DISTRIBUTION AND ORIGIN OF GASEOUS HYDROCARBONS
AND CARBON DIOXIDE IN THE QUATERNARY SEDIMENTS
AT STARUNIA PALAEONTOLOGICAL SITE AND VICINITY
(CARPATHIAN REGION, UKRAINE)
Ma ciej J. KO TARBA, Hen ryk SECH MAN & Marek DZI ENIEWICZ
Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal 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: ko tarba@agh.edu.pl
Ko tarba, M. J., Sech man, H. & Dzi eniewicz, M., 2009. Dis tri bu tion and ori gin of gase ous hydrocarbons and car bon di ox ide in the Qua ter nary sedi ments 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: 403–419.
Ab stract: Near- surface gases were in ves ti gated in Sta runia, where re mains of mam moth and woolly rhi noc er oses were dis cov ered at the be gin ning of the 20th cen tury. Sam ples were col lected at 30 meas ure ment sites, from 0.8, 1.6, 2.4, 3.2 and 4.0 m depths and ana lysed for their mo lecu lar and iso topic com po si tions. Most of the meth ane and all higher gase ous hy dro car bons ac cu mu lated within the near- surface zone are of ther mo genic ori gin. The dis tri bu tion of ther mo genic meth ane and higher gase ous hy dro car bons in Qua ter nary sedi ments is vari able and, gen er ally, in creases with depth. Mi cro bial meth ane or a mix ture of mi cro bial and ther mo genic meth ane also oc curs at the near- surface zone. The pres ence of hy dro gen and al ke nes in the near- surface gases also sug gests a con tri bu tion from re cent, mi cro bial pro cesses. He lium of crus tal ori gin mi grated through the Rinne Fault and other tec tonic zones lo cated in the study area, and was sub se quently dis persed dur ing mi gra tion through the litho lo-gi cally di ver si fied Pleis to cene and Holo cene sedi ments. The high vari abil ity of the car bon di ox ide con cen tra tion and sta ble car bon iso tope com po si tion at vari ous depths in the near- surface zone is caused by its poly ge netic ori gin (ther mo genic, mi cro bial, and oxi da tion of hy dro car bons and Qua ter nary or ganic mat ter), and also by its dis so lu tion in wa ter and oil dur ing mi gra tion through litho logi cally di ver si fied Pleis to cene and Holo cene sedi -ments. Zones of ther mo genic meth ane oc cur rence within Pleis to cene sedi ments, domi nated by muds satu rated with bi tu men and brines, pro vide the most fa vour able en vi ron ment for pres er va tion of large ex tinct mam mals. It is likely that dur ing the Pleis to cene win ters, when thick ice and snow cov ered the tun dra lake and swamp, zones of out flow of brines, oils, he lium, and ther mo genic gase ous hy dro car bons had a higher tem pera ture, which could have re sulted in melt ing and crack ing of the ice cover and drown ing of large mam mals. The most fa vour able con di tions for pres er va tion of large, ex tinct mam mals proba bly still ex ists in the vi cin ity of sam pling probe sites Nos 21, 22, 23 and 36. Zones of domi nance of mi cro bial meth ane con cen tra tions are less fa vour able sites for pres er va tion of ex tinct mam mals, be cause this meth ane was gen er ated within re cent swamps and also within re cent de scend ing me te oric wa ters which in fil trated into Pleis to cene and Holo cene sedi ments. Older Pleis to cene mi cro bial gases have al ready es caped to the at mos phere.
Key words: near- surface geo chemi cal sur vey, depth gas dis tri bu tion, sta ble car bon iso topes, gase ous hy dro car bon ori gin, car bon di ox ide ori gin, Qua ter nary sedi ments, woolly rhi noc eros, Sta runia pa lae on to logi cal site, Ukrain ian Car pa thi ans.
Manu script re ceived 5 May 2009, accepted 10 September 2009
IN TRO DUC TION
At the end of the 19th and in the first half of the 20th cen tu ries, ozo ke rite (earth wax) was mined in the Sta runia area, about 130 kilo me tres south east of Lviv, Ukrane (Fig. 1), from the rocks of the Boryslav- Pokuttya Unit of the Car pa thian Fore deep Ba sin (Al ex androwicz, 2004, 2005; Kol tun et al., 2005). In 1907, re mains of a mam moth and a woolly rhi noc eros were found in No. 4 (then called “Mam -moth”) shaft of the ozo ke rite mine at depths of 12.5 and
17.6 m, re spec tively. In 1929, in a spe cial shaft sunk at the ex pense of the Pol ish Acad emy of Arts and Sci ences, a unique, al most fully pre served woolly rhi noc eros was found at 12.5 m depth, within the Pleis to cene sedi ments satu rated with oil and brine (Ko tarba, 2002; Al ex androwicz, 2004, 2005).
