Abstract
Peatlands provide some of the most continuous and informative archives for reconstructing Holocene environmental and climatic changes. Their potential was recognised in the late 19th century, but systematic investigations began in the early 20th century, notably through the work of Swedish scientists. Among these, Erik Granlund made a particularly influential contribution by identifying a series of “recurrence surfaces” (rekurrensytor, RY) in southern Swedish peat bogs. These stratigraphic boundaries, marking transitions from highly humified to less humified peat, were interpreted as evidence of abrupt shifts from relatively dry and warm phases to wetter and cooler conditions. Based on extensive peatland surveys, Granlund proposed several such shifts during the middle and Late-Holocene. Two of his most distinct recurrence surfaces, RY V (~4250 cal a BP) and RY III (~2550 cal a BP), correspond closely to the globally recognised 4.2 and 2.8 ka climatic events, making his early identification of the 4.2 ka event particularly remarkable. The youngest surfaces, RY II (~1550 cal a BP) and RY I (~750 cal a BP), were less frequently recorded but appear to coincide with climatic fluctuations documented elsewhere in northwestern Europe. The uncertain RY IV (~3150 cal a BP), mainly observed in southwestern Sweden, may represent a local or regionally restricted event. This paper revisits Granlund’s concept of recurrence surfaces, evaluates their significance in light of modern paleoenvironmental approaches, and considers whether aspects of his interpretations may have been influenced by confirmation bias.
Introduction
Peat and peat bogs serve as critical archives for reconstructing past climate and environmental changes. Peatlands occur across all climatic zones but are particularly common in the Boreal and temperate regions of the Northern Hemisphere (Xu et al., 2018), where they are often subject to anthropogenic disturbances such as peat extraction and drainage (e.g. Turetsky and St. Louis, 2006). The use of peat as a climate archive has a long history, originating in the late 19th century, and peatland records in northwest Europe provided the foundation for the first systematic Holocene climatostratigraphy, the Blytt–Sernander system (e.g. Berglund, 1983; Birks and Seppä, 2010). Although its division into broad climatic periods is now regarded as overly simplistic, the terminology has remained influential in Scandinavian chronostratigraphy and European paleoenvironmental studies. Importantly, peat stratigraphy was among the first archives to demonstrate substantial Holocene climatic variability, challenging early interpretations from ice and marine cores that suggested relative stability (Chambers and Charman, 2004). The aim of this paper is to synthesise and critically evaluate classical studies of Swedish peatlands, with special reference to the concept of recurrence surfaces, and to contextualise these findings in relation to more recent peatland research and developments in stratigraphical and chronological methods.
Early research and the Blytt-Sernander system
The Blytt-Sernander system, developed in northwestern Europe, classified the Holocene into four (later five) climatic phases based on stratigraphic observations of peat bogs. The Norwegian botanist and geologist Axel Blytt proposed that darker peat layers formed during dry periods and lighter peat during wetter periods, introducing the terms Atlantic (warm and moist) and Boreal (cool and dry; Blytt, 1876). Swedish botanist and geologist Rutger Sernander continued Blytt’s work, refining and expanding Blytt’s system through studies conducted in the 1890s. His research culminated in his doctoral thesis (Sernander, 1894), in which he formally defined the Subboreal and Subatlantic phases.
An alternative interpretation was proposed by the Swedish botanist and geographer Gunnar Andersson (Andersson, 1892, 1902), who argued that the Holocene climate development followed a gradual cooling trend following a Holocene Climate Optimum rather than alternating wet and dry phases—a perspective that is now widely accepted (Wastegård, 2022). This stance led to a prolonged and contentious debate between Sernander and Andersson, with most contemporary geologists siding with Sernander. The Blytt-Sernander classification has since become less relevant and is now replaced by a three-part division of the Holocene: Early, Middle, and Late-Holocene (Walker et al., 2019), more in line with Andersson’s observations.
The boundary horizon and early peatland surveys
Late 19th and early 20th century peat bog investigations, particularly in northwestern Germany, revealed a recurring stratigraphic transition from highly decomposed to less decomposed Sphagnum peat, described as the “Grenzhorizont” or boundary horizon (Weber, 1900, 1926). Through archaeological dating, this shift was estimated to have occurred around 2500 BP. In 1911, Swedish peat researchers, including Sernander and Lennart von Post, were introduced to this concept during an excursion to northern Germany. Inspired by these findings, von Post (1912) identified similar stratigraphic transitions in several raised bogs in southern Sweden. He illustrated his results with several profiles through peat bogs, in all cases showing only a single distinct boundary between “older” and “younger” Sphagnum peat, which he correlated with Weber’s boundary horizon.
The Swedish peatland survey and Granlund’s contributions
The early 20th century was a pivotal era for Swedish lake and peatland research, marked by numerous seminal studies and the development of new methods and concepts, such as pollen analysis and peat humification analysis (von Post, 1916; von Post and Granlund, 1926). A large-scale survey of southern Sweden’s peatlands was conducted under the leadership of Lennart von Post at the Swedish Geological Survey (SGU). Initiated in 1916 during World War I, this survey aimed to assess peat resources for fuel production. The survey contributed to several doctoral theses and laid the foundation for Swedish peatland research (e.g. von Post and Granlund, 1926). While von Post was a key figure, Rutger Sernander played a significant role in initiating these studies (Lundqvist, 1958).
Erik Granlund (1892–1938) was an active participant in this survey, producing several scientific publications and maps. His most significant contribution to peat research is arguably his extensive 1932 monograph on the geology of Swedish raised bogs (Granlund, 1932). In this work, he describes both the lateral expansion and vertical growth of raised bogs in connexion to climate and environmental parameters such as precipitation changes and substrate. His most groundbreaking discovery was the identification of five so called recurrence surfaces—distinct stratigraphic boundaries where peat decomposition shifts abruptly. These transitions were interpreted as evidence of past climate changes, specifically shifts from dry and warm to wet and cold conditions. Granlund also recognised that the survey results could be used for, among other things, phytogeographical studies. Notably, he identified a distribution boundary running largely north-south, separating bogs in western Sweden characterised by dominance of Erica tetralix (cross-leaved heath) from Rhododendron thomentosum (wild rosemary/northern Labrador tea) in the east. Cross-leaved heath can be regarded as a western, oceanic indicator species, whereas wild rosemary/northern Labrador tea is an eastern, boreal indicator species and the boundary is gradual and climatically controlled rather than absolute.
