Fluvio-aeolian environments in the European Sand Belt (ESB) experienced significant changes during the last glacial-interglacial transition (Koster, 2005). Such sedimentary successions exhibit cyclical periods of cooling and warming that manifest themselves in two different ways. Cold periods resulted in increased sediment influx, leading to the accumulation of fluvial and aeolian sediments. Rapid aeolian accumulation, in particular, buried existing soil levels (Kasse and Aalbersberg, 2019). During warm periods, rivers incised and formed terrace systems (Vandenberghe
Aeolian cover sands and aeolian dunes in the central part of the ESB consist mainly of quartz with limited admixture of feldspars and other silicates as well as reveal good sorting of usually well-rounded material (Woronko
The large amount of charcoal of various sizes in the Late Glacial palaeosols indicates the large role of fires in the formation of vegetation cover and soils on the dune surface (Kaiser
Luminescence dating, predominantly using optically stimulated luminescence (OSL) on quartz grains, has greatly improved stratigraphic schemes by dating extensively exposed fluvial and aeolian deposits (Kaiser
One of the key sites where aeolian complex is intercalated with palaeosol and organic layers enriched in charcoals is the Korzeniew site, central Poland. The objectives of the present study are (i) to reconstruct chronology of the studied succession on the basis of OSL/14C dating results comparison; (ii) to establish a frequency of wildfires and their role in soil-forming processes; and (iii) to correlate chronology of wildfires with Late Glacial climate changes in the ESB.
The Korzeniew site is located in the eastern Wielkopolska region, central Poland (
The thickness of the Quaternary sediments is approximately 50 m. They are underlain by Miocene clay sediments. The Quaternary unit consists mainly of glacial tills of the Middle Pleistocene age (Trzmiel, 1995). During the LGM, glaciofluvial sand with gravels were deposited in front of an ice-sheet. In the same period, the extraglacial Prosna River and its tributaries deposited 5–10 m thick sandy-silty covers in the northerly direction. In the last glacial termination (LGT), two fluvial terraces were formed due to the Warta River incision and its tributaries (Kozarski
The studied outcrop was analysed in two parts. In the central part, where aeolian sediments reach maximum thickness (approximately 10 m) only general lithofacial analysis and palaeotransport direction was measured. Detailed studies were conducted in the eastern part of the outcrop on the lee-side of the dune foreset. In 2 profiles, 13 samples for OSL dating and 6 samples for radiocarbon dating were collected (
The lithological analyses were complemented by pedological observations. Soil horizons were identified along with their morphological features. Symbols of soil horizons were used in accordance with the international standards (Jahn
Luminescence dating samples were extracted from the most representative sections of a pristine vertical outcrop using thin-walled steel pipes. The luminescence measurements were conducted at the Gliwice luminescence dating laboratory (Moska
For OSL measurements, coarse quartz grains (125–200 μm) were isolated from sediment samples through a standard process involving treatment with 20% hydrochloric acid (HCl) and 20% hydrogen peroxide (H2O2). These quartz grains were then subjected to sieving and density separation using sodium polytungstate solutions, resulting in grains with densities ranging from 2.62 g/cm3 to 2.75 g/cm3. The final step involved etching the grains with concentrated hydrofluoric acid (HF).
Automated Risø TL/OSL DA-20 readers equipped with a calibrated 90Sr/90Y beta source were utilised for OSL measurements. This source delivered approximately 6.0 Gy/min to the grains at the sample position, with a 6 mm Hoya U-340 filter used during OSL measurements. The determination of equivalent doses employed the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000). The final equivalent dose (
Six samples were collected for radiocarbon dating. For all investigated samples, about 100 g of sediments that contain dozens of tiny pieces (about 1 mm or less) of charcoal were taken. In the Gliwice Radiocarbon Laboratory, it was possible to extract several small pieces of charcoal from this sediment, which were enough to create a graphite target for dating. Charcoal fragments for radiocarbon dating were selected from each post-fire level. Before charcoal extraction, all samples were analysed using optical microscope with attached high-quality digital camera, which made it possible to carry out a preliminary characterization of the organic material used in the 14C dates. As expected, all samples contained well-preserved charcoal material (see
For all samples, typical chemical pre-treatment based on the AAA (acid-alkali-acid) method was used before radiocarbon dating. The AAA pre-treatment involved rinsing the samples in hot HCl (0.5 M, 85°C, 1 h) followed by an NaOH (0.5 M, 85°C, 1 h) bath and final HCl wash (0.5 M, 85°C, 15 min.). Between treatments, the samples were rinsed with demineralised water. In the radiocarbon accelerator mass spectrometry (AMS) technique, the 14C concentration was measured in graphite prepared from the carbon contained in the sample. For this purpose, a line for preparation of graphite targets in the Gliwice 14C laboratory was used. All 14C results were calibrated using the OxCal program (Bronk Ramsey, 2009) and the IntCal120 calibration curve (Reimer
Radiocarbon dating results before and after calibration.