The near- surface geo chemi cal study down to 4.0 m depth was a part of an in ter dis ci pli nary re search proj ect fo
-Fig. 1. Sketch map of the Starunia area (Carpathian re gion, Ukraine) with the lo ca tion of sam pling sites. Lo ca tion of gas wells and seeps af ter Kotarba et al. (2005b). Oil and gas seeps: OE-U – “up per eye”; OE-L – “lower eye”
cused on the Sta runia area. Dur ing the pe riod 2006–2009, com pre hen sive in ves ti ga tions were car ried out at the Sta runia pa lae on to logi cal site (Ko tarba, 2009). Gen eral in for -ma tion cov er ing the his tory of pe tro leum ex ploi ta tion in the study area, de tails of the ge ol ogy, and the re sults of ear lier in ter dis ci pli nary re search were re ported in a spe cial mono -graph (Ko tarba, 2005) and in re cent pub li ca tions (Ko tarba
et al., 2008a; Ko tarba & Stachowicz- Rybka, 2008;
Soko-³owski et al., 2009; SokoSoko-³owski & Stachowicz- Rybka, 2009; Stachowicz- Rybka et al., 2009).
In 2004, a pre limi nary soil gas geo chemi cal sur vey was car ried out in the area of the aban doned ozo ke rite mine in Sta runia (Ko tarba et al., 2005a, 2008a), and in 2007 a de -tailed soil gas geo chemi cal sur vey (Sech man et al., 2009) was com pleted. The re sults of geo chemi cal re search to gether with com bined geoe lec tric (Moœcicki, 2009) and mi -cro grav ity sur veys (Porzucek & Madej, 2009a, b) en abled the authors to se lect the zones suit able for de tailed, deeper geo chemi cal sur vey. In to tal, 30 meas ure ment sites were se -lected for the near- surface geo chemi cal sur vey down to 4 m depth (Fig. 1).
The main aim of the geo chemi cal stud ies is to de ter -mine where the best con di tions within the Pleis to cene muds ex ist for the pres er va tion and con ser va tion of speci mens of other woolly rhi noc er oses and/or mam moths, apart from the ones dis cov ered in 1907 and 1929. There fore, the di rect ob jec tive of this study was to de ter mine the iso topic com po si tion, mo lecu lar con cen tra tion, and ori gin of gase ous al -kanes, al ke nes, car bon di ox ide, hy dro gen, and he lium in the near- surface zone at sam pling depths of 0.8, 1.6, 2.4 and 4.0 m. Links to the natu ral gases ac cu mu lated in deep struc -tures of the Boryslav- Pokuttya and the Skyba units of the Car pa thi ans and the re cently gen er ated mi cro bial gases have also been es tab lished. Moreo ver, an at tempt was made to de ter mine mi gra tion routes of natu ral gas from deep ac -cu mu la tions to the sur face, es pe cially along the thrust and fault zones, and then within litho logi cally di ver si fied Pleis -to cene and Holo cene sedi ments.
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 in five sites: Bo rys lav, Vol anka, Truskavets-Po-miarki, Dzviny ach and Sta runia (Al ex androwicz, 2005). The Sta runia ozo ke rite de posit oc curs in the Mio cene bearing Vo ro tyshcha beds of the Boryslav- Pokuttya Unit. This unit be longs to the in ter nal part of the Car pa thian Fore -deep Ba sin (Kol tun et al., 2005).
The top sur face of the Mio cene Vo ro tyshcha beds in Sta runia area, which un der lie 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 aban doned Sta runia ozo ke rite mine the Qua ter -nary sedi ments are mainly de vel oped as clayey muds with
plant re mains, peat, bio genic muds and peat muds (Soko-³owski et al., 2009; Soko(Soko-³owski & 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 and in fil trated to Qua ter nary sedi ments. Mio cene strata are rep -re sented by sandstone- claystone brec cias 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 con tains the main oil and gas res er voirs 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 (Œl¹czka et al., 2006) 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 Pokuttya and Skyba units (Ada menko et al., 2005; Kol tun et
al., 2005). The Mio cene Vo ro tyshcha salt bearing beds pro
-vide a per fect seal for hy dro car bon traps. How ever, these beds un der went con sid er able frac tur ing in the course of for ma tion of the Sta runia fold, dur ing the Car pa thian over -thrust move ments. As oil and gas were flow ing from the flysch strata to wards the sur face and most of gase ous hy dro car bons were emit ted to the at mos phere, liq uid hy dro car bons satu rated Qua ter nary sedi ments, and higher hy dro car -bons formed veins of ozo ke rite. These pro cesses con tinue even now, which is evi denced by oil and gas sur face seeps and mud vol ca noes in the Sta runia area.
