Investigated Sites

The study site is located near Biedrzykowice village (50°23'56.10"N; 20°18'44.26"E) in southern Poland, in the mesoregion named the Proszowice Plateau located on eastern part of the Małopolska Upland (Kondracki, 2002). (Figs. 1A–1D). The topographic surface of the Proszowice Plateau is situated between 220–390 m a .s.l., and the depth of the larger valleys ranges from 40 to 60 m. The mean slope inclination for a study plot is about 10°.

Fig. 1

A – Location of research area on the map of loess distribution (yellow patches) in southern Poland; B – DEM model; C – orthophotomap; D – aerial photo

The oldest rocks in the exposures are chalk marbles forming culminations. Miocene gypsum and clay occur under Quaternary formations. The cover of Pleistocene glacial formations has only been studied to a very limited degree. It consists of residual clays and sands of the oldest glaciation periods, whose stratigraphic affiliation is difficult to determine. Loess deposits are the most common Pleistocene sediments. Jersak (1973) assigned these areas to the transition loess formation. Here the loess cover can reach a thickness of 20 m and colluvial sediments on the footslope and toeslope as much as 7 m. The main part of the loess cover consists of the youngest Vistulian (=Weichselian) loess deposits accumulated mainly during younger part of MIS 2, i.e between 22 ka BP and 16 ka BP (e.g. Maruszczak, 1991, 2001; Jary, 2007).

Mean annual precipitation is 657 mm with the highest intensity in July. The mean value of July precipitation is 95 mm and the greatest recorded monthly precipitation (also recorded in July) was 226.1 mm (https://pl.climate-data.org/location/10413/#climate-graph) The average annual temperature is 8.2ºC, with a range from –4.4ºC (January) to 18.6ºC (July). The growing period is about 200 days of the year (Paszyński and Kluge, 1986). This area has been used as cultivated fields since the Neolithic (Kruk et al, 1996, 2016; Kruk and Milisauskas, 1999). Archaeological investigations in this area suggest a strong impact by farmers on the primary landscape in the years 3900–2500 cal. BC (Kruk et al, 1996, 2016). In Biedrzykowice, located almost 8 km north of the Neolithic site in Bronocice, a colluvial sediment profile preserved at the bottom of a dry valley (Sancygniówka River basin), which has been the subject of previous analyses (Śnieszko, 1985, 1995; Raven et al, 1999), was selected for detailed studies.

Sampling

Colluvial sediments and soil samples were collected from the wall of the gully as well as from the adjacent agricultural field (Fig. 1D). For the soil erosion and sedimentation study using fallout radionuclide, 34 cores were collected using a “Stiboka” motorized corer (inserting 10 cm diameter demountable steel tubes into the soil profiles) along five downslope transects from an agricultural field. The motorized sample corer allowed soil cores with a non-disturbed structure to be obtained. Core locations were recorded using an RTK GPS (Promark 220, about 0.1 m accuracy) and are presented in Fig. 1. The soil cores were sectioned into 10 cm intervals. Additional cores were collected from undisturbed reference sites and sectioned at 2, 3 or 5 cm increments. Reference sites were selected as places where soil erosion and sediment accumulation were not visible (mature and natural vegetation cover protects the soil surface from erosion). In the case of luminescence dating, two sets of samples were taken: the first from a gully wall and the second from an agricultural field close to the gully. We collected 13 samples for OSL dating and 50 samples for radionuclide analysis from the gully wall. The samples for optical dating were collected by using steel tubes driven into the sediment. The tubes were opened in a dark laboratory and about 1 cm of the sediment at each end was cut and removed before OSL measurement. Samples for activity measurement were taken from the surface to a depth of 500 cm. Samples for luminescence dating were also collected from three areas on the agricultural field: top of the slope, eroded area and base of the slope. This sampling was also performed using a light-protected procedure.

As well as the simultaneous application of both methods, 13 detailed micromorphological soil analyses were also carried out. The main goal of the micromorphological analyses was to determine litho- and pedological features (e.g. Kemp, 2001; Mroczek, 2008, 2013, 2018). The micromorphological samples were taken from the same horizons in the gully wall as the samples for OSL dating.

For dendrochronological analyses, we chose six larch trees (Larix decidua Mill.) growing on the edge of the gully under study. Due to erosion, the trees were tilted perpendicularly to the valley axis, and additionally their root system was partially exposed. We assumed that dendrochronological analyses would allow us to date erosion events and estimate the rate of gully hillslope retreat. Growth disturbances (tree ring reduction, tree ring eccentricity) formed after erosion events and recorded in wood of the studies trees were used to erosion dating. We collected two cores from each of the six trees with a Pressler borer at chest height, perpendicularly to the valley axis (6 cores) and parallel to the valley axis (6 cores). We also sampled by handsaw 4 exposed roots from tree number 6: root sample number 1 – sampled 5 cm from the gully edge, root sample number 2 – sampled 15 cm from the gully edge, root sample number 3 – sampled 20 cm from the gully edge, and root sample number 4 – sampled 50 cm from the gully edge. We assumed such a strategy would allow the rate of gully hillslope retreat to be checked.

