments.

Varudi was selected for study due to its proximity (7.5 km NW) to the Kunda Nordic Tsement Factory (henceforth Kunda cement factory) (Figs. 1 and 2), which has been operational since the 1870s. During the late 1970s, cement production peaked at ca. 1.2 million t yr⁻¹ (Fig. 3). The dust emissions from the cement plant have fluctuated between 45 000 and 99 000 t yr⁻¹ during the last decades of the 20th century, with the highest dust emissions recorded in 1991 (Ots and Mandre, 2012). In addition to emissions from the cement factory, the site has also received emissions from nearby alkaline generating industries and oil shale power plants, including the Balti power plant, located approximately 100 km west, and the Aru-Lõuna limestone quarry, situated 8 km to the east (Karofeld, 1996, Fig. 1) which supplies raw materials for cement production at Kunda. However, significant emission reductions occurred after 1996, following the installation of pollution control filters, which lowered emissions from 14 000 t in 1996 to 530 t in 2000 and 8 t in 2020 (Fig. 3).

The input of alkaline cement dust emissions generated drastic increases in porewater pH. During 1996–1997, the pH levels taken from peat pools at Varudi varied from 7.6 to 8.5, whereas in natural conditions bog pools have a pH in the range of 3–4. Disturbances at the site are compounded by drainage and peat harvesting for horticulture, which continues to the present day, with harvesting affecting around 40 % of the former centre of the site (Fig. 1C). There is limited documentary information available related to the timing of drainage and peat cutting at Varudi, although historic maps (EtoMesto, 2025; Maa-ja Rummiamet, 2025) indicate that most of the extensive drainage took place before 1977 and after 1944, probably relating to the large-scale drainage projects undertaken in Estonia in the 1960s under Soviet rule (Paavilainen and Päivänen, 1995). Peat extraction matching the areal extent of present-day works at the site by 1988. Prior to this, there is documentary evidence for drainage in the northern, eastern and western margins of the site having taken place around 1866–1911, likely indicative of the systematic forest drainage that took place in Estonia around this time (Paavilainen and Päivänen, 1995).

The chemical composition of the cement dust emissions from Kunda consists primarily of CaO (12 %–17 %), SiO₂ (6 %–9 %), and several other trace metals, including lead (Pb, ca. 60 mg kg⁻¹), cadmium (Cd, ca. 0.9 mg kg⁻¹), and zinc (Zn, 129 mg kg⁻¹) (Mandre and Ots, 1999). The pH of the cement dust in the water solution ranged between pH 12.3 to 12.6 (Mandre and Korsjukov, 2007).

In August 2022, an 86 cm peat core (VAR1) was extracted from Varudi peatland using a 1 m-long Wardenaar peat corer (Wardenaar, 1987, coring location: 59°26^′23^′′ N, 26°35^′55^′′ E, Fig. 1D). The core site was located near an actively harvested peat area but is within an intact section of the site in what was originally the bog's central raised dome. Vegetation was characterised by Pinus sylvestris, with the ground cover dominated by Eriophorum sp., Menyanthes trifoliata, Vaccinium sp. and scattered Betula nana. Both Sphagnum and brown mosses were also present. Despite its proximity to a drainage ditch, the coring location was representative of the overall condition of the site.

Following recovery, the core was wrapped in plastic and transported to the Faculty of Geographical and Geological Sciences at Adam Mickiewicz University, Poznań, Poland for analysis. The core was stored at 4 °C prior to sub-sampling.

Identifiable above-ground plant macrofossils were picked from 1 cm thick sub-samples taken at various depths throughout the core, following frameworks by Piotrowska et al. (2011) and Nilsson et al. (2001). Sphagnum stems, branches and leaves were preferentially used for dating where present. Where these were not available, above-ground remains of ericaceous plants (leaves, stems, seeds) were used instead. Initial samples were taken at 20 cm intervals throughout the core to establish a baseline chronology, which was then used to determine where additional samples would be selected. A total of 12 samples were sent for radiocarbon analysis. Each sample was pre-treated using the acid-base-acid approach and analysed by accelerator mass spectrometry (AMS) at the Poznań Radiocarbon Laboratory, Poland.

