Loess areas are susceptible to soil erosion, especially when under agricultural land use. Loess areas in southern Poland have been used for agriculture since the Neolithic (Kruk
The presence of sediments indicating both Neolithic and medieval water erosion of soil were observed as the infill relicts exposed in gully walls in the area where studies of modern soil erosion were carried out in the past (Śnieszko, 1995). The age of these colluvial sediments was documented by OSL dating. The aggradation of soil material eroded by water is synchronous with the archaeologically documented phases of agricultural colonization. The thickness of Holocene colluvial deposits accumulated in this area do not exceed 4 m. A comparison of these sediments resulting from soil erosion and characterized by limited thickness for the last 5–6 thousand years with the results of studies on the magnitude of contemporary soil erosion gives an idea of the difference in intensity of the erosion processes nowadays and in the past in this particular area (Śnieszko, 1985, 1987, 1995). One location where the impact of crop farming was significant for the natural environment in the past is near the Bronocice settlement in southern Poland. In the central part of the Nidzica Basin, one of the largest concentrations of funnel-bowl culture settlements was documented (Kruk
Archaeological investigation of this settlement and its neighbouring area has revealed no signs of agricultural pressure after the Neolithic until the Middle Ages, when settlements were established in locations where villages have persisted to this day (Kruk
The precise determination of the intensity of soil erosion that occurs today, as well as from the past, is a vital problem in palaeoenvironmental studies and still requires intensive research. Moreover, the precise age of sediment formation plays an important role in modelling past environmental dynamics. Each land cover change on the slope due to ploughing produces accelerated soil erosion on the slope itself, and sediment accumulation within mid-slope flat areas and on valley floors (Zadorova
The objective of this research was to use radioactive fallout, OSL and dendrochronology to further explore the potential for combining those dating methods to document soil redistribution over different time scales and human impact on the study area. In this study, we present the results of soil erosion analysis using the 137Cs and 210Pbex methods, OSL dating, sediment analysis, micromorphology analysis and dendrochronology analysis. The soil erosion during the last 50–100 years was determined in ploughed land using both the 137Cs and 210Pbex methods. To extend the study beyond the recent sediments to the Holocene colluvial package, luminescence dating method was also employed, specifically, the OSL dating of coarse-sized grains (Fuchs and Lang, 2009). Those dating methods, in connection with pedological and sedimentological analysis, provide information about the age of pre-historical and historical phases of intensified soil erosion. Additionally, dendrochronological analyses in our studies was used, mainly for checking whether erosion processes are currently active in the under study gully. Tree ring analyses can provide information about evolution of the gully morphology in a wide range of time from the last few decades to the hundreds of years (depending on the sampled tree age); (Vandekerckhove
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). (
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,
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 (
Colluvial sediments and soil samples were collected from the wall of the gully as well as from the adjacent agricultural field (
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.
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 (210Pbex ) 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
