Horizons of strongly decomposed peat (HSDP), known as moorsh horizons in the Polish soil classification system and muck horizons in the terminology of North American agriculture (Okruszko and Ilnicki, 2003), are a valuable indicator of paleohydrological changes in mires. They not only indicate hiatuses in peat accretion, but also indicate a long-term negative balance of peatland productivity in conditions in which there is a significant drop in the groundwater level.
On the territory of Poland, HSDP are usually observed in acrotelm layers in modern peatlands. In the case of minerotrophic mires, this usually applies to fens and wet meadows populated by phytocenoses that are dominated by sedges, and possibly to swamps overgrown with forest communities including common alder (e.g. Tomaszewska and Kołodziejczyk, 2010; Forysiak, 2012; Kołodziejczyk, 2013; Forysiak
The article presents the results of research on the chronology and determinants of HSDP development in peat sequences in the Racibórz Basin and analyses the relationships between plant succession and the productivity of the mires examined. In the context of the mechanism of HSDP formation and the conditions that determine the resumption of peat accumulation, the limitations of absolute dating of dry and wet phases with respect to mires are discussed.
The study sites discussed are located in the Racibórz Basin (
Location of the Ruda Kozielska (A), Łany Małe (B) Sławięcice 1 (C) and Sławięcice 2 (D) study sites.Fig. 1
During the field studies, HSDP were identified as intensively dark layers of amorphous peat (see Żurek
Laboratory methods were selected in order to verify the presence of HSDP in peat sequences (pollen analysis and radiocarbon dating) and to reconstruct the environmental conditions that accompanied their formation. Firstly, on the basis of lithologic features, the cores examined were divided into homogeneous sections, usually few centimetres thick. The origin of the deposits and the composition of the subfossil peat-forming communities were established on the basis of macrofossil analysis. The analysis covered both vegetative remains and carpological findings. Using the microscopic grid method, the percentage shares of amorphous organic matter and carbonised phytoclasts in peat were determined. Pollen analysis was used to describe vegetation changes on a regional scale and to assess the human impact on the environment in various phases of the Holocene. The presence of charcoal and carbonaceous dust that provided evidence of fires in the peatland area was observed during the analysis. Samples for pollen analysis were prepared using 10% KOH, 10% HCl, 40% HF and Erdtman’s acetolysis method (Faegri and Iversen, 1978). The calculation was based on the sum of tree and shrub pollen (AP) and of terrestrial herb and dwarf shrub pollen (NAP). Radiocarbon dating was used to determine the absolute age of deposits. Peat and wood samples as well as separate seeds were analysed by conventional beta counting or AMS methods at the Kyiv, Poznań and Gliwice Radiocarbon Laboratories (Table 1). The radiocarbon dates were calibrated using the OxCal 4.2.4 radiocarbon calibration programme (Bronk Ramsey, 2009) and IntCal13 atmospheric curve (Reimer
Dated material and results of radiocarbon dating of sediments from the sites Łany Małe (ŁM), Ruda Kozielska (RK), Sławięcice 1 (Sw1) and Sławięcice 2 (Sw2). OxCal 4.2.4 (Bronk Ramsey, 2009); IntCal13 atmospheric curve (Reimer et al., 2013)Core Depth (cm) Dated material Lab. code 14C age (BP) Cal. age (BC/AD) 36–38 seeds; i.a. Poz-23205 1975 ± 30 50 BC – AD 80 RK 51–55 alder peat Ki-7161 5660 ± 80 4690 BC – 4350 BC 85–89 alder peat Gd-10225 6010 ±120 5230 BC – 4610 BC 21–26 mainly roots of Ki-7583 1720 ± 80 AD 120 – AD 540 53–58 alder peat Ki-7569 2720 ± 70 1050 BC – 790 BC ŁM 58–63 alder peat Ki-7570 4530 ± 80 3510 BC – 2930 BC 86–91 alder peat Ki-7571 6240 ± 70 5360 BC – 5010 BC 33–38 alder peat Ki-7566 1305 ±70 AD 610 – AD 890 77–79 Poz-23206 2130 ± 30 350 BC – 50 BC Sw1 79–84 alder peat Ki-7567 3755 ± 70 2460 BC – 1960 BC 150–155 Ki-7568 8050 ± 80 7290 BC – 6690 BC 263–267 brown moss peat Ki-7157 9400 ± 90 9130 BC – 8350 BC 28–32 sphagnum moss-sedge peat Ki-11229 200 ± 60 AD 1520 – present SUI2 70–72 Poz-23211 1395 ± 30 AD 600 – AD 680 72–84 alder peat Ki-13092 4290 ± 60 3100 BC – 2690 BC 198–204 mainly roots and epidermis of reed Ki-11230 7480 ± 60 6450 BC – 6230 BC
In the Ruda Kozielska core, the greatest degree of decomposition is exhibited by peat at a depth of 60–51 cm. The deposits in question are black (10YR2/1 according to the Munsell Soil Color Charts) due to the presence of carbonised plant tissues and humic substances. A sedimentary discontinuity at a depth of 60–50 cm was reflected in the pollen analysis results. Abrupt changes in the percentage share of pollens (including that of
Simplified pollen diagram from the Ruda Kozielska core, Ruda River valley.Fig. 2
Macrofossil diagram from the Ruda Kozielska core, Ruda River valley.Fig. 3
