Chemical data on ashy soils as an information basis for dating archaeological sites

Graphical abstract Image, graphical abstract


Specifications
Soil Science Specific subject area Pedoarchaeology, Geochemistry Type of data Table  Image Figure How data were acquired XRF spectrometer (Spectroscan Max-GV, 'SPECTRON', Ltd). Emission spectrometer ICPE-90 0 0 with induction-bound plasma (Zn, Cu, Mn and Co), atomic absorption spectrometer Quant-2AT (Pb and C), spectrometer Quant-Z (Hg), photocolorimeter KFK-3-01 (As). Method of cluster analysis (hierarchic classification, unification by Ward's method). STATISTICA Advanced + QC for Windows v.10 Ru. Data format Raw Parameters for data collection The data derives from our study of 14 archaeological sites in the south of the East European Plain. The chemical composition of sampled ash deposits and soils formed on ash, as well as of plant samples (fossil and modern oak and grass plants), was analysed by 22 chemical elements, including 10 macroelements and 12 trace elements.

Description of data collection
The dataset includes the information on specific features of archaeological ash in relation to three other types of principal parent materials, on chemical properties of soils formed on ash, biogeochemical properties of soil formed on ash in the region with significant proportion of oak forests both in antiquity and today, and on the values of informative geochemical indicators for ash groups of different genesis for the regional reconstruction of palaeogeographic conditions. Data source location

Value of the Data
• Geochemical differences in soils formed on ash, which is reflected in an increased concentrations of Cu, As, SiO 2 , Al 2 O 3 and Pb and decreased concentrations of Sr, Ca, Co, Mg and Na 2 O, form the information basis for dating the termination of activities at archaeological sites and their economic zones. • In regions with different sources of available fuel (wood in the forest-steppe and dung in the steppe zone), fossil ash can aid the reconstructions of paleoclimate and living conditions, using the ensemble of chemical elements (P, K, Mn, Zn, Co, Pb, Sr and Zr) as an indicator. • Comparison of the chemical composition of experimentally ashed fossil oak wood and forbcereal hay made it possible to determine the elements indicative of wood ash and of ash resulting from the combustion of other fuels available in the steppe conditions, which makes it possible to ascertain the genesis of archaeological ash deposits.