Key studies and findings
Granlund’s initial studies in the 1920s examined peat bogs near Uppsala, including a study of Kungshamnsmossen, 10 km south of Uppsala (Figure 1; Granlund, 1931) followed by his comprehensive monograph in 1932 (Granlund, 1932). In Kungshamnsmossen, situated in a dead-ice hollow in the Uppsala esker, four desiccation boundaries (uttorkningshorisonter I–IV) were identified (Granlund, 1931). These boundaries, later termed recurrence surfaces, were correlated with periods of bog desiccation followed by renewed peat accumulation under wetter conditions, likely reflecting climate-induced fluctuations in groundwater levels within the esker (Granlund, 1931). Through pollen analysis and archaeological dating, Granlund preliminarily dated Boundary III to the Bronze-Iron Age transition (ca. 700–600 BCE), correlating it with Weber’s Grenzhorizont in Germany.
Bogs studied by Granlund (1931, 1932) in Uppland and near Stockholm.
Parallel investigations at Åkerlänna Stormosse (Figure 1), 25 km northwest of Uppsala, provided further evidence of recurrence surfaces. Initially, Granlund considered a desiccation boundary at this site to be the “true” boundary horizon, that is, Weber’s Grenzhorizont but he also identified a lower boundary, which he correlated with the oldest dessication boundary at Kungshamnsmossen (Granlund, 1931).
Definition and interpretation of recurrence surfaces
Already the year after his studies of Kungshamnsmossen and Åkerlänna Stormosse, Granlund presented his extensive survey of more than 160 peat bogs in southern and middle Sweden (Figure 2; Granlund, 1932). In this work, he presented for the first time the concept of recurrence surface to describe distinct changes in the degree of peat humification. His investigation was based partly on his own studies of bogs, but also incorporated results from the South Swedish peatland survey as well as other published and unpublished studies and geological maps. Within the survey, there is considerable variation, ranging from detailed studies of multiple cores with high-resolution pollen analyses and correlations with well-dated archaeological finds, to more general analyses based on single cores with low resolution. In some parts of the study area, particularly in the northern part where archaeological dating was not possible or incomplete, the recurrence surfaces remained undated. It is unclear how many of the 160 bogs Granlund actually visited in the field and only about 100 bogs are mentioned in his thesis (Figure 2; Supplemental Appendix 1). Bogs that were not named were likely included in the peatland survey, and the results were used by Granlund in the compilation of recurrence surfaces, although without any detailed indication of the reliability of the identifications and/or datings. A vast majority of the named bogs have been identified, although in several cases with other names (Supplemental Appendix 1). Approximately 25% of the bogs show today clear evidence of peat harvesting and/or extensive drainage, as indicated by aerial photographs.
Southern Sweden showing the bogs from Granlund’s study that have been identified by name and geographical location, marked with blue symbols.
The investigated bogs are well distributed across southern Sweden, although some areas were studied in greater detail than others, such as around Valdemarsvik in eastern middle Sweden (Figure 2) and south of Bällefors in the northeastern part of Västergötland (Figure 2), both areas hosting several undisturbed bogs. Entirely absent from the survey, however, were the western part of Västergötland, Bohuslän, and Halland, a part of Sweden where many recent peat stratigraphical investigations have been performed (e.g. De Jong et al., 2009; Sjöström et al., 2022). The reason for this is only briefly mentioned by Granlund but was partly related to avoiding conflicts and overlap with the investigations of other researchers at the time. In the case of Västergötland and Halland, this omission represents a significant limitation, as the region receives the highest precipitation in southern Sweden and undisturbed raised bogs are widespread.
Granlund described recurrence surfaces as follows: “The boundaries of this kind always signify a reversion to an earlier or less developed stage in the normal developmental sequence of a peatland. . . After each relapse, the normal developmental sequence is repeated, more or less in its entirety. Consequently, the entire peat stratigraphy is built up of recurrent layer sequences.”
Granlund argued that “recurrence” best captured the concept of these climatic relapses, while “surface” was chosen over terms like “horizon” to avoid ambiguity (Granlund, 1932).
Recurrence surfaces in peat are sharp, laterally traceable boundaries in peat stratigraphy, often associated with changes in humification, botanical composition, or mineral input (e.g. Blackford, 2000; Rundgren et al., 2023). They typically reflect shifts in the local hydrological balance, often caused by climate forcing. For example:
A wetter climate → higher water table → less decomposition → lighter, less humified peat.
A drier climate → lower water table → more decomposition → darker, more humified peat.
Methods for assessing humification have since evolved from qualitative field-based approaches (von Post, 1926) to quantitative laboratory techniques based on the chemical extraction and measurement of humic substances (e.g. Chambers et al., 2011). While most organic matter decomposition occurs under aerobic conditions within the surface acrotelm, decomposition processes also continue at depth under predominantly anaerobic conditions in the catotelm, albeit at substantially reduced rates (Chambers et al., 2011).
In the modern literature, transitions from relatively dry to wetter bog conditions are commonly described as wet shifts rather than recurrence surfaces (e.g. Mauquoy and Barber, 2002). Although recurrence surfaces or wet shifts are frequently interpreted as responses to regional hydroclimatic variability, alternative explanations must also be considered. Such stratigraphic transitions may arise from internal autogenic peatland processes (e.g. Ryberg et al., 2022) or from disturbance events such as wildfire, followed by vegetation recovery (e.g. Foster and Glaser, 1986). For example, localised ecohydrological feedbacks, including differential peat accumulation rates between rapidly expanding Sphagnum lawns and more slowly developing hollow communities, can generate stratigraphic boundaries resembling recurrence surfaces in the absence of external climatic forcing (Loisel and Yu, 2013).
Given that peat humification is influenced by multiple interacting factors, including vegetation composition and intrinsic peatland dynamics, robust paleoclimatic interpretations require careful evaluation alongside independent indicators. Consequently, multiproxy approaches are now widely employed to distinguish external climate forcing from internal peatland processes (e.g. Lamentowicz et al., 2008; Mauquoy et al., 2008).
