Buried Podzols as a pedostratigraphic marker for the Medieval Climatic Optimum: Grębociny soil in the dune deposits of the European Sand Belt

Abstract

The paleopedological record documented in aeolian dunes of the eastern European Sand Belt comprises predominantly Arenosols and only occasionally well-developed Podzols. There are several Late Pleistocene pedostratigraphic marker horizons of varied soil types designated in the European dune and loess deposits, but none falls within the range of the Holocene. Buried Podzol occurrences found recently in 10 inland dune sites dispersed throughout Central and Eastern Poland share similar pedological properties, geomorphological setting, and age in the 5th–15th century AD range of the historical Middle Ages. Therefore, they meet the criteria for distinction as a pedostratigraphic marker under the name Grębociny soil, after a locality with the most advanced podzolization of the paleosol dated to the High Middle Ages (1000–1300 AD). Preservation of the soils was enabled by burial during anthropogenically induced dune remobilization. At least some of the investigated dunes were used as pasture during soil development, as evidenced by tetrapod hoofprints recorded in and above the buried Podzols. Prevalent podzolization during the Middle Ages, in contrast to preceding and later times, could be facilitated by not only an impact of agriculture and forestry, but also relative warmth and humidity of the Medieval Climatic Optimum (ca. 900–1400 AD).

Keywords

dunes Late-Holocene paleosols pedology podzolization Poland

Introduction

Buried paleosols (Krasilnikov and Calderón, 2006; Muhs et al., 2013; Nettleton et al., 1998; Vancampenhout and Deckers, 2010) provide one of a few sources of information on the Quaternary local terrestrial paleoenvironment during the conditions of nondeposition and land surface stabilization (Catt, 1998; Kraus, 1999; Vancampenhout et al., 2013). In addition to that, paleosols occurring in aeolian deposits can serve as pedostratigraphic marker horizons, applicable for stratigraphic correlations at various spatial scales (Costantini, 2018; Hirsch et al., 2021; Lowe and Walker, 2014; Retallack, 2019).

The principles of designating paleosols as formal stratigraphic marker horizons (under the name Geosol) are elucidated in the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2021). However, these guidelines appear not to have gained general acceptance by the European Quaternary science community (Muhs et al., 2013). Although a number of pedostratigraphic marker horizons of varied age and extent are distinguished in Europe, no convention for naming such marker paleosols has yet been adopted. Therefore, all stratigraphically relevant late Pleistocene/early Holocene European marker horizons are named on an informal basis and referred to as “soil” (most often), “layer,” “bed” or “horizon” (Haesaerts et al., 1999; Hirsch et al., 2021; Kaiser et al., 2009; Sauer et al., 2016; Vancampenhout and Deckers, 2010; Van der Hammen, 1952). Pedostratigraphic nomenclature remains likewise beyond the scope of the International Stratigraphic Guide (Salvador, 1994). Notwithstanding the informal status of a pedostratigraphic unit, its type profile should in principle represent a distinct and typical development of a particular paleosol (Retallack, 2019). Independent of time concepts by definition, pedostratigraphic units may be diachronous, but may also comprise a record of uniform or similar-age soil development (Palmer, 2013).

Although in Europe the focus of much scholarly work to date has been on loess-paleosol sequences (Haesaerts et al., 1999; Kraus, 1999; Sauer et al., 2016; Vancampenhout et al., 2013), two Late Glacial (ca. 14.5–11.7 ka BP) pedostratigraphic marker horizons were also designated in aeolian sand sedimentary successions of periglacial environments belonging to the European Sand Belt (ESB; Figures 1a and 2; Koster, 1988; Zeeberg, 1998). Throughout the areas of the ESB, the distinct Usselo soil (Van der Hammen, 1952) is widely recognized in aeolian dune and coversand deposits. The Usselo soil is regarded to be a marker horizon for the Allerød (ca. 14–13 ka BP), although some cases are determined to correspond to the Younger Dryas (ca. 13–11.7 ka BP) and the Preboreal stage (ca. 11.7–10 ka BP). The Usselo soil occurs as weakly developed podzolic soil or podzolized Arenosol (Jankowski, 2012). Another widely documented paleopedological horizon that had been ascribed to the Late Glacial is the so-called Finow soil. However, it is currently regarded to be a product of early diagenesis, rather than the soil-forming processes, and its pedostratigraphic significance is contested (Hirsch et al., 2021). Therefore, Usselo soil remains the only unquestionable paleopedological horizon recognized in Quaternary dune sands throughout Europe. Other Late Pleistocene marker horizons of widely recognized stratigraphic importance, including the Lohne, Bryansk and Trubchevsk soils, are in finer-grained wind-blown (loess) deposits (Figure 2; Hirsch et al., 2021).

