SURFACE GEOCHEMICAL SURVEY AT STARUNIA
PALAEONTOLOGICAL SITE AND VICINITY
(CARPATHIAN REGION, UKRAINE)
Hen ryk SECH MAN, Ma ciej J. KO TARBA & Marek DZI ENIEWICZ
Fac ulty of Ge ol ogy, Geo phys ics and En vi ron mental Pro tec tion, AGH Uni ver sity of Sci ence and Tech nol ogy, Al. Mick iewicza 30, 30- 059 Kraków, Po land, e- mail: sech man@agh.edu.pl
Sech man, H., Ko tarba, M. J. & Dzi eniewicz, M., 2009. Sur face geo chemi cal sur vey at Sta runia pa lae on to logi cal site and vi cin ity (Car pa thian re gion, Ukraine). An nales So cie ta tis Ge olo go rum Po lo niae, 79: 375–390.
Ab stract: A sur face geo chemi cal sur vey was car ried out in an aban doned ozo ke rite mine in Sta runia, where rem nants of a mam moth and three woolly rhi noc er oses and one al most com pletely pre served rhi noc eros car cass were found in 1907 and 1929. In to tal, 689 meas ure ment sites were de fined within the se lected 300×350 m rec tan gle (ca. 10.5 ha). The ana lysed soil gases vary in their mo lecu lar and iso topic com po si tions. Sev eral zones with anoma lous meth ane con cen tra tions over 10 vol% were iden ti fied in the study area. These anoma lous con cen tra tions cor re late well with to tal C2-C5 al kane anoma lies (over 1 vol%). The zones with car bon di ox ide
con cen tra tions over 15 vol% are gen er ally shifted SW of those, where the meth ane and the sum of C2-C5 al kanes
anoma lies were iden ti fied. High he lium con cen tra tions of crus tal ori gin (over 0.001 vol%) are clus tered near fault zones. In a ma jor ity of cases meth ane is of ther mo genic ori gin, mi grat ing to the near- surface zone from deep- seated ac cu mu la tions. Mi cro bial meth ane, or meth ane with a high mi cro bial com po nent, is ge neti cally re lated to re cent swamps. The high vari abil ity of con cen tra tions of gase ous com po nents to gether with the highly vari able iso topic com po si tion of in di vid ual gases in the soil, in di cate their ther mo genic and/or mi cro bial ori gin, and point to the litho logi cal di ver sity of Qua ter nary sedi ments in the study area. Sev eral zones with ther mo genic gase ous hy dro car bons and he lium were de line ated. It is likely that dur ing the Pleis to cene win ters, un der a thick ice and snow cover, the tun dra lake and swamp zones around out flows of brines, oil, he lium and ther mo genic gases had a higher tem pera ture, which re sulted in melt ing and crack ing of the ice cover. These sites would be more pro spec tive for fu ture search of well- preserved large ver te brates.
Key words: sur face geo chemi cal sur vey, sta ble car bon iso topes, meth ane ori gin, gase ous hy dro car bon C2-C5
ori gin, car bon di ox ide ori gin, woolly rhi noc eros, Sta runia, Car pa thian re gion, Ukraine. Manu script re ceived 4 May 2009, ac cepted 1 October 2009
IN TRO DUC TION
A sur face geo chemi cal sur vey was a part of an in ter dis ci pli nary re search proj ect fo cused on the study of the Sta -runia area. Dur ing the pe riod 2006–2009, com pre hen sive in ves ti ga tions were car ried out in an aban doned ozo ke rite (earth wax) mine in Sta runia (Ko tarba, 2009), about 130 kilo me tres south east of Lviv, Ukraine (Fig. 1). Rem nants of a mam moth and woolly rhi noc eros were dis cov ered in 1907 in this place. Fur ther more, one nearly com pletely pre served rhi noc eros car cass (called “sec ond woolly rhi noc eros”) and rem nants of two other woolly rhi noc er oses were found in 1929. The dis cov ery of large Pleis to cene mam mals at the Sta runia ozo ke rite mine was a spec tacu lar sci en tific event on a world scale. An unique com bi na tion of oil and brine within the Pleis to cene clayey mud, into which the ani mals had sunk, re sulted in al most per fect pres er va tion of these speci mens.