Dating methods and chronology
A clear limitation of early peatland investigations was the absence of precise absolute dating methods. Like many of his contemporaries, Granlund relied on so-called archaeological dating, in which artefacts recovered from peat deposits were assigned ages based on typological classification and comparison with established chronological frameworks, most notably Montelius’ Bronze Age chronology (Montelius, 1885). This approach provided a relative chronological framework that could be transferred to peat sequences, often in combination with pollen stratigraphy. In many cases, artefacts dated within pollen-analysed stratigraphic contexts were used to constrain the chronology of adjacent peat deposits through palynological correlation. Subsequent archaeological investigations, particularly from Denmark, supported by extensive radiocarbon dating, have demonstrated that Montelius’ relative chronological framework is broadly robust. Discrepancies between conventional typological dating and radiocarbon determinations are generally limited and commonly fall within the range of a few decades (Olsen et al., 2011).
In some cases varve chronology was also employed to refine the dating of recurrence surfaces, and especially the Late-Holocene expansion of spruce, Picea abies, dated by varve chronology to c. 3000 cal a BP in central Sweden (Fromm, 1938).
By correlating pollen stratigraphy across multiple sites, Granlund demonstrated that a distinct recurrence surface in Åkerlänna Stormosse and Kungshamnsmossen was synchronous and correlated to the archaeologically dated Bronze-Iron Age transition (ca. 700–600 BCE).
Granlund’s identified recurrence surfaces
Granlund identified five recurrence surfaces in southern and middle Sweden (Figure 3):
Maps showing the distribution of recurrence surfaces I–V in Granlund’s study (Granlund, 1932).
Granlund included more than 160 peat bogs in his survey, and RY III was identified in over 90% of them. RY I and RY II were the least common and could be confirmed primarily in the northeastern, drier part of the study area. Most investigated bogs were reported as unaffected by human activities such as drainage or peat cutting, but all five recurrence surfaces were found only in one bog, Snöromsmossen, located approximately 7 km southeast of Stockholm (Figure 1). Today, this bog is partly degraded by drainage and peat cutting, making it unsuitable for renewed investigation. Later on, however, all five of Granlund’s recurrence surfaces were reported from other bogs in connexion with geological mapping, for example at Karinmossen in the southern part of the county of Gästrikland (Figure 1; Sandegren and Asklund, 1948). The peat humification changes were assessed exclusively in the field, and as far as is known, no more detailed laboratory analyses such as microfossil or macrofossil analyses were carried out. It is also unclear how pronounced a humification change had to be in order to be classified as a recurrence surface. Later studies have shown, however, that field assessments of peat humification do not always agree with laboratory measurements, where colorimetric analysis has been used to determine the degree of decomposition more precisely (Chambers et al., 2011; Kaila, 1956). In cases where bogs from the Swedish Geological Survey’s peatland surveys were included, sampling began at 50 cm depth in accordance with mapping protocols, which likely excluded humification changes from the most recent centuries, possible excluding the identification of RY I in many bogs. Granlund nevertheless observed a highly humified surface layer in many bogs, particularly in southern and western Sweden where peat deposits are thick, which he interpreted as evidence of stagnation and reduced peat accumulation (Granlund, 1932: 58).
In particular the three older surfaces, RY III, IV and V were most frequently observed in the wetter southwestern part of Sweden (Figure 3). RY III was linked to increased annual precipitation by approximately 100 mm, coinciding with the formation of ombrotrophic bogs and ombrotrophication of fens in Sweden (Granlund, 1932). Granlund also identified four periods of intense peat initiation, dated at 6000 BCE (7950 cal a BP), 3000 BCE (4950 cal a BP), around 2000 BCE (after RY V; ca 3950 cal a BP) and between c. 500 and 1 BCE (c. 2450–1950 cal a BP; Granlund, 1932). He interpreted the first three periods as paludification of dry land due to increased precipitation and rising ground water tables. The fourth is especially pronounced in the northern part of the investigated area where ombrotrophic bogs start to develop mainly on overgrown topogenic fens following an increase in precipitation at the Subboreal-Subatlantic boundary (Granlund, 1932: 163–164).
That same year of 1932, one of Sweden’s leading Quaternary geologists, Gösta Lundqvist, immediately adopted Granlund’s concept of recurrence surfaces (Lundqvist, 1932). Through theoretical reasoning, he also predicted two additional older recurrence surfaces, RY VII, dated to approximately 3700 BCE (5650 cal a BP), and RY VI, dated to around 2800–2700 BCE (4750–4650 cal a BP), as well as a future surface, RY 0, projected to form around 2200 CE. Lundqvist acknowledged that climate variability is influenced by multiple interacting factors but placed particular emphasis on a proposed cosmic driver: long-term variations in tidal forcing as proposed by Pettersson (1913). According to this hypothesis, periodic increases in tidal intensity, occurring approximately every 1400–1700 years (tantalisingly similar to the periodicity of Bond events!), would enhance oceanic and atmospheric disturbances, leading to increased storminess and precipitation. These climatic effects were assumed to promote rising groundwater levels, paludification of peatlands, and ultimately the formation of recurrence surfaces. However, there are certain weaknesses in Lundqvist’s line of reasoning. Only three recurrence surfaces, RY I, III and V coincide with Pettersson’s proposed tidal maxima, and the suggested linkage between tidal forcing and recurrence surfaces appears tenuous, even from a 1930s perspective. Many years later, however, and particularly following the first radiocarbon analyses of recurrence surfaces, compiled by Lundqvist (1957), his father Gösta Lundqvist expressed increasing scepticism regarding their chronological significance and synchroneity of recurrence surfaces (Lundqvist, 1963).
Granlund died in 1938, but his work left a lasting impact on Swedish peat research in the following decades and played a significant role in the Geological Survey of Sweden’s (SGU) mapping of Quaternary deposits during the mid-20th century (e.g. Granlund, 1943; Lundqvist, 1951). Several contemporary studies, however, suggested that Granlund’s system was more complex than initially thought, and some authors suggested additional recurrence surfaces between some of Granlund’s five surfaces. One such was a humification change between RY V and RY IV, dated to approximately 1700 BCE (around 3650 a BP), which was designated as RY IV:1 (Wenner, 1939).