1 | KOR_01 | GdA-7147.1.1 | 120 | 11,090 ± 55 | 13,095–12,930 | 13,105–12,840 | 13,005 ± 65 |
2 | KOR_02 | GdA-7148.1.1 | 200 | 11,070 ± 55 | 13,085–12,925 | 13,100–12,840 | 12,995 ± 70 |
3 | KOR_03 | GdA-7149.1.1 | 225 | 11,475 ± 60 | 13,435–13,300 | 13,475–13,185 | 13,355 ± 65 |
4 | KOR_04 | GdA-7150.1.1 | 250 | 11,640 ± 60 | 13,590–13,445 | 13,605–13,350 | 13,500 ± 70 |
5 | KOR_05 | GdA-7151.1.1 | 270 | 11,680 ± 55 | 13,595–13,480 | 13,745–13,415 | 13,540 ± 65 |
6 | KOR_06 | GdA-7152.1.1 | 310 | 11,760 ± 55 | 13,745–13,510 | 13,760–13,500 | 13,620 ± 80 |
All-important data for investigated luminescence samples: code, depth, radionuclide concentration, dose rate, equivalent dose (CAM model) and final age.
GdTL-4593 | KOR_1 | 75 | 3.5 ± 0.2 | 3.5 ± 0.3 | 145 ± 11 | 0.81 ± 0.04 | 9.4 ± 0.2 | 11.6 ± 0.6 |
GdTL-4594 | KOR_2 | 125 | 4.5 ± 0.2 | 4.0 ± 0.4 | 172 ± 13 | 0.91 ± 0.04 | 10.7 ± 0.2 | 11.8 ± 0.6 |
GdTL-4595 | KOR_3 | 190 | 3.7 ± 0.2 | 3.2 ± 0.3 | 167 ± 13 | 0.85 ± 0.04 | 10.5 ± 0.2 | 12.4 ± 0.6 |
GdTL-4596 | KOR_4 | 225 | 3.1 ± 0.2 | 2.9 ± 0.3 | 174 ± 13 | 0.85 ± 0.04 | 11.4 ± 0.2 | 13.5 ± 0.7 |
GdTL-4597 | KOR_5 | 260 | 5.2 ± 0.2 | 4.0 ± 0.3 | 183 ± 14 | 0.92 ± 0.05 | 12.4 ± 0.2 | 13.5 ± 0.7 |
GdTL-4598 | KOR_6 | 300 | 4.9 ± 0.2 | 4.1 ± 0.3 | 183 ± 14 | 0.91 ± 0.04 | 12.2 ± 0.4 | 13.5 ± 0.7 |
GdTL-4599 | KOR_7 | 300 | 2.8 ± 0.2 | 2.7 ± 0.3 | 148 ± 12 | 0.75 ± 0.04 | 10.1 ± 0.3 | 13.6 ± 0.8 |
GdTL-4600 | KOR_8 | 400 | 3.3 ± 0.2 | 3.1 ± 0.3 | 167 ± 13 | 0.80 ± 0.04 | 10.8 ± 0.3 | 13.6 ± 0.7 |
GdTL-4601 | KOR_9 | 500 | 4.9 ± 0.2 | 3.7 ± 0.3 | 175 ± 14 | 0.85 ± 0.04 | 11.4 ± 0.3 | 13.5 ± 0.7 |
GdTL-4602 | KOR_10 | 575 | 8.8 ± 0.3 | 6.2 ± 0.4 | 249 ± 19 | 1.07 ± 0.05 | 14.1 ± 0.4 | 13.2 ± 0.7 |
GdTL-4603 | KOR_11 | 600 | 7.4 ± 0.3 | 6.0 ± 0.3 | 241 ± 18 | 1.02 ± 0.05 | 14.4 ± 0.5 | 14.2 ± 0.7 |
GdTL-4604 | KOR_12 | 630 | 3.9 ± 0.2 | 3.6 ± 0.3 | 154 ± 12 | 0.75 ± 0.04 | 12.3 ± 0.2 | 16.5 ± 0.8 |
GdTL-4605 | KOR_13 | 660 | 4.2 ± 0.2 | 3.3 ± 0.3 | 152 ± 12 | 0.74 ± 0.04 | 12.3 ± 0.2 | 16.7 ± 0.9 |
CAM, Central Age Model; OSL, optically stimulated luminescence.
Two depositional units were distinguished based on the lithofacial analysis (
Upper unit (U-2) is composed of fine- to coarse-grained sands with inclined stratification in large scale with reactivation surfaces (
Seven soil units were identified on the lee-slope of the analysed dune (
Among these palaeosols, six soils have a simple A–C horizon structure. Sharply discernible from the neighbouring layers, the thin (~10 to 20 cm) humic horizons are the only horizons with evidence of soil changes. These horizons are rich in chaotically distributed charcoals with diameters often exceeding 1 mm. They are accompanied by biogenic channels filled with lighter and darker material, linked by adjacent layers of sand or other soil horizons.