In the Western part of the ESB, Late Glacial – Early Holocene aeolian sands predominantly form flat coversand landscapes (Koster, 2009). Sub-Atlantic (ca. 2500 BP–present) aeolian remobilization of older deposits locally resulted in the formation of drift sand sheets and the development of dunes (Pierik et al., 2018). Toward the eastern part of the ESB, the Late Glacial – Early Holocene aeolian deposits increasingly occur in the form of inland dunes clustered into dune fields (Koster, 2009). However, in numerous locations, the Late Glacial – Early Holocene dunes were subjected to reactivation during the Sub-Atlantic anthropogenic dune-forming phase (Tolksdorf and Kaiser, 2012; Twardy et al., 2014). Since the post-glacial climate amelioration, sandy landscapes in Europe have become inactive, vegetated and stabilized by forests (Küster et al., 2014). Therefore, virtually all episodes of soil erosion and dune reactivation may be related to human-induced vegetation depletion. In turn, forest growth and successive soil development were restricted to periods of less intensive land use (Beerten et al., 2014).

The sedimentary record of dune reactivation episodes includes sand layers or lenses burying a soil developed before the dune reactivation. In numerous cases, the soil was subjected to winnowing and the contact between the younger and older sedimentary units is marked by an erosional truncation surface (Hsieh et al., 2023). Some studies ascribe Sub-Atlantic aeolian processes solely to spatial and temporal differentiation of land use (Küster et al., 2014; Reiß et al., 2009). In contrast, several researchers highlight the role of an interplay between anthropogenic pressure and climatic factors (Bateman and Godby, 2004; Clemmensen et al., 2007; Starkel, 2005).

On the surface of present-day dune landforms in the east of the ESB, Podzols only occur in constrained local areas. The direct reason for such distribution is the climatic and pedological conditions of the podzolization (Fritsch et al., 2009; Lundström et al., 2000; Mokma et al., 2004; Sanborn et al., 2011; Stützer, 1998). This process generally occurs in soils in humid climates that developed on coarse-grained parent materials that were deficient in alkaline components (Lundström et al., 2000). Also, the importance of vegetation is a significant factor in podzolization and Podzols formed on dune sands typically occur beneath coniferous forest vegetation, whose litter promotes the accumulation of a significant amount of organic matter required for the podzolization (Lundström et al., 2000; Rahmonov and Oleś, 2010; Rahmonov et al., 2006).

Buried soils in Holocene aeolian deposits within the ESB are developed as Arenosols or Podzols (Beerten et al., 2012; Hsieh et al., 2023; Jäger and Kopp, 1999; Jankowski, 2002; Jankowski et al., 2011; Jankowski, 2012; Kaiser et al., 2009; Kappler et al., 2019; Kasse and Aalbersberg, 2019; Moska et al., 2022; Schirmer, 1999; Wallinga et al., 2013). In the western part of the ESB (N Germany, the Netherlands, Belgium), occurrences of the latter predominantly represent the time of the Holocene climatic optimum (Beerten et al., 2012; Schirmer, 1999; Van Mourik et al., 2012; van Mourik et al., 2012). In the sites containing younger Sub-Atlantic paleosols, either these pre-Medieval (ancient/Roman period; before 500 AD) or post-Medieval (Modern period; after 1500 AD) are predominantly nonpodzolized Arenosols, inceptive soils comprising of organic matter-enriched (A) mineral horizon over parent sandy material (C) (Jäger and Kopp, 1999; Kappler et al., 2019; Kozarski and Nowaczyk, 1991; Moska et al., 2022; van Mourik et al., 2012). In turn, the majority of Sub-Atlantic buried Podzol occurrences appear to represent the Medieval period (Beerten et al., 2014; Behrendt et al., 2002; Dijkmans et al., 1992; Kappler et al., 2019; Lungershausen et al., 2018; Tolksdorf et al., 2009; van Mourik et al., 2012; Wallinga et al., 2013). In the east of the ESB, buried soils are most commonly Arenosols (Hsieh et al., 2023). As studies of buried soils have been less advanced there to date, the only chronometrically confirmed occurrence of Medieval buried Podzol was documented in Roztocze Upland, SE Poland (Balaga and Chodorowski, 2006). Ultimately, known occurrences of Medieval buried Podzols have not yet been incorporated into a pedostratigraphic marker framework. The present contribution aims at documenting 10 recently recognized occurrences of Medieval buried Podzols in the eastern part of the ESB (Poland). Based on common paleosol properties, geomorphological setting and age in all studied sites, the new pedostratigraphic marker horizon is proposed and named Grębociny soil, after the type locality.