Gen eral in for ma tion on the his tory of the Sta runia dis cov er ies and the re sults of ear lier re search, in clud ing a re -con nais sance sur face geo chemi cal sur vey car ried out in the area in 2004, are dis cussed in a spe cial mono graph de voted to Sta runia (Ko tarba, 2005). Based on the re sults of the pre -limi nary sur face sur vey (Dzi eniewicz et al., 2005; Ko tarba
et al., 2005a, 2008b), a de tailed sur face geo chemi cal sur vey
was car ried out over an area of about 10.5 ha us ing a rec tan -gu lar meas ure ment grid.
The main aim of the cur rent study was to iden tify po ten tial sites within the Pleis to cene muds fa vour ing the pres er va tion and con ser va tion of speci mens of other woolly rhi -noc er oses and/or mam moths. There fore, a high- resolution sur vey of con cen tra tion and iso topic com po si tion of soil gases at a depth of about 1.2 m was un der taken. Meas ured mix ing ra tios of gase ous al kanes, al ke nes, car bon di ox ide,
Fig. 1. Sketch map of the Starunia study area (Carpathian re gion, Ukraine) show ing the lo ca tion of sur face geo chem i cal sur vey. Deep wells for oil and gas exploitation are also shown (Kotarba et al., 2005b)
hy dro gen and he lium, and re la tions be tween these com po nents, as well as sta ble car bon iso tope com po si tion of meth ane, eth ane, pro pane and car bon di ox ide, al lowed us to in -ves ti gate the ori gin and ge netic con nec tions be tween the soil gases, the natu ral gases ac cu mu lated in deep struc tures and the re cently gen er ated mi cro bial gases. Moreo ver, an at tempt was made to de ter mine the ex tent of mi gra tion of natu ral gas from deep ac cu mu la tions to the sur face, es pe -cially along the thrust and fault zones.
GEO LOG I CAL SET TING
AND PE TRO LEUM OC CUR RENCE
The Ukrain ian Car pa thi ans be long to the larg est pe tro -leum prov inces of Cen tral Europe, con sti tut ing one of the old est petroleum producing re gions in the world. Ex ploi ta tion of oil and natu ral gas be gan in 1854 and 1921, re spec -tively (Ko tarba & Kol tun, 2006). The ozo ke rite it self is a rare pe tro leum sub stance, and in the Car pa thi ans it is only found at five sites: Bo rys lav, Vol anka, Truskavets-Po-miarki, Dzviny ach, and Sta runia (Al ex androwicz, 2005). At the end of the 19th cen tury and in first half of the 20th cen -tury, ozo ke rite was ex ploited in the Sta runia area from the Mio cene salt- bearing Vo ro tyshcha beds of the Pokuttya Unit of the Car pa thian Fore deep Ba sin (Alexan-drowicz, 2004, 2005; Kol tun et al., 2005).
The top sur face of the Mio cene Vo ro tyshcha beds in Sta runia, un der ly ing Qua ter nary de pos its, oc curs at a maxi -mum depth of 17 me tres (Soko³owski et al., 2009). In the area of the aban doned Sta runia ozo ke rite mine, Qua ter nary sedi ments mainly con tain clayey muds with plant re mains, peat, bio genic muds and peat muds (Soko³owski et al., 2009; Soko³owski & Stachowicz- Rybka, 2009). Within these sedi ments, many brine and salt wa ter springs oc cur in the Sta runia vi cin ity. These brines and salt wa ters mi grated along the Mio cene Vo ro tyshcha beds. Mio cene strata con -tain sandstone- claystone brec cia with hal ite, potassium- salt, gyp sum and cal cite lay ers, and veins of ozo ke rite (Ko rin, 2005).
The Boryslav Pokuttya Unit is the main oil and gas res -er voir in the Ukrain ian Car pa thi ans. Tec toni cally, the unit rep re sents a stack of su per im posed nap pes, each of them com pris ing a flysch se quence cov ered by mo lasse strata. The Oli go cene Menil ite beds oc cur in the top of the flysch suc ces sion and are con sid ered to be the most im por tant hy -dro car bon source rock with rela tively high or ganic mat ter con tent (up to 20 wt%; Ko tarba & Kol tun, 2006). South and south west of the Sta runia ozo ke rite de posit, six oil and gas fields were dis cov ered within the Palaeo gene and Neo gene res er voirs of the Boryslav Pokuttya and Skyba units (Ada -menko et al., 2005; Kol tun et al., 2005).