Activity measurement

Before the activity measurement, all samples were dried, placed in measurement containers and stored for a minimum of three weeks to ensure radioactive equilibrium in the decay series. The activities of ¹³⁷Cs and ²¹⁰Pb, as well as other isotopes, such as the ²³⁸U series, ²³²Th series and ⁴⁰K, were measured for dose rate determination by using low background high resolution gamma spectrometry analysis. The detector resolution (FWHM) was 1.8 keV and the relative efficiency was 40% at 1332 keV. The counting time was usually 8 0 ks and IAEA ( RGU, RGTh, RGK) standards were used for calibration. The IAEA Soil-375 standard was used as a reference material for ¹³⁷Cs activity. The IAEA-385 standard was used to check the quality of efficiency calibration. To calculate the ²³⁸U content in the sediment, the following gamma lines were taken: 295.1 keV (²¹⁴Pb), 352.0 keV (²¹⁴Pb), 609.3 keV (²¹⁴Bi) and 1120.3 keV (²¹⁴Bi). For the ²³²Th decay chain, the following gamma lines were considered: 583.0 keV (²⁰⁸Tl), 911.2 keV (²²⁸Ac) and 2614.4 keV (²⁰⁸Tl). To calculate the ⁴⁰K content, the 1460.8 keV gamma line was taken. To calculate the ¹³⁷Cs content, the 661.7 keV gamma line was used. The total activity of²¹⁰Pb in the samples was measured at 46.5 keV and the concentration of the supported ²¹⁰Pb was assayed by measuring the short-lived daughters of ²²⁶Ra. The unsupported ²¹⁰Pb activity (²¹⁰Pb_ex ) in the samples was calculated by subtracting the supported ²¹⁰Pb from the total concentration of ²¹⁰Pb. In the case of ²¹⁰Pb, the results were corrected for self-absorption, in accordance with Cutshall et al (1983).

Dose rate calculation

The measured activities of radioisotopes in the sediment and soil samples were converted into dose rates by using the conversion factors described by Guerin et al (2011). The dry dose rates (Adamiec and Aitken, 1998; Guerin et al, 2011) were adjusted for water content, following Aitken (1985). The water content of samples measured in the laboratory was no higher than 15%, so we used a value of 10 ± 5% for our calculations (Bluszcz, 2000). The cosmic ray dose-rate at the site follows the calculations suggested by Prescott and Hutton (1994).

The results of radionuclide analysis as results of dose rate calculation are summarized in Table 1. For the sediment samples, the dating of the ¹³⁷Cs activity was included in the calculation of the dose rate, according to the procedure described in (Moska et al, 2004; Poręba et al, 2006).

¹³⁷Cs and ²¹⁰Pb dating

Caesium-137 (half-life 30.1 years) is a radioisotope whose main source in the environment is above-ground nuclear weapon tests in the 1950s and 1960s. The ¹³⁷Cs fallout connected with nuclear weapon testing is known as global fallout, and its distribution depends on latitude and precipitation. The highest intensity of ¹³⁷Cs deposition occurred in the period 1961–1963. Additionally, another ¹³⁷Cs fallout connected with the Chernobyl accident of 1986 has taken place in Europe. ¹³⁷Cs is strongly adsorbed into soil particles (Ritchie and McHenry, 1990) after its deposition on the ground surface, In areas contaminated as a result of the Chernobyl accident, calculation of soil erosion based on the ¹³⁷Cs inventories can be difficult due to problems in distinguishing between global and Chernobyl ¹³⁷Cs fallouts (Golosov et al, 1999; Poręba and Bluszcz, 2007). In contrast to ¹³⁷Cs, ²¹⁰Pb (with a half-life of 22.2 years) is a natural radioactive isotope which is a part of the ²³⁸U decay chain. This isotope is derived from the radioactive decay of ²²²Rn, a daughter of ²²⁶Ra. ²¹⁰Pb produced from ²²⁶Ra in situ in soil is known as supported, whereas the ²¹⁰Pb originating from atmospheric fallout is named unsupported (or ²¹⁰Pb_ex), and is used widely to study the accumulation rates of various sediments. The activity of ²¹⁰Pb_ex is usually calculated by subtracting the supported ²¹⁰Pb established via the activity of the ²²⁶Ra daughters from the total ²¹⁰Pb in a given soil or sediment sample (Walling, 1998; Walling and He, 1999; Mabit et al, 2014).