Regional setting

The study area extends across the eastern part of the European Sand Belt in Central and Eastern Poland (Figure 1). Ten investigated outcrops of Medieval buried Podzols are situated in inland dunes belonging to dune fields of varied size and dispersed across physiographic regions of Central Poland Lowlands (sites 1, 2 and 6–10) and Lesser Poland Upland (sites 3–5) (Table 1). From the paleogeographic point of view, the study area is located in the old morainic belt beyond the extent of the last (Vistulian) ice sheet. The recognized Medieval Podzol occurrences are apparently clustered in two distinct regions (north-eastern and south-western), although the area in between was also discerned for their presence. The dunes of parabolic, longitudinal and complex morphologies (Table 1) are located on varied substrates, ranging from (glacio)fluvial and glaciolacustrine sand and mud, glacial till to older bedrocks. The aeolian sand in the east of the ESB was sourced primarily by winnowing from outwash plains and river valleys (Łapcik et al., 2021). Presumably, all studied dunes developed in periglacial climatic conditions during the Late Glacial – Early Holocene dune-forming phases (Nowaczyk, 1986) and then were subjected to anthropogenically induced remodeling during historical times (Tolksdorf and Kaiser, 2012; Twardy et al., 2014). Anthropogenic influence on the distribution and composition of vegetation in Central Europe has reportedly occurred with variable intensity for at least four millennia, with a significant peak of deforestation during the Roman Period (ca. 3^rd century BC – 5^th century AD; Tolksdorf and Kaiser, 2012). On the contrary, the Early Middle Ages (500–1000 AD) were characterized by relatively less intense agricultural and economic activity, resulting in partial restoration of natural forests (Tolksdorf and Kaiser, 2012; Twardy et al., 2014). From the High Middle Ages (1000 to 1300 AD) on, deforestation progressed continuously, which entailed an increase in aeolian morphodynamic activity (Twardy et al., 2014). Recently, the dunes are stabilized with either artificially planted forest or self-seeding shrubbery (Table 1). The present-day annual mean air temperature in central-eastern Poland is ca. 8°C and the annual mean precipitation is ca. 600 mm (Tomczyk and Bednorz, 2022).

Table 1.

List of study sites with background information and radiocarbon ages of charcoals from podzolic paleosols.

Site no.	Site name	GPS coordinates	Physiographic region	Present day morphology	Present day vegetation	₁₄C age BP	IntCal20 cal AD
1	Zwierzyniec	51°29′47.4″N 19°23'59.4″E	Southern Greater Poland Lowland, Central Poland	Arm of a parabolic dune	Pine forest	535 ± 30	1323–1439
2	Grębociny	51°25′41.0″N 19°15'35.5″E	Southern Greater Poland Lowland, Central Poland	Longitudinal dune	Birch and pine shrubbery	925 ± 30	1033–1206
3	Ochotnik	51°08′02.3″N 19°47'24.5″E	Przedbórz Upland, Central Poland	Central part of a parabolic dune	Grass, single pines and birches	1255 ± 30	671–876
4	Osiny	50°52′24.0″N 19°37'03.5″E	Przedbórz Upland, Central Poland	Arm of a parabolic dune	Birch and pine forest	370 ± 30	1450–1634
5	Chlewice	50°40′44.7″N 19°59'37.1″E	Nida Basin, Central Poland	Central part of a parabolic dune	Pine forest	1260 ± 30	668–874
6	Rytele–Olechny	52°40′35.7″N 22°09'55.5″E	Southern Podlasie Lowland, Eastern Poland	Central part of a parabolic dune	Pine forest	435 ± 30	1421–1615
7	Ostrowik	52°06′13.4″N 21°24'53.0″E	Central Masovia Lowland, Eastern Poland	Central part of a complex dune	Pine forest	620 ± 30	1296–1400
8	Unin	51°52′44.1″N 21°42'16.1″E	Southern Podlasie Lowland, Eastern Poland	Central part of a parabolic dune	Pine forest	1490 ± 30	545–642
9	Przyłęk	51°46′25.4″N 21°39'18.5″E	Southern Podlasie Lowland, Eastern Poland	Central part of a complex dune	Birch and pine forest	575 ± 30	1306–1422
10	Gołąb	51°30′35.7″N 21°55'20.7″E	Central Masovia Lowland, Eastern Poland	Slope of a complex dune	Pine forest	465 ± 30	1410–1464

For map locations, see Figure 1b. Cal AD ages are reported as min-max equal probability ranges at a 95.4% confidence level.

Materials and methods

Pedological field and laboratory analyses

To investigate the morphology and properties of the buried soils identified as Podzols, 10 soil profiles were selected and excavated in dune deposits of the European Sand Belt in Poland (Figures 3 and 4). The fieldwork conducted through 2018–2022 comprised detailed sedimentological documentation of the entire succession exposed in visited sand pits. All studied soils were buried under a layer of younger sandy sediments of various thicknesses, in most cases with weakly-developed sandy soils classified as Arenosols (Table 2; IUSS Working Group WRB, 2022). Soil profiles were described according to the Guidelines for Soil Description (Jahn et al., 2006), and then disturbed samples from each soil horizon were taken to the laboratory. The color of the collected samples was described in the moist state according to Munsell Soil Color Charts (Munsell, 1975).

Table 2.

Morphology, physical and chemical properties of the buried soils studied.