De tails of ge ol ogy and pe tro leum oc cur rence in the Sta -runia area were pub lished by Al ex androwicz (2004, 2005), Kol tun et al. (2005), Ko rin (2005), Ko tarba & Rybka (2008), Ko tarba et al. (2008a), Soko³owski et al. (2009), Soko³owski & Stachowicz- Rybka (2009) and Stachowicz- Rybka et al. (2009), and in ref er ences therein.
METH ODS
Field work and sam pling pro ce dure
The sur face geo chemi cal sur vey was car ried out within a rec tan gu lar, 300×350 m area us ing regu lar, 12.5×12.5 m meas ure ment grid. In this pat tern, 725 meas ure ment sites were de ter mined, of which 689 were sam pled (Fig. 1). The spe cific free- gas sam pling method that was used (Sok olov & Grigo riev, 1962; Philp, 1987; Te de sco, 1995) was de vel -oped at the De part ment of Fos sil Fu els, AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków (Dzi eniewicz & Sech man, 2001, 2002). The tar get sam pling depth was 1.2 me tres, how ever, on site litho logi cal vari abil ity re sulted in ac -tual sam pling depths from 1.1 to 1.3 me tres.
A spe cial geo chemi cal probe was ap plied to gether with gas- tight sy ringe and sam ple ves sel. The probe was driven down to the sam pling depth, then soil at mos phere was col lected with the gas tight sy ringe. The gas sam ple was in -jected into a ves sel com pletely filled with the ana lyti cally pure satu rated so lu tion of NaCl and then trans ported to the labo ra tory. The sam pling pro ce dure en sured “ster ile” sam pling of soil gases, free of con tami na tion from the at mos -pheric air (Dzi eniewicz & Sech man, 2001, 2002; Sech man & Dzi eniewicz, 2007).
An a lyt i cal meth ods
The mo lecu lar com po si tion of gase ous hy dro car bons in soil gases was ana lysed with the FIS SONS In stru ments GC 8160 flame ionisation gas chro ma tograph un der the fol low ing con di tions: me tal lic col umn of in ter nal di ame ter 4 mil li -me tres and length 1.3 -me tre filled with ac ti vated alu mina (mesh 100/120); car rier gas flow rate 60 ml/min (he lium); pro grammed tem pera ture of ther mo stat: 90°C for 3 min; tem pera ture in crease from 90 to 200°C (30°C/min) and held at 200°C for 3 min; tem pera ture of FID de tec tor 270°C; tem pera ture of in jec tion cham ber 120°C; vol ume of the ana -lysed sam ple 1 ml.
The con cen tra tions (mix ing ra tios) of car bon di ox ide, hy dro gen and he lium were meas ured us ing a Carlo Erba In -stru ments GC 6300 gas chro ma tograph equipped with a TCD de tec tor. Car bon di ox ide was de tected un der the fol -low ing con di tions: me tal lic col umn 2.5 me tres long filled with the HAY SEP Q 80/100 mesh; car rier gas flow 30 ml/min (ar gon), tem pera ture of ther mo stat – 65°C (con -stant), tem pera ture of TCD de tec tor (base – 60°C, fila ment – 180°C). Hy dro gen and he lium were de tected un der the fol low ing con di tions: me tal lic col umn 2.5 me tres long filled with mo lecu lar sieve 5A 80/100 mesh; car rier gas flow 16 ml/min (ar gon), tem pera ture of ther mo stat – 65°C (con -stant), tem pera ture of TCD de tec tor (base – 60°C, fila ment – 180°C), vol ume of ana lysed sam ple, 2 ml (VALCO dos -age valve).
Both the FID and TCD read ings were proc essed with the Peak Sim ple soft ware. The gas chro ma tograph was cali -brated with the gas stan dards ”Scott II” from Su pelco and All tech. The pre ci sion of con cen tra tion (mix ing ra tio) meas ure ments was equal: ±0.01 ppm for hy dro car bons; ±10 ppm for hy dro gen, ±10 ppm he lium, and ±0.01 vol% for car bon di ox ide.