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 near- surface gas sam ples were col lected with a meas ure ment probe and an air- tight sy ringe. The free- gas sam pling method was ap plied (Dzi eniewicz & Sech man, 2001, 2002). In com pari son with other meth ods, this pro ce -dure re flects the cur rent dy nam ics of mi gra tion of gase ous com po nents (Sok olov & Grigo riev, 1962; Philp, 1987; Te de sco, 1995). The de vice was sub se quently modi fied to en -able col lec tion of sam ples from greater depths. A probe of 7 mm ex ter nal di ame ter, 3 mm in ter nal di ame ter and 4.5 m length was used. This probe was driven into the surface lay ers with a ham mer, and gas sam ples were col -lected at the depths of 0.8, 1.6, 2.4, 3.2 and 4.0 m. In to tal, 150 sam ples of near- surface gases were taken from 30 meas ure ment sites. The vol ume of a sin gle sam ple ranged from 25 to 100 ml, de pend ing on the li thol ogy of the soils. Col lected gas sam ples were trans ferred with an air tight sy
ringe to bot tles filled with a satu rated NaCl so lu tion. Sam ples were then trans ported to the AGH Uni ver sity of Sci -ence and Tech nol ogy in Kraków, Po land, for analy ses.
An a lyt i cal meth ods
The mo lecu lar com po si tion in near surface gase ous hy -dro car bons was ana lysed with a FIS SONS In stru ments GC 8160 flame- ionisation gas chro ma tograph. The pres ence and con cen tra tions of car bon di ox ide, hy dro gen, he lium, oxy gen and ni tro gen were meas ured us ing a Carlo Erba In -stru ments GC 6300 gas chro ma tograph. De tails of the gas
chro ma tographic pro ce dure were de scribed in Sech man et
al. (2009). The de tec tion limit was equal: hy dro car bons ±0.01 ppm; hy dro gen ±10 ppm, he lium ±10 ppm; car bon dioxide ±0.01 vol%, oxy gen ±0.01 vol%, ni tro gen ±0.01 vol%.
Sta ble car bon iso tope anal y ses were car ried out on a Finnigan Delta Plus mass spec trom e ter. Sta ble car bon iso -tope data are pre sented in d-no ta tion rel a tive to the PDB stan dard. The an a lyt i cal pre ci sion is es ti mated to be about ±0.2 ‰. Meth ane, eth ane, pro pane and car bon di ox ide were iso lated chromatographically for sta ble car bon iso tope anal -y ses. Meth ane, eth ane and pro pane were, in turn, combusted
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 near-sur face gas sam ples col lected down to 4 m depth
Statistical
parameters Unit CH4 C2H6 C3H8 i-C4H10 n-C4H10 neo- C5H12 i-C5H12 n-C5H12
Total C2-C5 alkanes CHC Minimum (vol%) 0.0003 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 2.6 Maximum (vol%) 92.2 2.25 0.40 0.19 0.26 0.0023 0.19 0.12 2.93 94250 Median (vol%) 0.047 0.0004 0.0003 0.0002 0.0002 <0.0000 0.0005 0.0001 0.0036 44.6 Mean (vol%) 9.91 0.11 0.031 0.014 0.017 0.0002 0.014 0.0063 0.19 1168 Standard deviation (vol%) 21.2 0.34 0.087 0.036 0.051 0.0005 0.038 0.021 0.53 8003 Number of samples * 150 146 147 146 137 72 136 123 150 150 Percentage of samples (%) 100.0 97.3 98.0 97.3 91.3 48.0 90.7 82.0 100.0 100
* – num ber of sam ples with con cen tra tion of com po nent above de tec tion limit; b.d.l. – be low de tec tion limit
C CH C H C H HC b = + 4 2 6 3 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 near-sur face gas sam ples col lected down to 4 m depth
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. 0.108 b.d.l. b.d.l. 1.8 Maximum (vol%) 0.049 0.0001 0.0008 0.049 15.8 0.063 0.0048 99.8 Median (vol%) <0.0000 <0.0000 <0.0000 <0.0000 0.24 <0.000 <0.0000 84.8 Mean (vol%) 0.0017 <0.0000 <0.0000 0.0017 1.26 0.005 0.0003 63.6 Standard deviation (vol%) 0.0072 0.00001 0.00009 0.0071 2.56 0.010 0.0008 38.0 Number of samples* 98 25 52 118 150 62 20 150 Percentage of samples (%) 65.3 16.7 34.7 78.7 100 41.3 13.3 100
* – num ber of sam ples with con cen tra tion of com po nent above de tec tion limit; b.d.l. – be low de tec tion limit
CDMI CO CO CH = + × 2 2 4 100%
over hot cop per ox ide (850°C) and the car bon di ox ide pro -duced with the on-line sys tem was in tro -duced to the mass spec trom e ter.