Horizon	Depth	Structure^a	Roots^b	Consistence^c	Color	Sand	Silt + Clay		pH	SOC	SOC/N
	cm				moist	Sand	Silt + Clay		pH	SOC
	cm				moist	%		%	H₂O	%
No.1 Zwierzyniec, Ortsteinic Rustic Albic Podzol (Arenic, Epic, Eutric)^d
Soil above: approx. 10–20 cm of sand without pedological features
Ab	0–10	SG	F	VFR	10YR 4/1	94.5	5.5		4.3	2.94	25
Esb	10–33	SG	V	LO	10YR 5/2	98.3	1.7		5.0	0.18	4
Bhsmb	33–40	SG; GR	F	FI	10YR 4/6	97.3	2.7		4.2	0.92	15
Bsb1	40–60	SG	V	FR	10YR 5/6	100.0	0.0		4.8	0.19	6
Bsb2	60–90	SG	V	LO	10YR 6/4	97.5	2.5		4.9	0.14	5
BC	90–120	SG	V	LO	10YR 6/3	99.3	0.7		4.5	0.16	6
No. 2 Grębociny, Ortsteinic Rustic Albic Podzol (Arenic, Epic, Eutric, Hyperspodic)^d
Soil above: 10–20 cm of sand without pedological features
Ab	0–10	SG	V	LO	10YR 4/2	97.5	2.5		5.2	0.43	17
Esb	10–30	SG	V	LO	10YR 6/2	99.6	0.4		5.6	0.14	6
Bhsmb1	10–45	SG; GR	V	FI	7.5YR 3/4	97.3	2.7		4.6	0.41	11
Bhsmb2	45–75	SG; GR	V	FR	10YR 4/6	99.3	0.7		4.6	0.31	8
Bsb	75–120	SG	V	LO	10YR 5/4	99.0	1.0		4.9	0.25	7
C	120–150	SG	V	LO	10YR 6/4	99.5	0.5		5.0	0.24	5
No. 3 Ochotnik, Albic Podzol (Arenic, Epic, Eutric)^d
Soil above: 70 cm of typical Arenosol Ol-A-AC-C
AE	0–12	SG	V	LO	10YR 4/1	98.8	1.2		5.2	0.50	14
Eb	12–15	SG	V	LO	10YR 5/3	99.4	0.6		5.2	0.21	5
Bhsmb	15–25	SG; GR	V	LO	10YR 4/6	99.8	0.2		4.8	0.46	10
Bsb	25–45	SG; GR	N	FR	10YR 5/4	99.8	0.2		5.0	0.31	8
BC	45–60	SG	N	LO	10YR 5/4	99.5	0.5		5.2	0.30	10
C1	60–85	SG	N	LO	10YR 6/6	99.7	0.3		5.1	0.32	9
C2	85–110	SG	N	LO	10YR 5/4	99.7	0.3		5.2	0.26	8
No. 4 Osiny, Rustic Entic Podzol (Arenic, Epic)^d
Soil above: 35 cm of weakly developed Arenosol C-A-AC
AE	0–10	SG	F	VFR	10YR 4/3	97.3	2.7		4.9	0.47	20
Bsb	10–25	SG	F	VFR	10YR 4/6	97.8	2.2		4.8	0.50	14
BC	25–55	SG	V	VFR	10YR 5/6	98.8	1.2		4.8	0.17	6
C1	55–85	SG	V	VFR	10YR 5/6	99.2	0.8		4.9	0.23	6
C2	85–105	SG	N	VFR	10YR 5/6	99.3	0.7		4.9	0.27	6
No. 5 Chlewice, Ortsteinic Rustic Albic Podzol (Arenic, Epic)^d
Soil above: 60 cm of typical Arenosol Ofh-Oh-AC-C
Ab	0–12	SG	F	LO	10YR 4/1	97.2	2.8		4.7	1.51	18
Esb	12–50	SG	F	LO	10YR 5/2	98.6	1.4		4.5	0.20	7
Bhsmb1	50–70	SG; GR	V	FI	7.5YR 3/4	97.2	2.8		4.3	1.19	17
Bhsmb2	70–100	SG; GR	V	VFR	10YR 5/6	98.7	1.3		4.5	0.30	10
BC	100–140	SG	V	LO	10YR 6/4	98.5	1.5		4.6	0.18	5
C	140–170	SG	V	LO	10YR 6/3	98.9	1.1		4.7	0.11	5
No. 6 Rytele-Olechny, Albic Podzol (Arenic, Epic)^d
Soil above: approx. 70 cm sequence of Arenosols Ofh-AC-A-C
Ab	0–5	SG	F	LO	10YR 4/2	96.6	3.4		4.7	0.66	13
Eb	5–10	SG	V	LO	10YR 5/2	97.2	2.8		4.8	0.26	10
Bsb	10–55	SG	V	LO	10YR 5/4	94.3	5.7		4.6	0.18	4
BC	55–80	SG	V	LO	10YR 6/4	99.5	0.5		4.9	0.15	5
No. 7 Ostrowik, Ortsteinic Rustic Albic Podzol (Arenic, Epic)^d
Soil above: 65 cm of typical Arenosol with weak podzolization Ofh-Oh-AE-C
Ab	0–10	SG	F	VFR	10YR 3/1	98.6	1.4		4.8	2.17	27
Eb	10–20	SG	F	VFR	10YR 4/2	99.1	0.9		4.4	0.35	8
Bhsmb	20–30	SG; GR	F	FI	10YR 3/4	99.2	0.8		4.5	1.77	22
Bsb	30–65	SG	V	LO	10YR 4/6	99.8	0.2		4.6	0.39	8
BC	65–100	SG	V	LO	10YR 6/4	100.0	0.0		4.9	0.18	7
No. 8 Unin, Rustic Albic Podzol (Arenic, Epic)^d
Soil above: 80 cm of typical Arenosol Ofh-A-C
Ab	0–10	SG	F	VFR	10YR 4/1	96.3	3.7		4.7	0.50	15
Eb	10–18	SG	F	VFR	10YR 5/3	97.4	2.6		4.7	0.22	12
Bsb	18–40	SG	V	VFR	10YR 4/6	97.9	2.1		4.6	0.46	10
BC	40–80	SG	V	VFR	10YR 5/4	98.7	1.3		4.9	0.20	6
No. 9 Przyłęk, Albic Podzol (Arenic, Epic, Eutric)^d
Soil above: 25 cm of weakly developed Arenosol Ofh-A-C
AE	0–15	SG	V	LO	10YR 4/2	95.6	4.4		5.1	0.82	14
Bsb	15–35	SG	F	LO	10YR 4/6	97.5	2.5		5.1	0.64	16
BC	35–60	SG	V	LO	10YR 4/6	98.7	1.3		5.2	0.59	11
C1	60–100	SG	V	LO	10YR 5/6	99.2	0.8		5.3	0.29	7
C2	100–150	SG	N	LO	10YR 5/4	99.1	0.9		5.3	0.25	12
No. 10 Gołąb, Albic Podzol (Arenic, Epic)^d
Soil above: 70 cm of typical Arenosol Olf-A-C
AE	0–10	SG	N	LO	10YR 4/2	98.0	2.0		4.8	0.81	16
Bsb	10–30	SG	V	VFR	10YR 4/6	96.1	3.9		4.7	0.85	20
BC	30–60	SG	N	LO	2.5Y 5/4	98.6	1.4		4.9	0.40	17
C	60–90	SG	N	LO	2.5Y 5/4	99.5	0.5		4.8	0.56	13