Sta ble car bon iso tope anal y ses were car ried out with a Finnigan Delta Plus mass spec trom e ter. The sta ble car bon iso tope data are pre sented in the d-no ta tion rel a tive to the VPDB stan dard. The an a lyt i cal pre ci sion is es ti mated to be ±0.2‰. Meth ane, eth ane, pro pane and car bon di ox ide were iso lated chromatographically for car bon iso tope anal y sis. Meth ane, eth ane and pro pane were combusted over hot cop -per ox ide (850°C) and the car bon di ox ide was in tro duced to the mass spec trom e ter.
Sta tis ti cal pro ce dure
For the mea sured con cen tra tions of gas eous hy dro car -bons, he lium, hy dro gen, car bon di ox ide and the re sults of iso to pic anal y ses [d13
C(CH4), d13C(C2H6), d13C(C3H8),
d13
C (CO2)] ba sic sta tis ti cal pa ram e ters are de ter mined.
Quali ta tive evalua tion of the meas ured meth ane, to tal al kanes C2-C5, car bon di ox ide and he lium con cen tra tions
was based on his to grams and prob abil ity plots. In each plot the bound ary be tween dis tinc tive sub sets of data was marked for each stud ied set of con cen tra tion. De ter mi na tion of con cen tra tion bounda ries was guided by visi ble changes of the slope of cu mu la tive prob abil ity curves. The de ter mined sub set lim its were taken as con tour spac ing in dis tri -bu tion maps of meth ane, to tal C2-C5 al kanes and car bon di
-ox ide con cen tra tions. Due to the in suf fi cient number of meas ure ment sites, in which he lium con cen tra tions ex -ceeded the de tec tion limit, changes in con cen tra tion of this gas are dis played as cir cles of rele vant di ame ters.
Cor re la tions be tween meth ane and to tal C2-C5 al kanes,
meth ane and he lium, to tal C2-C5 al kanes and he lium, meth
-ane and car bon di ox ide, to tal C2-C5 al kanes and car bon di
-ox ide con cen tra tions were ana lysed. Scat ter-plots were made and cor re la tion co ef fi cients R2 cal cu lated for these pairs. Di rec tional cor re la tions be tween changes of con cen -tra tions of the ana lysed com po nents were in ves ti gated.
Ta ble 1
Prin ci pal sta tis ti cal pa ram e ters of al kanes con cen tra tions and hy dro car bon in dex for soil gas sam ples
Statistical
parameters Unit CH4 C2H6 C3H8 i-C4H10 n-C4H10 neo-C5H12 i-C5H12 n-C5H12
Total C2-C5 alkanes CHC Minimum (vol%) 0.0001 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 1.6 Maximum (vol%) 82.6 2.6 0.407 0.2018 0.2515 0.0332 0.1763 0.1148 3.21 404990.6 Median (vol%) 0.0553 0.0002 0.0001 0.0001 0.0001 0.0 0.0002 0.0 0.0015 55.2 Mean (vol%) 8.06 0.0894 0.0177 0.0099 0.0066 0.0002 0.0073 0.0017 0.133 2545.5
Stand. dev. (vol%) 16.8 0.284 0.0563 0.0270 0.0273 0.0013 0.0238 0.0108 0.396 26087.7
Number of
samples * 689 652 663 622 615 404 636 371 687 685
Percentage
of samples (%) 100.0 94.6 96.2 90.3 89.3 58.6 92.3 53.8 99.7 99.4
* – num ber of sam ples with con cen tra tion of the given com po nent over de tec tion limit; stand. dev. – stan dard de vi a tion; b.d.l. – be low de tec tion limit
C CH C H C H HC= + 4 2 6 3 8 Ta ble 2
Prin ci pal sta tis ti cal pa ram e ters of alkenes, car bon di ox ide, hy dro gen and he lium con cen tra tions, and car bon di ox ide – meth ane in dex for soil gas sam ples
Statistical parameters Unit C2H4 C3H6 1-C4H8 Total C2-C4 alkenes CO2 H2 He CDMI Minimum (vol%) b.d.l. b.d.l. b.d.l. b.d.l. 0.01 b.d.l. b.d.l. 0.7 Maximum (vol%) 0.335 0.0167 0.1110 0.4627 15.52 0.066 0.0156 100.0 Median (vol%) 0.0000 0.0000 0.0000 0.0000 0.69 0.0 0.0000 90.5 Mean (vol%) 0.0061 0.0001 0.0007 0.0068 1.98 0.004 0.0005 67.1
Stand. dev. (vol%) 0.0315 0.0013 0.0065 0.0370 2.60 0.008 0.0014 37.2
Number of
samples* 420 138 145 510 689 300 126 689
Percentage of
samples (%) 61.0 20.0 21.0 74.0 100.0 43.5 18.3 100.0
* – num ber of sam ples with con cen tra tion of the given com po nent over de tec tion limit; stand. dev. – stan dard de vi a tion; b.d.l. – be low de tec tion limit
CDMI CO CO CH = + × 2 2 4 100%
These cor re la tions il lus trate the per cent age of con gru ent and non-congruent changes of the measured concentrations.