Another complicating factor for many years was the lack of precise dating methods. As a result, many studies conducted during the mid-1900s were characterised by lengthy and often complex discussions, involving, among other things, archaeological dating, pollen-analytical lead horizons (especially the Late-Holocene expansion of spruce, Picea abies), and dates from shoreline displacement curves—yet without independent chronological determinations (e.g. Fries, 1951; Selling, 1940).
With the advent of radiocarbon (C-14) dating in the 1950s, Jan Lundqvist (1957) demonstrated that several recurrence surfaces appeared to exhibit a delay of 200–400 years in western Sweden compared with eastern Sweden, making them unsuitable for precise chronological correlation. However, Lundqvist also noted that C-14 dating is not an entirely exact method and did not fully dismiss the concept of recurrence surfaces. It is important to recognise that radiocarbon dating was still in its infancy in the 1950s. Many of the obtained dates had large error margins and were reported as conventional ages without calibration to calendar years, which was unknown at the time. Additionally, issues such as age reversals and plateaus were not known (this particularly affects attempts to date RY III more precisely) and some of Lundqvist’s “too young” dates can simply be explained by the fact that the radiocarbon dates were not calibrated to calendar years (cf. Figure 4).
Observations of recurrence surfaces and wet shifts in Swedish ombrotrophic bogs over the last 6000 years. Recurrence surfaces I–VII are indicated by vertical blue boxes (Granlund, 1932; Lundqvist, 1932) and black boxes show age intervals for wet shifts in calendar years. The data are compiled from several regional studies: (a) Bohuslän (Fries, 1951), dated mainly by pollen-analytical lead horizons, (b) Värmland (Lundqvist, 1957, Figure 1), partly dated by uncalibrated radiocarbon ages, (c) Store Mosse, Småland (Svensson, 1988), (d) Undarsmosse, Halland (Jong et al., 2006), (e) Värmland (Borgmark and Wastegård, 2008), (f) wet shifts and climate-induced peat initiation in northern and central Sweden where grey bars mark peat initiation maximum and regional wet shifts (Rundgren, 2008), (g) amoebae-inferred wet shifts in two western Swedish bogs (Mallon, 2012), (h) Store Mosse, south Sweden (Kylander et al., 2013), (i) peat- and tree ring-inferred wet shifts Åbuamossen, Skåne, (Edvardsson, 2016), and (j) Davidsmosse, Halland (Sjöström et al., 2022, Figure 5).
During the mid-20th century, extensive mapping efforts were conducted in the northern and central parts of Sweden by the Geological Survey of Sweden (e.g. Granlund, 1943; Lundqvist, 1951). These mapping projects typically included comprehensive peat stratigraphic investigations, in which recurrence surfaces served as an important tool for dating and correlation. Published and unpublished results from these regional mappings were thoroughly compiled by Rundgren (2008), who identified two peat initiation maxima, dated at 9500–8000 cal a BP and 6000–5500 cal a BP and several recurrence surfaces or wet shifts dated to the last 5000 years, especially frequent at 4500–4100, 3300, 2500–2100 and 1300 cal a BP (Figure 4). Several of these wet shifts coincided with climate-related wet shifts observed in southern Sweden by Granlund and others.
As noted in the introduction, the main aim of this paper is to thoroughly review and critically evaluate Granlund’s concept of recurrence surfaces in light of more recent peatland studies and other paleoclimatic data. A key objective is also to assess whether Granlund’s observations are robust and reproducible, or whether they may have been influenced by confirmation bias. In the following sections, Granlund’s results will be compared primarily with well-dated peatland investigations from Sweden and northwestern Europe. In addition, other types of climate archives and historical data will be examined and evaluated in relation to Granlund’s studies, particularly those with evidence for climatic events during the past c. 6000 years.
Recurrence surfaces and wet shifts in Swedish bogs
It is evident from Figure 4 and Table 1 that several wet shifts have occurred in Swedish bogs during the second half of the Holocene, from around 6000 cal a BP onwards, roughly from the period following the Holocene Climate Optimum to the present (Wastegård, 2022). A long-term trend towards wetter conditions after 5000 cal a BP is interpreted as reflecting progressively stronger westerly airflow and increasing precipitation (Rundgren, 2008). It is worth noting that most early studies were conducted at a time when radiocarbon dating was either unavailable or still in its infancy, making some of the age determinations uncertain (e.g. Fries, 1951; Lundqvist, 1957).
Comparison of the timing of recurrence surfaces/wet shifts inferred from bog investigations in across NW Europe (cal a BP).
Record spanning the last c. 3800 years.
Record spanning the last c. 4500 years.
Record spanning the last c. 4500 years, minor wet shifts are not shown.
Record spanning 900–4800 cal a BP, minor wet shifts are not shown.
Record spanning the last c. 4000 years.
Record spanning the last c. 4500 years.
This also include RY VI and RY VII that were proposed by Lundqvist (1932).
Wet shifts at Draved Mose compiled by Hughes et al. (2000).
There is clear evidence that certain periods were marked by more or less synchronous environmental changes across large regions. The most distinct examples occur around 4200 cal a BP and around 2600 cal a BP. A somewhat less distinct but still regionally coherent shift is observed between c. 1600 and 1350 cal a BP (Figure 4). In addition, many records document a wet shift between 3600 and 3500 cal a BP, while the oldest wet phase, dated to approximately 6000–5500 cal a BP, also appears to have had a broad regional extent. Other humification changes, however, display a more irregular and spatially variable pattern across Sweden (Figure 4).
The following chapter provides a brief overview of each recurrence surface, including intermediate surfaces, and explores their potential correlations with other climate archives, primarily from northwestern Europe and the North Atlantic region.