RE SULTS
The ana lysed near-sur face gases col lected from 0.8, 1.6, 2.4, 3.2 and 4.0 m sam pling depths are vari able in their mo lec u lar and iso to pic com po si tions. Mo lec u lar and iso to -pic com po si tions, 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 of ana -lysed gases vary within the fol low ing ranges (Ta bles 1 to 3): CH4 from 0.0003 vol% to 92.2 vol% (mean 9.91 vol%), to -tal C2-C5 al kanes from 0 to 2.93 vol% (mean 0.191 vol%), of which C2H6 from 0 to 2.25 vol% (mean 0.108 vol%), C3H8 from 0 to 0.403 vol% (mean 0.031 vol%), i-C4H10 from 0 to 0.192 vol% (mean 0.014 vol%), n-C4H10 from 0 to 0.262 vol% (mean 0.017 vol%), neo-C5H12 from 0 to 0.0023 vol% (mean 0.0002 vol%), i-C5H12 from 0 to 0.187 vol% (mean 0.014 vol%) and n-C5H12 from 0 to 0.119 vol% (mean 0.0063 vol%), to tal C2-C4 alkenes from 0 to 0.049 vol% (av er age 0.0017 vol%), CO2 from 0.108 to 15.8 vol% (mean 1.26 vol%), H2 from 0 to 0.0627 vol% (mean 0.0051 vol%), He from 0 to 0.0048 vol% (mean 0.0003 vol%), hy dro car bon (CHC) in dex from 2.6 to 94,252 (mean 1,168), car bon di ox ide meth ane (CDMI) in dex from 1.8 to 99.8 (mean 63.8), d13 C(CH4) from –64.4 to –29.1‰, d13 C(C2H6) from –29.3 to –23.7‰, d13C(C3H8) from –28.2 to –22.7‰ and d13 C(CO2) from –20.1 to 24.3‰.
A his to gram of meth ane con cen tra tion re veals a polymodal pat tern (Fig. 2A). Sev eral sub sets can be dis tin -guished with bound ary val ues con firmed by the cu mu la tive prob a bil ity plot (Fig. 2B). These are: <0.0013 vol%, 0.0013 to 0.004 vol%, 0.004 to 0.025 vol%, 0.025 to 0.1 vol%, 0.1 to 5 vol%, 5 to 20.0 vol%, and over 20.0 vol%.
The same sta tis ti cal pro ce dure re veals the fol low ing bound ary val ues for to tal C2-C5 al kanes: 0 to 0.000028 vol%, 0.000028 to 0.00025 vol%, 0.00025 to 0.0028 vol%, 0.0028 to 0.035 vol%, 0.035 to 2.5 vol% and over 2.5 vol%, and for car bon di ox ide: <0.23 vol%, 0.23 to 0.35 vol%, 0.35 to 0.63 vol%, 0.63 to 3.2 vol%, 3.2 to 3.7 vol% and over 3.7 vol%, re spec tively.
DIS CUS SION
The oc cur rence and or i gin of gas eous hy dro car bons, hy dro gen, he lium and car bon di ox ide as well as the in flu -ence of mix ing and sec ond ary pro cesses on mo lec u lar and sta ble car bon iso tope com po si tions are discussed below.
Hy dro car bons
For the iden ti fi ca tion of the or i gin of ana lysed hy dro -car bon gases, the ge netic di a grams (Figs 3, 4) de scribed by
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 near-sur face gas sam ples col lected down to 4 m depth Statistical parameters Unit d 13C(CH 4) d13C(C2H6) d13C(C3H8) d13C (CO2) Minimum (‰) -64.4 -29.3 -28.2 -20.1 Maximum (‰) -29.1 -23.7 -22.7 24.3 Median (‰) -46.5 -27.4 -27.6 -15.7 Mean (‰) -46.8 -27.3 -27.0 -8.0 Standard deviation (‰) 8.2 1.2 1.6 13.9 Number of samples 63 25 15 63 Percentage of samples (%) 42.0 16.7 10.0 42.0
Fig. 2. (A) His to gram and (B) cu mu la tive fre quency di a gram of meth ane con cen tra tions
Berner and Faber (1996), Schoell (1988), Whiticar (1994), and Whiticar et al. (1986) were used. An im por tant im pli ca -tion for the in ter pre ta -tion is that a lin ear re la -tion ship of meth ane, eth ane and pro pane car bon iso topes (Fig. 5) with their re cip ro cal car bon num ber is not an ex clu sive in di ca tor of a nat u ral gas from a sin gle source as some times as sumed (e.g., Chung et al., 1988; Rooney et al., 1995). Zou et al. (2007) and Kotarba et al. (2009a) sug gest that a “dog leg” trend, ex em pli fied by rel a tively 13C de pleted meth ane and en riched pro pane com pared to eth ane, in this type of plot is a re sult of a nat u ral gas that was not gen er ated from a sin gle source rock or that un der went post-gen er a tion al ter ation (e.g., sec ond ary gas crack ing, mi cro bial ox i da tion, thermo-chem i cal sul phate re duc tion). More over, rel a tively more and more 13C de pleted meth ane to eth ane can be ap plied to eval u a tion of mix ing pro por tion be tween mi cro bial meth -ane and thermogenic gases (Kotarba & Lewan, 2004; Kotarba et al., 2009a).