SG – single grain, GR – granular.

F – few, V – very few, N – absence.

LO – loose, VFR – very friable, FR – friable, FI – firm.

According to IUSS World Reference Base WRB 2022.

In the laboratory, soil samples were air dried and passed through a 2 mm steel sieve. All laboratory analyses were performed on fine earth material (fraction <2 mm). The soil texture was determined using calgon (mixture of sodium carbonate and sodium hexametaphosphate) as a dispersant (Gee and Bauder, 1986; Van Reeuwijk, 2002) without the removal of organic matter and secondary oxides. Soil organic carbon (SOC) and nitrogen (N) concentrations were determined by dry combustion gas chromatography using a CHN analyzer Vario MICRO cube (Nelson and Sommers, 1996). The soil pH was measured potentiometrically in distilled water using a 1:1 ratio (Thomas, 1996). The soil profiles were then classified according to the World Reference Base for Soil Resources (IUSS Working Group WRB, 2022).

Age determination

An approach was chosen to obtain ¹⁴C ages of charcoals to determine the age of the paleosols studied, since the dating of bulk samples or extracted organic fractions is recognized to provide unreliable results (Wallinga et al., 2013). In turn, dating the sand using optically stimulated luminescence (OSL) could have not provided adequate precision and would reflect the age of sand deposition, instead of soil development. For this reason, at least 20 charcoal grains of 2–4 mm diameter per sample dispersed in A or AE soil horizons were collected directly from each paleosol and stored in polyethylene bags. It should be noted that radiocarbon dating only allows for indirect determination of soil development timeframes, as the date obtained reflects the time of forest fires that were occurring either before or during the soil formation. Hovever, since radiocarbon dates are based on mixed material from numerous charcoals, they represent an averaged age of charcoal assemblage, and the effect of contamination by redeposited older charcoals is minimized.

The AMS ¹⁴C dating was carried out at Poznań Radiocarbon Laboratory (laboratory code designation Poz), Poland, based on samples prepared following the procedure used at the Oxford Radiocarbon Accelerator Unit. The calibration of conventional ¹⁴C ages was performed using OxCal v4.4.4 software (Bronk Ramsey, 2009) with the IntCal20 calibration curve (Reimer et al., 2020). Calibrated age ranges and distributions are reported at 95.4% confidence level (Table 1; Figure 5; Millard, 2014). Sample treatment and calibration results are reported in detail by Ninard et al. (2023).