The measured ¹³⁷Cs and ²¹⁰Pb_ex activities were converted to inventories according to the formula described by Sutherland (1992) and are presented in Figs. 2–3. The simple comparison between the measured values of the ¹³⁷Cs or ²¹⁰Pb_ex inventories and the reference inventory values of ¹³⁷Cs or ²¹⁰Pb_ex allows us to recognize erosion and deposition areas; however, to obtain quantitative estimates of soil erosion, one of several available models must be used (Walling and Quine, 1990; Walling and He, 1999).To estimate soil erosion based on ¹³⁷Cs and ²¹⁰Pb_ex, the proportional model (¹³⁷Cs) as well as two mass balance models (¹³⁷Cs, ²¹⁰Pb_ex) were applied (Walling and Quine, 1990; Walling and He, 1999; Poręba, 2006). The equation for the proportional model can be written as (Walling and Quine, 1990):

Fig. 2

The spatial patterns of ¹³⁷Cs and ²¹⁰Pb_ex inventories on the studied agricultural field. The data on the figure are expressed in Bq·m^–2.

Fig. 3

The downslope change of ¹³⁷Cs and ²¹⁰Pb_ex inventories. On the figure is marked the shape of the slope.

where:

The main assumption for the proportional model is that ¹³⁷Cs is completely mixed within the plough layer. If this is true, then the soil loss is directly proportional to the ¹³⁷Cs loss from the soil profile. Although this model is relatively easy to use, it has several limitations. For instance, this model assumes that caesium is uniformly distributed in the plough layer. Immediately after fallout, the surface contained more caesium than the underlying soil horizons due to agricultural mixing, e.g. ploughing. This could result in an overestimation of soil loss (Walling and Quine, 1990). Two kinds of mass balance models were also used to calculate soil erosion and deposition. The first mass balance model was described by Zhang et al (1990) and is referred to as the simplified mass balance model. The main assumption of this model is that the total ¹³⁷Cs fallout occurred in 1963. The mean annual soil loss rate can be expressed as follows:

where:

Although this model is very easy to use, the main assumption of this approach that the total ¹³⁷Cs fallout input occurred in 1963 seems to be an oversimplification. This model does not take into account the value of ¹³⁷Cs freshly removed from the soil surface before incorporation into the plough layer by ploughing. The problem with selective sorption of ¹³⁷Cs by soil particles could be solved by adding a particle size correction factor (Zhang et al, 1999). Another problem with the simplified mass balance model is the assumption of the ¹³⁷Cs fallout in 1963. This does not take into account areas contaminated by Chernobyl caesium, such as the study area.

To overcome the problem with selective sorption, and also removing the fresh deposition of ¹³⁷Cs before mixing by ploughing, the mass balance model was improved by He and Walling (1997). This model was also used to calculate soil erosion based on the ²¹⁰Pb_ex inventories. According to He and Walling (1997), this model could be written as follows:

where:

For the areas where deposition of soil occurs, the mass balance model could be written according to Walling and He (1999) to calculate the soil deposition rate:

where:

Luminescence dating

Samples for OSL dating were treated with 10% HCl and 10% H₂O₂ for 48 hours to remove carbonates and organic material, respectively. Quartz was extracted from the 90–125 μm grain fraction by using density separation (sodium polytungstate). Finally, quartz grains were etched using 40% HF to remove their outer layer (Aitken, 1985, 1998). After etching, they were washed in HCl (20%) to remove any precipitated fluorides. The grains were then mounted on stainless steel discs using silicone oil. All treatment was conducted under subdued red light.

Quartz from each sample was checked for purity by means of an I R-test. No samples showed a significant IRSL signal, i.e the natural and regenerated signal ratios of the IRSL to the blue-light stimulated luminescence were ≤ 5%. The SAR procedure which was used for equivalent dose determination did not contain IRSL step.

Optically stimulated luminescence measurements (OSL) were made using an automated Daybreak 2200 TL/OSL reader (Bortolot, 2000), equipped with a calibrated ⁹⁰Sr/⁹⁰Y beta source (dose rate of 2.95 ± 0.09 Gy·min^–1).

The determination of equivalent doses of quartz grains was performed using multi-grain aliquots, each containing ca 1 mg of quartz grains. A 6 mm mask was used to disk preparation, it means that on the single disk should be about 1500 quartz grains with 90–125 μm diameter (Heer et al, 2012). For stimulation, blue diodes (470 ± 4 nm) delivering 45 mW·cm^–2 at the sample position were used. Equivalent doses were determined using a single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000). Dose recovery and thermal transfer tests were performed for each sample and a preheat plateau test was performed to establish the most appropriate preheat temperature.

Micromorphological analyses

Micromorphological analyses were carried out on a selected sequence of colluvial sediments with intercalated palaeosoil horizons in the gully (Figs. 4–5). The micromorphological samples were taken from the same horizons in the vertical outcrop as the samples for OSL dating. The material was collected in Kubiena tins (8×6×4 cm) directly from selected colluvial layers and soil horizons. Thin sections were made of samples according to the methodology proposed by Lee and Kemp (1992) and modified by Mroczek (2008). The size of the individual thin sections was ~8×6 cm, and their thickness ranged from 20 to 30 μm. Thin sections were observed using a polarizing microscope with magnifications up to 200×.