Sta ble car bon iso tope anal y ses of meth ane, eth ane and pro pane from the near-sur face zone (Ta ble 3 and Figs 3, 4) re veal that both eth ane and pro pane, at all ana lysed sam -pling sites and at dif fer ent depths, and meth ane at ma jor ity of these sites are of thermogenic or i gin and mi grated to the near-sur face zone from deep-seated ac cu mu la tions (Fig. 5). Pop u la tions of gases from deep ac cu mu la tions and gases from two oil seeps (Kotarba et al., 2005b) are also dis played in the ge netic di a grams (Figs 3–5). It ap pears that deep gases, which have ac cu mu lated in Oligocene and Eocene res er voirs, as well as gases from oil seeps, were gen er ated by both low- and high-tem per a ture thermogenic pro cesses
Fig. 3. Hy dro car bon in dex (CHC) ver sus d13C(CH4) for
near-sur face gases at the se lected sam pling depths. Compositional fields af ter Whiticar (1994). Shaded area rep re sents gas from deep ac cu mu la tions and sur face seeps at the Starunia palaeontological site and sur round ing area af ter Kotarba et al. (2005b)
Fig. 4. d13C(C2H6) ver sus (A) d13C(CH4) and (B) d13C(C3H8) for near-sur face gases at the se lected sam pling depths. Po si tion of
vitrinite reflectance curves for Type II and III kerogens af ter Berner and Faber (1996). Curves were shifted based on av er age val ues of d13C = –27.1 ‰ for type II, IIS and I/II kerogens, and d13C val ues = –25.0 ‰ for type III kerogen from the Menilite Shales in the
Carpathians (Curtis et al., 2004). Shaded area rep re sents gas from deep ac cu mu la tions and seeps at the Starunia palaeontological site and sur round ing area af ter Kotarba et al. (2005b)
(“oil win dow”) (Kotarba et al., 2005b) at the ma tu rity level 0.7 to 0.9% on vitrinite reflectance scale, based on the as -sump tion that mixed type II/III kerogen is present.
In the near-sur face gases, prac ti cally all of the eth ane,
pro pane, and meth ane with d13
C-val ues higher than –45‰ (Figs 3–5, 6C) are of thermogenic or i gin. It is ap par ent (Fig. 6C), that the dis tri bu tion of thermogenic meth ane in Qua ter -nary sed i ments is vari able and gen er ally in creases with depth. At the sites where d13
C-val ues of meth ane are lower than –45‰ (Figs 3–5, 6C, 7) mi cro bial meth ane or a mix ture of mi cro bial and thermogenic methanes oc cur. Mi cro -bial meth ane was gen er ated within Qua ter nary sed i ments of the Starunia area, as doc u mented by anom a lous d13
C-val ues of meth ane be low –55‰ (Figs 3, 7). The near-sur face gases sam pled at dif fer ent depths at sites Nos 1, 3, 14, 17, 20, 30, 31 and 35 (Figs 3, 7) re veal high mi cro bial com po nent. Sometimes, an iso to pic shift to wards heavier d13
C val ues (Fig. 3) can be caused not only by ge netic ef fects but also by sec ond ary ox i da tion of mi cro bial and/or thermogenic meth -ane (Whiticar & Faber, 1986; Woltemate et al., 1984). Some in ac cu ra cies in the clas si fi ca tion of the ana lysed gases in ge netic di a grams (Figs 3, 4) can be caused by the in flu ence of var i ous sec ond ary pro cesses (ox i da tion, biode-gra da tion, etc.) on mo lec u lar and/or iso tope com po si tions (Ta ble 3).
Wide ranges of meth ane and to tal C2-C5 al kanes con -cen tra tions of the ana lysed gases and con sid er ably higher mean val ues com pared to me di ans for mea sured hy dro car -bon con cen tra tions (Tab. 4, Figs 6A, B, 8A, B) in di cate the pres ence of a large num ber of anom a lous val ues in the 4 m sam ple depth in ter val. The di ver sity of changes in hy dro car bon con cen tra tions at dif fer ent depths points to the ani so -tropy of the near-sur face zone. The re main ing re la tion ships, apart from ge netic and sec ond ary pro cesses, may sug gest the in flu ence of other fac tors: lithological vari abil ity of Qua ter nary sed i ments (Soko³owski et al., 2009), their sat u ra tion with bi tu men (Kotarba et al., 2009b), as well as mois -ture con tent and the ac tion of mi cro or gan isms (Barabasz et
al., 2009). Fig. 5. Sta ble car bon iso tope com po si tion of meth ane, eth ane
and pro pane from near-sur face gas sam ples col lected at sites Nos 4, 5, 12, 14, 17, 18, 21, 22, 23 and 24, from 0.8, 1.6, 3.2 and 4.0 m depths. Struc ture of the graph af ter Rooney et al. (1995). Shaded area rep re sents gas from deep ac cu mu la tions and seeps at the Starunia palaeontological site and sur round ing area af ter Kotarba et al. (2005b)
Fig. 6. (A) Changes of meth ane con cen tra tions, (B) dis tri bu tion of meth ane con cen tra tions dis played in the form of box plots, and (C) its sta ble car bon iso tope com po si tion at se lected sam pling depths. Blue num bers in di cate lower and up per quartiles, while black num ber cor re sponds to me dian
Tak ing into ac count the mi gra tion of gases from deep ac cu mu la tions to wards the sur face, a gen eral in crease in con cen tra tion with depth should be ex pected (in creas ing dis per sion to wards the sur face). For meth ane, the high est interquartile range, an in di ca tor of the vari abil ity of an an a -lyzed pa ram e ter, was found at a sam pling depth of 1.6 m (Fig. 6B). This im plies that at this par tic u lar depth meth ane is de rived from a mix ture of thermogenic and mi cro bial
sources. For to tal C2-C5 al kanes the high est inter-quartile range was ob served at a sam pling depth of 4.0 m (Fig. 8B). The val ues of the con tour lines in the maps of the dis tri bu -tion of meth ane and to tal C2-C5 alkane con cen tra tions for dif fer ent sam pling depths (Figs 9, 10) were de ter mined based on a sta tis ti cal pro ce dure. For ex am ple, a his to gram and cu mu la tive fre quency dis tri bu tion for meth ane are pre -sented in Fig. 2. The lat eral dis tri bu tion of meth ane and to tal
C2-C5 alkane con cen tra tions shows a com pli cated pat tern. Qual i ta tively, ar eas of in creased meth ane con cen tra tion cor -re spond to those of in c-reased to tal C2-C5 al kanes (Figs 9, 10). This is par tic u larly vis i ble for the sam pling depth of 4.0 m (Figs 9E, 10E). Such relationships indicate that most of the gaseous hydrocarbons originate from deep sources.