To provide a reliable chronology for recently accumulated peat, ²¹⁰Pb and ^239+240Pu analyses were used on material from the upper 40 cm of the core at 1 cm contiguous resolution, following methods outlined by Appleby (1998). Peaks in the activity of ^239+240Pu, which are linked to nuclear fallout events (e.g., 1950s atmospheric nuclear tests), were also measured as independent time-markers to validate and supplement the age-depth model (Mróz et al., 2017; Cwanek et al., 2021). Samples were processed at the Polish Academy of Sciences' Institute of Nuclear Physics, Kraków, Poland using an AlphaAnalyst™ 7200 spectrometer (Mirion Technologies). Quality control and accuracy was ensured by measuring blanks and certified reference material (IAEA 447, IAEA 385) alongside the core samples (results are provided in Table S1. The activity concentration of ²¹⁰Pb (T1/2=22.3 years) was estimated by measurement of its decay product – ²¹⁰Po (T1/2=138.4 d), while ^239+240Pu was measured directly. Activity concentrations of ²¹⁰Pb and ^239+240Pu are reported in units of Bq kg⁻¹.

The age-depth model was constructed by integrating all ¹⁴C, ²¹⁰Pb, and ^239+240Pu data within a Bayesian framework in R using the package “rplum” (rplum package, Aquino-López et al., 2018; R Core Team, 2023). rplum generates maximum age probabilities at user-defined intervals (here every 1 cm), together with maximum and minimum ages based upon calculated 95 % credible intervals. This method allows for the integration of ¹⁴C dates with the ²¹⁰Pb dates without the need for re-modelling (Aquino-López et al., 2018, 2020). Radiocarbon dates were calibrated in rplum using the INTCAL20 curve, with post-1950 samples using the BOMB1 curve for the Northern Hemisphere (Reimer et al., 2020; Uno et al., 2013). In this study, the resultant ages are expressed as calendar years (cal) CE, with 0 BP equal to 1950 cal CE.

A total of 13 radiocarbon dates were analysed from core VAR1, four of which were post-bomb dates extending to a depth of 19.5 cm. The oldest dated section of the core produced a date of >1000 years. Radiocarbon and calibrated dates are shown in Table 1.

The ²¹⁰Pb activity profile throughout core VAR1 (Fig. 4) declined throughout the core, although not in a typical monotonic pattern, with a substantial increase from 12.5 to 19.5 cm, indicating that the rate of ²¹⁰Pb accumulation was not constant throughout the core. This spike is due to the enrichment of ²¹⁰Pb from fly-ash fallout from the cement factory and other sources (Vaasma et al., 2014, 2017). Comparison of the ²¹⁰Pb activities and cement production rates applied to the age-depth model support this (Fig. 4) with a significant correlation (τb=0.487, p=<0.0001), indicating that ²¹⁰Pb accumulation in the core cannot be solely attributed to precipitation. From 32 cm core depth, ²¹⁰Pb activities achieve low levels, although they do not reach the background activity. Unsupported ²¹⁰Pb was calculated using linear regression of the last 5 samples, showing that the samples assumed background activities following the final measured samples.

The ^239+240Pu activity profile reflects the history of atmospheric deposition at the site (Fig. 5). A clear peak at 28.5 cm corresponds to the onset of nuclear testing in 1945, followed by a second, larger peak at 22.5 cm, likely associated with the peak in global fallout from bomb testing in 1963, before the signing of the Partial Test Ban Treaty (Cwanek et al., 2021). Smaller peaks at 16.5 cm suggest possibly the 1986 Chernobyl fallout (Ketterer et al., 2004), and at 5.5 cm potentially originate from the 2011 Fukushima disaster (Bossew, 2013).