RE SULTS AND DIS CUS SION
The ana lysed soil gases vary in their mo lec u lar com po -si tion, and hy dro car bon (CHC) [CHC = CH4/(C2H6 + C3H8)]
and car bon di ox ide-meth ane (CDMI) {CDMI = [CO2/(CO2
+ CH4)] 100 (%)} gas in di ces. The mo lec u lar com po si tion
of the main ana lysed gases var ies within the fol low ing ranges (Ta bles 1, 2): CH4 from 0.0001 to 82.6 vol% (av er
-age 8.06 vol% for 689 sam ples), sum of C2-C5 al kanes from
0 to 3.21 vol% (av er age 0.133 vol%), sum of C2-C4 alkenes
from 0 to 0.463 vol% (av er age 0.0068 vol%), CO2 from
0.01 to 15.5 vol% (av er age 1.98 vol%), H2 from 0 to 0.066
vol% (av er age 0.004 vol%), He from 0 to 0.0156 vol%, hy -dro car bon (CHC) in dex from 55.1 to 404,991, and car bon
di ox ide methane (CDMI) index from 0.7 to 100.
The ana lysed soil gases are also vari able in their car bon iso tope com po si tion. Sta ble car bon iso tope ra tios vary within the fol low ing ranges (Ta ble 3): d13C(CH4) from
–84.6 to –29.9‰, d13
C(C2H6) from –29.4 to –23.6‰,
d13
C(C3H8) from –28.4 to –22.9‰, and d13C(CO2) from
–29.2 to 22.9‰.
The dis tri bu tion of meth ane con cen tra tion is multi-modal (Fig. 2A, B). Six sub sets can be dis tin guished: 0.0001 to 0.0016 vol%, 0.0016 to 0.015 vol%, 0.015 to 0.25 vol%, 0.25 to 1.78 vol%, 1.78 to 20.0 vol%, and over 20.0 vol%.
The dis tri bu tion of to tal C2-C5 al kanes con cen tra tions
is bi modal (Fig. 3A). How ever, the dis tri bu tion plot re veals more than 7 sub sets (Fig. 3B). The sub set lim its are: 0 to 0.00005 vol%, 0.00005 to 0.00032 vol%, 0.00032 to 0.01 vol.%, 0.01 to 0.1 vol%, 0.1 to 0.32 %, 0.32 to 2.0 vol%, and more than 2.0 vol%.
The spa tial dis tri bu tions of mea sured meth ane (Fig. 4) and to tal C2-C5 al kanes con cen tra tions (Fig. 5) re veal a
com plex pic ture. Anom a lous con cen tra tions of meth ane
(>0.25 vol.%) oc cupy about 30% of the mea sure ment area and form an ir reg u lar pat tern, ex tend ing from SSE to NNW (Fig. 4). The larg est ar eas of re duced meth ane con cen tra tion oc cur in the SW and NE parts of the study area. The dis tri -bu tion of the sum of C2-C5 alkane con cen tra tions (Fig. 5) is
sim i lar to that of meth ane. This is con firmed by the cor re la tion of their con cen tra tions (Fig. 6A), and also the high con -gru ence of the trends of their changes (Fig. 7A).