The measured activities of radioisotopes in the sediment and soil samples were converted into dose rates by using the conversion factors described by Guerin
The results of radionuclide analysis as results of dose rate calculation are summarized in
Results of activity concentration measurement (based on low-level semiconductor γ-spectrometry), dose rate calculation, De estimation and OSL ages for sediment samples from Biedrzykowice. 1Doses for samples collected from a depth lower than 50 cm were corrected with respect to range of gamma rays. 2Dose rates in this column were obtained by using portable gamma spectrometer in situ
Sample |
Lab code | Depth |
U |
Th |
K |
Dose rate |
Dose rate2 |
Palaeodose |
OSL age |
---|---|---|---|---|---|---|---|---|---|
Bie_1_1 | GdTL-2906 | 11–15 | 31.84±0.44 | 39.09±0.78 | 508±17 | 2.84±0.211 | 2.74±0.20 | 1.831±0.037 | 0.645±0.049 |
Bie_1_2 | GdTL-2907 | 26–30 | 34.21±0.36 | 40.08±0.62 | 502±16 | 3.08±0.211 | 3.03±0.22 | 2.182±0.085 | 0.708±0.056 |
Bie_1_3 | GdTL-2908 | 55–59 | 32.71±0.39 | 38.35±0.65 | 527±17 | 3.07±0.22 | 3.04±0.23 | 2.29±0.14 | 0.746±0.070 |
Bie_1_4 | GdTL-2909 | 120–124 | 29.53±0.55 | 36.80±0.83 | 492±17 | 2.85±0.20 | 3.12±0.23 | 1.922±0.031 | 0.674±0.049 |
Bie_1_5 | GdTL-2910 | 151–155 | 30.82±0.36 | 38.15±0.63 | 559±18 | 3.09±0.22 | 2.94±0.22 | 3.052±0.068 | 0.988±0.074 |
Bie_1_6 | GdTL-2911 | 200–204 | 30.23±0.23 | 38.55±0.48 | 539±16 | 3.01±0.22 | 3.01±0.23 | 7.88±0.20 | 2.62±0.20 |
Bie_1_7 | GdTL-2912 | 231–235 | 30.11±0.39 | 37.68±0.69 | 532±17 | 2.96±0.21 | 2.96±0.23 | 11.11±0.14 | 3.75±0.27 |
Bie_1_8 | GdTL-2913 | 286–290 | 27.97±0.39 | 34.45±0.68 | 533±17 | 2.86±0.21 | 3.12±0.24 | 16.32±0.28 | 5.71±0.43 |
Bie_1_9 | GdTL-2914 | 356–360 | 31.51±0.42 | 34.57±0.70 | 532±17 | 2.92±0.21 | 3.03±0.23 | 17.67±0.28 | 6.05±0.45 |
Bie_1_10 | GdTL-2915 | 380–384 | 26.20±0.45 | 31.80±0.74 | 510±17 | 2.70±0.20 | 2.54±0.20 | 19.70±0.15 | 7.30±0.54 |
Bie_1_11 | GdTL-2916 | 406–410 | 30.63±0.48 | 34.93±0.77 | 534±18 | 2.90±0.21 | 2.72±0.21 | 26.19±0.23 | 9.03±0.66 |
Bie_1_12 | GdTL-2917 | 425–429 | 32.62±0.48 | 36.65±0.79 | 524±18 | 2.94±0.21 | 2.98±0.23 | 32.40±0.40 | 11.02±0.80 |
Bie_1_13 | GdTL-2918 | 471–475 | 35.15±0.29 | 40.70±0.53 | 483±15 | 2.93±0.21 | 2.93±0.23 | 35.8±1.0 | 12.22±0.94 |
Bie_2_1 | GdTL-3067 | 12–15 | 29.99±0.73 | 34.01±0.81 | 504±21 | 2.89±0.221 | - | 2.25±0.18 | 0.779±0.086 |
Bie_2_2 | GdTL-3068 | 52–57 | 30.08±0.72 | 32.46±0.76 | 508±21 | 3.01±0.22 | - | 42.1±0.95 | 14.0±1.0 |
Bie_3_1 | GdTL-3069 | 12–15 | 31.01±0.67 | 29.98±0.63 | 502±21 | 2.80±0.211 | - | 3.97±0.30 | 1.42±0.15 |
Bie_3_2 | GdTL-3070 | 52–57 | 24.94±0.65 | 27.26±0.72 | 455±19 | 2.64±0.19 | - | 44.52±0.68 | 16.9±1.2 |
Bie_4_1 | GdTL-3071 | 14–18 | 32.61±0.79 | 34.72±0.83 | 528±22 | 3.02±0.231 | - | 0.528±0.033 | 0.175±0.017 |
Bie_4_2 | GdTL-3072 | 54–57 | 31.76±0.76 | 32.62±0.82 | 519±22 | 3.08±0.22 | - | 2.58±0.18 | 0.838±0.084 |
Bie_4_3 | GdTL-3073 | 91–95 | 30.28±0.64 | 33.10±0.67 | 545±22 | 3.11±0.23 | - | 13.05±0.31 | 4.20±0.33 |
Bie_4_4 | GdTL-3074 | 110–114 | 30.26±0.75 | 33.84±0.84 | 558±24 | 3.16±0.23 | - | 19.83±0.61 | 6.28±0.50 |
Bie_4_5 | GdTL-3075 | 145–149 | 28.03±0.95 | 33.18±0.94 | 595±23 | 3.19±0.24 | - | 39.17±0.83 | 12.28±0.96 |
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
The measured 137Cs and 210Pbex activities were converted to inventories according to the formula described by Sutherland (1992) and are presented in
where:
Y – mean annual soil loss (t·ha–1·a–1),
B – bulk density of the soil (kg·m–3),
d – depth of the plough layer (m),
X – percentage reduction in total 137Cs inventory (defined as (Aref - A)/Aref ·100),
Aref – local 137Cs reference inventory (Bq·m–2),
A – measured 137Cs inventory at the sampling point (Bq·m–2),
T – time elapsed since initiation of 137Cs accumulation (a),
P – the particle size correction factor (possible difference between the grain size composition of the mobilized sediment and the original soil). The value of this factor could be calculated according to He and Walling (1996).