The age-depth relationship for core VAR1 (Fig. 6) was calculated by aligning ²¹⁰Pb and ^239+240Pu data with the calibrated radiocarbon dates. Model diagnostics are shown in Fig. S1. Considering the enrichment of ²¹⁰Pb from industrial fallout, the assumption of a constant unsupported ²¹⁰Pb supply was violated. Despite this, anchoring the model with the known 1963 peak in ^239+240Pu activity and adding a constant but uncertain reservoir effect of ca. 15 years for radiocarbon dates improved the model alignment with the peaks in ^239+240Pu. A radiocarbon reservoir effect is possible in Sphagnum peatlands, due to the recycling of “old” gaseous carbon by mycorrhizal fungi associated with ericaceous plants near the peat surface. However, this does not usually occur when individual plant remains are dated, as was the case for our study (Piotrowska et al., 2011). A more likely explanation could be the uptake of the uptake of old carbon via dissolved carbonates', particularly in the more recent samples (Madeja and Latowski, 2008) where old carbon may derive from the buried cement dust.

The resulting age-depth model (Fig. 6) indicates that peat accumulation rates were stable from the base of the core until around 1255 cal CE, after which accumulation slowed to around 0.22 yr cm⁻¹, remaining low until around 1940 cal CE, where there was a sharp acceleration to ca. 2 yr cm⁻¹.

Figure 7 shows that activities of elements associated with clinker dust pollution began to increase around ca. 1873 cal CE, marking the beginning of cement production in Kunda. There is evidence of increasing lithogenic dust deposition throughout the core, beginning as early as the mid-12th century. There is a clear negative correlation between the counts of lithogenic elements associated with cement dust (Ti, Pb, Ca, K, Fe, Cr) and organic matter content, most noticeably between ca. 1942–2006 cal CE. Organic matter content falls sharply until ca. 1988 cal CE, suggesting the accumulation of cement dust occurred during this period, mirroring the results of studies in NE Estonia lakes impacted by oil shale powerplants (Varvas and Punning, 1993; Koff et al., 2016).

The aCAR for the Varudi core is illustrated in Fig. 7, which also shows the trends in LOI %, bulk density, and carbon accumulation throughout the core. Over the past millennium, the average aCAR for the entire core is 148.7 g OC m² yr⁻¹. This is especially high in the upper section of the core, where carbon accumulation rates peak at 342.2±231.7 g OC m² yr⁻¹. For most of the record (ca. 1045 to 1910 cal CE), the mean aCAR is 72.9±28.9 g OC m² yr⁻¹, aligning more closely with average values reported for Northern peatlands (Roulet et al., 2007). The rate of carbon accumulation is relatively stable from the base of the core until ca. 1280 cal CE, with aCAR averaging 94.5±19.8 g OC m² yr⁻¹. After this point, aCAR falls to 51.3±19.7 g OC m² yr⁻¹, remaining low until ca. 1840 cal CE. After this date, aCAR increases significantly, reaching an average of 256.6±221.2 g OC m² yr⁻¹.

The broken stick model shows that the two first components together explain a significant proportion (48.3 %) of the total variance in the peat's chemical composition (PCA1: 25.5 %, PCA2: 22.8 %). These components highlighted elements associated with clinker dust deposition from the Kunda cement factory and are significantly negatively correlated with peat LOI content (Fig. 8).

The palaeoecological record of the Varudi peatland reveals three distinct phases occurring over the past 1000 years: an ombrotrophic bog phase (ca. 1000–1250 cal CE), a poor fen phase (ca. 1250–1570 cal CE), and a wetland forest phase that continues to the present day. Most contemporary research on peatland recovery has been conducted on historically drained, mined, or agricultural sites (Roderfeld, 1993; Graf et al., 2008; Wagner et al., 2008; Paal et al., 2010). In contrast to these studies, an advantage of our palaeoecological approach is that it provides a unique pre-disturbance context for understanding the effects of atmospheric pollution upon this site.