Fig. 4

Micromorphological features and genetic interpretation of upper part of soil-colluvial sequence in Biedrzykowice.

Fig. 5

Micromorphological characteristic of lower part of soil-colluvial sequence in Biedrzykowice. The microphotograph presents massive microstructure in clay-rich soil material.

Dendrochronology

Within cores we dated the eccentric growth of trees, which is formed due to trees tilting and is relatively easy to observe in the wood cross-sections (Fig. 6A). We used cores collected perpendicularly to the gully axis for eccentric growth analyses. After root exposure, a tree produces reduced rings, therefore we also analysed ring reduction in the sample cores (Fig. 6B). The eccentric growth of trees develops on the side in which the tree stem is tilted (in the case of conifers), while ring reduction occurs on the entire perimeter of the tree. Every core was glued to wooden holders and dried. Next, the cores and root samples were sanded by 200, 500, and 1000 sand paper to better reveal the ring structure. We measured the ring width in the collected cores using the LINTAB measuring system, and visually cross-dated ring series from individual trees. Eccentric growth was calculated using a formula proposed by Wistuba et al (2013). Based on previous studies, we presumed that the yearly variation of the eccentricity index should be more than 50% for dating mass-movements or erosion (Malik and Wistuba, 2012). We used cores collected parallel to the gully axis for ring reduction analyses. For ring reduction calculation, we modified the method proposed by Schwiengruber et al (1985). We assumed that reduction occurs when at least 3 successive rings are reduced by more than 75% in relation to the average ring width in an individual core.

Fig. 6

Core with eccentric growth of tree (A), and ring reduction (B)

Grain size distribution measurements

Grain-size distributions were determined using a Malvern Mastersizer 3000 laser diffractometer. Before measurement, organic matter was removed by H₂O₂. The samples were dispersed with sodium hexametaphosphate and shaken overnight in distilled water to disperse (Mason et al, 2003; Schaetzl et al, 2014).

Measured ¹³⁷Cs and ²¹⁰Pb_ex activities

The ¹³⁷Cs and ²¹⁰Pb_ex inventories measured for the soil cores from the agricultural field range from 730 to 7911 Bq·m^–2 and from 1615 to 11136 Bq·m^–2, respectively (Figs. 2–3). To calculate the rate of soil erosion and sediment accumulation, the reference inventories of ¹³⁷Cs and ²¹⁰Pb_ex need to be known. The reference inventories for ¹³⁷Cs and ²¹⁰Pb_ex were measured in undisturbed soil cores collected from the area surrounding the study field. Fig. 7 provides an example of the depth distributions of ¹³⁷Cs and ²¹⁰Pb_ex at the reference site. ¹³⁷Cs was found at a depth of about 25–30 cm and, simultaneously, ²¹⁰Pb_ex was found at a similar depth. Both depth distributions show an exponential decrease with depth, however, the maximum ¹³⁷Cs concentration was just below the surface. This behaviour of caesium in soil has also been described by Li et al (2009). Based on the reference cores, the reference inventories were estimated. The mean value of the total baseline inventory of ¹³⁷Cs for the study area is 2627 Bq·m^–2. This value is substantially higher than the mean value for Poland (Pietrzak-Flis, 2000; Strzelecki et al, 1994). This is because the study area is located in the region where larger ¹³⁷Cs concentrations in soil are expected due to the Chernobyl accident. The values of Chernobyl ¹³⁷Cs depend on the trajectories of the main radioactivity clouds, and on the precipitation in the area of interest at the time of the accident (Stach, 1996; Strzelecki et al, 1994). Increased values of Chernobyl ¹³⁷Cs concentration are found along the line of Warsaw – Opole – the Kłodzko Basin. The largest values of Chernobyl ¹³⁷Cs fallout were found in the Opole and Silesia regions (SW Poland), and in the Kłodzko Basin in the foreland of the Sudetes Mountains. In these areas, the values of Chernobyl ¹³⁷Cs concentration were between 15.6 kBq·m^–2 and 19.3 kBq·m^–2, with the maximum value of 96 kBq·m^–2 in the Nysa area (Stach, 1996; Strzelecki et al, 1992, 1994; Szewczyk, 1994). The strong spatial variability of the ¹³⁷Cs concentration from the Chernobyl accident makes it difficult to use ¹³⁷Cs in soil erosion studies (Golosow et al, 1999). The mean value of the¹³⁷Cs deposition for the Świętokrzyskie region is 1.28 kBq·m^–2, and the range is from 0.31 to 3.55 kBq·m^–2 (Issajenko et al, 2014). Unfortunately, the authors of that study collected only the top 10 cm of soil, thus this value could be systematically lower than in our case. The mean value of ¹³⁷Cs deposition for Poland is 1.53 kBq·m^–2, and the range is from 0.22 to 17.79 kBq·m^–2 (Issajenko et al, 2014).