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
where:
Y – mean annual soil loss (t·ha–1·yr–1),
d – depth of the plough layer (m),
B – bulk density of soil (kg·m–3),
X – the percentage reduction in total 137Cs inventory (defined as (Aref – A)/Aref ·100),
t – time since the year 1963.
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
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 210Pbex inventories. According to He and Walling (1997), this model could be written as follows:
where:
A(t) – cumulative 137Cs or 210Pbex activity per unit area (Bq·m–2),
R – soil erosion rate (kg·m–2·a–1),
d – the average plough depth (kg·m–2),
λ – the radioactive decay constant for 137Cs or 210Pbex (a–1),
I(t) – annual 137Cs or 210Pbex deposition flux (Bq·m–2 a–1),
Γ – the percent of the fresh deposition of 137Cs or 210Pbex removed by erosion before mixing into the plough layer,
P – the particle size correction factor (see footnotes for
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:
Aex – is the excess 137Cs or 210Pbex inventory (Bq·m–2) / total measured inventory minus reference value of inventory/,
Cd(t’) – is a concentration of deposited sediment which is a sum of activities of mobilized sediment from upper part of the slope.
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,
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
Micromorphological analyses were carried out on a selected sequence of colluvial sediments with intercalated palaeosoil horizons in the gully (
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 (
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
The 137Cs and 210Pbex 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 (
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
The 137Cs fallout calculations for the study area are presented in
The mean value for the 210Pbex 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
Based on the reference values of 137Cs, about 62% of the study area was defined as eroding while, based on the 210Pbex inventories, this figure was about 50%. The simple comparison between the measured value of the 137Cs or 210Pbex inventories and the value of the reference inventory of 137Cs or 210Pbex 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 210Pbex, the proportional model (137Cs) as well as mass balance models (137Cs, 210Pbex) 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
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 210Pbex 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 210Pbex. However, the accumulation in the case of the 210Pbex 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.
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
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;
quartz grains into a ploughed A-horizon,
In the case of the gully wall sampling point, 13 sediment samples from different sediment horizons were collected for the luminescence study (
The results of particle-size analysis, along with the depth distributions of 137Cs and 210Pbex and OSL ages for three locations on the slope (top, eroded part and foot of the slope) are presented in
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
The sequence presented in
Typical micromorphological features (
Micromorphology of colluvial-soil sequence at Biedrzykowice. Microstructure: ch – channel, co – coprolithic, ma – massive. Frequency of occurrence: 0 – none, 1 – single, 2 – few, 3 – common, 4 – very common.
Unit | Depth (cm) | Microstructures | Clay coatings and infillings | Fe and Fe-Mn microforms | Fecal pellets | Charcoals |
---|---|---|---|---|---|---|
I | 10–15 | ch-co | 0 | 0 | 3 | 2 |
25–30 | ch | 0 | 2 | 2 | 2 | |
55–60 | ch | 1 | 2 | 1 | 0 | |
120–124 | ch | 2 | 2 | 3 | 2 | |
II | 151–155 | ch | 2 | 2 | 2 | 2 |
200–205 | ch | 2 | 2 | 2 | 1 | |
231–235 | ch | 1 | 2 | 2 | 0 | |
285–290 | ch | 0 | 3 | 2 | 0 | |
355–360 | ch | 0 | 3 | 2 | 0 | |
III | 380–385 | ch | 0 | 3 | 4 | 1 |
405–410 | ch | 3 | 2 | 2 | 3 | |
425–430 | ch-ma | 4 | 3 | 2 | 2 | |
470–475 | ch-ma | 2 | 1 | 0 | 0 |
The middle unit (151–360 cm;
The main microfeatures of the upper unit (0–124 cm;
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.
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 (
The tree is tilted perpendicularly to the gully axis, about 25% of its root system is exposed. The distance between the furthest exposed root and the hillslope edge is 0.5 m. The tree we sampled has grown since at least 1960. Eccentric growth of the tree started in the following years: 1977, 1981, 1996, 1999, 2000, and 2014. Strong ring reductions were found in 1970–1978, 1986–1989, 2010–2013.
The tree is tilted perpendicularly to the gully axis, about 50% of its root system is exposed. The distance between the furthest exposed root and the hillslope edge is 0.9 m. The tree we sampled has grown since at least 1949. Eccentric growth of the tree started in the following years: 1978, 1979, 1983, 1984, 1996, 1998, 1999, 2002, 2010, 1015, 2016. Strong ring reductions were found in 1961–1963, 1968–1973, 1989–1994, 2005–2011 (
The tree is tilted perpendicularly to the gully axis, about 50% of its root system is exposed. The distance between the furthest exposed root and the hillslope edge is 0.6 m. The tree we sampled has grown since at least 1953. Eccentric growth of the tree started in the following years: 1957, 1963, 1975, 1976, 1981, 1987, 1996, 2008, 2014, 2016. Strong ring reductions were found in 1970– 1976, 1968–1973, 1998–2002, 2010–2017.