Data Description
Geochemical differences between the main parent rocks and ash and soils formed on ash (by 6 macroelements and 6 trace elements, as well as by the SiO 2 /(RO + R 2 O) ratio) reflect the specificity of ash as a parent material [1] . The hierarchical classification of ash and soils formed on ash by 12 most informative chemical elements (Sr, Ca, Co, Mg, Na, Cu, As, Si, Al, Pb, K, P) has made it possible to observe the geochemical transformation of ash in ashy deposits in the course of pedogenesis. Geochemical features of ash deposits and soils formed on ash for three regions that differ in shares of forest cover are presented in the form of input data and calculated coefficients. Oak wood was obtained from the excavations of burial mound Ak-Kaya 9, where 2325-2350 years ago, at a depth of 9.5 m, it has been used in the floor construction. As various plants and other fuels differ in their elemental composition, chemical analysis of ash is an important tool for identifying different ancient fuels [2] . In addition, experimental ashing of fossil wood ( Quercus pubescens ) and hay from steppe grasses with subsequent chemical analysis has been carried out to identify archaeological ashes obtained from various types of fuel.  1. Location of research objects (ashy soil and ash deposits) within the continental forest-steppe, Belgorod Oblast (A) and within the Plain (steppe) and Piedmont (forest-steppe) Crimea (B). Archaeological sites: Borisovka, mid-5th c. BC (1); Kalos Limen, 4th c. BC -2nd c. AD (2); former village of Saya, before 1944 (3); Chernomorskoe/Settlement S11-022, Bronze Age and Early Iron Age (4); Kunan, 2nd c. BC (5); Former village of Oirat, before 1944 (6); Vitino 4th -3rd c. BC (7); Airchi, 2nd c. BC -1st c. AD (8); Ak-Kaya, c. 225-250 AD (9); Borut-Khane, 1st c. BC -1st c. AD (10); Kermen-Kyr, 2nd -3rd c. AD (11); Zayachye, 2nd -3rd c. AD (12); Settlement of Mysovka (Mysovoe II), 4th-2nd c. BC -3rd c. AD (13); Kazantip Vostochny 1, 3rd -2nd c. BC -3rd c. AD (14). Natural areas (A): A -Typical foreststeppe; B -Southern forest-steppe; C -Steppe. Natural areas (B): A -North Crimean Lowland steppe; B -Tarkhankut elevated plain; C -Central Crimean Plain steppe; D -Kerch hilly-ridged steppe; E -Foothill forest-steppe; F -Main ridge, mountainous meadows and forests; G -Southern Coast. A screenshot of the Relief map was downloaded from the site: https://maps-for-free.com/ Table 1 Objects of study at archaeological sites from three research regions (soils formed on ash and ash from cultural strata  Fig. 1 shows location of research objects (ashy soil and ash deposits) in the continental foreststeppe, Belgorod Oblast, and the Plain (steppe) and Piedmont (forest-steppe) Crimea. Table 1 lists objects of study at archaeological sites within three research regions (soils formed on ash and ash from cultural strata). Fig. 2 provides examples of the location of ash deposits on positive and negative relief elements in archaeological landscapes. Table 2 shows geochemical features of the main parent rocks, ash and soils formed on ash. Table 3 displays chemical properties of soils of archaeological sites in the south of the Kazantip Peninsula. Table 4 shows geochemical transformation of soil formed on ash (Borisovka settlement) over 24 centuries. Table 5 comprises the chemical composition of soils and ash of different ages at archaeological sites of the North-West Crimea. Table 6 contains the chemical composition of soils and ash at archaeological sites of the Piedmont Crimea. Fig. 3 features a dendrogram for the classification of ash and soils formed on ash deposits. Table 7 shows colouring and chemical composition of ash in groups classified by cluster analysis. Table 8 reports the main geochemical indicators of group objects (soils and ash), resulted from the cluster analysis. Table 9 compares the chemical composition of oak wood ash (OWA) and ash from hay of mixed grasses (GMA), using two ranked lists. Fig. 4 shows correlation of the content of chemical elements in archaeological ash from the forest area and poorly forested foothill zone. Fig. 5 shows regression dependences in the contents of Zn, Cu, Mn, Cd, Pb, As, Hg, Mo, and Co in the ash datable to the 1st c. AD and in experimentally ashed organic samples (modern oak; fossil oak; feather grass; cow dung and horse dung).

Research objects
Objects of study (ashy soil and ash deposits) include 14 archaeological sites located within three research regions in the south of the East European Plain ( Fig. 1 , Table 1 ). Ash deposits can be found both on positive landforms (ash hills from a few meters to 10-13 m high) ( Fig. 2 [8] suggests that degree of humus enrichment with nitrogen (C:N) is high (C:N is 5-8), and medium (C:N is 8-11). and in subordinate positions, such as defensive ditches of archaeological sites ( Fig. 2 , B). The soil section made in the periphery of the Borisovka site has revealed the ashy profile to a depth of 1 m, which corresponds to the mid-5th c. BC phase of the settlement. The archaeological sites of the North-West Crimea, in whose cultural strata ash has been sampled, range in date from the Late Bronze Age (second half of the 10th c. BC) to the Early Iron Age (3rd c. AD). In addition, at the two Tatarian settlements abandoned in 1944 (former villages of Saya and Ojrat) ash pits were studied. In the foothill Crimea, among the Late Scythian settlements (Ak-Kaya, Kermen-Kyr, and the village of Zayachye), Borut-Khane stands out with the ash hills ranging in height from 0.5 to 2.0 m. The earliest phase of this site dates to the 3rd-2nd c. BC, while its termination     [9] , as well as the dating of ceramics found when was the pit laying. date does not exceed the 1st century AD. For the sake of ash identification, the purest layer of ash at a depth of 46-98 cm in the profile of the site's defensive earthwork was selected, which then has been sampled at a depth of 63-73 cm for chemical analyses. At present, the nearest oak woodlands are located at a distance of 4.3 km from the settlement of Borut-Khane.

, A)
Colours (dry and moist) were described using the Munsell system [3] . This system uses cylindrical coordinates, which hinders its use for statistical calculations [4] . The conversion of Munsell soil colour values into the RCIE redness index of the CIE-L * a * b * system based Table 7 Colour and chemical composition of ash in the groups identified by the cluster analysis.

Objects
Group А x ±S x Group В x ±S x   Table 9 Comparison of the chemical composition of oak wood ash (OWA) and ash from hay of mixed grasses (GMA) using two ranked lists. on a universal colour space in Cartesian coordinates was carried out using the formula [5] : RCIE = [1010 × a(a2 + b2)0.5]/(bL6). The values of parameters L, a, b were obtained from the Munsell/CIE-L * a * b * system conversion table [4] . RCIE values shown in Table 1 are for dry soil.