The different phases of development exhibited by the site in contrast with the different periods of anthropogenic activity and disturbance are summarised in Fig. 14. During the early ombrotrophic phase, low palynological richness, minimal microcharcoal counts and abundant peat organic matter indicate a largely forested landscape with limited human impact near the site, consistent with other palaeoecological records throughout Estonia during this period (Veski et al., 2005). The dominance of Sphagnum mosses and mixotrophic testate amoeba species such as Archerella flavum and Hyalosphenia papilio reflect the relatively undisturbed and productive nature of the bog during this phase (Marcisz et al., 2020; Łuców et al., 2022). During this period, water table depths were high, and pH levels were low, consistent with natural conditions for ombrotrophic bogs in the Northern Hemisphere (Warner and Asada, 2006).

The beginning of agricultural activity around ca. 1250 cal CE is broadly coincident with the German-Danish crusaders occupation of Estonia starting in 1227 CE (Veski et al., 2005). From ca. 1250 to 1570 cal CE, increased Poaceae, Secale cereale, and field weed pollen indicates a shift toward more open, meadow-like conditions (Poska and Saarse, 2002; Niinemets and Saarse, 2009), which may relate to the environmental effects resulting from economic changes resulting from the Teutonic order crusades, similar to those identified by Brown and Pluskowski (2011) in a peatland record from northern Poland.

Declines in Pinus sylvestris and Picea abies, alongside increases in early successional species such as Betula and Alnus, and minute increases in microcharcoal accumulation rates (Fig. 11) suggest land clearance using slash-and-burn methods in the surrounding landscape (Jääts et al., 2010). While arboreal pollen remained dominant, indicating continued forest cover around the Varudi peatland, mineral soil enrichment of the peat suggests overland flow or aeolian deposition resulting from land clearance. Similar patterns of early human-induced soil erosion have been recorded in peatland records from central Estonia (Heinsalu and Veski, 2010). This is reflected by decreasing organic matter content and increasing concentrations of lithogenic elements such as Ca and Ti, patterns like those observed in Estonian lake sediment records following land clearance events (Koff et al., 2016; Vandel and Vaasma, 2018). Ti, a conservative lithogenic element, indicates soil erosion from the surrounding catchment (Boyle, 2001; Boës et al., 2011).

By ca. 1570 cal CE, the site had transitioned into a wetter, more minerotrophic fen, indicated by the gradual replacement of Sphagnum by monocots such as Carex sp. and Eriophorum sp., increases in Sphagnum sect. Cuspidata and subsequent declines in Sphagnum sect. Acutifolia and Sphagnum in the plant macrofossil record (Fig. 9). This shift towards wetter conditions is also supported by the testate amoebae reconstruction, possibly indicating permanent or seasonal standing water at this time (Fig. 10), and the presence of Scheuchzeria palustris pollen between ca. 1500–1800 cal CE (Fig. 11), a flood-tolerant species (Tallis and Birks, 1965). This interpretation is consistent with observations from contemporary bog pools, where Sphagnum is often rare or absent in shallow or temporary water bodies on a peatland's surface, while monocots such as Eriophorum species are abundant, as exemplified by the British National Vegetation Classification (NCV) bog pool community classification (M3) (Rodwell, 1998). Nutrient enrichment due to soil erosion resulting from forest clearance and agricultural activities in the landscape at this time likely contributed to this shift (Hölzer and Hölzer, 1998).

The establishment of the Kunda cement factory in 1871 brought significant ecological changes to the site. Substantial increases in Ti, Ca, Fe, and other elements associated with cement dust fallout occurred along with rapidly increasing pH and declining organic matter content, indicating the chemical influence of cement dust pollution upon the peat (Figs. 7 and 8). This pollution appears to have driven rapid changes in testate amoeba communities by ca. 1890 cal CE, as seen in the PrC record (Figs. 13 and 14). These results are consistent with previous palaeoecological studies on alkaline pollution impacts in Estonian peatlands and lakes (Varvas and Punning, 1993; Koff et al., 2016; Vandel and Vaasma, 2018; Vellak et al., 2014). The enrichment of the peat by cement dust is reported to have had a fertiliser-like effect on ligneous and ericaceous species, increasing tree cover, size, and stand density within the industrial region of Estonia at this time (Pensa et al., 2004; Ots et al., 2011).