Sta ble car bon iso tope anal y ses of meth ane, eth ane and pro pane in soil gases (Ta ble 3) re veal that meth ane from a ma jor ity of ana lysed sam pling sites (Figs 8, 9A, 10) and eth -ane and pro p-ane from all ana lysed sam pling sites (Fig. 9B) are thermogenic in or i gin and mi grated to the near-sur face zone from deep-seated ac cu mu la tions. A pop u la tion of gas sam ples from deep ac cu mu la tions and gases from two sur -face oil seeps are also dis played in the ge netic di a grams (Figs 8, 9A, B) (Kotarba et al., 2005b). Deep gases ac cu mu -lated in both the Oligocene and Eocene res er voirs as well as
Ta ble 3
Prin ci pal sta tis ti cal pa ram e ters of sta ble car bon iso tope com po si tion of meth ane, eth ane, pro pane and car bon
di ox ide for soil gas sam ples
Statistical parameters Unit d 13 C(CH4) d13C(C2H6) d13C(C3H8) d13C (CO2) Minimum (‰) -84.6 -29.4 -28.4 -29.2 Maximum (‰) -29.9 -23.6 -22.9 22.9 Median (‰) -49.4 -26.7 -25.3 -17.3 Mean (‰) -51.2 -26.6 -25.7 -13.4 Stand. dev. (‰) 10.5 1.1 1.7 11.6 Number of samples 147 83 21 150 Percentage of samples (%) 21.3 12.0 3.0 21.8
Stand. dev. – stan dard de vi a tion
Fig. 2. (A) His to gram and (B) cu mu la tive fre quency di a gram of meth ane con cen tra tion
gases as so ci ated with two sur face oil seeps in the Starunia area were gen er ated mainly by low-tem per a ture thermo-genic pro cesses (“oil win dow”) at a ma tu rity level of 0.8 to 1.0% on the vitrinite reflectance scale un der the as sump tion that mixed type II/III kerogen occurs (Figs 8, 9A, B).
In places, where d13C-val ues of meth ane are less than –55‰ (Figs 8, 9A, 10) mi cro bial meth ane or a mix ture of mi cro bial and thermogenic meth ane oc curs. The pres ence of hy dro gen and alkenes in the soil gases also sug gests a con tri bu tion from mi cro bial pro cesses (Ta ble 2). Large quan ti ties of mi cro bial meth ane were gen er ated within the Qua ter nary sed i ments of the Starunia area, as doc u mented by de pleted (“anom a lous”) d13C-val ues in meth ane, be low –55‰ (Fig. 10). Un like the sites of he lium and thermogenic (hy dro car bons and car bon di ox ide) gases fluxes, such lo ca -tions would be less likely sites for fu ture dis cov ery of wellpre served, ex tinct mam mals be cause mi cro bial meth -ane was gen er ated lo cally, in re cent swamps.
The dis tri bu tion of he lium con cen tra tions is mono-modal, asym met ric, pos i tively skewed with anom a lous tail (Fig. 11A). From the prob a bil ity plot the fol low ing con cen -tra tion ranges were de ter mined: 0 to 0.003 vol%, 0.003 to 0.006 vol%, 0.006 to 0.008 vol% and over 0.008 vol% (Fig. 11B). Gen er ally, the in creased he lium con cen tra tions fol -low those of meth ane (Fig. 4) and higher C2-C5 al kanes
(Fig. 5). This is con firmed by the rel a tively high sim i lar ity of changes of meth ane and he lium as well as the to tal C2-C5
al kanes and he lium con cen tra tions (Fig. 7). The low cor re -la tion be tween the ana lysed com po nents (Fig. 6B, C) points to dif fer ent or i gins for al kanes and he lium. In creased he lium con cen tra tions oc cur mainly in the cen tral and north -ern parts of the study area, where he lium and thermogenic gas eous hy dro car bons mi grate along the Rinne fault and other tec tonic zones and within lithologically dif fer en ti ated Pleis to cene sed i ments. He lium is gen er ally of crustal or i gin with more than 99.2% ra dio genic 4He, but an ap pre cia ble amount of 3He is probably derived from the upper mantle (Kotarba & Nagao, 2008).