RY VII (ca 5650 cal a BP)
The oldest surface was hypothesised by Lundqvist (1932) and a wet shift and peat initiation is indeed present in many southern Swedish bogs at this time, provided that the bog is located at an elevation high enough for it to have formed at that time (Table 1; Borgmark and Wastegård, 2008; Kylander et al., 2013; Mallon, 2012; Sandegren, 1937). A peat initiation maximum at 6000–5500 cal a BP is evident in western Sweden and Rundgren (2008) hypothesised that this peak may have resulted from a combination of decreasing temperatures and increasing precipitation. In the soil map description for Västerbotten County, published after Granlund’s death, it was noted that only two distinct recurrence surfaces could be identified in this northern region: one corresponding chronologically to RY VII and a later, much more commonly occurring RY III. An even older recurrence surface, RY VIII was also briefly mentioned, although without a more precise chronological determination (Granlund, 1943). A period of high lake levels are also inferred from a dendroclimatological study in Jämtland, central Sweden at c. 5500–5350 cal a BP (Figure 5; Gunnarson, 2008). It is likely that RY VII reflects a real climatic event in the North Atlantic region, as evidenced by an IRD peak around 5900 ca BP (Figure 6; Bond et al., 1997), a significant cold period in Greenland between approximately 6000 and 5000 a BP (O’Brien et al., 1995), first Holocene episode of increased storminess in the North Atlantic at 5800–5500 cal a BP (Figure 6; Sorrel et al., 2012), advancing glaciers in Norway (Figure 6; Nesje, 2009) and a wet-shift in bogs in northern Norway at ca 5900 cal a BP and in Finland at c. 5500 cal a BP (Table 1; Tuittila et al., 2007; Vorren et al., 2012). In continental Europe, a climate reversal, driven by fluctuations in solar activity, begins around 5600 cal a BP and persists until approximately 5300 cal a BP (Figure 6; Magny et al., 2006).
Observations of wet and/or cold events in Sweden from non-peat records over the last 6000 years.
Observations of wet and/or cold events in Northern Europe and the North Atlantic region over the last 6000 years.
RY VI (ca 4700 cal a BP)
This hypothetical recurrence surface was identified in only a few bogs in western Sweden (Sandegren, 1937). Fries (1951) describes a wet shift around 4950 cal a BP but broadly speaking, the period between approximately 5500 and 4200 cal a BP appears, with some exceptions, to have been characterised by stable or gradually decreasing temperatures in Scandinavia and the North Atlantic region, but without any distinct climate events (e.g. Seppä et al., 2005). In the Scandinavian mountains, glaciers began to advance again after having been largely melted away between roughly 6600 and 6000 cal a BP (Figure 6; Nesje, 2009), while lake levels in southern Sweden were slowly increasing until ca 4100 cal a BP (Figure 5; Digerfeldt, 1988). There is also some agreement with high lake-level records in European lakes (Figure 6; Magny, 2004; Pleskot et al., 2018) and a few Scandinavian bog records show a wet shift around 4800–4600 cal a BP (Table 1).
RY V (ca 4250 cal a BP)
RY V was the oldest recurrence surface identified by Granlund, archaeologically dated to approximately 2300 BCE (c. 4250 cal a BP). It is widespread across southern Sweden in bogs at higher elevation, with a predominantly western distribution, although the exact timing varies between c. 4500 and 4200 cal a BP (Figure 3; Granlund, 1932). However, RY V was also documented in bogs south of Stockholm that are at high enough elevations butisabsent in southern Sweden. Granlund also described that climate-driven peat initiation occurred in many places at this time. Several other studies of Swedish bogs also show a clear humification shift around 4200 cal a BP, indicating that this was a regional event that affected bogs in south and middle Sweden (Figure 4; Table 1; Borgmark and Wastegård, 2008; Edvardsson, 2016; Fries, 1951; Jong et al., 2006; Lundqvist, 1957; Mallon, 2012) and in many areas this is the oldest wet shift found.
Several other climate records from Sweden also indicate a climate event around 4500–4000 cal a BP, including advancing glaciers in the mountains (Figure 5; Rosqvist et al., 2004), reactivation of sand dunes (Figure 5; Alexanderson and Bernhardson, 2016), pollen-inferred temperatures (Seppä et al., 2005) and even a small transgression of Lake Vättern at the time of RY V (Norrman, 1964).
The recently established Mid-to-LateHolocene boundary is based on extensive compilations of global climate proxy data suggesting a significant climatic shift around 4.2 ka BP (e.g. Walker et al., 2019). However, other large-scale syntheses of proxy records do not consistently support a globally synchronous climate event at this time (e.g. Wanner et al., 2008). This will be discussed in more detail later in the paper.
One of the most widely dispersed Icelandic tephra layers, Hekla-4, has considerable potential for investigating climate variability associated with the 4.2 ka event. It has been identified in numerous peat sequences and lake sediment records across northwest Europe, and has recently been found and dated in the NGRIP ice core (4325 ± 8 cal a BP; Davies et al., 2024). It has, however, only in a few cases been found in studies specifically addressing hydroclimatic changes in peatlands (e.g. Borgmark and Wastegård, 2008; Vorren et al., 2007). Nevertheless, Hekla-4 offers significant potential for assessing whether the climatic shifts around 4200 BP occurred synchronously across regions or with temporal offsets.
RY IV (ca 3150 cal a BP) and intermediate humification shifts between RY V and RY IV
RY IV was first identified in bogs near Uppsala as the oldest recurrence surface, distinctly positioned beneath the so-called boundary horizon, later designated as RY III. The absence of RY V in these records was likely due to the fact that the bogs had not yet developed at the time of its formation (Granlund, 1931). Archaeological dating places RY IV between approximately 3300 and 3150 cal a BP, distinctly before the rise in Picea pollen dated with varve chronology to c. 3000 BP (Fromm, 1938).
RY IV is widespread across southern Sweden but occurs much less frequently than RY III (Figure 3). It is best developed in bogs on the Västergötland Plains in western Sweden. Granlund (1932) estimated that RY IV corresponds to an increase in annual precipitation of about 50 mm. However, he also noted that distinguishing RY IV from RY V and RY III in the field was often difficult, particularly in areas with slow peat accumulation. Later studies indicate that the time period between RY V and RY IV was characterised by multiple shifts in peat humification (Fries, 1951; Selling, 1940), a pattern subsequently confirmed by Borgmark and Wastegård (2008).
In many cases, both in Sweden and other parts of northwestern Europe, a marked shift towards wetter and/or colder conditions appears to have occurred around 3600–3500 cal a BP (Table 1; e.g. Charman et al., 2006; Edvardsson, 2016; Väliranta et al., 2007; Vorren et al., 2012). Whether this shift corresponds to the event Granlund identified as RY IV but with an incorrect age determination is difficult to determine without re-examining the original bogs where he documented a distinct recurrence surface.