Fig. 7

The example of depth distributions of ¹³⁷Cs and ²¹⁰Pb_ex for a reference site.

The calculated ¹³⁷Cs reference inventory, based on latitude and mean annual precipitation for the study area, is 1269 Bq·m^–2. To use a more sophisticated model to calculate soil erosion based on the ¹³⁷Cs inventories, knowledge of the annual fallout of ¹³⁷Cs is needed. Unfortunately, the approach described above does not provide this information. There are few places in the world where the deposition of ¹³⁷Cs has been measured since nuclear weapon tests started, and the information about the rate of fallout is essential to use ¹³⁷Cs to study soil erosion. In Poland, the deposition of ¹³⁷Cs has been measured since 1970 (Stach, 1996). The concentration of Chernobyl ¹³⁷Cs varies widely even within one field (Dubois and Bossew, 2003; Strzelecki et al, 1992, 1994; Szewczyk, 1994). The percentage of the Chernobyl deposition in the global deposition of ¹³⁷Cs must be known to be able to use the models to calculate soil erosion from the ¹³⁷Cs activity data. Nowadays, there is a problem with separating the Chernobyl fallout from global ¹³⁷Cs fallout. A valuable tracer of the Chernobyl fallout was another caesium isotope ¹³⁴Cs, but this isotope has a relatively short half-life (approx. 2 yr) and is no longer present in the environment. The annual fallout of ¹³⁷Cs could be obtained according to calculations proposed by Walling and He (1999), but this approach seems to be an over-simplification due to changes in the ¹³⁷Cs fallout in the past and local rainfall distribution. In the present study, the reference inventory of the ¹³⁷Cs fallout was separated from the Chernobyl fallout and from the global weapon test fallout by estimating the baseline inventory from atmospheric records. Moreover, this method is useful to obtain information about the variability of the ¹³⁷Cs deposition during the period of the fallout accumulation (Lu and Higgitt, 2000; Poręba and Bluszcz, 2007). This method allows us to estimate an initial inventory of ¹³⁷Cs connected with nuclear weapon tests, based on the precipitation data (IMGW-PIB data - Institute of Meteorology and Water Management - National Research Institute) and data provided by Sarmiento and Gwinn (1986) (Poręba and Bluszcz, 2007). The global ¹³⁷Cs fallout calculated by this model is 1456 Bq·m^–2 (Fig. 8a). This value is connected with nuclear weapon tests and is also called the global caesium fallout. The difference between this value and the value of caesium fallout measured at the reference site is associated with the Chernobyl fallout. For the study area, about 45% of the total caesium fallout is connected with the Chernobyl accident.

Fig. 8

a) The calculated annual ¹³⁷Cs inventories based on precipitation data and model Sarmiento-Gwinn for a study site; b) The calculated monthly ¹³⁷Cs inventories based on precipitation data and model Sarmiento-Gwinn for a study site for the period 1962–1964

The ¹³⁷Cs fallout calculations for the study area are presented in Fig. 8b. For the study area, about 44% of the global caesium fallout occurred in the period 1962–1964. These calculations take into account both the nature of rainfall and seasonal variations in rainfall over the period of fallout.

The mean value for the ²¹⁰Pb_ex reference inventories is 4835 Bq·m^–2. This value was obtained from the measurement of reference soil cores and is similar to values obtained by other authors (Porto et al, 2006; Kato et al, 2010). In this case, ²¹⁰Pb_ex constant deposition flux was assumed and the annual fallout of ²¹⁰Pb for the study area was estimated to be 150 Bq·m^–2·a^–1. Compared to ¹³⁷Cs, which is frequently used to study soil erosion, ²¹⁰Pb_ex is rarely used for this purpose. ²¹⁰Pb_ex is mainly used to study the sedimentation rate of lake sediments or peat accumulation. Nowadays, knowledge concerning the global values of ²¹⁰Pb_ex is also limited (Baskaran, 2011; Mabit et al, 2014) and the range of the ²¹⁰Pb_ex fallout is relatively large.