The concurrent hydrological changes experienced by the site, characterised by persistent water table lowering after ca. 1960, were likely driven by drainage and enhanced transpiration rates following tree and shrub expansion onto the site (Stelling et al., 2023). The simultaneous compounding effect of hydrological change upon the site confounds interpretation of the effects of alkaline pollution at Varudi peatland, as it is likely that the changes that occurred at this time were driven by the combined effects of both disturbances.

From 1955 CE onward, cement production increased, reflected in peaks of Ca, Ti, and K, alongside rising bromine (Br) levels, coincident with increased microcharcoal accumulation rates (increasing from ca. 11 to 58 particles yr⁻¹. The halogen Br is often linked to changes in storminess in peat palaeoecological studies due to being commonly sourced from marine aerosols (e.g., Shotyk, 1997; Turner et al., 2014). Here, elevated Br levels correlate strongly with cement dust indicators, suggesting that increased Br in the core corresponds with enhanced plant decay and oxidation of abiotic organic matter, resulting in the formation of methyl bromide (Lee-Taylor and Holland, 2000; Keppler et al., 2000). The increased presence of fungi in addition to microalgae species Botryococcus braunii in the palynological and plant macrofossil records further suggests that the soil decomposition rate increased during this period, coincident with periods of increased seasonal wetness (Barthelmes et al., 2012; Defrenne et al., 2023; Buttler et al., 2023; Thormann et al., 2003) (Fig. 12).

Following the restoration of Estonia's independence in 1991, more rigorous environmental regulations resulted in a reduction of industrial emissions (Kask et al., 2008). The effect of this decline is evident in our palaeoecological record, with declining pollution markers and pH beginning to return towards pre-disturbance levels after 1996 (Figs. 7 and 10), as well as the apparent (albeit limited) recovery of testate amoeba communities shown by the PrC curve (Fig. 13). Despite these improvements, the site has remained densely forested and enriched, with a persistently lowered water table. We find no evidence of botanical succession toward pre-disturbance conditions in the plant macrofossil record (Fig. 9). The presence of the calciphilous moss Tomentypnum nitens at the core surface likely reflects the legacy of alkaline pollution at the site (Maimer et al., 1992; Hájek et al., 2006; Apolinarska et al., 2024). These cement deposits may still influence surface water chemistry and vegetation through the upward movement of water enriched by the buried dust. Nonetheless, Sphagnum has re-established at the site since its disappearance in the 1960s, suggesting a degree of recovery. Identification and monitoring of the present-day flora could provide additional insights into the extent of this recovery.

Our study found evidence of recovery in PrC of the sub-fossil testate amoeba assemblages (Fig. 13). The community composition of these environmentally sensitive proxies showed signs of turnover nearly 30 years after installing filters at the Kunda cement factory. However, the extent of recovery indicated by the testate amoebae is limited, as the community composition in the top sediment layer more closely resembles conditions from ca. 1977 cal CE, the peak period of cement production at Varudi (Figs. 3 and 13).

We used adaptive spline GAMs to analyse our data, which while suitable for datasets exhibiting abrupt changes such as the example from Varudi (Burge et al., 2023), they cannot yet be incorporated into the Generalized Additive Mixed Model (GAMM) framework described by Simpson (2018) and are more susceptible to issues such as temporal autocorrelation and boundary effects (Simpson, 2018; Burge et al., 2023). While the model best fit our data, more evidence is needed to draw conclusive interpretations about recovery than these results alone. More recently accumulated samples may be more influenced by temporal autocorrelation (Burge et al., 2023). Some testate amoeba species may infiltrate the sub-fossil community composition downcore, making near-surface communities appear more similar (Liu et al., 2024). Another limitation of this study is that our results are based upon reconstructions taken from a single core and as such do not account for the spatial heterogeneity characteristic of peatlands. Although palaeoecological trends tend to be well replicated across multiple cores within a given site (e.g., Barber et al., 1999; Hendon et al., 2001), it may be that the trends reconstructed from VAR1 represent conditions specific to the sampling site, rather than the whole site (Bacon et al., 2017). Paal et al. (2010) examined peatland vegetation communities in eleven bogs in northeastern Estonia at varying distances from sources of atmospheric alkaline pollution, finding evidence of Sphagnum re-establishment and the recovery of other bog-specific plant species at several sites as surface waters re-equilibrated to pre-pollution conditions, particularly within microforms that are less affected by contaminated groundwater. Therefore, recovery at Varudi may be spatially heterogenous, with some areas recovering more quickly depending upon their contact with polluted peat layers.