The tree is tilted perpendicularly to the gully axis, no roots were exposed, tree growing at the distance of 1 m to the gully hillslope edge. The tree we sampled has grown since at least 1949. Eccentric growth of the tree started in the following years: 1957, 1963, 1975, 1976, 1981, 1987, 1996, 2008, 2014, 2016. No strong reductions were found.
The tree is tilted perpendicularly to the gully axis, about 50% of its root system is exposed. The distance between the furthest exposed root and the hillslope edge is 1 m. The tree we sampled has grown since at least 1948. Eccentric growth of the tree started in the following years: 1961, 1963, 1971, 1973, 1978, 1980, 1986, 1988, 1993, 1996, 1999, 2002, 2014, 2016. Strong ring reductions were found in 1983–1998, 2010–2015. Additionally from this site, the exposed roots of the studied tree were collected. Roots were exposed: no. 1 – 1993, no. 2 – 1998, no. 3 – 2008, no. 4 – 2014.
The calculated sedimentation rate for modern soil erosion, based on the 137Cs and 210Pbex measurements, ranges from almost 0 to 4.5 mm·a–1. The calculated total thickness of soil eroded obtained by the 137Cs and 210Pbex measurements is quite similar and ranges from 0 .3 to 23 cm and from 0.1 to 22 cm, respectively. In the case of sediment accumulation, the calculated total thickness of the accumulated sediments ranges from 4 to 46 cm in the case of the 137Cs measurement and from 1 to 33 cm in the 210Pbex measurements. The apparent similarity of the results obtained with both isotopes is somewhat surprising, taking into account the range in time of both methods. One possible explanation is the possible change in land use that took place after World War II. There are also visible differences in distribution on the surface of the field between the results of soil erosion and sediment accumulation obtained using the 137Cs and 210Pbex methods. The 210Pbex method indicates that the upper parts of the slope are eroded and a more gradual increase in accumulation takes place from more or less half of its length. In the case of 137Cs, the spatial distribution of erosion and accumulation intensity is slightly different. Rather, it remodels the surface of the slope, and the largest accumulation takes place higher than in the 210Pbex analyses. The eroded material probably does not reach the edge of the gully (former valley bottom) from the change in land use. Additionally, for the upper part of the colluvial sediment samples collected from the wall of the gully, a limited sediment accumulation was observed for the last 50–100 years. Both the isotope depth distributions, as well as the values of isotope inventories, confirmed this hypothesis. For those sampling points, 137Cs as well as 210Pbex are present only at the top of the modern A-horizon, which confirms that no mechanical mixing has occurred in this location over the last 50–100 years. The depth distribution of both isotopes, together with the inventories of 137Cs and 210Pbex , suggest that limited sediment delivery during the last 100 years occurred to the site at the edge of the gully. This delivery was probably before the fallout of 137Cs which occurred after the Chernobyl accident. Samples B_1_1 to B_1_4 (gully wall) were dated by the OSL method as medieval colluvial sediment (
For the sediment core located at the foot of the slope, the OSL age of the sediment is 0.175 ka. Although this layer contains 137Cs activity, its depth distribution and inventory suggest no sediment delivery for at least 60 years. The 137Cs and 210Pbex inventories also confirm the lack of modern sediment. Thus, this OSL age result seems to be reliable. For samples from the deeper layers, the OSL ages correspond to the ages of samples collected from the gully wall. Gully erosion and surface erosion after the deforestation of dry valleys took place simultaneously. The formation of a gully means that agrotechnical treatments on the slopes of the dry valley in which the gully has formed no longer reach its edge. If the area of a gully and its periphery is re-forested, the flow of material eroded away from the farmed slope of the dry valley to the edge of the gully ceases. Hence, in the Holocene water erosion soil profile that was exposed in the gully wall, no record of sediment caused by erosion of the dry valley slope used for agriculture has been found since the time of afforestation of the gully began.
