Investigation of the fingerprint of climate changes in the Tinovul Apa Roșie peat bog (central Romania) by using 210Pb dating method

The purpose of this research is to evaluate the fragility of the ecosystem of the Tinovul Apa Roșie peat bog, and the presence of the extreme weather events that this ecosystem has survived. Bogs are one of the most distinctive and specifi c habitat types amongst wetlands. The ombrotrophic (rain-fed) bogs get all their moisture from precipitation (rain, snow). Temperature and precipitation have a major impact on the hydrology of the bog. While precipitation induces the growth of the bog, changes of temperature affect the speed of its growth, meaning that the accumulation of bog is directly dependent on climate [1]. The vegetation composition, especially the dominant Sphagnum species signifi cantly infl uence the peat accumulation rates. Peatlands are carbon accumulating ecosystems, where the primary production exceeds the organic matter decomposition rate in the soil, and therefore they perform an important sink function in global carbon cycle [2-4]. The dominant plants of A raised peat bog in a temperate climate zone are mosses of the genus Sphagnum that provide most of the organic matter, whereas the dominant plants on the fens (fed by mineral-rich surface water or groundwater) are represented by sedge [5,6].


Introduction
The purpose of this research is to evaluate the fragility of the ecosystem of the Tinovul Apa Roșie peat bog, and the presence of the extreme weather events that this ecosystem has survived.
Bogs are one of the most distinctive and specifi c habitat types amongst wetlands. The ombrotrophic (rain-fed) bogs get all their moisture from precipitation (rain, snow). Temperature and precipitation have a major impact on the hydrology of the bog. While precipitation induces the growth of the bog, changes of temperature affect the speed of its growth, meaning that the accumulation of bog is directly dependent on climate [1]. The vegetation composition, especially the dominant Sphagnum species signifi cantly infl uence the peat accumulation rates.
Peatlands are carbon accumulating ecosystems, where the primary production exceeds the organic matter decomposition rate in the soil, and therefore they perform an important sink function in global carbon cycle [2][3][4]. The dominant plants of A raised peat bog in a temperate climate zone are mosses of the genus Sphagnum that provide most of the organic matter, whereas the dominant plants on the fens (fed by mineral-rich surface water or groundwater) are represented by sedge [5,6]. • Reduced amount of snowfall at high altitudes, in mountain areas.
These changes will have a signifi cant impact on the peatland's carbon store, greenhouse gas fl uxes as well as biodiversity. A low water level leads to a decrease of peatland surface and an increase of carbon emissions into the atmosphere, whereas a high-water level leads to an increase of peatland surface and carbon sequestration in peatlands [4].
The method of sequential Loss Ignition (LOI) is commonly used to estimate the organic matter content and carbonate minerals in sediments. The LOI method demonstrates the linear relations between organic and inorganic carbon content to identify the interface of peat sediment. The Loss on Ignition (LOI) of peat refl ects the input of clastic material from overbank fl ooding [7], which suggests that vertical changes of LOI in peat represent changes in sedimentation rate. High-resolution measurements of the LOI in peat are readily obtainable and which provide a record of changes in the overbank sedimentation rate. Where the organic matter is above 20% in the sampled material, that is considered peat [8]. The composition of peat is highly variable [2]. It is generally composed of at least 65% organic matter (dry weight basis) and less than 20%-35% inorganic material, and it may be up to 95%-99% organic [9]. Taking this into consideration, LOI measurements helped us in the separation of the organic peat growth rate from the inorganic sediment accumulation rate ( Figure 1). 210 Pb dating is a key technique to study sedimentary records of environmental change in the Anthropocene over a time scale of 150-200 years [10]. The continuing existence of peatland ecosystems is endangered by climate change and direct anthropogenic pressures (e.g. drainage, pollution and extraction).
The radionuclides 210 Po (T 1/2 : 138 days) and 210 Pb (T 1/2 : 22.3 years) are the fi nal radioactive members of the 238 U series and are widely present in the environment. The presence of 210 Po and 210 Pb in the sediments may be due to the decay of radionuclides in the 238 U chain present in the sediment or due to the deposition of 222 Rn (T 1/2 : 3.84 days) decay products.
Unsupported 210 Pb is deposited by wet and dry fallout. Normally, the estimated unsupported 210 Pb from the total 210 Pb inventory of a sediment section presents high values at the surface, decreasing with depth as a result of radioactivity decay [11].
Consequently, it is possible to establish age-depth models in sediment core sections, by quantifying the unsupported 210 Pb inventory from the surface down to the bottom until the unsupported 210 Pb becomes undetectable or negligible ) [12].
Infl ux of 210 Pb to bog areas was determined to originate from natural and anthropogenic sources [13].
The constant rate of the 210 Pb supply (CRS) model is often used to determine the ages and sedimentation rates of subsamples of sediment layers. This model is used if the growth rate is not constant over time, but if it can be presumed that the rate of the unsupported 210 Pb deposition is permanent [14][15][16] . Previous results showed that the 210 Pb method is the most accurate technique for determining the age and accumulation rate of peat [17]. The artifi cial fallout radionuclide 137 Cs data representing an independent chronomarker is usually included into 210 Pb chronology graphs for validation [17][18][19][20]. Given the fact that radionuclides are signifi cant aerosol tracers, it is important that we understand the cycling of 222 Rn-210 Pb, as this can help in the study of global aerosol-climate interaction [21].