Hy dro gen
Hy dro gen con cen tra tions in the near-sur face gases vary from 0.00 to 0.063 vol% (Ta bles 2 and 5). Nat u ral hy dro gen is gen er ated by var i ous biogenic and abiogenic pro cesses: mi cro bial fer men ta tion of sed i men tary or ganic mat ter, mi -cro bial car bon di ox ide re duc tion, ther mal de com po si tion of sed i men tary or ganic mat ter, hy dro ly sis, wa ter radiolysis (dis so ci a tion of wa ter mol e cules bom barded by al pha par ti -cles), and nat u ral nu clear re ac tions (Zobell, 1947; Zinger, 1962; Hawkes, 1972; Dubessy et al., 1988; Whiticar et al., 1986; Savary & Pagel, 1997). Hy dro gen is a very re ac tive and mo bile gas, so its re ten tion in pe tro leum traps and in
sed i men tary rocks is rather ephem eral. Thus, its pres ence in nat u ral gases in di cates that it is ei ther re cently gen er ated in sec ond ary re ac tions within the res er voir and/or in the adjacent source beds, or it is ascending from deep-seated sources (Hunt, 1996).
In Pleis to cene and Ho lo cene sed i ments con sid er able quan ti ties of methanogenic and methylotrophic bac te ria were found (Barabasz et al., 2009). There fore, it seems very prob a ble that the pres ence of hy dro gen within the nearsur face zone of the Starunia area is re lated to mi cro bial pro -cesses. The pres ence of alkenes in the near-sur face gases (Ta bles 2 and 4) also sug gests a con tri bu tion from microbial reactions.
He lium
He lium con cen tra tions vary from 0.00 to 0.0048 vol% (Ta ble 5). Gen er ally, in creased he lium con cen tra tions fol -low those of meth ane (Fig. 6) and higher C2-C5 al kanes (Fig. 7), and in most cases its el e vated (“anom a lous”) con
-Ta ble 4
Prin ci pal sta tis ti cal pa ram e ters of meth ane, al kanes and alkenes con cen tra tions of 150 near-sur face gas sam ples col lected from the depths: 0.8, 1.6, 2.4, 3.2 and 4 m
Depth (m)
Methane (vol %) Total C2-C5 alkanes (vol%) Total C2-C4 alkenes (vol%)
Maximum Mean Median Maximum Mean Median Maximum Mean Median
0.8 68.6 6.95 0.024 2.93 0.18 0.0026 0.012 0.0008 <0.0000
1.6 77.1 17.0 0.59 2.41 0.29 0.0058 0.046 0.0032 <0.0000
2.4 28.3 1.14 0.0079 0.088 0.0091 0.0008 0.0008 0.0001 <0.0000
3.2 76.4 9.18 0.036 2.15 0.14 0.0024 0.045 0.0015 <0.0000
4.0 92.2 15.3 0.044 2.85 0.34 0.0099 0.049 0.0028 <0.0000
Fig. 8. (A) Changes of to tal C2-C5 al kanes con cen tra tions and (B) dis tri bu tion of to tal C2-C5 al kanes con cen tra tions dis played in the
cen tra tion is con nected with the thermogenic gases (Fig. 11). He lium is gen er ally of crustal or i gin, and only an in sig -nif i cant amount is de rived prob a bly from the up per man tle (Kotarba & Nagao, 2008). He lium and thermogenic gas -eous hy dro car bons mi grated up wards along the Rinne Fault and other tec tonic zones in the study area, and then within lithologically di ver si fied Pleis to cene and Ho lo cene sed i -ments (Sechman et al., 2009).