Studies investigating the recovery of vegetation communities following alkaline pollution show that ecosystem recovery may be limited due to the present, degraded condition of the ecosystem. For example, Vellak et al. (2014) studied bryophyte recovery following reductions in atmospheric fly-ash pollution across Estonia and northwestern Russian sites. They found that bryophyte species growing in more heavily affected sites tended to be adapted for growing in low light conditions, due to competing with the larger and more dense vascular plants that encroached on these sites, in response to the more enriched and alkaline conditions for which they are better adapted than bog-specific vegetation (Pärtel et al., 2004; Zvereva et al., 2008a, 2008b). Increased nutrient and litter supply and root exudates, coupled with the faster growth of vascular plants and trees may further hinder the reestablishment of bog and fen communities, delaying or preventing recovery (Konings et al., 2019). Gunnarsson et al. (2002) demonstrated that nutrient enrichment, often accompanied by higher pH, can give vascular plants a competitive advantage over bog-specific vegetation (Limpens et al., 2003; Dieleman et al., 2015).

Each of the phases in ecosystem development in the palaeoecological record corresponds with substantial changes in the apparent carbon accumulation rate (aCAR) throughout the Varudi record (Fig. 7). Between ca. 1000 to 1250 cal CE, when the site was in a relatively pristine, undisturbed condition, aCAR rates were higher (32.0±10.7 gCm2yr-1) than the average for northern hemisphere ombrotrophic peatlands (23±2 gCm2yr-1) (Korhola et al., 1995; Yu et al., 2009). Throughout the transition from bog to poor fen, from ca. 1360 to 1570 cal CE, average aCAR rates most likely initially increased (38.5±34.3 gCm2yr-1). However, they were highly variable, likely reflecting the incorporation of vascular plant roots from the overlying forested phase, introducing younger carbon into deeper sediments. Between ca. 1630 and 1800 cal CE, rates fell as low as 19.0±5.4 gCm2yr-1, aligning with reported values for Finnish fens (Korhola et al., 1995). Overall, minerotrophic fens typically exhibit lower carbon accumulation rates than ombrotrophic bogs (Loisel and Bunsen, 2020; Yang et al., 2023).

Following the succession from poor fen to forested fen from ca. 1871 cal CE to the present, there is an apparent increase in aCAR, especially in the most recently accumulated peat (280.7±211.3 gCm2yr-1). Due to the incomplete decomposition of labile organic matter at the peat surface, care must be taken when interpreting recently accumulated carbon from peat core records (Young et al., 2019, 2021). This increase is likely due to high litter deposition from trees and shrubs, and the rapid decline in aCAR downcore suggests that this labile material is rapidly cycled back into the atmosphere rather than sequestered in the soil. Our results suggest that the current vegetation composition, dominated by trees and shrubs, is less effective for long-term carbon sequestration compared to earlier phases. While the water table remains reduced, the site shows limited potential for recovery to its original ombrotrophic condition due to ongoing drainage. At the same time, nutrient inputs and evapotranspiration from the overlying trees compound this limiting factor.

By comparing our pH reconstruction with significant successional shifts at the site, we may infer the thresholds at which transitions in the steady state of the ecosystem occurred in response to changes in alkalinity at Varudi bog. Defining tipping points that may be broadly applied to other sites allows for predicting ecosystem shifts in response to global change, allowing for prediction of such shifts in the future (Munson et al., 2018). To our knowledge, our study represents the first step towards defining thresholds for tipping points in peatland ecosystems in response to changes in alkalinity.