Results

General characteristics of study sites

In all studied sites, the podzolic paleosols are covered with sand layers or clinoforms of varying thickness, ranging from tens of centimeters to ca. 2 m (Figures 3 and 4). On the top surface of these surficial sand bodies, recent incipient Arenosols are developed, apart from outcrops where the topsoil has been removed by mining. At sites 1 and 2 (see Table 1), buried Podzol is the only paleosol occurring in the exposed part of dune sedimentary succession. In sites 5, 6, 7 and 8 multistory sequences (Holliday et al., 2017) of several younger (Modern period) incipient buried Arenosols occur in the interval above the Podzol. In the remaining sites, either one Arenosol (site 4) or a multistory sequence of two or more Arenosols (sites 3, 9 and 10) of Late Glacial – early Holocene age (Ninard et al., 2022, 2023; Sokołowski et al., 2022) occur in the dune deposits overlain by the buried Podzol (Figure 4). At sites 3, 9 and 10, the buried Podzol and Late Glacial – early Holocene paleosols are separated by a sediment interval merely 1 m thick or less (Figures 3 and 4).

The intervals underlying the buried Podzols (C soil horizon) record bioturbation by roots and animals, evidenced by complete homogenization of the sediment or occurrence of individual traces (Figure 6). The thickness of the interval completely homogenized by bioturbation ranges from 0.7 m to 1.6 m below the base of the A horizon. Structures interpreted as individual invertebrate burrows of 1–2 cm width penetrate up to 2 m beneath (Figure 6). At sites 2, 3 and 8, both the top surface of the buried Podzol and its sandy overburden (up to ca. 1 m above the top surface of Podzol) display structures interpreted as cattle hoofprints and, in the case of site 8, human footprints (Hsieh et al., 2023). At site 1, although the top surface of the buried Podozol is not intensely bioturbated, the overlying sandy deposits contain layers with abundant structures interpreted as cattle hoofprints (cf. Hsieh et al., 2023).

Age of the paleosols

The calibrated radiocarbon ages of all examined buried Podzols are within 5th–15th century AD range of historical Middle Ages (Figure 5; Buko, 2007). The range of investigated Podzol ages extends from pre-Polish state early Slavic settlement (Unin site) up to Late Medieval (1300–1500 AD) heyday of the Polish state (Osiny and Rytele-Olechny sites), a ca. 1 kyr long period marked by social and economic development across Europe (Buko, 2007). The sites in Figure 5 are ordered according to spatial proximity, with the sites 1–5 belonging to the south-western region, and the sites 6–10 to the north-eastern region. Even though there is no evident age clustering with respect to geographic setting, Late Medieval Podzols are the majority (six out of ten) of studied cases, with four of them located in the north-eastern region (Figures 1 and 5). The same calibrated age ranges are observed in the case of sites 3 (Ochotnik) and 5 (Chlewice), located 52 km away. On the other hand, pairs of sites located even closer to each other (1 and 2–12 km; 3 and 4 –31 km; 7 and 8–32 km; 8 and 9–12 km) display markedly different age ranges.

Pedological properties

The studied paleosols were classified according to the occurrence of a spodic diagnostic horizon as Podzols according to the World Reference Base 2022 (Table 2). Morphologically, eluvial (Eb, Esb) and illuvial (Bsb or Bshmb) horizons of varying thickness are present in the soils. The greatest differences between the studied soils were observed in the occurrence of well-developed eluvial horizons along with partially cemented illuvial (orstein) horizons, which were present in profiles from Zwierzyniec, Grębociny, Chlewice, and Ostrowik (Table 2, Figure 6). These horizons were characterized by a granular structure and significantly higher organic matter content compared to the other iluvial horizons where cementation were not detected. Morphologically, the studied soils were dominated by a single grain structure, which is characteristic of soil formations where loose sands predominate. The granular structure appeared only in the Bhsmb and Bsb horizons, associated with a higher concentration of illuviated organic matter. In terms of particle size composition, the sand fraction clearly dominated, while the content of silt and clay fractions combined never exceeded 6%. Increased proportion of these fractions occurred in all studied soils in the humus horizons and in the Bhsb and Bsb horizons in profiles 1, 2, 5, 6, and 10, or decreased evenly with depth, as in the case of profiles 3, 7, 8, and 9 (Table 2).The consistency of the soil was loose and became more compacted in spodic horizons, especially in soils with the occurrence of ortstein (profiles 1, 2, 5, and 7). The root content was rather low and decreased with depth of the soil. The reaction of the studied paleosols was acidic to very acidic, ranging from pH values 4.2 to 5.6. Typically, the lowest pH values were found in buried spodic horizons with a high concentration of organic carbon (Bhsb). In comparison to the other studied soils, profiles from Zwierzyniec, Grębociny, Ochotnik, and Przyłęk exhibited pH values indicating the presence of eutric properties. Furthermore, the paleosols from Ochotnik and Przyłęk showed significantly higher pH values exceeding 5.0 (Table 2). The organic matter accumulated in the analyzed soils was either strongly or moderately decomposed, as indicated by low values of the SOC/N ratio, mostly not exceeding 12 (Table 2, Figure 6). Exceptions were the humus or spodic horizons where the organic matter was less decomposed compared to the rest of the soil (higher SOC/N values). In the humus horizons, the SOC concentration did not exceed 1%, except in the sites Zwierzyniec, Chlewice, and Ostrowik, where the organic carbon content was 2.94%, 1.51%, and 2.17%, respectively (Table 2, Figure 6). The same sites also exhibited the highest downward movement of organic matter within the studied paleosoils, confirmed by the highest SOC concentration in the Bhsb horizons, which were 0.92% (Zwierzyniec), 1.19% (Chlewice), and 1.77% (Ostrowik).