Loss on Ignition (LOI)
1.5g aliquots of homogenized peat sample were measured into ceramic crucibles, weighted together, then they were heated to 350°C and kept there for 3 hours. After 3 hours the weight of the crucibles were measured again followed by heating to 750°C for 3 hours and fi nally their weight was measured for the third time. LOI analysis shows that we have peat accumulation above 22 and 25 cm in the sampling columns respectively (Figure 1).

Results and discussion
The 210 Pb dating method and the 137 Cs validation was carried out on peat bog samples from Tinovul Apa Roșie, Romania.
This area was assessed for the fi rst time by alpha and gamma spectrometric analysis, which were carried out to measure the radionuclides of interest ( 210 Pb/ 210 Po, 226 Ra and 137 Cs) in the peat sediments. The Chernobyl maximum was identifi ed in the normalized 137 Cs activity profi le. The activity ratios of anthropogenic radionuclides allow for the identifi cation of sources. The geochronology of the sediment layers and the sedimentation rates were assessed to characterize the peat growth processes involved in the evolution of the Tinovul Apa Roșie. Our results also confi rmed that the 210 Pb method is an accurate technique for determining the age and accumulation rate of a peat.

Sedimentation rates
The peat mass accumulation rate is considered an indicator of wet and warmer periods, which stimulate the accumulation rate of peat bog production, and colder, dry periods, when the peat growth is stagnant. In warm and dry periods, the peat Core C was taken from a higher elevation than Core A. In Core C the warming tendency in the local micro climate is clearly visible, which corresponds to an increasing growth rate of the peat, starting from 1900 from a value of 0.01±0.0009 to 0.04±0.008 g/cm 2 y. Global and regional data on temperature and precipitation to be compared is available only from the beginning of the 20th century, according to which in both cases the average temperature rise was between 0.5 and 1.1°C [22][23][24][25][26][27][28], (NOAA). In the last decades in the summer vegetative period there was an increase in temperature and precipitation, which is refl ected in the peat growth rate.
The radionuclide concentration (Figure 2) of 210 Pb shows an exponential decrease with the depth and can be used for radiometric age determination. For age calculation the Constant Rate of Supply model was used with minor modifi cations. The age obtained from 210 Pb dating for 1986 are clearly proved by the 137 Cs concentration originating from the Chernobyl event found in the sampled peat columns (Figure 3a,b). The peat growth rate was separated (extracted from the total value) from inorganic sediment deposition using the LOI results. Some fl ooding periods can be identifi ed in Core A, which was taken close to the Apa Roșie (Red Water). Core A shows relatively constant peat growth, except one time interval. In Core C the peat accumulation constantly increases starting from 1900, which is a clear consequence of micro climate changes in the Tinovul Apa Roșie (Red Water) peat bog.

Conclusion
Based on our research the relationship between climate and peatland distribution suggests a strong possible infl uence of future climate change on peatlands.
The higher sedimentation rate of 1940±5 is the result of a fl ood that modifi ed the growth of peat in Core A. This growth is not present in Core C due to the difference in altitude. (Core A 990m and Core C 1000m). In the Core C the warming tendency in the local micro climate is clearly visible, which corresponds to an increasing peat growth rate. The results of the last 100-150 years suggest fast and signifi cant changes in the fl ora and fauna of the geographical location, showing a pronounced degradation effect on the peatland ecosystem. Therefore, this unique geographical location deserves full scientifi c interest. The obtained high-resolution data on the Tinovul Apa Roșie (Red Water) peat bog, including the radioisotope concentrations, can serve as a regional benchmark for similar studies, and these data also showcase how peat bogs can constitute powerful tools to study atmospheric deposition.