Around ca. 1910 cal CE, the reconstructed pH for Varudi showed an increasing trend from the previous, relatively stable conditions, reaching a maximum of 7.2±0.6 by ca. 1985 cal CE (Fig. 10). This value aligns with previous pH measurements for forest litter in the zone surrounding the Kunda cement factory of 7.1–7.4, supporting the reliability of our reconstruction (Paal and Degtjarenko, 2015). We find that the transition from an ombrotrophic bog to fen-like conditions during the mid 13th century, driven by mineral soil enrichment resulting from land clearance for agriculture in the surrounding landscape, was associated with an increase of average pH levels from ca. pH 3.9 to pH 4.1 which represents (due to the logarithmic scale of pH) more than 1.5 times increase in alkalinity. In contrast, the subsequent shift from fen-like to forested conditions corresponded with a small average increase in pH to ca. 5.7, over 40 times more alkaline than the previous state and more than 60 times more alkaline than during the bog phase. Our results suggest that while substantial increases in pH can drive rapid and substantial ecosystem regime shifts, relatively small increases in pH (increases in pH of 0.2–0.3) may also drive critical ecosystem transitions that may be more gradual but can have equally as significant and long-lasting impacts upon peatland ecosystem functioning, such as the capacity for carbon accumulation. However, we stress that the threshold values defined here remain uncertain, owing to the relatively significant uncertainties associated with transfer function models in general (Amesbury et al., 2016). Higher pH values measured from bog pools at the site in 1996–1997 of 7.6–8.5 are slightly higher than our estimates for this period (6.1–6.6), and by 2022 pH had only fallen to 6.3–6.8 (Pärtma, 2023) although our reconstruction indicates pH had fallen to 4.8 by this time, suggesting that our reconstruction, despite having a strong predictive power, underestimates pH levels experienced by the site. Alternatively, this may simply reflect local variation in pH within Varudi, as the pH measurements were taken from bog pools in the NW margin of the bog, not adjacent to the coring location. This may warrant future re-evaluation of our data using transfer functions specifically developed for reconstructing pH in polluted peatlands. In addition, we cannot rule out the possibility that other impacts of cement deposition (e.g., reduced photosynthesis rates due to dust deposition) and, significantly – drainage; may have also played a role in driving local environmental changes at the site.

Contemporary field and lab experiments that manipulate the alkalinity of peatland soils directly may provide more precise values for ecosystem thresholds in response to alkaline pollution. However, to date few studies have done this for alkalinisation: one example is Kang et al. (2018) who conducted a series of field and laboratory pH manipulation experiments across seven peatlands in the UK, Japan, Indonesia and South Korea, finding that increases in pH resulted in significant changes in microbial composition, resulting in increased phenol oxidase activity and enhanced DOC releases. Another is the long-term (active since 2002) Whim Bog experiment near Edinburg, Scotland (Sheppard et al., 2011; Levy et al., 2019), where the effects of enrichment upon a peatland by gaseous ammonia (NH₃) and wet-deposited ammonium (NH4+) and nitrate (NO3-) are compared. Analysis by Sutton et al. (2020) indicates that vegetation declined three times more quickly when exposed to gaseous ammonia and three times for ammonium than for a similar dose of nitrate.

A significant limitation of these studies in defining tipping points is that the long timescale necessary for critical transitions to occur in some cases typically exceeds the lifespan of most observational and experimental studies (Taranu et al., 2018; Lamentowicz et al., 2019). Therefore, further palaeoecological work across a more extensive range of sites or with multiple cores from within a single site may also advance our understanding of ecosystem tipping points in response to alkalinisation. Furthermore, disentangling the effects of alkalinisation from those of drainage is challenging, as both factors may influence plant community composition, carbon accumulation rates, overall ecosystem functioning (Word et al., 2022) and possibly resilience to alkalinisation in peatlands. Since significant drainage and cement production appear to have begun at roughly the same period in the site's history, some of the ecological variability attributed here to cement dust pollution may instead be the result of drainage impacts. However, it is notable that despite reconstructed pH values decreasing following the cessation of significant emissions from Kunda, water table depths have not recovered. This indicates that drainage is unlikely to have influenced the apparent modest ecosystem recovery observed at Varudi in recent decades.