Discussion

Kappler et al. (2019) analyzed the relative probability distribution of radiocarbon ages from 19 Late Glacial – Holocene buried Podzols found in various sites in NE Germany. Although it was concluded that the development of Podzols was not restricted to any specific period of the Holocene, in eight of the sites (42%) buried Podzols were developed during the period of 500–1500 AD. The dating results in the present study, considered together with those published from the western part of the ESB (Kappler et al., 2019; Küster et al., 2014; Lungershausen et al., 2018; Van Mourik et al., 2012; van Mourik et al., 2012), show spatial and temporal heterogeneity of the Podzol development. In view of this, the timeframes of dune stabilization and remobilization could be subjected to influence by local human activities, for example, settlement development, deforestation, and agriculture. The six-fold growth of the European population between 650 AD and 1350 AD (Williams, 2000) entailed prevalent colonization of forested areas. The need for settlement and arable areas led to the most extensive clearing of central European forests up to that time (Williams, 2000). The record of these clearances in various sedimentary and dendrochronological geoarchives is an important field of Medieval environmental history studies (Kaiser et al., 2019, 2021).

However, it is likely that a number of biotic processes and human factors interacted to change contribution and in some cases increase the prevalence of Podzols during the Medieval ages. Podzolized soils occur in humid climatic zones characterized by a wide range of thermal conditions, yet exclusively these experiencing sufficient rainfall and predominance of precipitation over evapotranspiration (Fritsch et al., 2009; Lundström et al., 2000; Mokma et al., 2004; Sanborn et al., 2011; Stützer, 1998).

Medieval Climatic Optimum (MCO; Figure 2; Mann, 2002), known also as Medieval Climatic Anomaly (Bradley et al., 2003), was reportedly characterized by not only milder climate, but relatively higher precipitation and with less pronounced thermal anomalies compared to the preceding and following periods. High-resolution Medieval paleoclimate reconstructions for Central Europe are based on numerical modeling (Figure 5; Gómez-Navarro et al., 2014). Although exact paleotemperature and paleoprecipitaton values vary depending on the model adopted and are still subject to discussion, there is consensus that MCO in Europe lasted from ca. 900 to ca. 1400 AD (Büntgen et al., 2011; Graham et al., 2011; Waltgenbach et al., 2021) with an apogee in ’the High Medieval’ from ca. 1100 to ca. 1200 AD period (Bradley et al., 2003). MCO was preceded by the Dark Ages Cold Period (from ca. 400 to ca. 770 AD; Helama et al., 2017) and followed by the Little Ice Age (from ca. 1500 to ca. 1850; Graham et al., 2011). The described milder climatic conditions with higher precipitation could have determined more intensive development of vegetation and, as a result, the accumulation of larger amounts of organic matter in the soil during the decomposition of plant remains, which movement in the next stage promoted the podzolization process in the susceptible, sandy parent material (Fritsch et al., 2009; Mokma et al., 2004; Stützer, 1998). This may be demonstrated by the relatively high SOC concentration values found in most of the buried humus (A) or illuvial (Bsb or Bshmb) horizons in studied Podzols (Table 2, Figure 7).

Although Podzol development is initiated by coniferous vegetation, podzolization can also occur under the influence of deciduous trees (Lundström et al., 2000; Rahmanov and Oles, 2010; Rahmanov et al., 2006). The differentiation into coniferous or deciduous forests has the one of the greatest effects on the formation of Podzols due to the different chemical properties of the plant remains (Berg, 2000). Compared to deciduous litter, the litter found in coniferous forests consists of components less susceptible to biological degradation. As the process of decomposition slows, this results in accumulation of thick acidic organic horizons with rather poorly decomposed organic matter (Buurman and Jongmans, 2005; Schulp et al., 2008), which can be a simple source of mobile organic acids redistributed during podzolization. However, it can be anticipated that the development of Medieval Podzols formed under coniferous forests, which local growth on the dunes could possibly have resulted either from earlier clearances or under an indirect anthropogenic influence (Balaga and Chodorowski, 2006; Williams, 2000). The prevalent occurrence of cattle hoofprints above buried Podzols suggests that deforestation could have been caused by the need for pasture areas. Possibly, the feedback mechanism of cattle grazing-driven recurring dune reactivation and stabilization in the aftermath of Podzol burial might be responsible for the occurrence of intensely trampled layers sandwiched between intact layers in sand bodies overlying the Podzols (Hsieh et al., 2023).