Based on the reference values of ¹³⁷Cs, about 62% of the study area was defined as eroding while, based on the ²¹⁰Pb_ex inventories, this figure was about 50%. The simple comparison between the measured value of the ¹³⁷Cs or ²¹⁰Pb_ex inventories and the value of the reference inventory of ¹³⁷Cs or ²¹⁰Pb_ex allows us to recognize erosion and deposition areas; however, to obtain quantitative estimates of soil erosion, one of several available models must be used (Walling and Quine, 1990). To estimate soil erosion based on ¹³⁷Cs and ²¹⁰Pb_ex, the proportional model (¹³⁷Cs) as well as mass balance models (¹³⁷Cs, ²¹⁰Pb_ex) were applied (Walling and Quine, 1990; Walling and He, 1999; Poręba, 2006). The results of calculations for soil erosion for the valley section in question, using the four models, are presented in Fig. 9. The calculated soil erosion for the studied area shows significant variation. In the case of the ¹³⁷Cs measurement and the proportional model, the obtained soil loss results vary from 0.49 to 3.99 kg·m^–2·a^–1, whereas in the case of the simplified mass balance model the range is from 0.61 to 7.89 kg·m^–2·a^–1. The soil erosion rates calculated by the

Fig. 9

The spatial patterns soil erosion and sediment accumulation calculated by different models based on radionuclide inventories on the studied agricultural field: a) proportional model based on ¹³⁷Cs inventories, b) simplified mass balance model based on ¹³⁷Cs inventories, c) improved mass balance model based on ¹³⁷Cs inventories, d) improved mass balance model based on ²¹⁰Pb_ex inventories.

improved mass balance model for the cultivated slopes vary from 0.35 kg·m^–2·a^–1 to 5.98 kg·m^–2·a^–1. The mean soil erosion rate calculated for the proportional model is 2.09 kg·m^–2·a^–1, the mean soil erosion rate for the simplified mass balance model is 3.35 kg·m^–2·a^–1, with 2.18 kg·m^–2·a^–1 in the case of the improved mass balance model. The results obtained by the proportional model and improved mass balance model are similar, while the results of soil erosion loss obtained using the simplified mass balance model are substantially higher. The simplified mass balance model assumes that the entire caesium fallout occurred in one year, which might be a source of error in the case of areas contaminated by the Chernobyl caesium fallout. In the case of the improved mass balance model, the monthly (or at least annual) values of caesium fallout, as well as the initial distribution of fresh fallout of caesium in the top soil, are considered. Still, the improved mass balance model probably provides the most reliable results, but requires several additional parameters (Walling and He, 1999; Poręba and Bluszcz, 2008).

The soil erosion estimation based on the ²¹⁰Pb_ex measurements and the improved mass balance model ranged from 0.22 kg·m^–2·a^–1 to 3.07 kg·m^–2·a^–1, with a mean value of 1.38 kg·m^–2·a^–1.The range of calculated soil erosion is similar to that obtained by the proportional model or improved mass balance model in the case of caesium measurement, but the mean value of soil erosion is slightly smaller than the values obtained in those latter cases. That difference can be attributed to different periods covered by measurements of caesium and lead. The results suggest an intensification of soil erosion after 1950. The change in land use is also supported by a different distribution of the intensity of the erosion and accumulation processes within the examined slope. In the case of ¹³⁷Cs, large soil erosion values were obtained in the middle part, but in the upper part of the slope for ²¹⁰Pb_ex. However, the accumulation in the case of the ²¹⁰Pb_ex isotope analysis is greatest for the zone just above the edge of the gully (mean value 26 cm during 100 years) while the greatest accumulation for the ¹³⁷Cs isotope measurement is approximately 10–15 m from the edge of the ravine (mean value 25 cm during 54 years). This may be the result of changes in land use after World War II. As a result of intensive mechanical cultivation, the slope has been remodelled.

It is quite clear that the soil erosion rate calculated on the basis of radioisotope data depends on the model used; still we can note that the proportional model and the improved mass balance model provide similar results. Similar discrepancies between the results obtained by different models are also present when calculating sediment accumulation. In this case, the results using the proportional model and the improved mass balance model are also similar, whereas those of sediment accumulation obtained using the simplified mass balance model are slightly higher. Surprisingly, sediment accumulation at the foot of the slope for two sampling points by ¹³⁷Cs measurements was close to zero, which suggests no sediment delivery to those locations. This is also confirmed by the depth distribution of isotopes in the sediment profiles.

Results of OSL dating

OSL samples from four locations within the study area were analysed: one sampling location at the top of the slope, one located in the eroded part of the slope, one at the foot of t he slope and one from the wall of a gully located at the foot of the studied slope. The locations of sampling points for luminescence dating on the slope are marked in Fig. 3. Figs. 10–13 present the results of OSL dating for soil samples collected from the sampling positions on the slope (Figs. 10B–12B), a s well a s the samples collected from the gully wall (Fig. 13B). The results of OSL dating for the Biedrzykowice study site are also given in Table 1. This table also presents the results of radionuclide analysis in the dated samples, dose rate calculations and doses. To determine OSL ages, the central age model (CAM) proposed by Galbraith et al (1999) was used.

Fig. 10

The depth distribution of the ¹³⁷Cs and ²¹⁰Pb_ex (A), SAR OSL ages (B), grain size composition (C, D) and stratigraphy (E) for the soil core collected from the upper part of the slope.