The HSDP roof in the Łany Małe core occurs at a depth of 58 cm. The use of the radiocarbon method confirmed the discontinuity in deposit accumulation. The dating of a sample from a depth of 63–58 cm yielded the result of 3510–2930 cal BC while the overlying peat layer from a depth of 58–53 cm was dated to 1050–790 cal BC. The presence of a stratigraphic gap is underscored in the pollen diagram by the occurrence of very large quantities of carbonaceous dust and just a few unidentifiable pollen grains at a depth of about 58 cm (Fig. 4). Pollen spectra from a depth of 75–60 cm still show the broad development of pine communities with a small share of oak (
Simplified pollen diagram from the Łany Małe core, Kłodnica River valley.Fig. 4
Macrofossil diagram from the Łany Małe core, Kłodnica River valley.Fig. 5
A layer of dark (5Y2.5/1), highly decomposed peat is present in the Sławięcice 1 core at a depth of 108–79 cm. The presence of a hiatus was confirmed by palynological means. The analysis of a sample from a depth of 80 cm revealed the presence of a non-pollen layer containing large quantities of carbonaceous dust (Fig. 6). The pollen spectrum of deposits from a depth of 85 cm indicates a significant share of pine in the forest communities, which is characteristic of the early Holocene, and insignificant shares of alder, hazel and oak. On the other hand, the presence of
Simplified pollen diagram from the Sławięcice 1 core, Kłodnica River valley.Fig. 6
Macrofossil diagram from the Sławięcice 1 core, Kłodnica River valley.Fig. 7
The highest share of amorphous matter (63%), accompanied by a high share of carbonised phytoclasts (7%), was recorded in a level of dark (5Y 2.5/1) alder peat at a depth of 84–72 cm. Macrofossil analysis demonstrated the domination of common alder wood in this level, accompanied by
Simplified pollen diagram from the Sławięcice 2 core, Kłodnica River valley.Fig. 8
Macrofossil diagram from the Sławięcice 2 core, Kłodnica River valley.Fig. 9
The mid-Holocene peat decay was not recorded in all peat sequences in the Racibórz Basin. In the light of radiocarbon dating, it is evident that during the Atlantic Period a relatively high accumulation rate of
Paleobotanical analyses demonstrate that HSDP could develop in shallow mires formed both as a result of the paludification of distal floodplains (as in the case of the Łany Małe site) and the terrestrialisation of relatively deep oxbow lakes (as at the Sławięcice 1 and Sławięcice 2 sites). A common feature of all the study sites is that peat decay took place at the stage when a forest swamp community dominated by common alder developed. These observations contribute to the broader debate concerning the productivity of alder swamps (see Tobolski, 2000). While the peat-forming abilities of alder forest are unquestionable (Marek, 1965), in the forest phase of mire development a decrease in accumulation rate or even the decay of previously accumulated deposits is usually observed (e.g. Grosse-Brauckmann, 1986; Harmata, 1995). The reasons for this state of affairs can be complex. Alder swamps receive direct precipitation as well as groundwater inflow, with a small role of inundation waters (Matuszkiewicz, 2005). The consequence of such water relations may be the periodic advantage of aerobic processes in the acrotelm layer, with peat aeration in forest habitats potentially increased due to the penetration range of tree root systems (Tobolski, 2003). Either way, the key issue for the development of HSDP is the fact that the spread of alder forests in river valleys and the hydrological changes observed in North-Western and Central Europe (within the zone affected by the North Atlantic) were synchronous events. The results of paleohydrological studies provide ever more evidence that in the more recent part of the Atlantic period, water levels lowered in both lakes (e.g. Ralska-Jasiewiczowa and Starkel, 1988; Magny
After a period of peat decay, a return to peat accumulation was observed during the late Holocene in all the cores examined. Macrofossil analysis shows that changes in the peatland balance of productivity had already started at the time when swamp forests dominated by alder prevailed. The peat layers directly overlying the HSDP roof in the Łany Małe and Sławięcice 1 cores contain even more tree remains. The increase in the share of periderm could have been the result of the accumulation of above-ground remains on a long-term stable mire surface. Over time, the composition of the remains preserved in the peat, which resumed its accumulation, changed, reflecting the adaptation of plant communities to more humid habitats. In the sequences of deposits analysed, a shift was observed towards non-forest, sedge-moss communities belonging to the
Age-depth models of the studied cores.Fig. 10
The mechanisms that determine mire productivity sometimes make it difficult to date wet and dry phases in mires since 14C dating results are inconsistent with pollen data.