On the other hand, changing vegetation to deciduous, for example, as a result of anthropogenic reforestation, can cause a depodzolization effect in well-developed Podzols (Barrett and Schaetzl, 1998; Nørnberg et al., 1993). In general, the disappearance of coniferous forests that determine the acidic soil environment can slow or even reverse the process of podzolization and also affect the soil tropism. Therefore, the abrupt change in vegetation may explain the presence of eutric properties and higher pH values in soils from the Ochotnik and Przyłęk sites (Table 2). It should be noted that the ages obtained from radiocarbon dating reflect a particular instant in a prolonged process of Medieval Podzol development. The time factor, while directly related to climate and vegetation, is by itself crucial in podzolization. However, the degree of formation of the examined buried Podzols, expressed in the thickness of the eluvial and illuvial horizons, and the occurrence of ortstein seem to be uncorrelated with the time factor (Table 2, Figures 5 –7). Soils characterized by the most visible morphological features associated with the process of podzolization (Zwierzyniec, Grębociny, Ostrowik and Chlewice – Figures 6 and 7) are dated throughout the Medieval Climatic Optimum period (Figure 5). Nevertheless, the duration of the podzolization process is difficult to assess considering its dependence also on multiple other locally acting variables. In a relatively humid and mild temperate climate, the development of Podzol may be expected to have taken at least several hundred up to over a thousand years (Gus and Drewnik, 2017; Lundström et al., 2000).

Despite conspicuous development and, as evidenced in the present work, widespread occurrence of Medieval Podzols, there was only one study of such Podzol in Poland to date (Balaga and Chodorowski, 2006). Unspecified buried paleosols dated to the Medieval period have been reported from numerous sites in the east of the ESB (Tolksdorf and Kaiser, 2012 and references therein), but soil type determination is precluded by the lack of adequate paleosol descriptions from these sites. However, Sokołowski et al. (2022) covered Osiny and Gołąb sites in their pedostratigraphic study not taking into account the buried Podzols there. Two occurrences of Podzols buried in dune deposits of Central Brandenburg Plateaux and Lowlands region (NE Germany) were documented in Wutzetzer Heide (Behrendt et al., 2002) and Glashütte sites (Hirsch et al., 2017). In both cases, even though an overall geomorphic setting and height of dune landforms are comparable with those in the present study, the cases from Central Brandenburg are distinguished by significantly thicker dune sand succession overlying the Podzols (7–10 m – Behrendt et al., 2002; ca. 3.6 m – Hirsch et al., 2017). While the buried Podzol described by Behrendt et al. (2002) displayed an Early Medieval age (7th –9th century AD), the case studied by Hirsch et al. (2017) was dated at 1st –2nd century AD. Both Central Brandenburg sites display moderately advanced podzolization with eluvial and illuvial horizons present, resembling those in the present study, except for the sites where the orstein horizon is developed. From the point of view of pedological characteristics, the pre-Medieval paleosol at Glashütte site, determined by Hirsch et al. (2017) as Entic Rustic Podzol (Arenic), most resembles the Late Medieval paleosol at site 4 (Osiny) in the present study. Moreover, soil chemical properties (pH and SOC concentration) at Glashütte site correspond to those observed in the majority of the investigated paleosols (Table 2), suggesting probably similar environmental conditions under which they were formed.

According to Sevink et al. (2018), the prevalence of Early Holocene buried incipient Podzols in the Netherlands may have remained long unnoticed due to their visual resemblance to the Late Glacial Usselo soil. Similarly, Medieval buried Podzols might have been overlooked despite the fact most of them are visually distinguished by eluvial and illuvial horizons developed better than in older paleosols.

Conclusion

To date, Late Glacial – Early Holocene Usselo soil remained the only unquestionable pedostratigraphic marker horizon distinguished in the aeolian dune deposits of the European Sand Belt. The present study brings forward a new pedostratigraphic marker, for which a name Grębociny soil is proposed after a type locality in Grębociny site (Central Poland). Buried Podzols classified as Grębociny soil display similar age, geomorphologic setting, as well as characteristic, uniform properties. Such paleosols were documented in 10 inland dune sites dispersed over a vast area of central and eastern Poland. Although the exact time frames of the development of Podzols vary according to local conditions, all studied cases fall within the 500–1500 AD range. Such a precisely determined time frame distinguishes it from existing marker paleosols recognized in aeolian deposits. Given the above, documented Medieval paleosols meet the criteria of a reliable pedostratigraphic marker. At the same time, they provide insights on the anthropogenic influence on aeolian sand remobilization, which resulted in burial of soils during or shortly after the Middle Ages. Therefore, it is proposed that the Grębociny soil is established as a new marker horizon corresponding to the Medieval Climatic Optimum in the eastern part of the European Sand Belt.

Footnotes

Acknowledgements

Magdalena Gus-Stolarczyk is thanked for assistance in fieldwork and valuable suggestions. The authors wish to thank two anonymous reviewers for their comments which improved the manuscript.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financed by the National Science Centre, Poland, from the programme Daina 1, grant agreement No 2017/27/L/ST10/03370.

ORCID iD

Krzysztof Ninard

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