Dating of archaeological sites
The age of each site was established archaeologically (coins, amphora stamps or other narrowly datable pottery) or based on historical record [6] . Based on archaeological date of each site, the age of soils was determined. The date at which human activity at settlements ceased and pedogenesis began was controlled by the method of pedogenetic chronology, which is based on the chronosequence showing the dependence of the humus horizon thickness on soil age [7] . Ash layer from the Bronze Age and Early Iron Age site of Chernomorskoe/S11-022 (No 4) has been radiocarbon dated. A sheep/goat bone, collected for this purpose at a depth of 62 cm from the top surface, inside a 60-90 cm-thick layer of ash, has been dated to 1130-760 ВС (95.4%) (Kyiv Radiocarbon Laboratory, Ki-19342).  (2); feather grass ( Stipa capillata ) (3); cow dung (4); horse manure (5).

Comparison of main parent rocks and ash
The chemical composition of main parent rocks (carbonate loam and limestone eluvium) was compared with that of archaeological ash and soils formed on it ( Table 2 ). Concentration of macro and microelements within ash deposits and soils was determined by technique of measuring metals mass fraction and oxides in powder samples using an XRF spectrometer (Spectroscan Max-GV).
As a special type of parent material, ash differs significantly in its chemical composition from the more widely distributed rocks (loam and eluvium of carbonate rocks): it has a higher content ( > 30%) of P, Ca, Sr (as compared to loam) and P, Co, Sr, Zn, Pb, Cu (as compared to eluvium) and smaller concentrations of Cu, Pb, Fe, Ni (as compared to loam) and Ca (as compared to eluvium).

Chemical properties of soils of archaeological sites in the dry steppe
Ash pits and soils formed on ashy cultural strata were studied on the Kazantip Peninsula ( Table 3 ), which is an area completely devoid of forests due to climatic conditions (annual precipitation 329 mm). The ash accumulation in this area stops in the 3rd century AD but the life here continued also later (7th-9th centuries AD).
The titrimetric version of the determination of C org after I. V. Tyurin was carried out by oxidation of the organic substance with a solution K 2 Cr 2 O 7 in sulphuric acid in a thermostat (at Т = 140 °С )), which is accompanied by the reduction of Cr (VI) to Сг 3 + . Nitrogen total (N) was determined using the Kjeldahl method (GOST 26107-84, Last Modified: 09.12.2018) "Soils. Methods for determination of total nitrogen". In titration method, nitrogen is calculated from the amount of sulphuric acid spent for titrating ammonium borate. The data obtained for C org and N were used to assess the degree of enrichment of humus with nitrogen (C:N) according to the scale [8] . Acidimetric method used to determine the content of carbonates ( СО 2 ) in the soil is based on their destruction with hydrochloric acid solution followed by titration of its residue with sodium hydroxide solution.
As judged by regression lines in relation to complete junction, the ash from the settlement of Borut-Khane, 1st c. BC -1st c. AD ( Fig. 5 ), situated in the region which in antiquity and nowadays was short of firewood (red arrow), differs significantly from oak ash (in excess of As, Hg, Mn, Co), being most similar to the ash of feather grass (the main steppe grass) and cow dung, i.e. in composition it is close to kizyak which was probably the main fuel at that time.

Data analysis
Macro-elements and trace elements were determined by wavelength-dispersion (XRF analyser Spectroscan Max-GV). The results were quantitatively calibrated using a set of state (GOST) standard samples of soil composition. The geochemical composition of each soil sample was determined by two repeats. In case of unacceptable discrepancies between the measurements, which were detected using the spectrometer software, additional replicates were performed until an acceptable result was achieved (usually in the third repeat).
CO 2 , C org , total N (data presented in Table 3 ). C org in soils was determined by the titrimetric version of Tyurin's method with oxidation in a thermostat at 140 ˚С , СО 2 by acidometry [15] . Total nitrogen (N) was estimated by Kjeldahl's procedure. For the accuracy control, every tenth sample was measured in two repeats. In addition, after each ten measurements, a device control was carried out by measuring standard reference samples.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.

Data for Reference
Archaeological ash deposits and soils formed on ash in the south of the East European Plain (Original data) (Mendeley Data).