The dis tri bu tion of car bon di ox ide con cen tra tions is multimodal (Fig. 12A). The lim its of the con cen tra tion sub -sets are de ter mined from the prob a bil ity plot: 0.01 to 0.1 vol%, 0.1 to 0.22 vol%, 0.22 to 0.4 vol%, 0.4 to 2.0 vol.%, 2.0 to 6.0 vol%, and over 6.0 vol% (Fig. 12B). Changes in car bon di ox ide con cen tra tions (Fig. 13) gen er ally cor re -spond to those of meth ane (Fig. 4) and to tal C2-C5 al kanes
(Fig. 5), but de tailed pat terns are com pli cated. Con cen tra -tions of CO2 form ir reg u lar, highly frag mented fields. Con
-cen tra tions over 2 vol% clus ter in both the cen tral and southeast ern parts of the study area. The sim i lar ity of dis -tri bu tion of al kanes and car bon di ox ide con cen tra tions is con firmed by the con gru ent trends of their changes. For meth ane, the de gree of con gru ence with car bon di ox ide may reach even 70%, and in the case of to tal C2-C5 al kanes
al most 67% (Fig. 7). The pres ence of anom a lous car bon dioxide con cen tra tions at the same mea sur ing sites, where higher con cen tra tions of meth ane homo logues were de -tected, may sug gest its endogenic or i gin. How ever, the lack of cor re la tion in scat ter plots (Fig. 6D, E) between alkanes and carbon dioxide concentration suggests another source of the latter.
The sta ble car bon iso tope com po si tion of car bon di ox -ide (Figs 14, 15) sug gest a va ri ety of or i gins: thermogenic (most fre quent), re cent mi cro bial pro cesses, and sec ond ary mi cro bial ox i da tion of hydrocarbons.
The val ues of pH and chlo ride con cen tra tions in the ground wa ters are vari able (Kotarba et al., 2009). Be cause of the me tab o lism of meth ane bac te ria (methanogens), which de pends on these con di tions (Woltemate et al., 1984; Zhang & Chen, 1985), de tailed iden ti fi ca tion of sites where mi cro bial meth ane oc curs, may help to iden tify places un fa vour able to burial and pres er va tion of Pleis to cene ver te brates. Lo cally, in re cent swamps, high quan ti ties of mi cro -bial meth ane were gen er ated. The Qua ter nary sed i ments of the Starunia study area mainly con tain clayey muds with plant rem nants, peats, biogenic muds, peat muds, sands, and grav els (Soko³owski et al., 2009; Soko³owski & Stacho-wicz-Rybka, 2009). In the area of the aban doned ozokerite mine, apart from oil and gas also brines oc cur, which are ge
-Fig. 3. (A) His to gram and (B) cu mu la tive fre quency di a gram of to tal C2-C5 al kanes con cen tra tion
Fig. 5. Con tour map of to tal C2-C5 al kanes mea sured in soil gas sam ples and lo ca tion of anom a lous he lium con cen tra tions mea sured in
net i cally re lated to the un der ly ing Mio cene Vorotyshcha salt-bear ing beds (Duliñski et al., 2005; Korin, 2005). The re dox environment within the Quaternary sediments is diversified.
The dis tri bu tions of the mix ing ra tios of the ana lysed gases can be strongly af fected by a high num ber of closed and re culti vat ed shafts. The ex ist ing and re culti vat ed dug-wells and old bore holes, num ber ap prox i mately 500 (Kotarba & Stachowicz-Rybka, 2008; Kotarba, 2009) in the study area (Fig. 1). Such sites can pro vide ad di tional mi gra -tion routes for ascending gases.
CON CLU SIONS
The wide range of con cen tra tions of the ana lysed gases, the pres ence of sev eral pop u la tions of three ana lysed gas -eous com po nents (meth ane, to tal C2-C5 al kanes and car bon
di ox ide), as well as vari able car bon iso tope ra tios of meth -ane, eth -ane, pro pane and car bon di ox ide point to mul ti ple or i gins and a com plex his tory of mi gra tion of these gases to and within the nearsur face zone. In the ma jor ity of the in ves ti gated sites, meth ane is of thermogenic or i gin and it mi -grated to the near-sur face zone from deep ac cu mu la tions.