The fact that even early studies, conducted with the same dating limitations as Granlund (Fries, 1951; Wenner, 1939), questioned both the timing and synchronicity of RY IV suggests that, along with RY I, it remains the most uncertain of Granlund’s recurrence surfaces. Rundgren (2008), however, noted a peak in the frequency of wet shifts at c. 3300 cal a BP in bogs in central and northern Sweden and Edvardsson (2016) noted three periods of wet shifts in a bog in Skåne, dated to 4100–4050, 3500 and 3180–3000 cal a BP. It is thus possible that the period between approximately 4200 and 2550 cal a BP, that is, RY V and RY III is best characterised as a time of frequent and recurring wet shifts, rather than a single, well-defined regional climate event (e.g. Borgmark and Wastegård, 2008; Edvardsson, 2016). Interestingly, however, Jessen et al. (2005) noted a rapid major climate event of high magnitude around 3100 a BP in a lake sediment study from Västergötland, occurring within an otherwise stable period between approximately 3450 and 2700 cal a BP. The cause of this climate event is unknown, but it is described as an anoxic phase. Whether this event is directly linked to the formation of a distinct RY IV in several bogs in Väsergötland remains uncertain, but it is a possibility that merits further investigation.
Most other European and North Atlantic records do not exhibit a distinct climatic event contemporaneous with RY IV (cf. Figure 6), with the exception of some bog records showing a wet shift around 3200–3300 cal a BP (Table 1). However, it is noteworthy that the “Bond event 2” at around 2800 cal a BP actually comprises two peaks of similar magnitude in drift-ice indexes, the first occurring at approximately 3300 cal a BP and the second at 2800 cal a BP (Bond et al., 2001; Figure 2). An abrupt cold event has also been described from the North-West Atlantic at 3046–3018 cal a BP (Klus et al., 2018).
RY III (ca 2550 cal a BP)
In Granlund’s study, RY III is the recurrence surface with the widest geographical distribution and is generally the most well-developed (Figure 3). It is also the recurrence surface believed to correspond to Weber’s boundary horizon and appears to be the one unanimously accepted by contemporary Quaternary geologists in Sweden. It also appears to be the most reliably dated recurrence surface, occurring at the transition between the Bronze Age and Iron Age in Swedish archaeology, well after the rise in Picea pollen, traditionally dated to 3000 cal a BP in central Sweden (e.g. Fromm, 1938). Also in the northern county of Västerbotten, RY III was noted in virtually all investigated bogs as the only clear humification change (Granlund, 1943).
A globally recognised climate event around 2800 cal a BP is well documented in the literature and has been linked to a sudden decrease in solar activity around 2900–2800 cal a BP (e.g. van Geel et al., 2000). This period also coincided with a peak in ice-rafted debris (IRD) deposition in the North Atlantic (Figure 6; Bond et al., 1997) and several peat records show pronounced changes to wet conditions at this time, not only in Europe (Figure 6, Table 1; Charman et al., 2006) but also in global records (e.g. Chambers et al., 2007). Some investigations, however, question the link to solar activity (Plunkett, 2006). In Sweden, numerous peat records indicate a shift towards wetter conditions during this time (Figure 4, Table 1; Andersson and Schoning, 2010; Jong et al., 2006; Mellström et al., 2015; Sjöström et al., 2022; Svensson, 1988). Evidence of a wetter climate is further supported by rising lake and groundwater levels in southern Sweden (Figure 5; Berglund et al., 1991; Digerfeldt, 1988) and increased intensity of precipitation in the Scandes Mountains (Berntsson et al., 2015).
Granlund dated RY III to approximately 2550 BP, but there are strong reasons to believe it corresponds to a calibrated age of around 2800–2700 cal a BP. The variability of wet shifts in Swedish bogs at this time, as shown in Figure 4, likely reflects differences in dating methods used before and after the advent of radiocarbon dating. Additionally, the challenge of precisely dating records around the so-called Hallstatt Plateau (c. 2450 14C year BP) contributes to this spread of several 100 years (e.g. Reimer et al., 2020).
RY II (c. 1550 cal a BP) and humification shifts between RY II and RY I
The second-youngest recurrence surface, RY II, was found across large parts of the area studied by Granlund but was largely absent in southern Sweden. It appears to be most common in the drier eastern middle Sweden (Figure 3). Granlund dated RY II to the mid-Iron Age, around 400 CE (c. 1550 BP).
However, numerous studies from Sweden and the broader North Atlantic region suggest a significant climate shift occurred during the middle centuries of the first millennium CE. This period is often referred to as the
In more recent Swedish studies using modern dating techniques, the climate changes of the first millennium CE, predating the Mediaeval Warm Period, emerge as among the most pronounced of the Late-Holocene, despite considerable variation in age estimates. These changes may even represent two separate climate events, separated by a few centuries. An older phase, dated to approximately 1700–1500 cal a BP, coincides with RY II and the onset of the DACP. This period is recorded as a wet shift in peat sequences in south-western Sweden (Figure 4; Borgmark and Wastegård, 2008; Jong et al., 2006; Sjöström et al., 2022), marked by increased wind erosion in Dalarna county (Figure 5; Alexanderson and Bernhardson, 2016), and reflected in wetter conditions in Jämtland county, as inferred from lacustrine stable isotope data (Figure 5; Andersson and Schoning, 2010). A younger phase, occurring around 1300–1000 cal a BP, is characterised by glacier advances in the Scandinavian mountains (Figure 5; Rosqvist et al., 2004), narrow tree rings in the Torneträsk chronology (Grudd et al., 2002), and increased storm frequency in southern Sweden (Figure 5; Kylander et al., 2023). This period is also associated with higher precipitation, as indicated by submerged tree trunks and peat proxies (Figures 4 and 5; Gunnarson, 2008; Rundgren et al., 2023; Svensson, 1988).