The medieval colluvial sediment is about 150 cm thick, while the thickness of the Neolithic colluvial sediment is about 180 cm. Unfortunately, there are difficulties in precisely establishing the age of the latest Neolithic colluvial sediment due to pedoturbation. In the case of medieval sediments, the upper layer might have been rejuvenated as well, however, it is significantly less pronounced than in the case of the Neolithic colluvial sediment. While 50 cm of sediment has accumulated over the last 50 years, only about 180 cm of sediment accumulated during the few hundred years of the Neolithic. Thus, in the case of medieval sediment, it can be concluded that the layer of about 150cm thickness accumulated in a very short time. The relatively small age range of medieval sediment suggests that the intensity of erosion at that time exceeded even what we are seeing nowadays. Similar results indicating an increase in medieval erosion have been noted by Fuchs
This resulted in the abandonment of this area as farmland, an invasion by woody vegetation, and consequently in the stoppage/minimization of erosion on the slope. Erosion processes re-started about 50–60 years ago, as indicated by the 137Cs and 210Pbex isotope measurements, probably due to land use change. The oldest sampled tree, one of the biggest in the studied gully started growing in the 1930’s. This indicates the period when in the gully began to grow trees.
Long-term research on the Neolithic settlement in Bronocice, and its immediate surroundings in the vicinity of the specific site investigated by the authors, commenced in 1981. Studies on Holocene sediments in river valleys and dry valleys started shortly thereafter and the joint results of archaeological and geological research were published in 1996 (Kruk
The intensification of agricultural colonization in the studied area in the Middle Ages, now confirmed in radiometric dating, has been documented historically as well (Śnieszko, 1985). As in the case of research on the Neolithic colluvium, prior to these measurements there was similarly no direct indisputable evidence of the presence of medieval colluvial sediments corresponding to agricultural land use.
Radiometric measurements of the sediments in Biedrzykowice also confirm another well-established hypothesis in the studies of local pre-history: that of significantly decreased agricultural pressure in the Bronze Age and in the period of Roman influence, as settlements were dispersed and agriculture poorly advanced during that time.
Colluvial sediment dating and erosion rate calculations based on radionuclide analysis were used to perform a detailed study of the Holocene sediment budget of a slope in a loess area in Poland. As shown by the analysis, the area has been contaminated by Chernobyl 137Cs fallout – although this contamination is not strong, the SMBM model is not reliable in this case. The IMBM model, although more complicated, seems a much better choice for the study area. Similar results were obtained using the proportional model. The results of soil erosion obtained with the 210Pb isotope measurement method are smaller and have different spatial variability. This probably reflects the changing agricultural use after World War II.
Modern soil erosion based on results from the 137Cs method is 2.1 kg·m–2·a–1, whereas that obtained by the 210Pbex method equals 1.4 kg·m–2·a–1. The rate of erosion on the slope is quite variable. About 100% of the eroded soil has accumulated at the base of the slope. The mean thickness of modern sediment is 30 cm, the thickness of medieval sediment is about 155 cm, with Neolithic sediment about 180 cm thick. Sedimentation rates for the Neolithic sequences compared to those of the medieval periods are lower. The results suggest that the increased intensity of water soil erosion on the sl opes was one of the results of human impact on the environment. The start of sediment accumulation is correlated with the beginning of Neolithic farming. Although the thickness of medieval sediment is less than that of Neolithic sediment, the intensity of soil erosion was probably highest shortly after the reintroduction of farming in the Middle Ages, when gully formation had already started. The current intensity of soil erosion is probably lower than in the past and equals a few cm (up to 23 cm) for the last 50 years. The pattern of soil erosion on the slope suggests that we are currently witnessing slope sediment water transport with redeposition still on the slope.
The precise dating of colluvial sediment suggests that the sedimentation rate and thus also the erosion intensity for the Middle Ages was probably higher than nowadays. The OSL dating results show that colluvial sediments could indeed be dated by this method with sufficient precision, despite the rather short transport of quartz grains on the slope - mainly because the bleaching of the OSL signal begins even before the transport of grains on the slope, and finishes after burial by the next sediment layer at the accumulation site. Simultaneous use of the OSL dating method, the 137Cs and 210Pbex isotopic methods and of detailed pedological analysis allowed us to establish a reliable sediment chronology and reconstruct the history of soil erosion and sediment accumulation.
Gully erosion and surface erosion after the deforestation of dry valleys take place simultaneously. The formation of a gully means that agrotechnical treatments on the slopes of the dry valley in which the gorge has formed no longer reach its edge. If the area of a gully and its periphery is re-forested, the flow of material eroded away from the farmed slope of the dry valley to the edge of the gully ceases. Hence, in the Holocene water erosion soil profile that was exposed in the gully wall, no record of sediment caused by erosion of the dry valley slope used for agriculture has been found since afforestation of the gully began.