Fig. 6. Scat ter-plots of: (A) meth ane ver sus to tal C2-C5 al kanes, (B) meth ane ver sus he lium, (C) to tal C2-C5 al kanes ver sus he lium, (D)
Ei ther the mi cro bial meth ane or the thermogenic meth ane mixed with a high con tri bu tion of mi cro bial component occurs at a few measurement sites only.
Car bon di ox ide most fre quently is of thermogenic or i gin, but only some times is re cent mi cro bial or i gin or a prod -uct of sec ond ary mi cro bial ox i da tion of hydrocarbons.
High he lium con cen tra tions (over 0.001 vol%) oc cur near the fault zones and within lithologically dif fer en ti ated Pleis to cene sed i ments. He lium is gen er ally of crustal origin. Sites with anom a lous (high) con cen tra tions of meth ane and to tal C2-C5 al kanes may sug gest the ex is tence of a
Ple-istocene wa ter res er voir (”palaeoswamp”), which prob a bly de vel oped over fault zones and may have had a rect an gu lar (350 per 150 m) shape and NW–SE trend. The her biv o rous mam mals mi grat ing in search of food and wa ter might have been trapped in such places and drowned in the clayey mat -ter sat u rated with brine and oil. It seems pos si ble that dur ing the Pleis to cene tun dra win ters, when a thick ice and snow cover was rest ing on the tun dra ”palaeoswamp”, the seeps of brine, oil, he lium and thermogenic gases had a higher temperature, which re sulted in melt ing and crack ing of the ice cover. Iden ti fi ca tion of sev eral sites within the ”palaeo-swamp”, where thermogenic gas eous hy dro car bons, car bon di ox ide and he lium oc cur may lead to the dis cov ery of zones fa vour able for burial and preservation of Pleisto-cene extinct mammals.
Large quan ti ties of mi cro bial meth ane were gen er ated within the Qua ter nary sed i ments of the Starunia area. Un -like the sites of sig nif i cant fluxes of he lium and
thermo-Fig. 7. Re sults of di rec tional cor re la tions be tween mea sured com po nents
Fig. 8. Hy dro car bon in dex (CHC) ver sus d13C(CH4) for soil
gases. In cluded for com par i son nat u ral gases from deeper ac cu mu -la tions and sur face seeps from Kotarba et al. (2005b). Compo-sitional fields af ter Whiticar (1994). See Fig. 1 for lo ca tions and sam ple key of nat u ral gases from deep ac cu mu la tions and sur face seeps. Explantions of sym bols as in Fig. 9
genic gases (hy dro car bons and car bon di ox ide), such lo ca -tions would be less likely lo ca -tions for fu ture dis cov ery of well-pre served large ver te brates be cause mi cro bial meth ane was gen er ated lo cally, in re cent swamps. More over, mi cro -bial meth ane gen er ated from Pleis to cene swamps escaped to the atmosphere.
The dis tri bu tions of the mea sured mix ing ra tios of the ana lysed gases can be sig nif i cantly in flu enced by fault zones and lithologically dif fer en ti ated Pleis to cene sed i -ments as well as by nu mer ous closed and re culti vat ed shafts, ex ist ing and re culti vat ed dugwells and old bore holes. Such sites pro vide ad di tional mi gra tion routes for as -cend ing, thermogenic and microbial gases.
Ac knowl edge ments
Fi nan cial sup port from the Min is try of Sci ence and Higher Ed u ca tion (grant No. 139/UKR/2006/01) is kindly ac knowl edged. Re view com ments and sug ges tions by Andrzej Zuber of the Pol ish Geo log i cal In sti tute in Kraków and Kazimierz Ró¿añski of the AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków were very help ful. We would like to ex press our grat i tude to Geoffrey S. Ellis of the U.S. Geo log i cal Sur vey in Den ver for his crit i cal com ments, which im proved this pa per. An a lyt i cal iso tope work by Tomasz Kowalski from the AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków is grate fully ac knowl edged.
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