However, uncertainties in age models make it difficult to determine whether these records reflect two distinct climate events or a single, prolonged shift, especially since some studies possibly capture delayed responses (e.g. glacier advances). Some records also suggest unusually warm summers around 1500–1400 cal a BP, which may separate the wet/cold events at 1700–1500 and 1300–1000 cal a BP (Linderholm and Gunnarson, 2005). A second cold or wet phase is also visible in other European climate archives, such as Danish peat deposits around 950 cal a BP (Figure 6; Aaby, 1976; Mauquoy et al., 2008), glacier advances in Norway 1200–1000 cal a BP (Figure 6; Nesje, 2009), and rising lake levels in Europe 1300–1100 cal a BP (Figure 6; Magny, 2004; Pleskot et al., 2018).
Borgmark and Wastegård’s stacked peat humification record from Värmland, southwestern Sweden (Borgmark and Wastegård, 2008) shows two regional humification shifts in the first millennium in the common era: the first at approximately 1550 cal a BP, corresponding in time to RY II (Borgmark and Wastegård, 2008, Figure 6), and the second at around 1250 cal a BP, marking the onset of one of the wettest periods of the last 4000 years in their record.
Granlund, however, observed only a single, distinct recurrence surface between RY III and RY I in a limited number of bogs. Whether these recurrence surfaces truly formed simultaneously is difficult to determine, especially given the large uncertainties associated with individual age estimates. One example is Snöromsmossen, SE of Stockholm where Granlund estimated RY II to have formed between 1850 and 1250 cal a BP (Granlund, 1932: 83). It is thus possible that Granlund grouped together several asynchronous wet shifts under RY II without definitive supporting evidence.
RY I (c. 750 cal a BP)
Granlund identified only one recurrence surface formed during the last millennium, and only in a few bogs, primarily in the drier eastern part of the studied area (Figure 3). In bogs from the Swedish Geological Survey’s peatland surveys, however, sampling started at 50 cm depth according to mapping protocols, likely overlooking humification changes from the past centuries and thereby missing RY I in many cases. However, later investigations in western Sweden (Figure 2a; Fries, 1951; Lundqvist, 1957), as well as Rundgren (2008) in northern and central Sweden, noted a shift towards wetter bog conditions in several bogs around 750–500 cal a BP, though not as pronounced as during earlier parts of the Holocene. The dating of this wet shift aligns well with the onset of the Little Ice Age (LIA), which marks the culmination of the cooling trend of the Middle and Late-Holocene. RY I is separated from RY II and associated humification shifts between c. 1700 and 1000 cal a BP by the Mediaeval Warm Period (MWP) with predominantly dry and warm conditions in Europe and the North Atlantic region, dated to around 1150–700 cal a BP (e.g. Gauld et al., 2024). The Mediaeval Climate Anomaly and the Little Ice Age are well-studied climatic events from the later part of the Late-Holocene, with comprehensive summaries provided by authors such as Ljungqvist (2009) and Gauld et al. (2024) and several proxy and historical records in Sweden show evidence of a warm period followed by a cooling, although the timing of the events differ between different archives and regions (Grudd, 2008; Hormes et al., 2004; Leijonhufvud et al., 2008; Rosqvist et al., 2004, 2013; Sundqvist et al., 2010). Most of these studies show a clear transition to a colder climate, while the results regarding changes in precipitation during the LIA are more divergent (e.g. Edwards et al., 2017; Gunnarson, 2008; Retsö and Leijonhufvud, 2020; Rosqvist et al., 2013). However, there is substantial evidence suggesting that northern Europe experienced a significant increase in precipitation around 750 cal a BP (Helama et al., 2009). Dendrochronological analyses indicate that a mediaeval megadrought lasting several centuries between approximately 1150 and 750 cal a BP was followed by a period of markedly increased rainfall, beginning around 750 cal a BP. Wet shifts are also identified in several European bogs around 700–600 cal a BP (Table 1). Other European records, however, date the onset of colder and more variable conditions to slightly later, around 500 cal a BP (Figure 6; e.g. Seppä et al., 2009; Sorrel et al., 2012; Tuittila et al., 2007).
There are relatively few Swedish studies of peat sequences from the last millennium with the same temporal resolution as, for example, investigations of Lille Vildmose in Denmark, which have shown a clear connexion between sunspot minima and wet shifts during the last millennium (Mauquoy et al., 2008). However, in general, many peatlands in Europe appear to have experienced a drying trend over the past 300 years due to climate drying and warming, as well as human impact (Swindles et al., 2019), something that Granlund noted in Swedish bogs already in his thesis (Granlund, 1932). Finally, it remains to be seen whether RY 0 will develop in Swedish bogs around 2200 CE or not, as predicted by Gösta Lundqvist in 1932.
Discussion
Granlund’s investigations, despite being nearly a century old, represent a vast and unique body of field data covering large parts of southern Sweden, and that his large-scale regional surveys of peat records remain unparallelled today. Many of his findings and conclusions have proven accurate and have been validated by later studies. However, some results can be questioned and may have been influenced by confirmation bias, or simply by the fact that the dating methods available at the time were not precise enough to distinguish between closely spaced changes in peat humification, such as around RY II and RY IV.
The strength of Granlund’s investigations lies primarily in the sheer volume and geographical breadth of the material he collected. Rundgren’s compilation of peat stratigraphic data from the Geological Survey of Sweden (SGU) should also be acknowledged in this context (Rundgren, 2008). More recent studies have often focussed on individual peatlands which, although analysed using a wider range of proxies and more advanced dating methods than were available in Granlund’s time are not necessarily representative of large-scale regional climate variation (e.g. Borgmark and Wastegård, 2008; Edvardsson, 2016; Kylander et al., 2013; Sjöström et al., 2022). Today, several of the bogs investigated by Granlund have been subject to peat cutting and drainage and are not suitable for paleo climatic research (Supplemental Appendix 1).
Two of Granlund’s most widespread recurrence surfaces, RY V and RY III, correspond to well-known and widespread Holocene climate events: the so-called 4.2 and 2.8 ka BP events, although the significance of the 4.2 ka BP event has been questioned by some authors (e.g. Bradley and Bakke, 2019; McKay et al., 2024). These shifts have left clear imprints in the peat stratigraphies and other hydroclimatic records around central/northern Europe (Figures 4–6). In particular, RY III is widely distributed across southern Sweden and appears to reflect a significant increase in precipitation across much of the country, including the Scandinavian mountain range (Berntsson et al., 2015; Jong et al., 2006; Sjöström et al., 2022; Svensson, 1988). Other climate archives in Sweden show little or no change around 2.8 ka BP, suggesting that this event primarily involved increased precipitation rather than a change in temperature (e.g. Antonsson et al., 2008; Engels, 2021; Rosqvist et al., 2004) .