Car bon di ox ide
Car bon di ox ide which oc curs in the nat u ral gases of deep ac cu mu la tions and “up per oil eye” (OE-U) seep (Fig. 11) was gen er ated by thermogenic pro cesses (Kotarba et al., 2005b). How ever, car bon di ox ide from the “lower oil eye” (OE-L) sur face seep and from var i ous sam pling depths of the near-sur face zone is iso to pi cally very heavy [d13
C(CO2)
up to 24.3‰] (Ta ble 3, Fig. 11). This iso to pic ef fect is most prob a bly con nected with mi cro bial pro cesses and/or iso tope frac tion ation caused by dis so lu tion of car bon di ox ide in wa -ter and oil dur ing mi gra tion. The high vari abil ity of sta ble car bon iso tope com po si tion of car bon di ox ide at var i ous depths in the near-sur face zone (Figs 12, 13C) could be caused by the fol low ing fac tors: sep a rate oc cur rence of thermogenic and mi cro bial car bon di ox ide, mix ing of these
gases, sec ond ary ox i da tion of hy dro car bons, as well as car -bon di ox ide dis so lu tion in wa ter and oil dur ing mi gra tion through lithologically di ver si fied Pleis to cene and Ho lo cene sed i ments.
The con tour map of car bon di ox ide con cen tra tion for dif fer ent depths (Fig. 14) is based on the same sta tis ti cal pro ce dure as the con tour maps for meth ane and to tal C2-C5 al kanes (Figs 7, 9).
Fig. 10. Con tour maps of to tal C2-C5 al kanes con cen tra tion at the near-sur face zone at sam pling depths: (A) 0.8, (B) 1.6, (C) 2.4, (D)
The ver ti cal dis tri bu tion of car bon di ox ide con cen tra tion (Ta ble 5, Figs 13, 14) shows re la tion ships that are sim i -lar to those for meth ane (Figs 4, 6) and to tal C2-C5 al kanes (Figs 5, 7) con cen tra tions. A dis tinct in crease in car bon di -ox ide con cen tra tion was iden ti fied at sam pling depth 1.6 m (Figs 13A, B, 14B). These com pli cated depth vari a tions in con cen tra tion and sta ble iso tope com po si tion of car bon di ox ide also in di cate both the mul ti ple or i gins of this com po -nent of the ana lysed gases and the influence of secondary processes.
Or i gin of near-sur face gases and sites of pres er va tion of ex tinct large mam mals
Zones of higher thermogenic meth ane con cen tra tion to -gether with Pleis to cene sed i ments dom i nated by clayey muds highly sat u rated with bi tu men and brines (Kotarba et
al., 2009b) seem to be the most fa vour able en vi ron ment for
pres er va tion of large mam mal fos sils.
It is likely that dur ing the Pleis to cene tun dra win ters, when thick ice and snow cov ered the tun dra ”palaeoswamp”, the ar eas where the out flows of brines, oils, he -lium, and thermogenic gas eous hy dro car bons took place were char ac ter ized by higher tem per a tures, which re sulted in melt ing and crack ing of ice and snow and drown ing of large mam mals (Kotarba et al., 2008b).
Chlo ride ion con tent in the Qua ter nary sed i ments is also vari able (Kotarba et al., 2009b). Be cause the me tab o lism of meth ane bac te ria (methanogens) de pends on these fac tors (Woltemate et al., 1984; Zhang & Chen, 1985), de tailed iden ti fi ca tion of such sites may help to iden tify the sites un fa vour able for burial and pres er va tion of Pleis to cene ver te -brate fos sils. Zones of mi cro bial meth ane’s oc cur rence within Pleis to cene sed i ments would be less prom is ing sites for fu ture dis cov ery of wellpre served, large, ex tinct mam -mals. The older Pleis to cene and Ho lo cene mi cro bial gases have al ready es caped to the at mo sphere. Mi cro bial meth ane re cently de tected in the nearsur face zone was gen er ated lo -cally, in re cent swamps and in re cent de scend ing me te oric wa ters which in fil trated Pleis to cene and Ho lo cene sed i -ments. Mo lec u lar and iso to pic com po si tions of ana lysed gases ac cu mu lated in the Qua ter nary sed i ments in the near-sur face zone en abled the de ter mi na tion of the area of large mam mals pros pect ing (yel low zone in Fig. 1). Bear ing in mind the lithological cri te ria, where the to tal thick ness of Pleis to cene muds ex ceeds 2 me ters (Soko³owski et al., 2009), the most fa vour able con di tions for pres er va tion of large, extinct mammals still exist in the vicinity of sampling probe sites Nos 21, 22, 23 and 36 (Fig. 1).