Particular methodological problems are related to the dating of dry phases during which HSDP form. Pollen spectra from a depth of 75–60 cm in the Łany Małe core enable the correlation of sediments with the Boreal period, which is inconsistent with the date of 3510–2930 cal BC (at a depth of 63–58 cm). A similar situation occurs at the Sławięcice 1 site where the date of 2460–1960 cal BC was obtained at a depth of 84–79 cm. While at a depth of 80 cm a non-pollen layer was present, just below it (at a depth of 85 cm) pollen spectra indicated the Boreal period. In the Sławięcice 2 core, pollen spectra from a depth of 100–90 cm suggest also an early Holocene age, which is inconsistent with the date of 3100–2690 cal BC (at a depth of 72–84 cm).The above results demonstrate that HSDP contain organic matter of different ages – both from the period in which the decay process took place (mainly intrusive alder wood) and older, usually strongly decayed matter from the degraded peat layer. The significant age difference between pollen analysis results and the 14C dates obtained indicates that the postsedimentological decay processes involved ever deeper levels of previously accumulated peat. Given the negative balance of peat productivity, the mire surface probably became lower (Fig. 11).
Model explaining the causes of inconsistencies between 14C dating and pollen analysis as a result of HSDP development.Fig. 11
The situation at the Ruda Kozielska site could have been slightly different. There, pollen analysis demonstrates that deposit continuity was probably disturbed at a depth of 60–50 cm. The underlying deposits (at a depth of 70–60 cm) may be associated with the Boreal period, and the overlying ones (at a depth of 50–40 cm) – with the Subboreal period. Deposits from the top HSDP layer, at a depth of 55–51 cm, were dated to 4690–4350 cal BC, i.e. to the Atlantic period. In general, pollen spectra from that depth are not inconsistent with that date. On this basis, it can be assumed that at the end of the Atlantic period, the productivity of the Ruda Kozielska mire could have been balanced. Organic matter decomposition processes were mainly synsedimentological ones and complete mineralisation of the previously accumulated peat, which would have resulted in the lowering of the mire surface, did not occur.
Radiocarbon dating of the samples that directly overlie HSDP, and in particular the application of the AMS method to the carpological remains deposited under conditions of a positive balance of productivity, should in theory enable the determination of the age at which the wet phase began in the mire in question. However, the verification of the dates obtained using pollen analysis is not an easy matter, and in some places it is ambiguous due to the absence of clear diagnostic features, sedimentation discontinuity and limited deposit thickness.
Most doubts arise in connection with the Sławięcice 1 core where the resumption of peat accumulation was dated to 350–50 cal BC using the AMS method. On the other hand, the analysis results of pollen deposits lying at that depth, e.g. high levels of
At the remaining sites, pollen analysis results appear to be broadly consistent with 14C dates. In the Sławięcice 2 core, the resumption of peat accumulation occurred in the 7th century CE according to AMS dating. In the light of pollen analysis, a correlation between the deposits examined and the Subatlantic period cannot be ruled out, e.g. on the basis of the increase in the level of
The studies conducted in the Racibórz Basin confirmed that fossil HSDP, which are characterised by a dark colour, a high proportion of humic substances and a decrease in the concentration and preservation levels of pollen grains, are evidence of long-term overdrying of the acrotelm layer. This is demonstrated by the presence of hiatuses, whose occurrence is indicated by sudden spikes in the pollen curves and by radiocarbon dating results that point to breaks of the order of 103 years in the accumulation of deposits, and also by traces of peat fires in the form of charcoal and carbonaceous dust. Peat cores from the Racibórz Basin contain records of wet and dry phases, which are increasingly frequently identified in the history of Central European mires. Pollen analysis demonstrates that the hiatuses cover the Atlantic period and at least partly the Subboreal one. The resumption of peat accumulation occurred in the late Holocene, although both pollen data (which point to the Subboreal or Subatlantic period at individual sites) and radiocarbon dating (ranging from 1050–790 cal BC to cal AD 600–680) appear to indicate that the process was not synchronous. In the light of the results obtained, it appears that the condition for HSDP development was the simultaneous presence of endogenous (the advanced development stage of mires with forest communities of the The dating of phases with a negative balance of mire productivity should be approached with caution, since HSDP contain organic matter of different ages. Studies have shown that in the case of post-sedimentological peat decay involving ever deeper peat layers, 14C dates differ significantly from the chronozones determined using palynological methods. This indicates that when determining the age of dry phases, the dating of pollen, carpological finds or above-ground vegetation remains (e.g. mosses) is not useful since they could have been preserved from the earlier phases of mire development. The dating of intrusive wood enables the determination of the development age of the forest formation, which, however, is not necessarily the same as the HSDP development age. The correlation between pollen analysis and 14C dating results indicates that it is possible to reliably determine the age of the boundary between dry and wet phases in mires.