Granlund’s early identification of a climatic event around 4200 cal BP is particularly notable in light of the broader scientific recognition of the 4.2 ka event as a global marker of climatic transition, although most records, especially from the Eastern Mediterranean to India show that it was a dry event rather than a wet event (e.g. Helama, 2024). Granlund’s RY V may represent one of the earliest regionally explicit interpretations of a climatic shift near 4.2 ka BP This climatic event, now commonly referred to as the 4.2 ka BP event, was first framed as a global phenomenon by Weiss et al. (1993) and popularised by Staubwasser et al. (2003) who discussed Holocene monsoon variability and drought.
In recent years, particularly since the 4.2 ka event was proposed as the boundary between the Middle and Late-Holocene, there has been ongoing debate about whether this event truly represents a global climatic phenomenon. Recent studies have both supported and contested the idea that the 4.2 ka event was globally synchronous, highlighting the complexity of regional climate responses during this period (e.g. Helama, 2024; McKay et al., 2024; Nan et al., 2025). It lies beyond the scope of this article to discuss whether the 4.2 ka BP event constitutes a significant global climate event; however, it is worth noting that several studies of climate archives from the North Atlantic region, including Sweden, indicate a distinct climatic shift at this time, primarily towards colder and wetter conditions (Alexanderson and Bernhardson, 2016; Andresen et al., 2006; Bebchuk et al., 2024; Bond et al., 1997; Candy et al., 2025; Jalali et al., 2019; Jong et al., 2006; Klus et al., 2018).
With RY V and III as clear examples of regionally widespread evidence of synchronous or near-synchronous regional climate changes, there is greater uncertainty regarding the other recurrence surfaces. This is particularly true for RY VI (c 4700 cal a BP), which has only been identified in a few bogs in Scandinavia, but also for the later-formed RY IV, II, and I. In the case of RY IV, it is quite possible that the period between RY V and RY III was characterised by several, but not necessarily synchronous, changes in humification, something that was already suggested during Granlund’s time by other researchers and later confirmed in more recent studies (e.g. Borgmark and Wastegård, 2008; Fries, 1951; Wenner, 1939).
As for RY II, it is clear that multiple climate changes occurred in Northern Europe during the first millennium of the Common Era (Figure 6), and that it is more a question of two distinct phases rather than a single synchronous climate change.
RY I was identified in only a few of the bogs examined by Granlund, yet it may reflect a broader regional shift in precipitation patterns across Northern Europe during the onset of the Little Ice Age. This interpretation is consistent with dendroclimatological evidence from Finland and associated proxy records indicating a prolonged mediaeval megadrought, followed by a marked increase in precipitation around 750 cal BP (Helama et al., 2009).
Relatively few peatland studies focussing on the last millennium have been conducted in Sweden compared with other parts of NW Europe. High-resolution, well-dated investigations, such as the studies from Lille Vildmose in Denmark (Mauquoy et al., 2008) are almost entirely absent. This represents a significant knowledge gap, particularly given the potential of such records to capture detailed climate variability, hydrological changes, and human–environment interactions during the Late-Holocene.
Conclusion, critical evaluation and future outlook
Granlund’s concept of recurrence surfaces offer valuable insights into Holocene climate variability, despite being nearly a century old. The correspondence of RY V and RY III with the 4.2 and 2.8 ka BP events confirm the potential of peat stratigraphy to register regionally expressed climate shifts. While modern techniques allow for finer resolution and multi-proxy approaches, the extensive spatial coverage of Granlund’s surveys remains unparallelled. Future research that integrates these historical data with contemporary methods could not only improve regional chronologies but also help resolve questions about the spatial coherence and climatic drivers of key Holocene transitions. It is, in fact, highly impressive that Granlund, nearly a century ago, was able to draw conclusions regarding the use of peat as a high-resolution climate archive, conclusions that remain broadly valid despite the absence of modern methods such as radiocarbon dating and tephrochronology.
Several of the peat bogs investigated by Granlund remain unaffected by human activities such as drainage and peat cutting, and have not been examined with modern methods. This applies, for example, to the area south of Bällefors in northeastern Västergötland (Figure 2), which hosts numerous undisturbed bogs with up to four recurrence surfaces. Peatlands just north of Granlund’s study area, such as Jordbärsmuren and Karinmossen in the southern part of Gästrikland (Figure 1; Sandegren and Asklund, 1948), also merit renewed investigation. From these sites, multiple recurrence surfaces have been described, along with the presence of volcanic ash deposits that enable precise chronological correlations between different peat stratigraphies (Svensson, 2013).
A key question remains: do recurrence surfaces genuinely reflect Holocene climate shifts, or were they influenced by confirmation bias? For example, several temporally close humification changes may have been grouped together and interpreted as a single, synchronous climate event, as appears to be the case between RY V and RY IV, and around the time of RY II. It may also be noted that many bogs displayed only one distinct recurrence surface, which Granlund consequently identified as RY III, even in cases where the age determination was uncertain or entirely lacking.
Granlund’s survey clearly focussed primarily on the older part of the RY system, spanning approximately 5000–2000 cal. years BP. In contrast, the younger period (c. 2000–0 cal. years BP) received less attention. This may have led to the potentially misleading conclusion that fewer humification shifts occurred during the Common Era than in the preceding three millennia. One possible reason for this gap was the limited availability of well-dated sites for the later period, as well as the fact that many recent peat records had already been affected by peat cutting.
Supplemental Material
sj-xlsx-1-hol-10.1177_09596836261440848 – Supplemental material for Granlund’s recurrence surfaces revisited and reanalysed
Supplemental material, sj-xlsx-1-hol-10.1177_09596836261440848 for Granlund’s recurrence surfaces revisited and reanalysed by Stefan Wastegård in The Holocene
Footnotes
Acknowledgements
I am very grateful for the comments provided by two anonymous reviewers, which substantially improved the first version of the article. ChatGPT was used for translating Swedish texts and, in some cases, for improving sentence flow and avoiding linguistic errors and mistakes.
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Declaration of conflicting interests
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References
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