Only the third soil from the top of the exposure has a much better developed profile: 3Ab-3Eb-3Bsb-3Cb with a total thickness of 50–60 cm (
In the simple palaeosols, a large number of well-preserved charcoals (
The two youngest results represent the same soil horizon, which underwent stratification. Both dating results GdA-7147.1.1 and GdA-7148.1.1 show astonishing agreement at 13,005 ± 65 calBP and 12,995 ± 70 calBP. The highest layer is considered to be a layer that was reworked from the same fire event as the layer below. This layer was deposited on a soil cover with active podzolization soil processes. The subsequent post-fire layers in the central part are represented by the results GdA-7149.1.1 at 13,355 ± 65 calBP, GdA-7150.1.1 at 13,500 ± 70 calBP, and GdA-7151.1.1 at 13,540 ± 65 calBP. In our opinion, the horizon represented by the result GdA-7149.1.1 is an independent event, while the horizons below may represent the same past fire event. From a statistical point of view, both results are similar, and their presence can be explained in a similar way to the two highest layers.
No charcoal fragments could be extracted from the lowest palaeosol (8Ab), so we have no results for that layer. This palaeosol has a completely different character, as evidenced by its increased Fe and Mn hydroxides content, which is evident in the abruptly higher R index value relative to the adjacent sand layers (
Samples for OSL dating were collected from two closely spaced profiles (
Dose distributions have unimodal character, suggesting that the quartz bleaching process during redeposition occurred correctly (
The investigated profile in the Korzeniew site contains remarkably interesting aeolian succession enriched in charcoal layers, presumably of post-fire origin (Van Hoesel
The initial aeolian deposition represents U-1 unit. Rhytmically stratified sandy and silty sediments were deposited under changeable conditions. Sandy lithofacies (
Reduction of available capacity of sandy-silty material and relative improvement of environmental conditions led to initial pedogenic processes and formation of the first soil. Absence of organic material limited possibility of direct dating of such processes. We can assume that the palaeosol formed under harsh conditions between the very end of the Late Pleniglacial and first part of the Late Glacial period. OSL dating result obtained directly from the palaeosol refers to the Bølling interstadial 14.2 ± 0.7 ka (GdTL-4603), but it should be noted that this palaeosol is a natural boundary between aeration and saturation zones of the first ground-water level. We assume that the vertical migration of fine-grained material (particularly clay minerals) and precipitation of Fe–Mn hydro-oxides played an important role. Nevertheless, weak pedogenic processes are visible. Such morphological position of the soil (at the top of aeolian sand cover and beneath the migrating dune foreset) is often for palaeosols of the Bølling age (Kaiser
The main deposition in the site is represented by U-2 unit. Lithofacies
The cross-checking of OSL and radiocarbon dating results from the U-2 unit reveals their consistency and correct chronology of depositional processes (
Radiocarbon dating results allow to estimate chronology of local wildfires. At least four fires took place between 13 620 ± 80 calBP (GdA-7152) and 12 995 ± 70 calBP (GdA-7148). The result obtained from the first palaeosol (13 005 ± 65 calBP, GdA-7147) suggests at least partial redeposition of charcoals. Nevertheless, an increase of fires number is reported also from the central and western parts of the ESB and related to expansion and removal of pine during the Late Glacial climate oscillations, particularly in the Allerød–Younger Dryas transition (Kaiser
Vegetation played a crucial role in stabilising dunes, so events like wildfires drastically changed the environmental conditions on dunes. In the Korzeniew site, the fire processes are not correlated with climate deterioration, typical for the Allerød–Younger Dryas threshold (Jankowski, 2012; Hošek
Despite being considered a period of significant climate warming, the Allerød interstadial also had at least two short-lived colder events (Rasmussen
The youngest part of the Korzeniew profile can be correlated with the Younger Dryas period, based on the luminescence results 11.6 ± 0.6 ka (GdTL-4593) and 11.8 ± 0.6 ka (GdTL-4594). This aeolian phase also contains material from the fire event described by the radiocarbon result 13,005 ± 65 calBP (GdA-7147.1.1), but in this case, it is much more likely that these charcoals were reworked from the same fire event as the layer below. Therefore, it is easy to explain their significantly older age than the neighbouring luminescence dating results.
Investigated succession in the Korzeniew site led to reconstructing sedimentary and palaeoenvironmental processes. Such reconstruction is enhanced by cross-checking radiocarbon-OSL dating results. The research carried out revealed that:
Simultaneous using of two independent dating methods and dense sampling is appropriate to construct a detailed chronostratigraphic model. Successfully reconstructed sequence showing silt-sand aeolian deposition from the very end of the Late Pleniglacial gave way to dune deposition with palaeosols within the Allerød interstadial and Younger Dryas. Wildfires controlled the development of the vegetation cover on and in the immediate vicinity of the dunes and the activity of aeolian processes. Despite the relatively warm climatic conditions during the Allerød interstadial, an increased number of fires were observed during this period. This is linked to the expansion of pine trees and their high fire potential. Short-termed climate oscillations in the Allerød interstadial might have destabilised vegetation conditions and facilitated the development of fires on the one hand and facilitated the re-colonisation of dune areas by plants on the other.