How ever, it must be em pha sized that spa tial dis tri bu -tions of con cen tra -tions 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, which num ber ap prox i mately 500 in the study area (Kotarba
Ta ble 5
Prin ci pal sta tis ti cal pa ram e ters of car bon di ox ide, hy dro gen and he lium con cen tra tions mea sured in 150 near-sur face gas sam ples col lected from the depths: 0.8, 1.6, 2.4, 3.2 and 4 m
Depth (m)
Carbon dioxide (vol%) Hydrogen (vol%) Helium (vol%)
Maximum Mean Median Maximum Mean Median Maximum Mean Median 0.8 7.99 1.02 0.24 0.016 0.0033 <0.000 0.0048 0.0004 <0.000
1.6 15.8 2.49 0.47 0.017 0.0044 0.003 0.0040 0.0004 <0.000
2.4 4.65 0.41 0.22 0.053 0.0037 <0.000 0.0005 <0.0000 <0.000 3.2 12.0 1.18 0.23 0.063 0.0068 <0.000 0.0024 0.0002 <0.000 4.0 11.8 1.73 0.28 0.056 0.0074 <0.000 0.0033 0.0003 <0.000
Fig. 11. d13C(CH4) ver sus d13C(CO2) for near-sur face gases
from five sam pling depths be tween 0.8 and 4.0 m. Compositional fields mod i fied from Gutsalo and Plotnikov (1981) and Kotarba (1988). Shaded area rep re sents gas from deep ac cu mu la tions, “up -per oil eye” (OE-U) and “lower oil eye” (OE-L) seeps at the Starunia palaeontological site and the sur round ing area af ter Kotarba et al. (2005b)
& Stachowicz-Rybka, 2008; Kotarba, 2009). Such sites can be lo cated at ad di tional mi gra tion routes for as cend ing gases as well as de scend ing me te oric waters and microbial gases.
CON CLU SIONS
The re sults of mo lec u lar and iso to pic anal y ses of near-sur face gases down to 4.0 m of depth in the Starunia
area were used for es tab lish ing the most fa vour able zone for pres er va tion of large, ex tinct mam mals within the Pleis to -cene sediments.
Highly vari able con cen tra tions of meth ane (up to 92.2 vol%), to tal C2-C5 al kanes (up to 2.93 vol%) and car bon dioxide (up to 15.8 vol%), iden ti fied at dif fer ent depths, in di cate a di ver si fied or i gin of the ana lysed gases and the in -flu ence of sec ond ary pro cesses (ox i da tion, biodegradation, etc.). A ma jor part of the meth ane and all of the higher gas -eous hy dro car bons ac cu mu lated within the near-sur face
zone are of thermogenic or i gin. The dis tri bu tion of thermogenic meth ane and higher gas eous hy dro car bons in Qua ter -nary sed i ments var ies and, gen er ally, in creases with depth. He lium of crustal or i gin mi grated along the Rinne Fault and through other tec tonic zones in the study area, and was sub -se quently dis per-sed dur ing mi gra tion through the litholo-gically di ver si fied Pleis to cene and Ho lo cene sed i ments. Car bon di ox ide has a poly gen etic or i gin: thermogenic, mi -cro bial, and gen er ated by ox i da tion of mi -cro bial meth ane and/or thermogenic hy dro car bons. Apart from ge netic fac -tors, the high vari abil ity of car bon di ox ide con cen tra tion ob served at var i ous depths was caused by its dis so lu tion in water and oil during migration through litholog i cally diver-sified Pleistocene and Holocene sediments.
Zones of thermogenic meth ane ac cu mu la tions within Pleis to cene sed i ments dom i nated by clayey muds highly sat u rated with bi tu men and brines are the most fa vour able pres er va tion sites for large, ex tinct mam mals. It is likely that dur ing the Pleis to cene tun dra win ters thick ice and snow cov ered the tun dra lake and swamp ar eas. Only in lo -ca tions, where out flows of brines, oils, he lium and thermogenic gas eous hy dro car bons took place, due to higher tem -per a ture, would melt ing and crack ing of ice and snow and drown ing of large mam mals pos si bly have oc curred. Bear -ing in mind the geo chem i cal and lithological cri te ria, the most fa vour able con di tions for the pres er va tion of large, ex -tinct mam mals still ex ist in the vicinity of sampling probe sites Nos 21, 22, 23 and 36.
Zones of the oc cur rence of mi cro bial meth ane within Pleis to cene sed i ments would be less fa vour able for fu ture dis cov ery of well-pre served, large, ex tinct mam mals be -cause this meth ane was gen er ated within the re cent swamps and also in re cent de scend ing me te oric wa ters, which in fil trated Pleis to cene and Ho lo cene sed i ments. The older Pleis -to cene and Ho lo cene mi cro bial gases have al ready es caped to the at mo sphere. More over, dis tri bu tion of the ana lysed
gases within the nearsur face zone can be sig nif i cantly in flu enced not only by fault zones and lithologically di ver si -fied Pleis to cene sed i ments, but also by nu mer ous, closed and re culti vat ed shafts, by the ex ist ing and re culti vat ed dug-wells and by old bore holes. Such sites can de ter mine ad di tional mi gra tion routes for as cend ing gases, de scend ing me te oric wa ters and mi cro bial gases, and the sec ond ary processes taking place then may negatively affect the preservation of organic matter.
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 and Jan Golonka of the AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków for their 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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