Fig. 11

The depth distribution of the ¹³⁷Cs and ²¹⁰Pb_ex (A), SAR OSL ages (B), grain size composition (C, D) and stratigraphy (E) for the soil core collected from the middle part of the slope.

Fig. 12

The depth distribution of the ¹³⁷Cs and ²¹⁰Pb_ex (A), SAR OSL ages (B), grain size composition (C, D) and stratigraphy (E), for the soil core collected from the lower part of the slope.

Fig. 13

The results of OSL SAR dating for the side of the gully (B), results of U-238, Th-232 (C) and K-40 (D) analysis as well as grain size analysis (E, F) in samples from the wall of gully. On the figure also shown the results of activity measurements by portable gamma spectrometer (B) and stratigraphy (A).

In the case of ploughed land, the luminescence ages of the deeper layer (loess) are between 14–17 ka (Bie_2_2 and Bie_3_2; GdTL-3068, GdTL-3070; Table 1). The OSL distributions are almost symmetric, thus the results from CAM model probably represent the true depositional age. The luminescence ages of the layers closer to the soil surface are between 0.779–1.42 ka (Bie_2_1 and Bie_3_1; GdTL-3067, GdTL-3069), and OSL age distributions are positively skewed and asymmetric. Those distributions could be a result of incomplete bleaching or post depositional processes, particularly as in this case we are dealing with both biological and mechanical mixing (in the form of the incorporation of

quartz grains into a ploughed A-horizon, Fig. 14) (Bateman et al, 2003). Due to pedoturbation (both biological and mechanical mixing) a part of quartz grains move up to the topographic surface where the luminescence signal may be bleached. Just before the sediment transport on the slope, or at the beginning of this process, more than 90% of the initial luminescence signal is bleached. Similar results have been obtained for sandy loess in the south of Poland (Poręba et al, 2015). In the case of sampling points located at the base of the slope, 5 sediment samples were OSL dated (Fig. 12). A ll OSL ages are consistent with a stratigraphic order. The age of the sediment sample from the top soil is equal to 0.175 ka (Bie_4_1; GdTL-3071). Below, very close to the surface, the age of the sediment sample equals 0.838 ka (Bie_4_2; GdTL-3072). The next part of the sediment core is represented by two luminescence results within the range 4.20–6.28 ka (Bie_4_3 and Bie_4_4; GdTL-3073, GdTL-3074). The younger results are probably due to bioturbation before covering by the next sedimentation. The result of OSL dating of the lowermost sample is equal to 12.28 ka (Bie_4_5; GdTL-3075). This is significantly younger than the expected age of loess cover formation, which may be the result of bioturbation before covering by Neolithic sediment.

Fig. 14

The examples of D_e distributions.

In the case of the gully wall sampling point, 13 sediment samples from different sediment horizons were collected for the luminescence study (Fig. 13). The age of the sediment sample from the top soil (modern humus soil-horizon) corresponds with the age of sediments collected on the eroded slope. The whole profile could be divided into three main parts. The upper part of the colluvial sediment is clearly the youngest part of the analysed profile, with four luminescence results within the range 0.645–0.746 ka (Bie_1_1 - Bie_1_4; GdTL-2906, GdTL-2907, GdTL-2908 GdTL-2909; Table 1). It can also be seen that all 4 dates are within a very small range. For the OSL samples of the middle part of the sediment profile, the results are within the range of 6.05–0.988 ka (Bie_1_5 – Bie_1_9; GdTl-2910, GdTL2911, GdTL2912, GdTL2913). The lowest dated samples of this part of sediment are connected with the beginning of Neolithic colluvium accumulation (6.05–5.71 ka). The sediment samples B_1_5-B_1_7 are visibly younger than the expected age. This could be a result of the post-sedimentation pedoturbation processes. The visible fossil soil at the ceiling of the top of this sediment profile could confirm this hypothesis. The results of OLS dating for the lowest part of this sediment profile are within the range from 12.22 ka to 7.30 ka (Bie_1_9 - Bie_1_13; GdTL-2914, GdTL-2915, GdTL-2916, GdTL-2917). Those samples were taken from the Late Pleistocene-early Holocene fossil soil. The lowest sample is from the B horizon of fossil soil, whereas the rest of the sample comes from the A horizon of fossil soil. The results of OSL dating for the humus horizon are visibly younger due to the rejuvenation by pedoturbation during the Holocene. The results of OSL ages for the lower part of the A horizon of the Late Pleistocene – Holocene fossil soil are comparable with the previous results of ¹⁴C dating (Śnieszko, 1995).

Results of grain size analysis

The results of particle-size analysis, along with the depth distributions of ¹³⁷Cs and ²¹⁰Pb_ex and OSL ages for three locations on the slope (top, eroded part and foot of the slope) are presented in Figs. 10–13. The studied sediment is characterized by only slight variations in grain-size distributions in the zones where the studied radionuclides (¹³⁷Cs and ²¹⁰Pb_ex) occur. Two profiles presented in Figs. 10C and 11C have similar grain-size distributions. Silt is the dominant fraction varying between 77 and 80% and fine silt is the larger subfraction and increases relatively to coarse silt at the base of the solum.

The clay content (<4 μm) is constant and fluctuates between 13.0 and 15.5%. Sand (>63 μm) content also varies little and is less than 10% of the total sediment. The sediments are very poorly sorted (1.56–1.8 ϕ) and the mean grain size (Mz) varies little (5.04–5.6 ϕ). The profile presented in Fig. 12C shows an increasing clay fraction with depth through the colluvial sediment. Below the colluvial layer, Mz decreases slightly (Fig. 12D).

The sequence presented in Fig. 13E has a particle-size distribution which reflects the lithogenic and pedogenic character of particular layers. Shifts between neighbouring units are visible, as is the base of the solum. Clay content is very high (9.5–21.3%) in the middle layer (older colluvium) and lo wer in the lower part of the oldest soil. Maximum clay content is in the Bt horizon of the lower unit, and the minimum is in the uppermost A horizon (10%) and the lower part of the lower colluvial layer. The biggest fluctuations in particle size distribution are recorded in clay and sand content (9.5–21.3% and 0.5–12.0%, respectively). The whole sequence is poorly sorted (δ = 1.6–1.9 ϕ), mean grain size (Mz) varies between 5.0 and 6.0 φ (15–29 μm) and average grain size is 21 μm (Fig. 13F).

Micromorphology

Typical micromorphological features (Table 2, Fig. 5) of the lowest soil unit (depth 380–475 cm) are: channel microstructure (common in the whole sequence), clay microfeatures and Fe-Mn nodules (especially common in the middle part). Additionally, the upper part is rich in faecal pellets and pieces of charcoal. Microscopic observations of thin sections showed a “typical” record of occurrence of pedogenic microforms. The clay coatings, infilling recorded inside the channels and fissures are the effect of illuviation processes and the formation of well-developed Bt-argic horizons. The other microfeatures mentioned are also typical pedogenic features, confirming a high level of soil development – Fe-Mn nodules are redoximorphic features; while charcoal pieces – pedogenic ones. According to palaepedological criteria for the area of the Southern Polish Loess Upland, this type of palaeosol can be described as a buried mature soil developed at the end of the Late Glacial and Early Holocene (e.g. Jersak et al, 1992; Konecka-Betley, 2002).

The middle unit (151–360 cm; Table 2; Fig. 4) has very similar micromorphological characteristics, but the distribution of diagnostic features is very specific. In the thin sections, all the features mentioned above were also identified. The Fe-Mn microfeatures and fecal pellets were recorded in the whole unit as common and very common forms. The unit is clear ly two-parted, with a division according to the presence of charcoal and clay microfeatures. The upper part is rich in illuvial microforms and also in – but less common – charcoal. Angular clay concentrations are very specific – their distribution is not associated with commonly present biogenic channels. Chaotically arranged clay particles are frequently present inside the groundmass. The micromorphological characteristics suggest this unit is a colluvial soil developed on older, redeposited soil material. The presence of clay features testifies to the parent material of this soil coming from a redeposited Bt soil horizon.

The main microfeatures of the upper unit (0–124 cm; Table 2) correspond with those of the middle unit, but their degree of development is much weaker. The top horizon clearly has pedogenic features – modern faecal pellets and charcoals are common. There are no traces of illuviation and redoximorphic features. The middle and especially lower units have clay and Fe-Mn features (single to common) and faecal pellets (common). The unique features are clay coatings – thin and continuous on the walls of channels. This set of micromorphological features testifies to young pedogenic processes. The lack of older, redeposited clay microforms indicates that the parent material was not directly connected with the older and well developed Bt soil horizon. According to our results of micromorphology analysis, it cannot be excluded that the redeposited material of upper soil horizons of Holocene soil was the parent material of the young soil.

Dendrochronology

Below, the results of dendrochronological analyses are presented for samples taken from trees growing at the base of the slope near the edge of the gully.

Tree no. 1

The tree is tilted perpendicularly to the gully axis, about 75% of its root system is exposed. The distance between the furthest exposed root and the hillslope edge is 1 .5 m. The tree we sampled has grown since at least 1938. It was not possible to determine eccentricity because it was not possible to sample a core from the side of the tree exposed to the gully. Strong ring reductions were found in 1970–1978, 1986–1989, 2010–2013 (Fig. 15A).

Fig. 15

Graphs showing strong ring reductions in tree no. 1 signed by arrows and grey surfaces (A), tree ring curves form opposite site of the tree no. 3 (B), and yearly variation of eccentricity index of the tree no. 3 (C)